Expanding the Application of Sentinel-2 Chlorophyll Monitoring across United States Lakes.

Eutrophication of inland lakes poses various societal and ecological threats, making water quality monitoring crucial. Satellites provide a comprehensive and cost-effective supplement to traditional in situ sampling. The Sentinel-2 MultiSpectral Instrument (S2 MSI) offers unique spectral bands positioned to quantify chlorophyll a, a water-quality and trophic-state indicator, along with fine spatial resolution, enabling the monitoring of small waterbodies. In this study, two algorithms-the Maximum Chlorophyll Index (MCI) and the Normalized Difference Chlorophyll Index (NDCI)-were applied to S2 MSI data. They were calibrated and validated using in situ chlorophyll a measurements for 103 lakes across the contiguous U.S. Both algorithms were tested using top-of-atmosphere reflectances (ρ t), Rayleigh-corrected reflectances (ρ s), and remote sensing reflectances (R rs ). MCI slightly outperformed NDCI across all reflectance products. MCI using ρ t showed the best overall performance, with a mean absolute error factor of 2.08 and a mean bias factor of 1.15. Conversion of derived chlorophyll a to trophic state improved the potential for management applications, with 82% accuracy using a binary classification. We report algorithm-to-chlorophyll-a conversions that show potential for application across the U.S., demonstrating that S2 can serve as a monitoring tool for inland lakes across broad spatial scales.

Eutrophication impacts the distribution and functional traits of viral communities in lakes.

Viruses play a crucial role in aquatic ecosystems by regulating microbial composition and impacting biogeochemical cycling. While the response of viral diversity to the trophic status has been preliminarily explored in lake ecosystems, there is limited integrated exploration of the biogeography of viruses, host associations, and the auxiliary metabolic genes (AMGs), particularly for plateau lakes. Therefore, this research investigated the viral biogeography, virus-host association, and AMGs in the surface waters of 11 lakes varying in trophic levels (eutrophic and oligo-mesotrophic) in the Yunnan-Guizhou plateau region of China. A total of 73,105 viral operational taxonomic units were obtained from 11 samples, with 84.8 % remaining unannotated at the family level, indicating a predominance of novel viruses within these lakes. The most abundant viral family was Kyanoviridae (24.4 %), recognized as a common cyanophage. The vast majority of cyanobacteria and several eukaryotic algae were predicted as hosts for the viruses, with a lytic lifestyle predominating the life strategy of these cyanophages, implying the potential influence of the virus on algae. The viral community structure significantly correlated with both trophic status and the bacterial community. The structure equation model analysis revealed chlorophyll a was the primary factor affecting viral communities. Moreover, numerous AMGs linked to carbon metabolism, phosphorus metabolism, sulfur metabolism, and photosynthesis were found in these lakes, some of which showed virus preference for the trophic statuses, suggesting a vital role of the virus in driving biogeochemical cycling in the lake crossing different nutrient levels. In addition, a restricted presence of viruses was found to infect humans or harbor antibiotic resistance genes in the lakes, suggesting a subtle yet potential link to human health. Overall, these findings offer insights into the response of viral communities to eutrophication and their potential role in biogeochemical cycling and controlling algal propagation.

From disruption to adaptation: Response of phytoplankton communities in representative impounded lakes to China's South-to-North Water Diversion Project.

Impounded lakes are often interconnected in large-scale water diversion projects to form a coordinated system for water allocation and regulation. The alternating runoff and transferred water can significantly impact local ecosystems, which are initially reflected in the sensitive phytoplankton. Nonetheless, limited information is available on the temporal dynamics and assembly patterns of phytoplankton community in impounded lakes responding to continuous and periodic water diversion. Herein, a long-term monitoring from 2013 to 2020 were conducted to systematically investigate the response of phytoplankton community, including its characteristics, stability, and the ecological processes governing community assembly, in representative impounded lakes to the South-to-North Water Diversion Project (SNWDP) in China. In the initial stage of the SNWDP, the phytoplankton diversity indices experienced a decrease during both non-water diversion periods (8.5 %∼21.2 %) and water diversion periods (5.6 %∼12.2 %), implying a disruption in the aquatic ecosystem. But the regular delivery of high-quality water from the Yangtze River gradually increased phytoplankton diversity and mediated ecological assembly processes shifting from stochastic to deterministic. Meanwhile, reduced nutrients restricted the growth of phytoplankton, pushing species to interact more closely to maintain the functionality and stability of the co-occurrence network. The partial least squares path model revealed that ecological process (path coefficient = 0.525, p < 0.01) and interspecies interactions in networks (path coefficient = -0.806, p < 0.01) jointly influenced the keystone and dominant species, ultimately resulting in an improvement in stability (path coefficient = 0.878, p < 0.01). Overall, the phytoplankton communities experienced an evolutionary process from short-term disruption to long-term adaptation, demonstrating resilience and adaptability in response to the challenges posed by the SNWDP. This study revealed the response and adaptation mechanism of phytoplankton communities in impounded lakes to water diversion projects, which is helpful for maintaining the lake ecological health and formulating rational water management strategies.

Unexpected increase of sulfate concentrations and potential impact on CH4 budgets in freshwater lakes.

The continuous increase in sulfate (SO42-) concentrations discharged by anthropogenic activities lacks insights into their dynamics and potential impact on CH4 budgets in freshwater lakes. Here we conducted a field investigation in the lakes along the highly developed Yangtze River basin, China, additionally, we analyzed long-term data (1950-2020) from Lake Taihu, a typical eutrophic lake worldwide. We observed a gradual increase in SO42- concentrations up to 100 mg/L, which showed a positive correlation with the trophic state of the lakes. The annual variations indicated that eutrophication intensified the fluctuation of SO42- concentrations. A random forest model was applied to assess the impact of SO42- concentrations on CH4 emissions, revealing a significant negative effect. Synchronously, a series of microcosms with added SO42- were established to simulate cyanobacteria decomposition processes and explore the coupling mechanism between sulfate reduction and CH4 production. The results showed a strong negative correlation between CH4 concentrations and initial SO42- levels (R2 = 0.83), indicating that higher initial SO42- concentrations led to lower final CH4 concentrations. This was attributed to the competition for cyanobacteria-supplied substrates between sulfate reduction bacteria (SRB) and methane production archaea (MPA). Our study highlights the importance of considering the unexpectedly increasing SO42- concentrations in eutrophic lakes when estimating global CH4 emission budgets.

Watershed hydrology mediates the recovery of an arsenic impacted subarctic landscape.

A holistic understanding of the chemical recovery of lakes from arsenic (As) pollution requires consideration of within-lake biogeochemical cycling of As and processes occurring in the surrounding catchment. This study used a watershed mass balance approach, complemented by experimental sediment incubations, to assess the mobility and transport of As within a subarctic watershed (155 km2) impacted by more than 60 years of atmospheric mining emissions. The period of record spanned a transition from drought to high streamflow between September 2017 and September 2019, which yielded insights into the interacting effects of hydrology and within-lake biogeochemical cycling of As. Internal loading of As from contaminated lake sediments (25 - 46 kg As year-1) and contributions from terrestrial sources (16 - 56 kg As yr-1) continue to negatively impact lake water quality (19 - 144 µg As L-1), but the relative importance of these loads varies seasonally and inter-annually in response to changing hydrological conditions. Wet conditions resulted in greater transport of As from terrestrial reservoirs and upstream areas, shorter lake water retention time, and increased the downstream export of As. During dry periods, the lake was disconnected from the surrounding watershed resulting in limited terrestrial contributions and longer lake water residence time, which delayed recovery due to the greater relative influence of internal loading from contaminated sediments. This study highlights that changing hydroclimatic regimes will alter trajectories of chemical recovery for arsenic impacted lakes through the coupling of within-lake and watershed transport processes.

Water quality dynamics and underlying controls in the Halton Region, Ontario.

We assessed the hydrochemistry of 15 watersheds in the Halton Region, southern Ontario, in high resolution (n > 500 samples across n > 40 streams) to characterize water quality dynamics and governing controls on major and trace element concentrations in this rapidly urbanizing region. In 2022, major water quality parameters were generally in line with historic monitoring data yet significantly different across catchments, e.g., in specific conductance, turbidity, phosphate and chloride, and trace element concentrations. Distinct hydrochemical signatures were observed between urban and rural creeks, with urban stream sections and sites near the river mouths close to Lake Ontario having consistently higher chloride (up to 700 mg/L) and occasional enrichment in nutrients levels (up to 8 and 20 mg/L phosphate and nitrate, respectively). Particularly upper reaches exhibited hydrochemical signatures that were reflective of the catchment surface lithologies, for instance through higher dissolved Ca to Mg ratios. Unlike for chloride and phosphate, provincial water quality guidelines for trace elements and heavy metals were seldom surpassed (on < 10 occasions for copper, zinc, cadmium, and uranium). Concentrations of other trace elements (e.g., platinum group elements or rare earth elements) were expectedly low (< 0.3 µg/L) but showed spatiotemporal concentration patterns and concentration-discharge dynamics different from those of the major water quality parameters. Our results help improve the understanding of surface water conditions within Halton's regional Natural Heritage Systems and demonstrate how enhanced environmental monitoring can deliver actionable information for watershed decision-making.

A Decade of Data and Hundreds of Analytes: Legacy and Emerging Chemicals in North American Herring Gull Plasma.

Between 2010 and 2021, 199 herring gull serum samples were collected from Lake Michigan, Lake Huron, and Lake Erie, including two Areas of Concern: Saginaw Bay and the River Raisin. They were analyzed for 21 polybrominated diphenyl ether congeners, 10 non-PBDE flame retardants, 85 polychlorinated biphenyls, 17 legacy organochlorine pesticides, and 36 per- and polyfluoroalkyl substances. Σ36PFAS, Σ85PCB, Σ21PBDE, and Σ17Pesticide concentrations comprised 41-74%, 17-50%, 3-4%, and 5-9% of the total concentration, respectively. Median concentrations of the chemical groups ranged from 81.5 - 129 ng/g ww for PFAS, 26.3 - 158 ng/g ww for PCBs, 4.26 - 8.89 ng/g ww for PBDEs, and 8.08 - 23.0 ng/g ww for pesticides. The regional concentrations of all four classes of compounds are significantly decreasing when sites are combined with halving times of 11.3 ± 4.8, 8.2 ± 4.3, 5.9 ± 3.1, and 8.3 ± 4.2 years for Penta-BDE mixture, ΣDDTs, Σ85PCBs and Σ36PFAS, respectively. These results suggest that while PFAS has emerged as the dominant group of chemicals in the plasma, legacy pollutants continue to represent a threat to herring gulls and wildlife in the Great Lakes basin. PCBs were the largest contributor to the chemical load in plasma of birds whose colonies are located near the River Raisin, and continue to pose a threat to herring gulls within the two Areas of Concern.

Do new lakes behave like natural lakes regarding sediment composition and phosphorus fluxes?

Numerous new lakes have been established during the last few decades. Lakes established on former agricultural soils often have high legacy phosphorus (P)-content, which constitutes a risk for potential internal P-loading after the lake is formed. In this study, we compared the P release and sediment P-pools from 31 new lakes and 31 natural lakes, to assess their similarities and differences. A suite of other sediment characteristics was identified and compared for both natural and new lakes; catchment characteristics of the new lakes also were analyzed. P release from the sediment of new lakes was significantly lower than from natural lakes (13.2 mg P m-2 d-1) compared to new lakes (6.9 mg P m-2 d-1). The P release was found to be low when molar Fe:P ratios were above 10. A significant correlation was found between the content of mobile-P (loosely adsorbed P, iron-bound P, and leachable organic P) and TP in the sediment, irrespective of lake type. The composition of the mobile P-pool also differed, with the new non-excavated lakes showing a higher proportion of RP-BD; both new lake types had significantly (p = 0.021) lower proportions of nrP, compared to natural lakes in the uppermost 10 cm sediment. In addition, variance in P release and mobile-P content of new lakes could be explained in terms of the land use of the catchments. Most sediment characteristics of new lakes established without topsoil excavation reached the average levels of natural Danish lakes with respect to density, organic content and P content within 20-30 years, while excavated lakes showed no such tendencies.

Water residence time is an important predictor of dissolved organic matter composition and drinking water treatability.

Freshwater ecosystems are critical resources for drinking water. In recent decades, dissolved organic matter (DOM) inputs into aquatic systems have increased significantly, particularly in central and northern Europe, due to climatic and anthropogenic drivers. The associated increase in dissolved organic carbon (DOC) concentration can change lake ecosystem services and adversely affect drinking water treatment processes. In this study, we examined spatial and temporal patterns of DOM treatability with granular activated carbon (GAC) and biological reactivity based on 14-day bacterial respiration incubations at 11 sites across Mälaren during six-time points between July 2019 and February 2021. Mälaren is the third largest lake in Sweden and provides drinking water for over 2 million people including the capital city Stockholm. In our spatio-temporal analysis, we assessed the influence of phytoplankton abundance, water chemistry, runoff, and climate on DOM composition, GAC removal efficiency, and biological reactivity. Variations in DOM composition were characterized using optical measurements and Orbitrap mass spectrometry. Multivariate statistical analyses indicated that DOM produced during warmer months was easier to remove by GAC. Removal efficiency of GAC varied from 41 to 87 %, and the best predictor of treatability using mass spectrometry was double bond equivalents (DBE), while the best optical predictors were specific UV absorbance (SUVA), and freshness index. The oxygen consumption rate (k) from the bacterial respiration incubations ranged from 0.04 to 0.71 d-1 and higher in warmer months and at deeper basins and was associated with more aliphatic and fresh DOM. The three deepest lake basins with the longest water residence time (WRT) were temporally the most stable in terms of DOM composition and had the highest DOC removal efficiency and k rates. DOM composition in these three lake basins was optically clearer than in basins located closer to terrestrial inputs and had a signature suggesting it was derived from in-lake processes including phytoplankton production and bacterial processing of terrestrial DOM. This means that with increasing WRT, DOM derived from terrestrial sources shifts to more aquatically produced DOM and becomes easier to remove with GAC. These findings indicate WRT can be highly relevant in shaping DOM composition and thereby likely to affect its ease of treatability for drinking water purposes.

Mapping global lake aquatic vegetation dynamics using 10-m resolution satellite observations.

Aquatic vegetation is crucial for improving water quality, supporting fisheries and preserving biodiversity in lakes. Monitoring the spatiotemporal dynamics of aquatic vegetation is indispensable for the assessment and protection of lake ecosystems. Nevertheless, a comprehensive global assessment of lacustrine aquatic vegetation is lacking. This study introduces an automatic identification algorithm (with a total accuracy of 94.4%) for Sentinel-2 MSI, enabling the first-ever global mapping of aquatic vegetation distribution in 1.4 million lakes using 14.8 million images from 2019 to 2022. Results show that aquatic vegetation occurred in 81,116 lakes across six continents over the past four years, covering a cumulative maximum aquatic vegetation area (MVA) of 16,111.8 km2. The global median aquatic vegetation occurrence (VO, in %) is 3.0%, with notable higher values observed in South America (7.4%) and Africa (4.1%) compared with Asia (2.7%) and North America (2.4%). High VO is also observed in lakes near major rivers such as the Yangtze, Ob, and Paraná Rivers. Integrating historical data with our calculated MVA, the aquatic vegetation changes in 170 lakes worldwide were analyzed. It shows that 72.4% (123/170) of lakes experienced a decline in aquatic vegetation from the early 1980s to 2022, encompassing both submerged and overall aquatic vegetation. The most substantial decrease is observed in Asia and Africa. Our findings suggest that, beyond lake algal blooms and temperature, the physical characteristics of the lakes and their surrounding environments could also influence aquatic vegetation distribution. Our research provides valuable information for the conservation and restoration of lacustrine aquatic vegetation.

A Broad-Scale Look at Nutrient Limitation and a Shift toward Co-limitation in United States Lakes.

There is a longstanding debate about the role of nitrogen (N) vs phosphorus (P) in limiting primary production in lakes and whether co-nutrient limitation should be considered for managing eutrophication. We evaluated nutrient limitation and eutrophication at a subcontinental scale. Using U.S. Environmental Protection Agency National Lakes Assessment data, we assessed broad-scale patterns in nutrient limitation and compared samples of all surveyed lakes and lakes resurveyed in multiple surveys. We found that N correlated more strongly with productivity in the western U.S., while P correlated more strongly in the eastern U.S. The aggregated subcontinental effect suggests the importance of factors like N-deposition, terrestrial vegetation, underlying geology, and land use for understanding drivers of nutrient dynamics in lakes. Our study showed how patterns can aggregate across subcontinental scales yet still demonstrate considerable variation when more deeply examined on an individual lake level. Overall, we found that nutrient limitation is dynamic over space and time, with most lakes being co-limited. The prevalence of co-limitation also increased from 2007 to 2017. Trophic states within each limitation category varied substantially. Our findings underscore the need for combined N and P reductions to mitigate accelerated eutrophication.

Composite water value: A way forward to balance the development and protection of transboundary lakes.

Transboundary lakes are shared by multiple administrative regions. The key to balance the development and protection of transboundary lakes is to properly measure the value of water resources. Most of previous studies on the measurement of lake water resources value have not fully considered the ecosystem service function. This paper proposes a new concept "composite water value" to measure the value of transboundary lakes by integrating the external runoff value and the internal runoff value of water resources. The study constructs a composite water value measurement system for transboundary lakes, further analyzes its influencing factors,and applies the system to the case of Nansi Lake, a representative transboundary lake in eastern China. The results show that: (1) The composite water value of lakes is influenced by various factors, including industrial structure, water withdrawal, and water use methods, which impact the external runoff water value; meanwhile, the composite water quality and fluctuations in lake level are closely associated with the internal runoff water value. From 2008 to 2021, the average annual composite water value of Nansi Lake was 39.628 billion yuan, exhibiting a "rising-falling-fluctuating rising" trend due to pollution control policies, reduced precipitation, and enhanced water-saving technologies successively. (2) From a long-term perspective, it is necessary to focus on the internal runoff water use value of lakes. The internal runoff water value of Nansi Lake has been over 75% of the composite water value, and flood storage and water conservation are important manifestations of its ecosystem service value. (3) The external runoff water value of lake is closely related to the internal runoff water value, and relevant departments need to consider the balance between the water withdrawal of multiple cities along the lake and the retained water volume of the lake to achieve the maximum benefit of composite water value.

Ice phenology interactions with water and air temperatures in high mountain lakes.

Ice phenology is of great importance for the thermal structure of lakes and ponds and the biology of lake species. Under the current climate change conditions, ice-cover duration has been reduced by an advance in ice-off, and a delay in ice-on, and future projections foresee this trend as continuing. Here, we describe the current ice phenology of Pyrenean high mountain lakes and ponds, including ice-cover duration and ice-on and ice-off dates. We used mixed models to identify the variables that explained the observed patterns, extrapolated them across all water bodies in the mountain range, and related the seasonality of air and water temperatures with ice phenology using structural equation models. Ice phenology was obtained from the temperature series of 85 lakes and ponds for fourteen years, including 2001 to 2004 and 2009 to 2019. We discovered that high autumn precipitation was related to earlier ice-on dates, and that earlier ice-off dates were associated with higher following-summer water temperatures. We found a greater predictability of ice-off dates and ice-cover duration than ice-on dates. Altitude was the most important variable explaining the variation in ice phenology, followed by latitude, which was related to climatic differences among the northern and southern slopes of the mountain range. The lake area was significant for ice-on dates and ice-cover duration. The interannual variability in air temperature and radiation was remarkable for the ice-off date and ice-cover duration but not for the ice-on date. In contrast, wind speed was related to an earlier ice-off date and shorter ice-cover duration. All the measured lakes and ponds froze in winter during the studied period, a feature maintained in the extrapolation to the whole set of water bodies.

Vertical Distribution and Seasonal Patterns of Candidatus Nitrotoga in a Sub-Alpine Lake.

The nitrite oxidizing bacterial genus Ca. Nitrotoga was only recently discovered to be widespread in freshwater systems; however, limited information is currently available on the environmental factors and seasonal effects that influence its distribution in lakes. In a one-year study in a dimictic lake, based on monthly sampling along a vertical profile, the droplet digital PCR quantification of Ca. Nitrotoga showed a strong spatio-temporal patchiness. A correlation ana-lysis with environmental parameters revealed that the abundance of Ca. Nitrotoga correlated with dissolved oxygen and ammonium, suggesting that the upper hypolimnion of the lake is the preferred habitat.

Spatial distribution of sediment dissolved organic matter in oligotrophic lakes and its binding characteristics with Pb(II) and Cu(II).

Dissolved organic matter (DOM), the most active component in interstitial waters, determines the stability of heavy metals and secondary release in sediments. However, little is known about the composition and metal-binding patterns of DOM in interstitial water from oligotrophic lakes affected by different anthropogenic perturbations. Here, 18 interstitial water samples were prepared from sediments in agricultural, residential, tourist, and forest regions in an oligotrophic lake (Shengzhong Lake in Sichuan Province, China) watershed. Interstitial water quality and DOM composition, properties, and Cu(II)- and Pb(II)-binding characteristics were measured via physicochemical analysis, UV-vis spectroscopic, fluorescence excitation-emission matrix-parallel factor analysis (EEM-PARAFAC), and fluorescence titration methods. The DOM, which was produced mainly by microbial activities, had low molecular weights, humification degrees, and aromaticity. Based on EEM-PARAFAC results, the DOM was generally composed of tryptophan- (57.7%), terrestrial humic- (18.7%), microbial humic- (15.6%), and tyrosine-like (8.0%) substances. The DOM in the metal complexes was primarily composed of tryptophan-like substances, which accounted for ~42.6% of the DOM-Cu(II) complexes and ~72.0% of the DOM-Pb(II) complexes; however, microbial humic-like substances primarily contributed to the stability of DOM-Cu(II) (logKCu = 3.7-4.6) and DOM-Pb(II) (logKPb = 4.3-4.8). Water quality parameters did not significantly affect the stability of DOM-metal complexes. We demonstrated that the metal-binding patterns of DOM in interstitial water from oligotrophic lakes are highly dependent on microbial DOM composition and are affected by anthropogenic perturbations to a lesser extent.

Analyzing variation of water inflow to inland lakes under climate change: Integrating deep learning and time series data mining.

The alarming depletion of global inland lakes in recent decades makes it essential to predict water inflow from rivers to lakes (WIRL) trend and unveil the dominant influencing driver, particularly in the context of climate change. The raw time series data contains multiple components (i.e., long-term trend, seasonal periodicity, and random noise), which makes it challenging for traditional machine/deep learning techniques to effectively capture long-term trend information. In this study, a novel FactorConvSTLnet (FCS) method is developed through integrating STL decomposition, convolutional neural networks (CNN), and factorial analysis into a general framework. FCS is more robust in long-term WIRL trend prediction through separating trend information as a modeling predictor, as well as unveiling predominant drivers. FCS is applied to typical inland lakes (the Aral Sea and the Lake Balkhash) in Central Asia, and results indicate that FCS (Nash-Sutcliffe efficiency = 0.88, root mean squared error = 67m³/s, mean relative error = 10%) outperforms the traditional CNN. Some main findings are: (i) during 1960-1990, reservoir water storage (WSR) was the dominant driver for the two lakes, respectively contributing to 71% and 49%; during 1991-2014 and 2015-2099, evaporation (EVAP) would be the dominant driver, with the contribution of 30% and 47%; (ii) climate change would shift the dominant driver from human activities to natural factors, where EVAP and surface snow amount (SNW) have an increasing influence on WIRL; (iii) compared to SSP1-2.6, the SNW contribution would decrease by 26% under SSP5-8.5, while the EVAP contribution would increase by 9%. The findings reveal the main drivers of shrinkage of the inland lakes and provide the scientific basis for promoting regional ecological sustainability.

Phytoplankton biomass in northern lakes reveals a complex response to global change.

Global change may introduce fundamental alterations in phytoplankton biomass and community structure that can alter the productivity of northern lakes. In this study, we utilized Swedish and Finnish monitoring data from lakes that are spatially (135 lakes) and temporally (1995-2019, 110 lakes) extensive to assess how phytoplankton biomass (PB) of dominant phytoplankton groups related to changes in water temperature, pH and key nutrients [total phosphorus (TP), total nitrogen (TN), total organic carbon (TOC), iron (Fe)] along spatial (Fennoscandia) and temporal (25 years) gradients. Using a machine learning approach, we found that TP was the most important determinant of total PB and biomass of a specific species of Raphidophyceae - Gonyostomum semen - and Cyanobacteria (both typically with adverse impacts on food-webs and water quality) in spatial analyses, while Fe and pH were second in importance for G. semen and TN and pH were second and third in importance for Cyanobacteria. However, in temporal analyses, decreasing Fe and increasing pH and TOC were associated with a decrease in G. semen and an increase in Cyanobacteria. In addition, in many lakes increasing TOC seemed to have generated browning to an extent that significantly reduced PB. The identified discrepancy between the spatial and temporal results suggests that substitutions of data for space-for-time may not be adequate to characterize long-term effects of global change on phytoplankton. Further, we found that total PB exhibited contrasting temporal trends (increasing in northern- and decreasing in southern Fennoscandia), with the decline in total PB being more pronounced than the increase. Among phytoplankton, G. semen biomass showed the strongest decline, while cyanobacterial biomass showed the strongest increase over 25 years. Our findings suggest that progressing browning and changes in Fe and pH promote significant temporal changes in PB and shifts in phytoplankton community structures in northern lakes.

A regional examination of the footprint of agriculture and urban cover on stream water quality.

Freshwater systems in cold regions, including the Laurentian Great Lakes, are threatened by both eutrophication and salinization, due to excess nitrogen (N), phosphorus (P) and chloride (Cl-) delivered in agricultural and urban runoff. However, identifying the relative contribution of urban vs. agricultural development to water quality impairment is challenging in watersheds with mixed land cover, which typify most developed regions. In this study, a self-organizing map (SOM) analysis was used to evaluate the contributions of various forms of land cover to water quality impairment in southern Ontario, a population-dense, yet highly agricultural region in the Laurentian Great Lakes basin where urban expansion and agricultural intensification have been associated with continued water quality impairment. Watersheds were classified into eight spatial clusters, representing four categories of agriculture, one urban, one natural, and two mixed land use clusters. All four agricultural clusters had high nitrate-N concentrations, but levels were especially high in watersheds with extensive corn and soybean cultivation, where exceedances of the 3 mg L-1 water quality objective dramatically increased above a threshold of |∼30 % watershed row crop cover. Maximum P concentrations also occurred in the most heavily tile-drained cash crop watersheds, but associations between P and land use were not as clear as for N. The most urbanized watersheds had the highest Cl- concentrations and expansions in urban area were mostly at the expense of surrounding agricultural land cover, which may drive intensification of remaining agricultural lands. Expansions in tile-drained corn and soybean area, often at the expense of mixed, lower intensity agriculture are not unique to this area and suggest that river nitrate-N levels will continue to increase in the future. The SOM approach provides a powerful means of simplifying heterogeneous land cover characteristics that can be associated with water quality patterns and identify problem areas to target management.