Absorption and attenuation of visible and near-infrared light in water: dependence on temperature and salinity

The apparent quantum yield matrix (AQY-M) of CDOM photobleaching in estuarine, coastal, and oceanic surface waters

The photochemical degradation of chromophoric dissolved organic matter (CDOM) upon solar exposure, known as photobleaching, can significantly alter the optical properties of the surface ocean. By leading to the breakdown of UV- and visible-radiation-absorbing moieties within dissolved organic matter, photobleaching regulates solar heating, the vertical distribution of photochemical processes, and UV exposure and light availability to the biota in surface waters. Despite its biogeochemical and ecological relevance, this sink of CDOM remains poorly quantified. Efforts to quantify photobleaching globally have long been hampered by the inherent challenge of determining representative apparent quantum yields (AQYs) for this process, and by the resulting lack of understanding of their variability in natural waters. Measuring photobleaching AQY is made challenging by the need to determine AQY matrices (AQY-M) that capture the dual spectral dependency of this process (i.e., magnitude varies with both excitation wavelength and response wavelength). A new experimental approach now greatly facilitates the quantification of AQY-M for natural waters, and can help address this problem. Here, we conducted controlled photochemical experiments and applied this new approach to determine the AQY-M of 27 contrasting water samples collected globally along the land-ocean aquatic continuum (i.e., rivers, estuaries, coastal ocean, and open ocean). The experiments and analyses revealed considerable variability in the magnitude and spectral characteristics of the AQY-M among samples, with strong dependencies on CDOM composition/origin (as indicated by the CDOM 275-295-nm spectral slope coefficient, S275-295), solar exposure duration, and water temperature. The experimental data facilitated the development and validation of a statistical model capable of accurately predicting the AQY-M from three simple predictor variables: 1) S275-295, 2) water temperature, and 3) a standardized measure of solar exposure. The model will help constrain the variability of the AQY-M when modeling photobleaching rates on regional and global scales.

Photobiomodulation for knee osteoarthritis: a model-based dosimetry study

LED-based photobiomodulation (LED-PBM) for the treatment of knee osteoarthritis (KOA) is a promising technology. However, the light dose at the targeted tissue, which dominates the phototherapy effectiveness, is difficult to measure. This paper studied the dosimetric issues in the phototherapy of KOA by developing an optical model of the knee and performing Monte Carlo (MC) simulation. The model was validated by the tissue phantom and knee experiments. In the study, we investigated the effect of luminous characteristics of the light source, such as divergence angle, wavelength and irradiation position, on the treatment doses for PBM. The result showed that the divergence angle and the wavelength of the light source have a significant impact on the treatment doses. The optimal irradiation location was on both sides of the patella, where the largest dose could reach the articular cartilage. This optical model can be used to determine the key parameters in phototherapy and help the phototherapy of KOA patients.

Manifestation of hydration of Na+ and Cl- ions in the IR spectra of NaCl aqueous solutions in the range of 2750-4000 cm-1

This work is aimed at the study at studying the influence of the interaction of solvate shells on the profiles of the IR spectra of sodium chloride solutions in the 2750-4000 cm^-1 range. The IR spectra of distilled water and sodium chloride solutions were obtained with the limit (0.356 g per 100 g of water) and 50 % of the limit (0.178 g per 100 g of water) concentrations at a temperature of 21˚. Theoretical methods based on the use of the DFT approach with the XLYP exchange-correlation potential are used to calculate the profiles of the IR spectra of clusters containing 9 water molecules per one NaCl molecule at the limit concentrations of the solution. In the case when the cluster contained a NaCl molecule, the spectra were calculated for interacting and non-interacting solvate shells in which the number of H₂O molecules varied from 3 to 6. The expansion of the experimental band profile on a basis containing the profiles of the theoretical bands made it possible to study the features of NaCl hydration with a change in the concentration of solutions. It was found that the IR spectrum band is formed mainly by interacting Na⁺ and Cl^- solvation shells, each containing 4 H₂O molecules, while the ninth H₂O molecule provides the bond between the solvated ions. As the salt concentration increases, the contribution of the solvation shells to the band profile increases too. The agreement reached in the positions and profiles of experimental and theoretical water bands at different solution concentrations substantiates the adequacy of the theoretical description of NaCl hydration. Theoretical studies explained the effect of a decrease in the band width, an increase in the peak intensity, and a shift of its maximum toward higher wavenumbers with increasing solution concentration.

Aquaphotomics for monitoring of groundwater using short-wavelength near-infrared spectroscopy

Water spectrum of any aqueous system contains information about OH covalent and hydrogen bonds that are highly influenced by the environment and the rest of the molecules in the system. When aquaphotomics is used to analyze the water near infrared (NIR) spectra, the information about the water molecular structure can be obtained as a function of internal and external factors. The objective of this research is to apply aquaphotomics analysis to evaluate different groundwaters by using their NIR unique spectral pattern, robust to external influences of temperature and humidity, that can potentially be used for water type identification and screening practice. Two groundwaters obtained at different depths and their mixture, differing in mineral content and molecular structure were monitored on a daily basis using portable visible/NIR (vis/NIR) spectrometer during three consecutive years. The spectra were pre-processed by smoothing and multiplicative scatter correction (MSC) to remove noise and baseline effects. Results showed that NIR spectral patterns of groundwater samples were affected by changes in environmental factors - temperature, humidity, time and others. The water absorbance bands which are highly influenced by humidity and temperature in short wavelength NIR region were identified. Their avoidance resulted in obtaining consistent spectral patterns during the entire monitoring period, unique for each groundwater, that can be used as its fingerprint and monitored over time. Consistency and uniqueness of the spectral pattern for each groundwater provide a potential to use the deviation of spectral pattern as an indicator of changes in the water. These results confirm that vis/NIR spectral pattern can be used as an integrative marker of water status, stable over time, providing the basis for an efficient cost-effective method for monitoring of water functionality.

A Shipborne Photon-Counting Lidar for Depth-Resolved Ocean Observation

Depth-resolved information is essential for ocean research. For this study, we developed a shipborne photon-counting lidar for depth-resolved oceanic plankton observation. A pulsed fiber laser with frequency doubling to 532 nm acts as a light source, generating a single pulse at the micro-joule level with a pulse width of less than 1 ns. The receiver is capable of simultaneously detecting the elastic signal at two orthogonal polarization states, the Raman scattering from seawater, and the fluorescence signal from chlorophyll A. The data acquisition system utilizes the photon-counting technique to record each photon event, after which the backscattering signal intensity can be recovered by counting photons from multiple pulses. Benefitting from the immunity of this statistical detection method to the ringing effect of the detector and amplifier circuit, high-sensitivity and high-linearity backscatter signal measurements are realized. In this paper, we analyze and correct the after-pulse phenomenon of high-linearity signals through experiments and theoretical simulations. Through the after-pulse correction, the lidar attenuation coefficient retrieved from the corrected signal are in good agreement with the diffuse attenuation coefficients calculated from the in situ instrument, indicating the potential of this shipborne photon-counting lidar for ocean observation applications.

Classification of Eurasian Watermilfoil (Myriophyllum spicatum) Using Drone-Enabled Multispectral Imagery Analysis

Remote sensing approaches that could identify species of submerged aquatic vegetation (SAV) and measure their extent in lake littoral zones would greatly enhance SAV study and management, especially if these approaches can provide faster or more accurate results than traditional field methods. Remote sensing with multispectral sensors can provide this capability, but SAV identification with this technology must address the challenges of light extinction in aquatic environments where chlorophyll, dissolved organic carbon, and suspended minerals can affect water clarity and the strength of the sensed light signal. Here, we present an uncrewed aerial system (UAS)-enabled methodology to identify the extent of the invasive SAV species Myriophyllum spicatum (Eurasian watermilfoil, or EWM), primarily using a six-band Tetracam multispectral camera, flown over sites in the Les Cheneaux Islands area of northwestern Lake Huron, Michigan, USA. We analyzed water chemistry and light data and found our sites clustered into sites with higher and lower water clarity, although all sites had relatively high water clarity. The overall average accuracy achieved was 76.7%, with 78.7% producer’s and 77.6% user’s accuracy for the EWM. These accuracies were higher than previously reported from other studies that used remote sensing to map SAV. Our study found that two tested scale parameters did not lead to significantly different classification accuracies between sites with higher and lower water clarity. The EWM classification methodology described here should be applicable to other SAV species, especially if they have growth patterns that lead to high amounts of biomass relative to other species in the upper water column, which can be detected with the type of red-edge and infrared sensors deployed for this study.

Comparing the Use of Red-Edge and Near-Infrared Wavelength Ranges for Detecting Submerged Kelp Canopy

Kelp forests are commonly classified within remote sensing imagery by contrasting the high reflectance in the near-infrared spectral region of kelp canopy floating at the surface with the low reflectance in the same spectral region of water. However, kelp canopy is often submerged below the surface of the water, making it important to understand the effects of kelp submersion on the above-water reflectance of kelp, and the depth to which kelp can be detected, in order to reduce uncertainties around the kelp canopy area when mapping kelp. Here, we characterized changes to the above-water spectra of Nereocystis luetkeana (Bull kelp) as different canopy structures (bulb and blades) were submerged in water from the surface to 100 cm in 10 cm increments, while collecting above-water hyperspectral measurements with a spectroradiometer (325–1075 nm). The hyperspectral data were simulated into the multispectral bandwidths of the WorldView-3 satellite and the Micasense RedEdge-MX unoccupied aerial vehicle sensors and vegetation indices were calculated to compare detection limits of kelp with a focus on differences between red edge and near infrared indices. For kelp on the surface, near-infrared reflectance was higher than red-edge reflectance. Once submerged, the kelp spectra showed two narrow reflectance peaks in the red-edge and near-infrared wavelength ranges, and the red-edge peak was consistently higher than the near-infrared peak. As a result, kelp was detected deeper with vegetation indices calculated with a red-edge band versus those calculated with a near infrared band. Our results show that using red-edge bands increased detection of submerged kelp canopy, which may be beneficial for estimating kelp surface-canopy area and biomass.

Modeling benthic solar exposure (UV and visible) in dynamic coastal systems to better inform seagrass habitat suitability

Seagrass meadows worldwide provide valuable ecosystem services but have experienced sharp declines in recent decades. This rapid loss has prompted numerous restoration efforts with variable levels of success, often depending on the suitability of the restoration sites. The selection of sites can be guided by simple habitat suitability models driven with environmental variables deemed critical to the successful growth of new transplants. Habitat suitability models typically consider the influence of bathymetry, sediment type, salinity, wave exposure, and water quality. However, they typically do not explicitly include benthic exposure to ultraviolet (UV) and commonly use depth as a coarse proxy for photosynthetically active radiation (PAR). Benthic exposure to UV and PAR are both key parameters for habitat suitability but can be challenging to determine, especially in coastal environments influenced by rivers and tides where they are extremely variable. Here, we demonstrate the development of a simple but effective model of spectrally-resolved benthic solar irradiance for a dynamic marsh-influenced mesotidal estuary in Massachusetts. In-situ measurements were used to develop and validate an empirical model predicting the UV-visible vertical diffuse attenuation coefficient spectra of downwelling irradiance, K_d(λ), from simple physical parameters about tides, river discharge and location. Spectral benthic solar irradiances (280-700 nm) were calculated hourly for 3 years (2017-2019) using modeled and validated cloud-corrected surface downwelling irradiances, estimates of water depth, and the modeled K_d(λ) spectra. The mapped irradiances were used to provide improved seagrass habitat suitability maps that will guide future restoration efforts in the estuary. We expect the approach presented here can be adapted to other dynamic coastal environments influenced by tides and rivers and/or applied to other light-dependent organisms and biogeochemical processes.

Correction of Temperature Variation with Independent Water Samples to Predict Soluble Solids Content of Kiwifruit Juice Using NIR Spectroscopy

Using the framework of aquaphotomics, we have sought to understand the changes within the water structure of kiwifruit juice occurring with changes in temperature. The study focuses on the first (1300-1600 nm) and second (870-1100 nm) overtone regions of the OH stretch of water and examines temperature differences between 20, 25, and 30 °C. Spectral data were collected using a Fourier transform-near-infrared spectrometer with 1 mm and 10 mm transmission cells for measurements in the first and second overtone region, respectively. Water wavelengths affected by temperature variation were identified. Aquagrams (water spectral patterns) highlight slightly different responses in the first and second overtone regions. The influence of increasing temperature on the peak absorbance of the juice was largely a lateral wavelength shift in the first overtone region and a vertical amplitude shift in the second overtone region of water. With the same data set, we investigated the use of external parameter orthogonalisation (EPO) and extended multiple scatter correction (EMSC) pre-processing to assist in building temperature-independent partial least square regression models for predicting soluble solids concentration (SSC) of kiwifruit juice. The interference component selected for correction was the first principal component loading measured using pure water samples taken at the same three temperatures (20, 25, and 30 °C). The results show that the EMSC method reduced SSC prediction bias from 0.77 to 0.1 °Brix in the first overtone region of water. Using the EPO method significantly reduced the prediction bias from 0.51 to 0.04 °Brix, when applying a model made at one temperature (30 °C) to measurements made at another temperature (20 °C) in the second overtone region of water.

Safe Hb Concentration Measurement during Bladder Irrigation Using Artificial Intelligence

We have developed a sensor for monitoring the hemoglobin (Hb) concentration in the effluent of a continuous bladder irrigation. The Hb concentration measurement is based on light absorption within a fixed measuring distance. The light frequency used is selected so that both arterial and venous Hb are equally detected. The sensor allows the measurement of the Hb concentration up to a maximum value of 3.2 g/dL (equivalent to ≈20% blood concentration). Since bubble formation in the outflow tract cannot be avoided with current irrigation systems, a neural network is implemented that can robustly detect air bubbles within the measurement section. The network considers both optical and temporal features and is able to effectively safeguard the measurement process. The sensor supports the use of different irrigants (salt and electrolyte-free solutions) as well as measurement through glass shielding. The sensor can be used in a non-invasive way with current irrigation systems. The sensor is positively tested in a clinical study.

Inelastic hyperspectral Scheimpflug lidar for microalgae classification and quantification

An inelastic hyperspectral Scheimpflug lidar system was developed for microalgae classification and quantification. The correction for the refraction at the air-glass-water interface was established, making our system suitable for aquatic environments. The fluorescence spectrum of microalgae was extracted by principal component analysis, and seven species of microalgae from different phyla have been classified. It was verified that when the cell density of Phaeocystis globosa was in the range of ${{1}}{{{0}}^4}\sim{{1}}{{{0}}^6}\;{\rm{cell}}\;{\rm{m}}{{\rm{L}}^{- 1}}$, the cell density had a linear relationship with the fluorescence intensity. The experimental results show our system can identify and quantify microalgae, with application prospects for microalgae monitoring in the field environment and early warning of red tides or algal blooms.

Electromagnetic Dosimetry for Isolated Mitochondria Exposed to Near-Infrared Continuous-Wave Illumination in Photobiomodulation Experiments

This paper presents results on the electromagnetic field computed inside isolated mitochondria when they are exposed to near-infrared illuminations with reference to photobiomodulation experiments. The accurate calculation of the electromagnetic dose is considered to be important for a better understanding of the mechanism of interaction of light with these organelles and to improve the reliability and repeatability of the experiments. To get such results, we introduce several models. Even though they refer to a well-defined experimental setup, different models are necessary to take into account the possible different dispositions of the mitochondria, and of the differences in their dimensions and in their constitutive parameters. Different wavelengths and polarizations are considered as well. The effects of all parameters on the electromagnetic field inside mitochondria are discussed. © 2021 Bioelectromagnetics Society.

Vibrational modes of water predict spectral niches for photosynthesis in lakes and oceans

Stretching and bending vibrations of water molecules absorb photons of specific wavelengths, a phenomenon that constrains light energy available for aquatic photosynthesis. Previous work suggested that these absorption properties of water create a series of spectral niches but the theory was still too simplified to enable prediction of the spectral niches in real aquatic ecosystems. Here, we show with a state-of-the-art radiative transfer model that the vibrational modes of the water molecule delineate five spectral niches, in the violet, blue, green, orange and red parts of the spectrum. These five niches are effectively captured by chlorophylls and phycobilin pigments of cyanobacteria and their eukaryotic descendants. Global distributions of the spectral niches are predicted by satellite remote sensing and validated with observed large-scale distribution patterns of cyanobacterial pigment types. Our findings provide an elegant explanation for the biogeographical distributions of photosynthetic pigments across the lakes and oceans of our planet.

Simple Method to Determine the Apparent Quantum Yield Matrix of CDOM Photobleaching in Natural Waters

The photobleaching of chromophoric dissolved organic matter (CDOM) is considered an important loss process for CDOM absorption in sunlit natural waters, where it can regulate the biota's exposure to sunlight, surface solar heating, and dissolved organic matter dynamics. Despite its importance, this sink remains poorly quantified, primarily because of the difficulty of determining photobleaching apparent quantum yields (AQYs) that capture the dual spectral dependency of this process and are applicable to polychromatic sunlight. Here, we present a simple method to determine a CDOM photobleaching AQY matrix (AQY-M) for natural water samples that does not require any a priori assumptions about the spectral dependency of photobleaching. It combines controlled irradiation experiments, a partial least-square regression, and an optimization procedure to produce AQY-Ms that are spectrally coherent and optimized for modeling accurate photobleaching rates in natural waters. Water temperature and the solar exposure history of CDOM had a major influence on the magnitude and spectral characteristics of the AQY-M. These factors should be considered when determining the AQY-M of samples and provide constraints when modeling photobleaching rates in natural waters. We expect that this effective method will provide future studies with a robust means to characterize and understand the variability of AQY-M in natural waters.

Submerged macrophyte assessment in rivers: An automatic mapping method using Pléiades imagery

Submerged macrophyte monitoring is a major concern for hydrosystem management, particularly for understanding and preventing the potential impacts of global change on ecological functions and services. Macrophyte distribution assessments in rivers are still primarily realized using field monitoring or manual photo-interpretation of aerial images. Considering the lack of applications in fluvial environments, developing operational, low-cost and less time-consuming tools able to automatically map and monitor submerged macrophyte distribution is therefore crucial to support effective management programs. In this study, the suitability of very fine-scale resolution (50 cm) multispectral Pléiades satellite imagery to estimate submerged macrophyte cover, at the scale of a 1 km river section, was investigated. The performance of nonparametric regression methods (based on two reliable and well-known machine learning algorithms for remote sensing applications, Random Forest and Support Vector Regression) were compared for several spectral datasets, testing the relevance of 4 spectral bands (red, green, blue and near-infrared) and two vegetation indices (the Normalized Difference Vegetation Index, NDVI, and the Green-Red Vegetation Index, GRVI), and for several field sampling configurations. Both machine learning algorithms applied to a Pléiades image were able to reasonably well predict macrophyte cover in river ecosystems with promising performance metrics (R² above 0.7 and RMSE around 20%). The Random Forest algorithm combined to the 4 spectral bands from Pléiades image was the most efficient, particularly for extreme cover values (0% and 100%). Our study also demonstrated that a larger number of fine-scale field sampling entities clearly involved better cover predictions than a smaller number of larger sampling entities.

An improved spectrophotometric method tests the Einstein-Smoluchowski equation: a revisit and update

Theoretical prediction and experimental measurements of light attenuation in chemically pure and optically transparent solvents have attracted continuous attention, due in part to their curious nature, and in part to the increasing requirements of solvent-related applications. Yet hitherto, a majority of accurate spectrophotometric measurements of transparent solvents upon visible light radiation often end up using long-path-length cells, usually over dozens of cm, rendering the measure costly and complex; meanwhile, the guidance for choosing the Einstein-Smoluchowski equation or its variants as the best formula to predict the light scattering in solvents has remained elusive. Here we demonstrate a simple, versatile and cost-effective spectrophotometric method, enabling a sensitivity of 10-4 dB cm-1 over a 0.5 cm differential path length based on using standard double-beam spectrophotometer. We prove that this method reduces the path length by a factor of 100 while still making its closest approach to the record-low measurement of solvent extinction. We also validate that all the present equations used for predicting the light scattering in the solvent possess similar capacities, suggesting that the criterion for the choice of the appropriate formula simply depends on the equation's practicability. Following the elucidation of the wavelength range where the light scattering dominates the extinction, we further identify differences between scattering coefficients via the theoretical predictions and experimental measures, exposing the need for an improved theory to account for the solvent scattering phenomenon.

Biological rhythms in the deep-sea hydrothermal mussel Bathymodiolus azoricus

Biological rhythms are a fundamental property of life. The deep ocean covers 66% of our planet surface and is one of the largest biomes. The deep sea has long been considered as an arrhythmic environment because sunlight is totally absent below 1,000 m depth. In the present study, we have sequenced the temporal transcriptomes of a deep-sea species, the ecosystem-structuring vent mussel Bathymodiolus azoricus. We reveal that tidal cycles predominate in the transcriptome and physiology of mussels fixed directly at hydrothermal vents at 1,688 m depth at the Mid-Atlantic Ridge, whereas daily cycles prevail in mussels sampled after laboratory acclimation. We identify B. azoricus canonical circadian clock genes, and show that oscillations observed in deep-sea mussels could be either a direct response to environmental stimulus, or be driven endogenously by one or more biological clocks. This work generates in situ insights into temporal organisation in a deep-sea organism.

Little is known about gene expression of organisms in the deep sea, partially owing to constraints on sampling these organisms in situ. Here the authors circumvent this problem, fixing tissue of a deep-sea mussel at 1,688 m in depth, and later analyzing transcriptomes to reveal gene expression patterns showing tidal oscillations.

Improving Underwater Continuous-Variable Measurement-Device-Independent Quantum Key Distribution via Zero-Photon Catalysis

Underwater quantumkey distribution (QKD) is tough but important formodern underwater communications in an insecure environment. It can guarantee secure underwater communication between submarines and enhance safety for critical network nodes. To enhance the performance of continuous-variable quantumkey distribution (CVQKD) underwater in terms ofmaximal transmission distance and secret key rate as well, we adopt measurement-device-independent (MDI) quantum key distribution with the zero-photon catalysis (ZPC) performed at the emitter of one side, which is the ZPC-based MDI-CVQKD. Numerical simulation shows that the ZPC-involved scheme, which is a Gaussian operation in essence, works better than the single photon subtraction (SPS)-involved scheme in the extreme asymmetric case. We find that the transmission of the ZPC-involved scheme is longer than that of the SPS-involved scheme. In addition, we consider the effects of temperature, salinity and solar elevation angle on the system performance in pure seawater. The maximal transmission distance decreases with the increase of temperature and the decrease of sunlight elevation angle, while it changes little over a broad range of salinity

Remote Sensing Retrieval of Total Phosphorus in the Pearl River Channels Based on the GF-1 Remote Sensing Data

Total phosphorus (TP) concentration is one of the indicators for surface water quality evaluation. In this study, an indirect algorithm was proposed to retrieve TP concentration. This algorithm retrieves the TP concentration in urban waters based on Gaofen-1 (GF-1) remote sensing data. The algorithm uses the correlation between remote-sensing reflectance, optically significant constituents of water (chlorophyll, suspended sediment, and organic matter (excluding algae)), and TP to establish a retrieval model. First, the concentrations of optically active components are retrieved using a semi-analytical model. Second, the correlation between TP and optically active components is used to retrieve the TP concentration in waters. The GF-1 remote sensing data for 7 August 2015 were used to perform remote sensing retrieval of TP concentration in the Pearl River channels in Guangzhou, China. The results show that the TP concentration in most areas of the Front Channel, Western Channel, Guangzhou Channel, and the western part of the Back Channel was higher than 0.2 mg/L, while the TP concentration in the middle and eastern parts of the Back Channel was generally lower than 0.2 mg/L. The mean absolute percentage error of the retrieval is 24.18%. The experimental results show that the model is suitable for remote sensing retrieval of TP in urban waters in Guangzhou.

Designing a Smart Bath Assistive Device Based on Measuring Inner Water Temperature for Bathing Temperature Monitoring

Today, taking a bath is not only a means to keep clean, but also to reduce fatigue and stress. However, taking a bath with hot water for a long time can also be dangerous, leading to scalding or even a heart attack. To prevent these risks, several studies based on measuring bio-signals have been conducted, but due to high prices, difficulty of use, and restricted functions, these studies' recommendations cannot be easily adopted by the public. Therefore, developing accurate methods to measure bathing temperature and bathing time should be the most direct approach to solve these problems. In this study, a smart bath assistive device based on an inner water temperature measurement function is proposed. Prior to development of the device, a bathing environment was emulated with six temperature sensors affixed to different depths to find the optimal depth for measuring bathing temperature. According to the measurement results, the device was designed in a mushroom shape with the cap part floating on the water's surface and housing the electronic components, and temperature sensors within the stem part were immersed in the water approximately 5 cm below the surface to measure the inner water temperature. Due to the low-power consuming Advanced RISC Machine (ARM) processor and waterproof design, the device is able to float in hot water and monitor the bathing temperature variation over a long period of time. The device was compared alongside a commercial analog bathing thermometer to verify the performance of temperature measurements. In addition, a compensation algorithm was developed and programmed into the device to improve the accuracy of measurements. Processed data is transmitted by Bluetooth to a dedicated Android app for data display and storage. The final results show that the proposed device is highly accurate and stable for monitoring bathing temperature.

Evaluation of glint correction approaches for fine-scale ocean color measurements by lightweight hyperspectral imaging spectrometers

Low-power, lightweight, off-the-shelf imaging spectrometers, deployed on above-water fixed platforms or on low-altitude aerial drones, have significant potential for enabling fine-scale assessment of radiometrically derived water quality properties (WQPs) in oceans, lakes, and reservoirs. In such applications, it is essential that the measured water-leaving spectral radiances be corrected for surface-reflected light, i.e., glint. However, noise and spectral characteristics of these imagers, and environmental sources of fine-scale radiometric variability such as capillary waves, complicate the glint correction problem. Despite having a low signal-to-noise ratio, a representative lightweight imaging spectrometer provided accurate radiometric estimates of chlorophyll concentration-an informative WQP-from glint-corrected hyperspectral radiances in a fixed-platform application in a coastal ocean region. Optimal glint correction was provided by a spectral optimization algorithm, which outperformed both a hardware solution utilizing a polarizer and a subtractive algorithm incorporating the reflectance measured in the near infrared. In the same coastal region, this spectral optimization approach also provided the best glint correction for radiometric estimates of backscatter at 650 nm, a WQP indicative of suspended particle load.

Empirical Formulas for Estimating Backscattering and Absorption Coefficients in Complex Waters from Remote-Sensing Reflectance Spectra and Examples of Their Application

Many standard methods used for the remote sensing of ocean colour have been developed, though mainly for clean, open ocean waters. This means that they may not always be effective in complex waters potentially containing high concentrations of optically significant constituents. This paper presents new empirical formulas for estimating selected inherent optical properties of water from remote-sensing reflectance spectra R_rs(λ), derived, among other things, for waters with high concentrations of dissolved and suspended substances. These formulas include one for estimating the backscattering coefficient b_b(620) directly from the magnitude of R_rs in the red part of the spectrum, and another for estimating the absorption coefficient a(440) from the hue angle α. The latter quantity represents the water's colour as it might be perceived by the human eye (trichromatic colour vision); it is easily calculated from the shape of the R_rs spectrum. These new formulas are based on a combined dataset. Most of the data were obtained in the specific, optically complex environment of the Baltic Sea. Additional data, taken from the NASA bio-Optical Marine Algorithm Dataset (NOMAD) and representing various regions of the global oceans, were used to widen the potential applicability of the new formulas. We indicate the reasons why these simple empirical relationships can be derived and compare them with the results of straightforward modelling; possible applications are also described. We present, among other things, an example of a simple semi-analytical algorithm using both new empirical formulas. This algorithm is a modified version of the well-known quasi-analytical algorithm (QAA), and it can improve the results obtained in optically complex waters. This algorithm allows one to estimate the full spectra of the backscattering and absorption coefficients, without the need for any additional a priori assumptions regarding the spectral shape of absorption by dissolved and suspended seawater constituents.

Retrieving Total and Inorganic Suspended Sediments in Amazon Floodplain Lakes: A Multisensor Approach

Remote sensing imagery are fundamental to increasing the knowledge about sediment dynamics in the middle-lower Amazon floodplains. Moreover, they can help to understand both how climate change and how land use and land cover changes impact the sediment exchange between the Amazon River and floodplain lakes in this important and complex ecosystem. This study investigates the suitability of Landsat-8 and Sentinel-2 spectral characteristics in retrieving total (TSS) and inorganic (TSI) suspended sediments on a set of Amazon floodplain lakes in the middle-lower Amazon basin using in situ Remote Sensing Reflectance (Rrs) measurements to simulate Landsat 8/OLI (Operational Land Imager) and Sentinel 2/MSI (Multispectral Instrument) bands and to calibrate/validate several TSS and TSI empirical algorithms. The calibration was based on the Monte Carlo Simulation carried out for the following datasets: (1) All-Dataset, consisting of all the data acquired during four field campaigns at five lakes spread over the lower Amazon floodplain (n = 94); (2) Campaign-Dataset including samples acquired in a specific hydrograph phase (season) in all lakes. As sample size varied from one season to the other, n varied from 18 to 31; (3) Lake-Dataset including samples acquired in all seasons at a given lake with n also varying from 17 to 67 for each lake. The calibrated models were, then, applied to OLI and MSI scenes acquired in August 2017. The performance of three atmospheric correction algorithms was also assessed for both OLI (6S, ACOLITE, and L8SR) and MSI (6S, ACOLITE, and Sen2Cor) images. The impact of glint correction on atmosphere-corrected image performance was assessed against in situ glint-corrected Rrs measurements. After glint correction, the L8SR and 6S atmospheric correction performed better with the OLI and MSI sensors, respectively (Mean Absolute Percentage Error (MAPE) = 16.68% and 14.38%) considering the entire set of bands. However, for a given single band, different methods have different performances. The validated TSI and TSS satellite estimates showed that both in situ TSI and TSS algorithms provided reliable estimates, having the best results for the green OLI band (561 nm) and MSI red-edge band (705 nm) (MAPE < 21%). Moreover, the findings indicate that the OLI and MSI models provided similar errors, which support the use of both sensors as a virtual constellation for the TSS and TSI estimate over an Amazon floodplain. These results demonstrate the applicability of the calibration/validation techniques developed for the empirical modeling of suspended sediments in lower Amazon floodplain lakes using medium-resolution sensors.

Three-dimensional confocal Raman temperature characterization of electrokinetically pumped microchannels

A novel method for noninvasive, three-dimensional temperature characterization in microfluidic devices is presented. A specially designed confocal microscope was built and used to measure water temperature by sensing the Raman spectrum variations of the liquid. This is achieved by splitting the spectrum in the isosbestic point and detecting it with two photon counters. The difference between the signals of each detector divided by their sum shows a linear dependence with temperature. A fiber-coupled laser beam is used to pump the sample with 25 mW of optical power at 405 nm. This allows a 0.8 K temperature precision and a 9 μm axial resolution using a 1 s integration time. These features make temperature profiling in all dimensions possible, in contrast with previous methods, where the information present in the height of the channel is lost and the whole spectrum needs to be recovered before computing the sample temperature. Using this technique, different geometries of polydimethylsiloxane microchannels sealed with a 150 μm thick glass coverslip were studied, showing that heat flow through the glass is the dominating dissipation mechanism and defines the maximum temperature in the channel. The results show good agreement with previous work found in the literature.

Under the spotlight: mechanisms of photobiomodulation concentrating on blue and green light

Photobiomodulation (PBM) describes the application of light at wavelengths ranging from 400-1100 nm to promote tissue healing, reduce inflammation and promote analgesia. Traditionally, red and near-infra red (NIR) light have been used therapeutically, however recent studies indicate that other wavelengths within the visible spectrum could prove beneficial including blue and green light. This review aims to evaluate the literature surrounding the potential therapeutic effects of PBM with particular emphasis on the effects of blue and green light. In particular focus is on the possible primary and secondary molecular mechanisms of PBM and also evaluation of the potential effective parameters for application both in vitro and in vivo. Studies have reported that PBM affects an array of molecular targets, including chromophores such as signalling molecules containing flavins and porphyrins as well as components of the electron transport chain. However, secondary mechanisms tend to converge on pathways induced by increases in reactive oxygen species (ROS) production. Systematic evaluation of the literature indicated 72% of publications reported beneficial effects of blue light and 75% reported therapeutic effects of green light. However, of the publications evaluating the effects of green light, reporting of treatment parameters was uneven with 41% failing to report irradiance (mW cm^-2) and 44% failing to report radiant exposure (J cm^-2). This review highlights the potential of PBM to exert broad effects on a range of different chromophores within the body, dependent upon the wavelength of light applied. Emphasis still remains on the need to report exposure and treatment parameters, as this will enable direct comparison between different studies and hence enable the determination of the full potential of PBM.

Remote Sensing Estimation of Sea Surface Salinity from GOCI Measurements in the Southern Yellow Sea

Knowledge about the spatiotemporal distribution of sea surface salinity (SSS) provides valuable and important information for understanding various marine biogeochemical processes and ecosystems, especially for those coastal waters significantly affected by human activities. Remote-sensing techniques have been used to monitor salinity in the open ocean with their advantages of wide-area surveys and real-time monitoring. However, potential challenges remain when using satellite data with coarse spatiotemporal resolutions, leading to a loss of valuable information. In the current study, based on the local dataset collected over the southern Yellow Sea (SYS), a region-customized algorithm was developed to estimate SSS by using the remote sensing reflectance. The model evaluations indicated that our algorithm yielded good SSS estimation, with a root-mean-square error (RMSE) of 0.29 psu and a mean absolute percentage error (MAPE) of 0.75%. Satellite-derived SSS results compared well with those derived from in situ observations, further suggesting the good performance of our developed algorithm for the study regions. We applied this algorithm to Geostationary Ocean Color Imager (GOCI) data for the month of August from 2011 to 2018 in the SYS, and produced the spatial distribution patterns of the SSS for August of each year. The SSS values were high in offshore waters and lower in coastal waters, especially in the Yangtze River estuary. The negative correlation between the monthly Changjiang River discharge (CRD) and SSS (R = −0.71, p < 0.001) near the Yangtze River estuary was observed, suggesting that the SSS distribution in the Yangtze River estuary was potentially influenced by the CRD. In offshore waters, the correlation between SSS and CRD was weak (R < 0.2), suggesting that the riverine discharge’s effect might be weak.

Absorption of light by water in the region of high transparency: recommended values for photon-transport calculations

Prompted by data requirements for photon-transport simulation, we have surveyed measurements of the absorption coefficient of light by water in the wavelength range 171-2000 nm, covering the whole region where the absorption length exceeds 1 mm. Absorption spectra were compiled for pure water, clean seawater, and seawater containing a moderate amount of organic matter. The intensive and detailed nature of the laboratory and at-sea observations leading to the pure-water and clean-seawater spectra lend confidence to these results. As to dissolved organic matter in seawater, this is so highly variable in composition and concentration that this spectrum can only be considered as indicative of its effects.

Fabrication of Chlorophyll-Incorporated Nanogels for Potential Applications in Photothermal Cancer Therapy

Nanogels have been widely used in biomedical applications, such as carriers for hyperthermia cancer treatment, drug delivery, and imaging. Owing to the enhanced permeability and retention effect, nanogels have shown a great potential in cancer therapy. In this study, sodium copper chlorophyllin (SCC), a low cytotoxicity and biodegradable photothermal agent, was copolymerized with a nanogel of N-[3-(dimethylamino)propyl]methacrylamide. The nanogels could produce heat under exposure to a green laser with a 532 nm wavelength. The positively charged nature of the nanogels enhanced the endocytosis of the nanogels. The cell mortality was greatly enhanced with the treatment of the SCC-containing nanogels and green light illumination. Our results suggest the potential of SCC-containing nanogels in photothermal cancer therapy.

Fabrication of Photothermo-Responsive Drug-Loaded Nanogel for Synergetic Cancer Therapy

Temperature stimulus, easy modulation in comparison to other environmental stimuli, makes thermo-responsive nanocarriers popular in the applications of controlled drug release for cancer therapy. In this study, photosensitive sodium copper chlorophyllin (SCC) was incorporated into thermo-responsive polymeric nanogels consisted of N-isopropylacrylamide and N-(hydroxymethyl)acrylamide. Significant heat was generated from the SCC-containing nanogels under the exposure to 532-nm green laser, and resulted in cell mortality. The thermo-responsive nanogel loaded with 5-FU, an anti-cancer drug, released the drug explosively when exposed to green laser. The combination of hyperthermia and temperature-induced drug release via green laser irradiation greatly enhanced cell mortality to a maximal extent. Such photothermo-responsive nanogel possesses a great potential in anti-cancer treatment.

Proof of concept for a model of light reflectance of plastics floating on natural waters

Remote sensing of plastic littering natural waters is an emerging field of science with the potential to provide observations on local to global scales. We present the verification of a theoretical reflectance model of sunlight interacting with a water surface littered with buoyant plastic objects. We measured a few common litter items of different polymers as well as shapes, transparencies, and surface roughnesses. Spectral reflectance measurements in the field were backed up with measurements in the laboratory of coefficients of total and diffuse reflectance, transmittance and absorption. We evaluated a single-band algorithm for 850 nm wavelength and a dual-band algorithm using a second wavelength at a polymer absorption band between 1660 and 1730 nm. Both algorithms were plastic litter type specific. Our findings show that for interpreting spectral remote sensing of floating plastic, physical properties that control geometrical optics should complement information about the absorption spectra of the polymer.

Role of temperature in de-mixing absorbance spectra composed of compound electrolyte solutions

This work is focused on the role of temperature in the de-mixing of absorbance spectra measured in mixed aqueous Na₂SO₄ and NaNO₃ solutions. First, the influence of temperature on the absorbance spectrum of demineralized water was determined. Second, the absorbance spectra of five separate electrolytes (NaNO₂, NaNO₃, CaCl₂, K₂CO₃, and NaOH) at three temperatures (4°C, 25°C, and 50°C) for concentrations ranging from 0.0625 M to 0.5 M were examined. These five electrolytes show similar temperature dependencies. Finally, absorbance spectra of mixed solutions were investigated at temperatures of 5°C, 15°C, 25°C, 35°C, and 45°C for concentrations ranging from 0.0625 M to 0.5 M per electrolyte in the mixture. The spectral window from 650 to 1100 nm was utilized to observe the ionic and temperature influences on the vibrational modes of the OH bond in the solvent molecules. The effects of dissolving Na₂SO₄ and NaNO₃ are nonlinearly cumulative at lower temperatures indicating extended alteration of the water structure beyond the first hydration shell. A similar trend was observed for a mixture of Na₂CO₃ and NaCl. Furthermore, it was found that higher temperatures are better for recovering the separate component absorption signatures of an electrolyte mixture. The near-infrared spectral regime is well suited for integrated sensing, and therefore these results can help in designing an integrated sensor to identify inorganic species in water.

Monte Carlo simulations of the backscattering measurements for associated uncertainty

Three-dimensional simulations using the Monte Carlo method are implemented to analyze and quantify the uncertainty and the influence of absorption on the measurement of light backscattering by ECO-BB9 (WET Labs) sensor for a wide variety of optically complex and open ocean waters. The analytical investigation of the geometrical configuration revealed a distinct effective path length which contributes towards an accurate assessment of absorption effect on the backscattering measurement. The present study proposes the application of a non-linear relationship to determine the measured parameter from the detector counts more accurately than the conventional method that applies the scale factor. It was found that the mean centroid angle of the instrument shows marginal variations for varying absorption and backscattering coefficients. Nevertheless, the mean centroid angle for the instrument over the entire course of the simulation study was found to be 124° which conforms well with the study of Doxaran et al. [Opt. Express 24, 3615 (2016)].

Low Temperature-Mediated Enhancement of Photoacoustic Imaging Depth

We study the temperature dependence of the underlying mechanisms related to the signal strength and imaging depth in photoacoustic imaging. The presented theoretical and experimental results indicate that imaging depth can be improved by lowering the temperature of the intermediate medium that the laser passes through to reach the imaging target. We discuss the temperature dependency of optical and acoustic properties of the intermediate medium and their changes due to cooling. We demonstrate that the SNR improvement of the photoacoustic signal is mainly due to the reduction of Grüneisen parameter of the intermediate medium which leads to a lower level of background noise. These findings may open new possibilities toward the application of biomedical laser refrigeration.

An overview of approaches and challenges for retrieving marine inherent optical properties from ocean color remote sensing

Ocean color measured from satellites provides daily global, synoptic views of spectral waterleaving reflectances that can be used to generate estimates of marine inherent optical properties (IOPs). These reflectances, namely the ratio of spectral upwelled radiances to spectral downwelled irradiances, describe the light exiting a water mass that defines its color. IOPs are the spectral absorption and scattering characteristics of ocean water and its dissolved and particulate constituents. Because of their dependence on the concentration and composition of marine constituents, IOPs can be used to describe the contents of the upper ocean mixed layer. This information is critical to further our scientific understanding of biogeochemical oceanic processes, such as organic carbon production and export, phytoplankton dynamics, and responses to climatic disturbances. Given their importance, the international ocean color community has invested significant effort in improving the quality of satellite-derived IOP products, both regionally and globally. Recognizing the current influx of data products into the community and the need to improve current algorithms in anticipation of new satellite instruments (e.g., the global, hyperspectral spectroradiometer of the NASA Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission), we present a synopsis of the current state of the art in the retrieval of these core optical properties. Contemporary approaches for obtaining IOPs from satellite ocean color are reviewed and, for clarity, separated based their inversion methodology or the type of IOPs sought. Summaries of known uncertainties associated with each approach are provided, as well as common performance metrics used to evaluate them. We discuss current knowledge gaps and make recommendations for future investment for upcoming missions whose instrument characteristics diverge sufficiently from heritage and existing sensors to warrant reassessing current approaches.

Determination of the absorption coefficient of chromophoric dissolved organic matter from underway spectrophotometry

Measurements of the absorption coefficient of chromophoric dissolved organic matter (ay) are needed to validate existing ocean-color algorithms. In the surface open ocean, these measurements are challenging because of low ay values. Yet, existing global datasets demonstrate that ay could contribute between 30% to 50% of the total absorption budget in the 400-450 nm spectral range, thus making accurate measurement of ay essential to constrain these uncertainties. In this study, we present a simple way of determining ay using a commercially-available in-situ spectrophotometer operated in underway mode. The obtained ay values were validated using independent collocated measurements. The method is simple to implement, can provide measurements with very high spatio-temporal resolution, and has an accuracy of about 0.0004 m^-1 and a precision of about 0.0025 m^-1 when compared to independent data (at 440 nm). The only limitation for using this method at sea is that it relies on the availability of relatively large volumes of ultrapure water. Despite this limitation, the method can deliver the ay data needed for validating and assessing uncertainties in ocean-colour algorithms.

Assessing uncertainties in scattering correction algorithms for reflective tube absorption measurements made with a WET Labs ac-9

In situ absorption measurements collected with a WET Labs ac-9 employing a reflective tube approach were scatter corrected using several possible methods and compared to reference measurements made by a PSICAM to assess performance. Overall, two correction methods performed best for the stations sampled: one using an empirical relationship between the ac-9 and PSICAM to derive the scattering error (ε) in the near-infrared (NIR), and one where ε was independently derived from concurrent measurements of the volume scattering function (VSF). Application of the VSF-based method may be more universally applicable, although difficult to routinely apply because of the lack of commercially available VSF instrumentation. The performance of the empirical approach is encouraging as it relies only on the ac meter measurement and may be readily applied to historical data, although there are inevitably some inherent assumptions about particle composition that hinder universal applicability. For even the best performing methods, residual errors of 20% or more were commonly observed for many water types. For clear ocean waters, a conventional baseline subtraction with the assumption of negligible near-IR absorption performed as well or better than the above methods because propagated uncertainties were lower than observed with the proportional method.

Estimation of underwater visibility in coastal and inland waters using remote sensing data

An optical method is developed to estimate water transparency (or underwater visibility) in terms of Secchi depth (Z _sd ), which follows the remote sensing and contrast transmittance theory. The major factors governing the variation in Z _sd , namely, turbidity and length attenuation coefficient (1/(c + K _d ), c = beam attenuation coefficient; K _d  = diffuse attenuation coefficient at 531 nm), are obtained based on band rationing techniques. It was found that the band ratio of remote sensing reflectance (expressed as (R _rs (443) + R _rs (490))/(R _rs (555) + R _rs (670)) contains essential information about the water column optical properties and thereby positively correlates to turbidity. The beam attenuation coefficient (c) at 531 nm is obtained by a linear relationship with turbidity. To derive the vertical diffuse attenuation coefficient (K _d ) at 531 nm, K _d (490) is estimated as a function of reflectance ratio (R _rs (670)/R _rs (490)), which provides the bio-optical link between chlorophyll concentration and K _d (531). The present algorithm was applied to MODIS-Aqua images, and the results were evaluated by matchup comparisons between the remotely estimated Z _sd and in situ Z _sd in coastal waters off Point Calimere and its adjoining regions on the southeast coast of India. The results showed the pattern of increasing Z _sd from shallow turbid waters to deep clear waters. The statistical evaluation of the results showed that the percent mean relative error between the MODIS-Aqua-derived Z _sd and in situ Z _sd values was within ±25%. A close agreement achieved in spatial contours of MODIS-Aqua-derived Z _sd and in situ Z _sd for the month of January 2014 and August 2013 promises the model capability to yield accurate estimates of Z _sd in coastal, estuarine, and inland waters. The spatial contours have been included to provide the best data visualization of the measured, modeled (in situ), and satellite-derived Z _sd products. The modeled and satellite-derived Z _sd values were compared with measurement data which yielded RMSE = 0.079, MRE = -0.016, and R ²  = 0.95 for the modeled Z _sd and RMSE = 0.075, MRE = 0.020, and R ²  = 0.95 for the satellite-derived Z _sd products.

Blue Laser Diode Enables Underwater Communication at 12.4 Gbps

To enable high-speed underwater wireless optical communication (UWOC) in tap-water and seawater environments over long distances, a 450-nm blue GaN laser diode (LD) directly modulated by pre-leveled 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) data was employed to implement its maximal transmission capacity of up to 10 Gbps. The proposed UWOC in tap water provided a maximal allowable communication bit rate increase from 5.2 to 12.4 Gbps with the corresponding underwater transmission distance significantly reduced from 10.2 to 1.7 m, exhibiting a bit rate/distance decaying slope of -0.847 Gbps/m. When conducting the same type of UWOC in seawater, light scattering induced by impurities attenuated the blue laser power, thereby degrading the transmission with a slightly higher decay ratio of 0.941 Gbps/m. The blue LD based UWOC enables a 16-QAM OFDM bit rate of up to 7.2 Gbps for transmission in seawater more than 6.8 m.

Special properties of luminescent magnetic NaGdF4:Yb3+, Er3+ upconversion nanocubes with surface modifications

Gadolinium-based upconversion nanocubes with amine surface modification are made by a one-pot process. The interfacial effect on their behaviors are observed.

Polarized transfer functions of the ocean surface for above-surface determination of the vector submarine light field

A method is developed to determine the underwater polarized light field from above sea surface observations. A hybrid approach combining vector radiative transfer simulations and the Monte Carlo method is used to determine the transfer functions of polarized light for wind-driven ocean surfaces. Transfer functions for surface-reflected skylight and upward transmission of light through the sea surface are presented for many common viewing and solar geometries for clear-sky conditions. Sensitivity of reflection matrices to environmental conditions is examined and can vary up to 50% due to wind speed, 25% due to atmospheric aerosol load, and 10% due to radiometer field-of-view. Scalar transmission is largely independent of water type and varies a few percent with wind speed, while polarized components can change up to 10% in high winds. Considerations for determining the water-leaving radiance (scalar or vector) are discussed.

Optical closure in marine waters from in situ inherent optical property measurements

Optical closure using radiative transfer simulations can be used to determine the consistency of in situ measurements of inherent optical properties (IOPs) and radiometry. Three scattering corrections are applied to in situ absorption and attenuation profile data for a range of coastal and oceanic waters, but are found to have only very limited impact on subsequent closure attempts for these stations. Best-fit regressions on log-transformed measured and modelled downwards irradiance, Ed, and upwards radiance, Lu, profiles have median slopes between 0.92 - 1.24, revealing a tendency to underestimate Ed and Lu with depth. This is only partly explained by non-inclusion of fluorescence emission from CDOM and chlorophyll in the simulations. There are several stations where multiple volume scattering function related data processing steps perform poorly which suggests the potential existence of unresolved features in the modelling of the angular distribution of scattered photons. General optical closure therefore remains problematic, even though there are many cases in the data set where the match between measured and modelled radiometric data is within 25% RMS%E. These results are significant for applications that rely on optical closure e.g. assimilating ocean colour data into coupled physical-ecosystem models.

Estimating the Underwater Diffuse Attenuation Coefficient with a Low-Cost Instrument: The KdUINO DIY Buoy

A critical parameter to assess the environmental status of water bodies is the transparency of the water, as it is strongly affected by different water quality related components (such as the presence of phytoplankton, organic matter and sediment concentrations). One parameter to assess the water transparency is the diffuse attenuation coefficient. However, the number of subsurface irradiance measurements obtained with conventional instrumentation is relatively low, due to instrument costs and the logistic requirements to provide regular and autonomous observations. In recent years, the citizen science concept has increased the number of environmental observations, both in time and space. The recent technological advances in embedded systems and sensors also enable volunteers (citizens) to create their own devices (known as Do-It-Yourself or DIY technologies). In this paper, a DIY instrument to measure irradiance at different depths and automatically calculate the diffuse attenuation K_d coefficient is presented. The instrument, named KdUINO, is based on an encapsulated low-cost photonic sensor and Arduino (an open-hardware platform for the data acquisition). The whole instrument has been successfully operated and the data validated comparing the KdUINO measurements with the commercial instruments. Workshops have been organized with high school students to validate its feasibility.

An optical method to assess water clarity in coastal waters

Accurate estimation of water clarity in coastal regions is highly desired by various activities such as search and recovery operations, dredging and water quality monitoring. This study intends to develop a practical method for estimating water clarity based on a larger in situ dataset, which includes Secchi depth (Z sd ), turbidity, chlorophyll and optical properties from several field campaigns in turbid coastal waters. The Secchi depth parameter is found to closely vary with the concentration of suspended sediments, vertical diffuse attenuation coefficient K d (m(-1)) and beam attenuation coefficient c (m(-1)). The optical relationships obtained for the selected wavelengths (i.e. 520, 530 and 540 nm) exhibit an inverse relationship between Secchi depth and the length attenuation coefficient (1/(c + K d )). The variation in Secchi depth is expressed in terms of undetermined coupling coefficient which is composed of light penetration factor (expressed by z(1%)K d (λ)) and a correction factor (ξ) (essentially governed by turbidity of the water column). This method of estimating water clarity was validated using independent in situ data from turbid coastal waters, and its results were compared with those obtained from the existing methods. The statistical analysis of the measured and the estimated Z sd showed that the present method yields lower error when compared to the existing methods. The spatial structures of the measured and predicted Z sd are also highly consistent with in situ data, which indicates the potential of the present method for estimating the water clarity in turbid coastal and associated lagoon waters.