Trophic state resilience to hurricane disturbance of Lake Yojoa, Honduras

Cyclones are a poorly described disturbance in tropical lakes, with the potential to alter ecosystems and compromise the services they provide. In November 2020, Hurricanes Eta and Iota made landfall near the Nicaragua-Honduras border, inundating the region with a large amount of late-season precipitation. To understand the impact of these storms on Lake Yojoa, Honduras, we compared 2020 and 2021 conditions using continuous (every 16 days) data collected from five pelagic locations. The storms resulted in increased Secchi depth and decreased algal abundance in December 2020, and January and February 2021, and lower-than-average accumulation of hypolimnetic nutrients from the onset of stratification (April 2021) until mixus in November 2021. Despite the reduced hypolimnetic nutrient concentrations, epilimnetic nutrient concentrations returned to (and in some cases exceeded) pre-hurricane levels following annual water column turnover in 2021. This response suggests that Lake Yojoa's trophic state had only an ephemeral response to the disturbance imposed by the two hurricanes, likely due to internal input of sediment derived nutrients. These aseasonal storms acted as a large-scale experiment that resulted in nutrient dilution and demonstrated the resilience of Lake Yojoa's trophic state to temporary nutrient reductions.

Delignified wood aerogels as scaffolds coated with an oriented chitosan–cyclodextrin co-polymer for removal of microcystin-LR

Nano-porous aerogels are an advantageous approach to produce low-density materials with high surface area, particularly when using biobased materials. Frequently, most biobased aerogels are synthesized through a bottom-up approach, which requires high energy inputs to break and rebuild the raw materials, and for elimination of water. To curb this, this work focused on generating aerogels by a top-down approach through the delignification of a wood substrate while eliminating water by solvent exchange. To diversify the surface chemistry for use in water treatment, the delignified wood–nanowood-was coated with a chitosan–cyclodextrin co-polymer and tested in the capture of microcystin-LR. The generated nanowood structure had 75% porosity after coating, with up to 339% water swelling and an adsorption capacity of 0.12 mg g⁻¹ of the microcystin. This top-down technique enables the generation of low-cost aerogels by reducing steps, using a biobased self-assembled coating with hydrophobic active sites, and avoiding costly energetic input.

Herein, a preassembled chitosan-cyclodextrin was used as a versatile coating onto delignified wood as an alternative for the removal of microcystin-LR. The addition of β-cyclodextrin proved to allow the nanowood scaffold to adsorb up to 0.12 mg g⁻¹.

Alterations to sediment nutrient deposition and transport along a six reservoir sequence

Reservoir presence and construction has become commonplace along rivers due to the multitude of ecosystem services they provide. Many services are well recognized, including the effectiveness of sequestering both sediments and sediment-bound nutrients such as silts and phosphorus (P). Reservoirs are also capable of transforming or sequestering significant quantities of nutrients with more complex biogeochemical pathways, like nitrogen (N). Reservoir assessments, independent of inflow-outflow models, have primarily focused on a small number of systems creating a growing need to understand how reservoirs function both individually and as reservoir sequences within large rivers and their watersheds. Models have simulated the overall efficiency and drivers of reservoir nutrient deposition, but few have considered how a sequence of reservoirs alters deposition as an interdependent watershed-sediment-transport-system. In this study, we collected sediment cores from a six-reservoir sequence along a 5th - 6th order stream receiving treated waters from a large metropolitan area in the subtropical southeastern United States. Paleolimnological studies of subtropical reservoirs are underrepresented and are needed to understand the history of reservoir development. Using paleolimnological techniques and a known 30 year flux of riverine nutrient loading from waste water treatment facilities, we compared nutrient deposition to reservoir morphological qualities and primary producer community structure during the past ~50 years. Our findings suggest phosphorus deposition is associated with reservoir order downstream of the primary nutrient source, nitrogen deposition is linked to reservoir water retention time, and N:P is most strongly linked to reservoir surface area and watershed population density. Our results were strongly influenced by a large upstream and metropolitan nutrient source, common in large rivers, but under different conditions of nutrient loading (i.e. nonpoint source), reservoirs may express other nutrient depositional patterns.

Environmentally dependent adsorption of 2,4-dichlorophenol on cellulose-chitosan self-assembled composites

With the increasing need for bio-based materials developed by environmentally friendly procedures, this work shows a green method to develop shape-controlled structures from cellulose dissolving pulp coated by chitosan. This material was then tested to adsorb a common and widespread pollutant, 2,4-dichlorophenol under different pH conditions (5.5 and 9). Herein it was noticed that the adsorption only occurred in acidic pH (5.5) where electrostatic interaction drove the adsorption, demonstrating the potential to tune the response under desired conditions only. The adsorption was successful in the hydrogel structure with an adsorption capacity of 905 ± 71 mg/g from a solution with 16.6 ppm; furthermore, adsorption was also possible with dried hydrogel structures, presenting a maximum of adsorption of 646 ± 50 mg/g in a similar 16.6 ppm solution. Finally, adsorbent regeneration was successfully tested for both, dry (rewetted) and never-dried states, showing improved adsorption after regeneration in the case of the never dried hydrogel structures.

A Review on the Study of Cyanotoxins in Paleolimnological Research: Current Knowledge and Future Needs

Cyanobacterial metabolites are increasingly studied, in regards to their biosynthesis, ecological role, toxicity, and potential biomedical applications. However, the history of cyanotoxins prior to the last few decades is virtually unknown. Only a few paleolimnological studies have been undertaken to date, and these have focused exclusively on microcystins and cylindrospermopsins, both successfully identified in lake sediments up to 200 and 4700 years old, respectively. In this paper, we review direct extraction, quantification, and application of cyanotoxins in sediment cores, and put forward future research prospects in this field. Cyanobacterial toxin research is also compared to other paleo-cyanobacteria tools, such as sedimentary pigments, akinetes, and ancient DNA isolation, to identify the role of each tool in reproducing the history of cyanobacteria. Such investigations may also be beneficial for further elucidation of the biological role of cyanotoxins, particularly if coupled with analyses of other abiotic and biotic sedimentary features. In addition, we identify current limitations as well as future directions for applications in the field of paleolimnological studies on cyanotoxins.

Organic carbon sequestration in sediments of subtropical Florida lakes

Recent studies have shown that sediments of temperate and tropical lakes are sinks for organic carbon (OC), but little is known about OC burial in subtropical lakes. There are questions regarding the ability of subtropical lakes to store OC, given their relatively warmwater temperatures, lack of ice cover, frequent water-column mixing, and labile carbon forms. We used 210Pb-dated sediment cores from 11 shallow Florida (USA) lakes to estimate OC burial, i.e. net OC storage, over the last ~100 years. Shallow Florida water bodies average ~30% OC content in their sediments and displayed rates of net OC accumulation (63-177 g C m-2 a-1) that are similar to natural temperate lakes, but lower than temperate agricultural impoundments. We considered the influence of lake morphometry on OC storage in our study lakes, but did not observe an inverse relationship between lake size and OC burial rate, as has been seen in some temperate lake districts. We did, however, find an inverse relation between mean water depth and OC sequestration. Despite recent cultural eutrophication and the associated shift from macrophyte to phytoplankton dominance in the Florida study lakes, overall OC burial rate increased relative to historic (pre-1950 AD) values. Lakes cover >9000 km2 of the Florida landscape, suggesting that OC burial in sediments amounts to as much as 1.6 Mt a-1. The high rate of OC burial in Florida lake sediments indicates that subtropical lakes are important for carbon sequestration and should be included in models of global carbon cycling.

Correction to: Chronic prescribed burning alters nutrient deposition and sediment stoichiometry in a lake ecosystem

The publication contained a dating model that was based on AD/BC dates instead of years before present (YBP) dates for the three C¹⁴ AMS values. As a result, dates reported as YBP should be reported as BC. While all of the dates for the prescribed burning period are correct given that they were based on the ²¹⁰Pb model, all dates reported as YBP should read BC. Specific changes to the manuscript are as follows: The abstract should read, "throughout the last 8000 years." The final paragraph in the introduction should read, "from the mid-Holocene (~ 6000 BC) to present." The end of the first paragraph in the Results section should read, "the sediment core represented the past ~ 8000 YBP and the core sections below the ²¹⁰Pb record averaged sedimentation rates of 1.73 ± 2.1 mg cm^-2 year^-1." All dates presented as Years Before Present or YPB for the remainder of the manuscript should be reported as BC. Added to Acknowledgements: "The authors would like to thank Sally Horn and Matt Boehm with help with the age model and dating." Table 1 has been updated with the "Calibrated Age" column reflecting the correct dates in YBP notation. Figure 2 has been updated to reflect the BC to YBP changes in the calibrated AMS C¹⁴ dates. Both panels have been changed to include the older dates.

Chronic prescribed burning alters nutrient deposition and sediment stoichiometry in a lake ecosystem

Prescribed fire is a common management practice for forests and other terrestrial environments. Following a prescribed burn, ash erodes into aquatic environments potentially altering terrestrial-aquatic connectivity and water quality. In this study, we collected a sediment core from Ocean Pond, FL, USA, a lake that has received fire ash from decades of prescribed burning events. Paleolimnological measurements of macrocharcoal, nutrients, stable isotopes (δ¹³C, δ¹⁵N), and photosynthetic pigments were used to reconstruct fire regimes, material inputs, and lake primary producer responses for periods of prescribed burns and other lake periods throughout the last 8000 years [corrected]. Results show that the period of repeated modern-prescribed fires coincided with decreased C and N depositions in the lake, while P deposition increased causing alterations to nutrient storage and stoichiometry. However, photosynthetic pigments indicated low primary producer abundance during the prescribed fire period. These changes in nutrient dynamics could provide new insights into biogeochemical pathways in land-water connected systems where burning has not been considered.

Avoiding timescale bias in assessments of coastal wetland vertical change

There is concern that accelerating sea-level rise will exceed the vertical growth capacity of coastal-wetland substrates in many regions by the end of this century. Vertical vulnerability estimates rely on measurements of accretion and/or surface-elevation-change derived from soil cores and/or surface elevation tables (SETs). To date there has not been a broad examination of whether the multiple timescales represented by the processes of accretion and elevation change are equally well-suited for quantifying the trajectories of wetland vertical change in coming decades and centuries. To examine the potential for timescale bias in assessments of vertical change, we compared rates of accretion and surface elevation change using data derived from a review of the literature. In the first approach, average rates of elevation change were compared with timescale-averaged accretion rates from six regions around the world where sub-decadal, decadal, centennial, and millennial timescales were represented. Second, to isolate spatial variability, temporal comparisons were made for regionally unique environmental categories within each region. Last, comparisons were made of records from sites where SET-MH stations and radiometric measurements were co-located in close proximity. We find that rates vary significantly as a function of measurement timescale and that the pattern and magnitude of variation between timescales are location-specific. Failure to identify and account for temporal variability in rates will produce biased assessments of the vertical change capacity of coastal wetlands. Robust vulnerability assessments should combine accretion rates from multiple timescales with the longest available SET record to provide long-term context for ongoing monitoring observations and projections.

Photosynthetic costs and benefits of abaxial versus adaxial anthocyanins in Colocasia esculenta 'Mojito'

Anthocyanins in upper (adaxial) leaf tissues provide greater photoprotection than in lower (abaxial) tissues, but also predispose tissues to increased shade acclimation and, consequently, reduced photosynthetic capacity. Abaxial anthocyanins may be a compromise between these costs/benefits. Plants adapted to shaded understory environments often exhibit red/purple anthocyanin pigmentation in lower (abaxial) leaf surfaces, but rarely in upper (adaxial) surfaces. The functional significance of this color pattern in leaves is poorly understood. Here, we test the hypothesis that abaxial anthocyanins protect leaves of understory plants from photo-oxidative stress via light attenuation during periodic exposure to high incident sunlight in the forest understory, without interfering with sunlight capture and photosynthesis during shade conditions. We utilize a cultivar of Colocasia esculenta exhibiting adaxial and abaxial anthocyanin variegation within individual leaves to compare tissues with the following color patterns: green adaxial, green abaxial (GG), green adaxial, red abaxial (GR), red adaxial, green abaxial (RG), and red adaxial, red abaxial (RR). Consistent with a photoprotective function of anthocyanins, tissues exhibited symptoms of increasing photoinhibition in the order (from least to greatest): RR, RG, GR, GG. Anthocyanic tissues also showed symptoms of shade acclimation (higher total chl, lower chl a/b) in the same relative order. Inconsistent with our hypothesis, we did not observe any differences in photosynthetic CO2 uptake under shade conditions between the tissue types. However, GG and GR had significantly (39 %) higher photosynthesis at saturating irradiance (A sat) than RG and RR. Because tissue types did not differ in nitrogen content, these patterns likely reflect differences in resource allocation at the tissue level, with greater nitrogen allocated toward energy processing in GG and GR, and energy capture in RG and RR (consistent with relative sun/shade acclimation). We conclude that abaxial anthocyanins are likely advantageous in understory environments because they provide some photoprotection during high-light exposure, but without the cost of decreased A sat associated with adaxial anthocyanin-induced shade syndrome.