Benthic nutrient cycling at the sediment-water interface in a lagoon fish farming system (northern Adriatic Sea, Italy)

Relationship between Nitrogen Dynamics and Key Microbial Nitrogen-Cycling Genes in an Intensive Freshwater Aquaculture Pond

Intensive aquaculture in high-density hybrid snakehead [Channa maculata (♀) × Channa argus (♂)] fishponds can lead to toxic conditions for fish. This study investigated nitrogen migration and transformation in these fishponds during different cultivation periods. Using qPCR technology, we analyzed the abundance variation of nitrogen-cycling microorganisms in water and sediment to reveal the nitrogen metabolism characteristics of hybrid snakehead fishponds. The results showed that fish biomass significantly impacts suspended particulate matter (SPM) flux. At the sediment-water interface, inorganic nitrogen fluxes showed predominant NO3--N absorption by sediments and NH4+-N and NO2--N release, especially in later cultivation stages. Sediments were rich in nirS and AMX 16S rRNA genes (ranging from 4.04 × 109 to 1.01 × 1010 and 1.19 × 108 to 2.62 × 108 copies/g, respectively) with nirS-type denitrifiers potentially dominating the denitrification process. Ammonia-oxidizing bacteria (AOB) were found to dominate the ammonia oxidation process over ammonia-oxidizing archaea (AOA) in both water and sediment. Redundancy analysis revealed a positive correlation between SPM flux, Chlorophyll a (Chl-a), and denitrification genes in the water, and between nitrogen-cycling genes and NH4+/NO2- fluxes at the interface. These findings provide a scientific basis for nitrogen control in hybrid snakehead fishponds.

Microplastics and trace metals in river sediment: Prevalence and correlation with multiple factors

Microplastics (MPs), which are ubiquitous, are no longer novel emerging pollutants, yet our knowledge of them is insufficient. This study investigates the prevalence of MPs and trace metals in sediment belonging to Ma River, Vietnam, and their interaction with various parameters, including nutrients such as total carbon (TC), total nitrogen (TN), and total phosphorus (TP), grain sizes, and MPs in surface water. The study revealed that the abundance of MPs in sediment (MPs/S) is relatively high (i.e., 1328.3 ± 1925.5 items.kg-1 dry weight), while the concentration of MPs in surface water (MPs/W) was relatively low (i.e., 57.3 ± 55.8 items.m-3) compared to other areas. Notably, the study found that arsenic and cadmium concentrations exceeded baseline levels, indicating their anthropogenic origin. To interpret the relationship between MPs/S, metals, and the aforementioned parameters, principal component analysis and Pearson correlation analyses were employed. The results demonstrated a significant correlation between metals and nutrients, as well as small grain sizes such as clay and silt. It was observed that the majority of metals displayed co-occurrence with one another but showed weak associations with the levels of MPs present in both water and sediment. Additionally, a weak correlation was observed between MPs/W and MPs/S. In conclusion, these findings suggest that the distribution and behavior of MPs and trace metals in aquatic systems are influenced by multiple factors, including nutrient levels, grain size, and other chemical and physical characteristics of the environment. While certain metals may have natural sources, others may result from human activities such as mining, industrial discharge, and wastewater treatment plants. As a result, understanding the sources and aspects of metal contamination are critical for determining their relationship with MPs and developing effective strategies for mitigating their impact on aquatic ecosystems.

Dissolved gaseous mercury production and sea-air gaseous exchange in impacted coastal environments of the northern Adriatic Sea

The northern Adriatic Sea is well known for mercury (Hg) contamination mainly due to historical Hg mining which took place in Idrija (Slovenia). The formation of dissolved gaseous mercury (DGM) and its subsequent volatilisation can reduce the amount of Hg available in the water column. In this work, the diurnal patterns of both DGM production and gaseous Hg fluxes at the water-air interface were seasonally evaluated in two selected environments within this area, a highly Hg-impacted, confined fish farm (VN: Val Noghera, Italy) and an open coastal zone less impacted by Hg inputs (PR: Bay of Piran, Slovenia). A floating flux chamber coupled with real-time Hg⁰ analyser was used for flux estimation in parallel with DGM concentrations determination through in-field incubations. Substantial DGM production was observed at VN (range = 126.0-711.3 pg L^-1) driven by both strong photoreduction and possibly dark biotic reduction, resulting in higher values in spring and summer and comparable concentrations throughout both day and night. Significantly lower DGM was observed at PR (range = 21.8-183.4 pg L^-1). Surprisingly, comparable Hg⁰ fluxes were found at the two sites (range VN = 7.43-41.17 ng m^-2 h^-1, PR = 0-81.49 ng m^-2 h^-1), likely due to enhanced gaseous exchanges at PR thanks to high water turbulence and to the strong limitation of evasion at VN by water stagnation and expected high DGM oxidation in saltwater. Slight differences between the temporal variation of DGM and fluxes indicate that Hg evasion is more controlled by factors such as water temperature and mixing conditions than DGM concentrations alone. The relative low Hg losses through volatilisation at VN (2.4-4.6% of total Hg) further confirm that static conditions in saltwater environments negatively affect the ability of this process in reducing the amount of Hg retained in the water column, therefore potentially leading to a greater availability for methylation and trophic transfer.

The SHAPE Project: An Innovative Approach to Understanding Seasonal and Diel Dissolved Oxygen Dynamics in the Marano and Grado Lagoon (Adriatic Sea) under the WFD/2000/60/CE

Dissolved oxygen (DO) is a key element for the survival of marine organisms and is a supporting element in the current Water Framework Directive (WFD). DO deficiency is a common event that occurs in coastal environments such as estuaries and lagoons, but a long-term DO database that helps detect daily and seasonal oscillations is difficult to obtain with commonly used sampling and analytical procedures. In this work, a network of multi-parametric probes was deployed in the Marano and Grado Lagoon (northern Adriatic Sea, Italy) in order to obtain a dataset from the continuous monitoring of DO and complementary parameters. DO showed a high degree of variability both in terms of spatial and seasonal distribution and was dependent on solar radiation and water temperature. During the summer and in areas characterised by scarce water renewal, DO was below the threshold set as the minimum requirement for aquatic life, thus some water bodies (WBs) were classified as moderate sensu WFD. The inputs of freshwater discharge from inland and marine waters during tides are, however, able to well oxygenate most of the lagoon. These results will be useful in supporting the management and protection of this vulnerable environment.

Algal blooms modulate organic matter remineralization in freshwater sediments: A new insight on priming effect

This study provides a novel insight into the degradation of sediment organic matter (SOM) regulated by algae-derived organic matter (AOM) based on priming effect. We tracked the dynamics of SOM mineralization products and pathways, together with priming effects (PE) using the compound-specific stable isotope (δ¹³C) technique following addition of low- and high-density algal debris in sediments. We found that algal debris increased the total carbon oxidation rate, and resulted in denitrification and methanogenesis-dominated SOM mineralization. While iron reduction and sulphate reduction played important roles in the early period of algal accumulation. Total carbon oxidation rate and anaerobic rates (R_anaerobic) were higher in the amended treatments compared with that in the control. Analysis indicated that algal debris had a positive PE on SOM mineralization, which caused an intensified mineralization in the initial phase with over 80% of dissolved inorganic carbon deriving from SOM degradation. Total carbon oxidation rate of SOM deduced from priming effect (R_TCOR-PE) was similar to R_anaerobic, further indicating SOM mineralization was a critical source of the end products. These findings deviate the causal focus from the decomposition of AOM, and confirm the accumulation of AOM as the facilitator of SOM mineralization. Our study offers empirical evidences to advance the traditional view on the effect of AOM on SOM mineralization.

Nitrogen budget at sediment-water interface altered by sediment dredging and settling particles: Benefits and drawbacks in managing eutrophication

Internal nitrogen (N) loading of lakes is commonly controlled by sediment dredging, although its comprehensive effect on internal N loading remains unclear. Herein, we examined the long-term effects of sediment dredging on internal N loading from a new perspective on the N budget at the sediment-water interface (SWI) through a simulation of field dredging performed by incubating intact sediment cores from a shallow eutrophic lake (Lake Taihu). We further evaluated the role of settling particles (SP) in the recovery of N cycle processes after dredging and its potential impact on the N budget. Our results demonstrated that dredging could help reduce organic matter and total N in sediments; improve the redox environment of the SWI; slow down N mineralization, N fixation, denitrification, and anaerobic ammonia oxidation (anammox); and alter the N budget at the SWI and the contribution of various N cycle processes. However, the input of SP enriched in fresh organic matter and N could accelerate the recovery of N cycle processes at the SWI, reducing the variation in the N budget and the contribution of each N cycle process caused by dredging. Dredging significantly reduced the N flux at the SWI, which was evident from the reduction of inorganic N release flux and N removal through denitrification and anammox. Therefore, sediment dredging has its advantages and disadvantages in managing internal N loading in lakes. To maintain a long-term control on the release of internal N through sediment dredging, measures should be taken based on the in-lake and watershed to inhibit the inflow and settlement of particulate matter.

Multivariate statistical analyses for water and sediment quality index development: A study of susceptibility in an urban river

In this study, multivariate statistical analyses were performed to develop water and sediment quality indexes, allowing us (i) to select with reliability the most appropriate chemical variables for the evaluation of river quality susceptibility; (ii) to weight the influence of each variable based on monitored data; (iii) to consider possible synergism or antagonism derived from the combined effect of several pollutants; and (iv) to express the quality as a deviation from selected site-specific reference conditions. For the establishment of these threshold/maximum values, combining two biological indicators related to denitrifying bacteria in sediments turned out to be applicable to ensure compliance with the European water quality standard. The joint implementation of water and sediment quality indexes assisted us in the rapid detection of the deleterious effect of different anthropogenic contamination sources, as well as the influence of hydrological regime seasonality on river quality. In addition, metal-dependent water quality appeared to be coupled to sediment dynamics, since they were preferentially adsorbed onto sediments during low flow seasons, whereas there was potential for metal mobilization to water during sediment resuspension in high flow seasons. Therefore, an annual determination of sediment quality index was also recommended as suitable tool for prospective monitoring water quality, identifying those sites which could deserve special attention during certain periods, and planning future strategies for river quality improvement. However, two limitations were found: (1) sediment was not appropriate for water physicochemical quality early monitoring due to organic matter and nutrient continuous transformation; and (2) a multimetric index did not provide a concise and definitive quality information, thus a new tool for combining with quality index was proposed for specifically evaluate the water and sediment quality by identifying pollutant/s of concern at each location.

Impact of finfish aquaculture on biogeochemical processes in coastal ecosystems and elemental sulfur as a relevant proxy for assessing farming condition

We conducted experiments to investigate the effects of finfish aquaculture and to propose appropriate proxies for assessing their environmental impact. Due to enhanced fish feed input, sulfate reduction (SR) and the resulting metabolic products (H₂S, NH₄⁺, PO₄^3-) were significantly greater at the farm than at the control site. Benthic release of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) from farm sediment accounted for 52-837% and 926-1048%, respectively, of the potential DIN and DIP demand for phytoplankton production. The results suggest that excess organic loading in fish farms induces deleterious eutrophication and algal blooms in coastal ecosystems via benthic-pelagic coupling. Direct SR measurement provided the most useful information of all the parameters on organic contamination in fish farms. However, given its abundance, relatively lower chemical reactivity and relative ease of analysis, elemental sulfur was regarded as the most appropriate proxy for assessing the environmental impacts of finfish aquaculture.

Organotin compounds in touristic marinas of the northern Adriatic Sea: occurrence, speciation and potential recycling at the sediment-water interface

Butyltin compound (BTC) contamination was evaluated in two north Adriatic marinas, San Rocco (Italy) and Lucija (Slovenia). BTC sedimentary concentrations (121 ± 46 and 352 ± 30 ng Sn g^-1 in San Rocco and Lucija, respectively) evidenced the past use of antifouling paints, confirmed by the reduced tributyltin content (~ 46%) with respect to the sum of BTC. Elemental and organic carbon isotopic (δ¹³C) analyses of bulk sediments and its lipid and humic substances were performed in order to evaluate their role in BTC partitioning and preservation. The δ¹³C of sedimentary bulk and refractory organic matter suggested that diagenetic processes could play a role in the preservation or release of pollutants. No contamination was found in water collected from the benthic chamber and thus, fluxes at the sediment-water interface were not assessed, except for MBT efflux at Lucija (28.9 ng Sn m^-2 day^-1). Nevertheless, BTC concentrations in porewaters (up to 75 ng Sn l^-1) and rather low sediment-porewater partitioning coefficients (K_d) with respect to the data reported in the literature would suggest a potential risk of the reintroduction of BTC into the water column at both sites: at Lucija, sedimentary contamination is high despite the greater Log K_d, whilst at San Rocco, the low BTC concentration is associated with a reduced sediment affinity.

Spatio-temporal Variation in Nutrient Profiles and Exchange Fluxes at the Sediment-Water Interface in Yuqiao Reservoir, China

Nutrients released from sediments have a significant influence on the water quality in eutrophic lakes and reservoirs. To clarify the internal nutrient load and provide reference for eutrophication control in Yuqiao Reservoir, a drinking water source reservoir in China, pore water profiles and sediment core incubation experiments were conducted. The nutrients in the water (soluble reactive P (SRP), nitrate-N (NO₃⁻-N), nitrite-N (NO₂⁻-N), and ammonium-N (NH₄⁺-N)) and in the sediments (total N (TN), total P (TP) and total organic carbon (TOC)) were quantified. The results show that NH₄⁺-N was the main component of inorganic N in the pore water. NH₄⁺-N and SRP were higher in the pore water than in the overlying water, and the concentration gradient indicated a diffusion potential from the sediment to the overlying water. The NH₄⁺-N, NO₃⁻-N, and SRP fluxes showed significant differences amongst the seasons. The NH₄⁺-N and SRP fluxes were significantly higher in the summer than in other seasons, while NO₃⁻-N was higher in the autumn. The sediment generally acted as a source of NH₄⁺-N and SRP and as a sink for NO₃⁻-N and NO₂⁻-N. The sediments release 1133.15 and 92.46 tons of N and P, respectively, to the overlying water each year.

Nutrients and metals interactions between water and sediment phases: An urban river case study

The provision of water to meet the needs of an ever increasing urban population is a significant challenge. This is because urban receiving waters are constantly at risk from pollutant inputs via stormwater runoff and wastewater discharge. This research study employed multiple approaches including principal component analysis, Bayesian Networks (BNs) modelling and geospatial analysis to identify patterns in the distributions of nutrients and metals in water and sediments in an urban river and the interactions between the two phases. In both, water and sediments, nutrient concentrations/loads varied in the order of total carbon (TC) > total nitrogen (TN) > total phosphorus (TP). The river sediments were found to contain the highest crustal metal loads, while in water, the marine-related metals had the highest concentrations. The BNs modelling of pollutant interactions between water and sediment phases indicated that nitrogen is more likely to be transferred from water to sediment than the opposite, while anthropogenic metals are more likely to be transferred from sediments to water. Further, geospatial analysis showed that TN, crustal metals and anthropogenic metal loads in sediments increased from upstream to downstream, while having a decreasing pattern in water. However, marine-related metals in both, water and sediments had increasing concentrations/loads from upstream to downstream. These spatial patterns are attributed to the interactions between water and sediment phases, sediment transport along the river and seawater intrusion in the estuarine area. The study outcomes are expected to contribute to enhancing the knowledge required for developing mitigation strategies to improve urban receiving water quality.

Dissolved Gaseous Mercury Production at a Marine Aquaculture Site in the Mercury-Contaminated Marano and Grado Lagoon, Italy

Dissolved gaseous mercury (DGM) production was examined in relation to ultraviolet radiation within a marine aquaculture site in the contaminated Marano and Grado Lagoon (Italy). The measured rates of DGM production relative to time elapsed (17.06 and 20.68 pg h^-1, respectively) were substantially (6-20 times) higher than what has been observed in other marine Hg studies. We measured similar levels of DGM relative to dissolved total mercury (THg_D) (0.84%-8.91%) at these sites in comparison to uncontaminated marine sites, however relative to other moderately-contaminated marine sites in Portugal the % DGM/THg_D was high. These results suggest a substantial capacity for Hg volatilization from these highly contaminated lagoons to the atmosphere due to photoreduction mechanisms.

Mobility of metal(loid)s at the sediment-water interface in two tourist port areas of the Gulf of Trieste (northern Adriatic Sea)

One of the main environmental issues affecting coastal marine environments is the accumulation of contaminants in sediments and their potential mobility. In situ benthic chamber experiments were conducted at two tourist ports (marinas) located in the Gulf of Trieste, one in Slovenia and one in Italy. The aim was to understand if and where recycling at the sediment-water interface (SWI) may affect metal(loid)s. Short sediment cores were also collected near the chamber to investigate the solid (sediments) and dissolved phases (porewaters). Both diffusive and benthic fluxes were estimated to elucidate the release of metal(loid)s at the SWI. Total element concentrations and their labile fractions were determined in sediments to quantify their potential mobility. The total element contents were found to be two orders of magnitude higher in the Italian marina than in the Slovenian one, especially for Hg (up to 1000 mg kg^-1), whereas the labile fraction was scarce or null. The opposite occurred in the Slovenian marina. Metal(loid)s in porewaters showed a clear diagenetic sequence and a close dependence upon the suboxic/anoxic conditions of sediments. The results suggest that although the sediments of the Italian marina exhibit the highest total metal(loid) concentration, these elements are scarcely remobilisable. Conversely, in the Slovenian marina, sediments seem to be comparatively more prone to release metal(loid)s at the SWI.