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•First analytical method for intracellular microcystins (MCs) in sediment.•Includes a suite of variants (LR, 7dmLR, RR, YR, WR, LA, LF, LY, LW) and nodularin.•Reports the first ...measurements of MCs in sediment pore waters.•MCs detected in >100 year old lake sediments suggesting long-term preservation.•Sediment-pore water distribution (Kd) differed between variants suggesting differences in environmental fate.
The fate and persistence of microcystin cyanotoxins in aquatic ecosystems remains poorly understood in part due to the lack of analytical methods for microcystins in sediments. Existing methods have been limited to the extraction of a few extracellular microcystins of similar chemistry. We developed a single analytical method, consisting of accelerated solvent extraction, hydrophilic–lipophilic balance solid phase extraction, and reversed phase high performance liquid chromatography-tandem mass spectrometry, suitable for the extraction and quantitation of both intracellular and extracellular cyanotoxins in sediments as well as pore waters. Recoveries of nine microcystins, representing the chemical diversity of microcystins, and nodularin (a marine analogue) ranged between 75 and 98% with one, microcystin-RR (MC-RR), at 50%. Chromatographic separation of these analytes was achieved within 7.5min and the method detection limits were between 1.1 and 2.5ngg−1 dry weight (dw). The robustness of the method was demonstrated on sediment cores collected from seven Canadian lakes of diverse geography and trophic states. Individual microcystin variants reached a maximum concentration of 829ngg−1 dw on sediment particles and 132ngmL−1 in pore waters and could be detected in sediments as deep as 41cm (>100 years in age). MC-LR, -RR, and -LA were more often detected while MC-YR, -LY, -LF, and -LW were less common. The analytical method enabled us to estimate sediment-pore water distribution coefficients (Kd), MC-RR had the highest affinity for sediment particles (log Kd=1.3) while MC-LA had the lowest affinity (log Kd=−0.4), partitioning mainly into pore waters. Our findings confirm that sediments serve as a reservoir for microcystins but suggest that some variants may diffuse into overlying water thereby constituting a new route of exposure following the dissipation of toxic blooms. The method is well suited to determine the fate and persistence of different microcystins in aquatic systems.
This review summarises knowledge on the ecology, toxin production, and impacts of toxic freshwater benthic cyanobacterial proliferations. It documents monitoring, management, and sampling strategies, ...and explores mitigation options.
Toxic proliferations of freshwater benthic cyanobacteria (taxa that grow attached to substrates) occur in streams, rivers, lakes, and thermal and meltwater ponds, and have been reported in 19 countries. Anatoxin‐ and microcystin‐containing mats are most commonly reported (eight and 10 countries, respectively).
Studies exploring factors that promote toxic benthic cyanobacterial proliferations are limited to a few species and habitats. There is a hierarchy of importance in environmental and biological factors that regulate proliferations with variables such as flow (rivers), fine sediment deposition, nutrients, associated microbes, and grazing identified as key drivers. Regulating factors differ among colonisation, expansion, and dispersal phases.
New ‐omics‐based approaches are providing novel insights into the physiological attributes of benthic cyanobacteria and the role of associated microorganisms in facilitating their proliferation.
Proliferations are commonly comprised of both toxic and non‐toxic strains, and the relative proportion of these is the key factor contributing to the overall toxin content of each mat.
While these events are becoming more commonly reported globally, we currently lack standardised approaches to detect, monitor, and manage this emerging health issue. To solve these critical gaps, global collaborations are needed to facilitate the rapid transfer of knowledge and promote the development of standardised techniques that can be applied to diverse habitats and species, and ultimately lead to improved management.
Cyanobacterial harmful algal blooms are an increasing threat to coastal and inland waters. These blooms can be detected using optical radiometers due to the presence of phycocyanin (PC) pigments. The ...spectral resolution of best-available multispectral sensors limits their ability to diagnostically detect PC in the presence of other photosynthetic pigments. To assess the role of spectral resolution in the determination of PC, a large (<inline-formula> <tex-math notation="LaTeX">N =905 </tex-math></inline-formula>) database of colocated in situ radiometric spectra and PC are employed. We first examine the performance of selected widely used machine-learning (ML) models against that of benchmark algorithms for hyperspectral remote sensing reflectance (<inline-formula> <tex-math notation="LaTeX">R_{\mathrm {rs}} </tex-math></inline-formula>) spectra resampled to the spectral configuration of the Hyperspectral Imager for the Coastal Ocean (HICO) with a full-width at half-maximum (FWHM) of < 6 nm. Results show that the multilayer perceptron (MLP) neural network applied to HICO spectral configurations (median errors < 65%) outperforms other ML models. This model is subsequently applied to <inline-formula> <tex-math notation="LaTeX">R_{\mathrm {rs}} </tex-math></inline-formula> spectra resampled to the band configuration of existing satellite instruments and of the one proposed for the next Landsat sensor. These results confirm that employing MLP models to estimate PC from hyperspectral data delivers tangible improvements compared with retrievals from multispectral data and benchmark algorithms (with median errors between <inline-formula> <tex-math notation="LaTeX">\sim 73 </tex-math></inline-formula>% and 126%) and shows promise for developing a globally applicable cyanobacteria measurement approach.
Human activities can cause large alterations in biogeochemical cycles of key nutrients such as carbon (C), nitrogen (N), and phosphorus (P). However, relatively little is known about how these ...changes alter the proportional fluxes of these elements across ecosystem boundaries from rivers to lakes. Here, we examined environmental factors influencing spatial and temporal variation in particulate C:N:P ratios across the Lake Erie watershed from its tributaries to its outflow. Throughout the study, particulate nutrient ratios ranged widely (C:N 2.0–25.8, C:P 32–530, N:P 3.7–122.9), but mean values were generally lower than previous estimates from different aquatic environments. Particulate C:N ratios varied the least across all environments, but C:P and N:P ratios increased between tributaries and coastal areas and throughout the growing season in coastal environments. These ratios also differed temporally in offshore waters as particulate C:P and N:P were higher in the spring and summer and lower in the fall and winter. Particulate C:P ratios also increased between the western/central and eastern basins indicating differential nutrient processing across the lake. These stoichiometric changes were associated with unique environmental factors among ecosystems as tributary stoichiometry was related to terrestrial land use and land cover, coastal ratios were a product of mixing between riverine and offshore waters, and offshore patterns were influenced by differences in temperature and particulate nutrient loading among basins. Overall, by studying changes in particulate C:N:P ratios across the Lake Erie watershed, our study demonstrates the power of using mass balance principles to study nutrient transformations along the aquatic continuum.
Lake Erie is subject to recurring events of cyanobacterial harmful algal blooms (cHABs), but measures of nutrients and total phytoplankton biomass seem to be poor predictors of cHABs when taken ...individually. A more integrated approach at the watershed scale may improve our understanding of the conditions that lead to bloom formation, such as assessing the physico-chemical and biological factors that influence the lake microbial community, as well as identifying the linkages between Lake Erie and the surrounding watershed. Within the scope of the Government of Canada's Genomics Research and Development Initiative (GRDI) Ecobiomics project, we used high-throughput sequencing of the 16S rRNA gene to characterize the spatio-temporal variability of the aquatic microbiome in the Thames River-Lake St. Clair-Detroit River-Lake Erie aquatic corridor. We found that the aquatic microbiome was structured along the flow path and influenced mainly by higher nutrient concentrations in the Thames River, and higher temperature and pH downstream in Lake St. Clair and Lake Erie. The same dominant bacterial phyla were detected along the water continuum, changing only in relative abundance. At finer taxonomical level, however, there was a clear shift in the cyanobacterial community, with
dominating in the Thames River and
and
in Lake St. Clair and Lake Erie. Mantel correlations highlighted the importance of geographic distance in shaping the microbial community structure. The fact that a high proportion of microbial sequences found in the Western Basin of Lake Erie were also identified in the Thames River, indicated a high degree of connectivity and dispersal within the system, where mass effect induced by passive transport play an important role in microbial community assembly. Nevertheless, some cyanobacterial amplicon sequence variants (ASVs) related to
, representing less than 0.1% of relative abundance in the upstream Thames River, became dominant in Lake St. Clair and Erie, suggesting selection of those ASVs based on the lake conditions. Their extremely low relative abundances in the Thames suggest additional sources are likely to contribute to the rapid development of summer and fall blooms in the Western Basin of Lake Erie. Collectively, these results, which can be applied to other watersheds, improve our understanding of the factors influencing aquatic microbial community assembly and provide new perspectives on how to better understand the occurrence of cHABs in Lake Erie and elsewhere.
Molot LA, Schiff SL, Venkiteswaran JJ, Baulch HM, Higgins SN, Zastepa A, Verschoor MJ, Walters D. 2021. Low sediment redox promotes cyanobacteria blooms across a trophic range: implications for ...management. Lake Reserv Manage. 37:120-142.
Field observations and experimental manipulations with different oxidizing agents including nitrate demonstrate that high sediment redox prevents cyanobacteria blooms in eutrophic freshwaters. Conversely, low sediment redox caused by depletion of dissolved oxygen and nitrate allows blooms to form. This explains why bloom risk increases with phosphorus levels: Higher productivity increases the spatial and temporal extent of low sediment redox. The intermediate link between low redox and cyanobacteria blooms appears to be internal loading of ferrous iron (Fe
2+
) from reduced sediments with diffusion to depths accessible to migrating cyanobacteria, providing a source for their high iron demand. Regardless of whether Fe
2+
release is the intermediate link, the concept of "low sediment redox as promoter" has major potential to improve bloom management if managers consider the impact of their nutrient management choices, nutrient targets, and in-lake methods on sediment redox. Phosphorus input targets can be adjusted as climate change alters the extent of anoxia, and short-term bloom prediction models that incorporate the sediment redox concept could predict onset of blooms earlier than current models that depend on detection of photosynthetic pigments associated with blooms.
Among its many impacts, climate warming is leading to increasing winter air temperatures, decreasing ice cover extent, and changing winter precipitation patterns over the Laurentian Great Lakes and ...their watershed. Understanding and predicting the consequences of these changes is impeded by a shortage of winter‐period studies on most aspects of Great Lake limnology. In this review, we summarize what is known about the Great Lakes during their 3–6 months of winter and identify key open questions about the physics, chemistry, and biology of the Laurentian Great Lakes and other large, seasonally frozen lakes. Existing studies show that winter conditions have important effects on physical, biogeochemical, and biological processes, not only during winter but in subsequent seasons as well. Ice cover, the extent of which fluctuates dramatically among years and the five lakes, emerges as a key variable that controls many aspects of the functioning of the Great Lakes ecosystem. Studies on the properties and formation of Great Lakes ice, its effect on vertical and horizontal mixing, light conditions, and biota, along with winter measurements of fundamental state and rate parameters in the lakes and their watersheds are needed to close the winter knowledge gap. Overcoming the formidable logistical challenges of winter research on these large and dynamic ecosystems may require investment in new, specialized research infrastructure. Perhaps more importantly, it will demand broader recognition of the value of such work and collaboration between physicists, geochemists, and biologists working on the world's seasonally freezing lakes and seas.
Plain Language Summary
The Laurentian Great Lakes are the world's largest freshwater ecosystem and provide diverse ecosystem services to millions of people. Affected by multiple interacting stressors, this system is the target of extensive restoration and management efforts that demand robust scientific knowledge. Winter limnology represents a key knowledge gap that limits understanding and prediction of the function of the Great Lakes and other large temperate lakes. Here, we summarize what is known about the Great Lakes during their 3–6 months of winter, identify key questions that must be addressed to improve understanding of the physical, chemical, and biological functioning of large lakes in winter, and suggest ways to address these questions. We show that ice cover is a “master variable” that controls numerous aspects of large temperate lake ecology and that the effects of the ongoing reduction in ice cover extent and duration cannot be predicted without improved knowledge of winter limnology.
Key Points
Winter limnology is a key knowledge gap that limits understanding and management of the Great Lakes and other large, seasonally frozen lakes
We review the winter physics, chemistry, and biology of the Great Lakes and identify priority questions for winter research on large lakes
Ice cover is a “master variable” for many large lake limnological processes, making a better understanding of its role a research priority
Understanding the role of seasonally ice-covered lakes in biogeochemical cycling is important for predicting the effects of changing winter conditions. However, little is known about phosphorus (P) ...cycling and the mechanisms of internal P release in seasonally ice-covered lakes. We investigated under-ice P cycling in a large and temperate lake composed of several basins with significantly different water depths. We used a multifaceted approach, combining analyses of P and total iron (Fe) distributions in sediments, pore water, and the water column. During the winter period, steep gradients of redox-sensitive parameters (i.e., O2 and Eh) formed at the sediment-water interface (SWI) due to minimal water movement and water column hypoxia. P diffusive fluxes during winter are substantial (0.2–4.8 mg P/m2/day). Short-term P release in shallow basins is strongly influenced by Fe reduction. In contrast, decoupling of Fe from P in deep basins suggests that hypoxia does not play a major role in short-term P mobilization. The differences in short-term P release mechanisms can be interpreted in terms of basin depth, stability of thermal stratification and time of water mixing. This ultimately influences the behavior of Fe and P, as well as their speciation and concentration in the sediment and water column. The deep basins in LOW contained higher total phosphorus (TP) and total iron (Fe) in sediments and soluble reactive phosphorus (SRP) in deep water compared to shallow basins. In surface sediments, P bound to the redox-sensitive P binding form (BD-P) is the diagenetically reactive P phase, emphasizing strong coupling with ferric Fe. In contrast, calcium carbonate-bound P in surface sediments indicates diagenetic sequestration of P with sediment burial. Overall, this study provides evidence that P cycling remains active in winter, and an understanding of its contribution to the overall ecosystem process is needed to predict how lake ecosystems will behave under climate change.
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•Under-ice P cycling is associated with spatial heterogeneity in sediment composition.•Steep gradients in redox-sensitive parameters were formed at the SWI.•Reductive dissolution of Fe(oxy)hydroxides drove P and Fe fluxes.•P bound to redox sensitive (FeBD-P) forms was the major source of P release.•P immobilization was controlled by redox-insensitive calcium (Ca)-P phases.
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