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Under‐ice photoautotrophs in lakes are generally considered to be limited by light rather than nutrients. Despite reduced light intensity under the ice, there is increasing evidence that suggests some lakes support high levels of photoautotrophs. We explored how snow cover (i.e., light) and nutrients (i.e., nitrogen and phosphorus) influence ice‐associated photoautotroph growth in a Minnesota, USA lake. Using a novel under‐ice approach, we deployed nutrient diffusing substrates (single or combined nutrient amendments) under two different light scenarios (snow covered, reduced light; snow removed, increased light) near the water‐ice interface to mimic a range of conditions ice‐associated photoautotrophs may be exposed to. Natural snow cover reduced light compared with snow removal, particularly early in the experiment before snow began to melt. When comparing photoautotroph chlorophyll a (Chl a) between snow treatments, we found a significant snow effect with higher concentrations in the snow removed treatment. We also found a significant nutrient effect, for all nutrient treatments, on Chl a concentrations in both snow conditions. The effect of any nutrient treatment on Chl a concentrations was similar. Our results suggest that ice‐associated photoautotrophs were able to grow in all snow conditions, but snow removal resulted in higher growth and nutrient availability also mediated responses. Thus, both light and nutrient conditions in the winter may strongly affect ice‐associated photoautotroph dynamics. [ABSTRACT FROM AUTHOR]

In lakes, the rates of gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP) are often controlled by resource availability. Herein, we explore how catchment vs. within lake predictors of metabolism compare using data from 16 lakes spanning 39°N to 64°N, a range of inflowing streams, and trophic status. For each lake, we combined stream loads of dissolved organic carbon (DOC), total nitrogen (TN), and total phosphorus (TP) with lake DOC, TN, and TP concentrations and high frequency in situ monitoring of dissolved oxygen. We found that stream load stoichiometry indicated lake stoichiometry for C : N and C : P (r2 = 0.74 and r2 = 0.84, respectively), but not for N : P (r2 = 0.04). As we found a strong positive correlation between TN and TP, we only used TP in our statistical models. For the catchment model, GPP and R were best predicted by DOC load, TP load, and load N : P (R2 = 0.85 and R2 = 0.82, respectively). For the lake model, GPP and R were best predicted by TP concentrations (R2 = 0.86 and R2 = 0.67, respectively). The inclusion of N : P in the catchment model, but not the lake model, suggests that both N and P regulate metabolism and that organisms may be responding more strongly to catchment inputs than lake resources. Our models predicted NEP poorly, though it is unclear why. Overall, our work stresses the importance of characterizing lake catchment loads to predict metabolic rates, a result that may be particularly important in catchments experiencing changing hydrologic regimes related to global environmental change. [ABSTRACT FROM AUTHOR]

Cyanobacterial blooms have substantial direct and indirect negative impacts on freshwater ecosystems including releasing toxins, blocking light needed by other organisms, and depleting oxygen. There is growing concern over the potential for climate change to promote cyanobacterial blooms, as the positive effects of increasing lake surface temperature on cyanobacterial growth are well documented in the literature; however, there is increasing evidence that cyanobacterial blooms are also being initiated and persisting in relatively cold‐water temperatures (< 15°C), including ice‐covered conditions. In this work, we provide evidence of freshwater cold‐water cyanobacterial blooms, review abiotic drivers and physiological adaptations leading to these blooms, offer a typology of these lesser‐studied cold‐water cyanobacterial blooms, and discuss their occurrence under changing climate conditions. [ABSTRACT FROM AUTHOR]

Winter conditions, such as ice cover and snow accumulation, are changing rapidly at northern latitudes and can have important implications for lake processes. For example, snowmelt in the watershed—a defining feature of lake hydrology because it delivers a large portion of annual nutrient inputs—is becoming earlier. Consequently, earlier and a shorter duration of snowmelt are expected to affect annual phytoplankton biomass. To test this hypothesis, we developed an index of runoff timing based on the date when 50% of cumulative runoff between January 1 and May 31 had occurred. The runoff index was computed using stream discharge for inflows, outflows, or for flows from nearby streams for 41 lakes in Europe and North America. The runoff index was then compared with summer chlorophyll‐a (Chl‐a) concentration (a proxy for phytoplankton biomass) across 5–53 years for each lake. Earlier runoff generally corresponded to lower summer Chl‐a. Furthermore, years with earlier runoff also had lower winter/spring runoff magnitude, more protracted runoff, and earlier ice‐out. We examined several lake characteristics that may regulate the strength of the relationship between runoff timing and summer Chl‐a concentrations; however, our tested covariates had little effect on the relationship. Date of ice‐out was not clearly related to summer Chl‐a concentrations. Our results indicate that ongoing changes in winter conditions may have important consequences for summer phytoplankton biomass and production. [ABSTRACT FROM AUTHOR]

Widespread long‐term increases in dissolved organic carbon (DOC) concentrations (i.e., "browning") have been observed in many lakes, but the ecological consequences are poorly understood. Some studies suggest a unimodal relationship between DOC and primary productivity, with peak productivity at intermediate DOC concentrations. This peak is hypothesized to result from the tradeoff between light absorbing properties of DOC, and increases in limiting nutrients with browning. Nevertheless, it is unclear whether nutrient stoichiometry is constant as lakes brown. Across both regional and national surveys, we found a positive linear relationship between DOC and both total and organic forms of nitrogen and phosphorus. However, long‐term data from a suite of browning lakes indicates that total nutrients do not increase as DOC increases through time. Our results show that DOC and limiting nutrients are coupled spatially, but not temporally, and that this temporal mismatch challenges previous conceptualizations of the long‐term effects of browning on productivity. [ABSTRACT FROM AUTHOR]

People extensively use lakes and rivers covered by seasonal ice. Although ice cover duration has been declining over the past 150 years for Northern Hemisphere freshwaters, we know relatively little about how ice loss directly affects humans. Here, we synthesize the cultural ecosystem services (i.e., services that provide intangible or nonmaterial benefits) and associated benefits supported by inland ice. We also provide, for the first time, empirical examples that give quantitative evidence for a winter warming effect on a wide range of ice‐related cultural ecosystem services and benefits. We show that in recent decades, warmer air temperatures delayed the opening date of winter ice roads and led to cancellations of spiritual ceremonies, outdoor ice skating races, and ice fishing tournaments. Additionally, our synthesis effort suggests unexploited data sets that allow for the use of integrative approaches to evaluate the interplay between inland ice loss and society. [ABSTRACT FROM AUTHOR]

Winter is an important season for many limnological processes, which can range from biogeochemical transformations to ecological interactions. Interest in the structure and function of lake ecosystems under ice is on the rise. Although limnologists working at polar latitudes have a long history of winter work, the required knowledge to successfully sample under winter conditions is not widely available and relatively few limnologists receive formal training. In particular, the deployment and operation of equipment in below 0°C temperatures pose considerable logistical and methodological challenges, as do the safety risks of sampling during the ice‐covered period. Here, we consolidate information on winter lake sampling and describe effective methods to measure physical, chemical, and biological variables in and under ice. We describe variation in snow and ice conditions and discuss implications for sampling logistics and safety. We outline commonly encountered methodological challenges and make recommendations for best practices to maximize safety and efficiency when sampling through ice or deploying instruments in ice‐covered lakes. Application of such practices over a broad range of ice‐covered lakes will contribute to a better understanding of the factors that regulate lakes during winter and how winter conditions affect the subsequent ice‐free period. [ABSTRACT FROM AUTHOR]

Abstract: Surface water temperatures are warming in many lakes across the globe, and this is widely attributed to warming air temperatures. Yet two lakes in Pennsylvania (USA) have shown long‐term increases in surface water temperatures over the past 27 summers during a period with no significant increase in regional air temperature. We examined the relationship between long‐term trends in seven metrics of whole‐lake thermal structure in two lakes and several potential driver variables. Driver variables included water transparency, lake pH, and meteorological variables. Both lakes exhibited significant surface warming and hypolimnetic cooling, resulting in stronger thermal stratification that further reduced mixing and heat transfer to deep waters. During this time period, there were no long‐term trends in solar radiation or in thawing degree days, but annual precipitation and lake pH increased. Water transparency greatly decreased due to increased dissolved organic matter quantity and color, most likely due to increased precipitation and recovery from anthropogenic acidification. In both lakes, the changes in lake thermal structure and heat distribution were strongly related to the decreases in water transparency and increases in dissolved organic matter. This transparency‐mediated mechanism may augment the effects of air temperature‐driven lake warming in other regions where decreasing transparency is also prevalent, further enhancing increases in surface water temperature and thermal stratification. These results have important ecological and biogeochemical implications, highlighting the need for investigations of multiple drivers to fully understand how lakes will respond to future climate change. [ABSTRACT FROM AUTHOR]

Lakes and reservoirs bury large quantities of organic carbon ( C) and nutrients (nitrogen, N; phosphorus, P) in their sediments, especially when expressed relative to the small area they occupy. Global estimates of C and nutrient burial rates in reservoirs require a quantitative understanding of the wide variation in the rates and ratios at which C, N and P are sequestered in sediments., We examined how catchment and reservoir characteristics relate to sediment organic C, N and P concentrations, stoichiometric burial ratios ( C : P, C : N, N : P) and burial rates in 13 small reservoirs across a catchment land use gradient in the Midwestern United States., Sediment P concentrations were positively correlated with urban catchment land use and negatively correlated with agricultural catchment land use. Stoichiometric burial ratios varied with catchment land use. Both N : P and C : P were positively correlated with agricultural land use, while these ratios were negatively correlated with urban land use and forested land use., In general, rates of C, N and P burial per unit lake area were not related to land use in the catchment, but were all positively correlated with catchment area to lake area ratios. Results from our study reservoirs suggest that reservoir burial rates are more tightly coupled with morphometric catchment characteristics than with land use., Our results suggest that small reservoirs are regionally and globally significant for biogeochemical processing. However, regional variation requires that much more comprehensive sampling is needed for accurate estimates of global element burial rates. [ABSTRACT FROM AUTHOR]

Sediment organic carbon (C) burial and CO2 fluxes in inland waters are quantitatively important in regional and global carbon budgets. Estimates of C fluxes from inland waters are typically based on limited temporal resolution despite potential large variations with season and weather events. Further, most freshwater C budget studies have focused on natural soft-water lakes, while reservoirs and hard-water systems are globally numerous. Our study quantifies C fluxes in two hard-water, human constructed reservoirs (Ohio, USA) of contrasting watershed land use (agriculture vs. forest) using high-resolution mass balance budgets. We show that during a dry summer, C retention and export via the dam were reduced compared to a wet summer. Both reservoirs were net CO2 sources during a wet summer, but CO2 sinks during a dry summer. Despite weather-related summer differences, annual C fluxes within each reservoir were similar between years. Both reservoirs appear to be net autotrophic despite often being CO2 sources based on budgets. This is likely because CO2 fluxes in our hard-water reservoirs were more strongly associated with DIC than DOC. Using our C fluxes and statewide watershed land use, we determined the regional importance of Ohio reservoirs in OC burial and CO2 emissions. We estimate that Ohio reservoirs bury up to 4 times more OC, but emit <25% of CO2, than predicted based on their area and recent global mean estimates in lentic ecosystems. Our results provide evidence that moderately old (~50 years), temperate hard-water reservoirs are important OC sinks but contribute little to CO2 emissions. [ABSTRACT FROM AUTHOR]

1. Elevated allochthonous inputs of nutrients and sediments to aquatic ecosystems are associated with eutrophication and sedimentation. Reservoirs receive substantial subsidies of nutrients and sediments from catchments due to their large catchment : lake area ratios. We examined the effect of elevated subsidies of sediments and/or dissolved nutrients on the success (survival, growth, biomass and condition factor) of larval gizzard shad ( Dorosoma cepedianum), a widespread and dominant omnivorous fish in reservoir ecosystems. 2. We simulated allochthonous agricultural subsides by manipulating dissolved nutrients and sediment inputs in a 2 × 2 factorial design in experimental mesocosms. We predicted that larval fish success would be greater under elevated nutrients. However, we propose two alternative hypotheses with respect to the overall effect of allochthonous sediment inputs. If sediment inputs negatively affect larval gizzard feeding success, larval success would be highest when only nutrients are added and lowest when only sediments are added (+N > +N+S ≥ C > +S). If high turbidity enhances larval foraging activity (due to greater contrast between prey and background), we predict that larval success would be highest when both subsidy types (nutrients and sediment) are elevated, intermediate when either nutrients or sediments are added and the lowest when no subsidies are added (+N+S > +N ≥ +S > C). 3. Our results indicate that elevated nutrient and sediment conditions enhanced larval gizzard shad biomass, but the overall nutrient addition effect was greater than the sediment addition effect (+N ∼ +N+S > +S > C). We observed differential effects of nutrient and sediment inputs on larval survival, growth and condition factors. 4. The enhancement of fish biomass in elevated nutrients (+N, +N+S) relative to control conditions was associated with improved gizzard shad survival and not greater growth. The enhancement of fish biomass in the elevated sediment treatment (+S) relative to the control conditions was caused by an increase in survival that more than compensated for a negative effect of sediment addition on growth. 5. Our findings support the recommendation that reservoir management practices must consider the links between land use practices and food web dynamics. Our results suggest that reduction of subsidies of nutrients and sediments to productive reservoirs would decrease survival of larval gizzard shad due to lower food availability. [ABSTRACT FROM AUTHOR]

Herbivores can have both direct (consumptive) and indirect (nutrient-mediated) effects on primary producer biomass and nutrient stoichiometry. Ecological stoichiometry theory predicts that herbivores of contrasting body stoichiometry will differentially remineralize nutrients, resulting in feedbacks on producer stoichiometry. We experimentally separated direct and indirect effects of aquatic vertebrate grazers on periphyton by manipulating grazer abundance and identity in mesocosms, and using grazer exclusion cages to expose periphyton to recycled nutrients in the absence of direct grazing. In experiment 1, we used a catfish with high body phosphorus (low body N:P), Ancistrus triradiatus, to assess consumptive versus nutrient-mediated effects of grazer density on periphyton. In experiment 2, we compared the nutrient-mediated effects of grazing by Ancistrus triradiatus and Rana palmipes, a tadpole with low body phosphorus and high body N:P. In experiment 1, we found that increasing catfish density led to lower biomass and particulate nutrients in periphyton through direct consumptive effects, but that nutrient-mediated indirect effects enhanced periphyton biomass when grazers were experimentally separated from direct contact with periphyton. As predicted by stoichiometry theory, nutrient recycling by this P-rich grazer tended to increase algal C:P and N:P (although effects were not statistically significant), while their consumptive effects reduced algal C:P and N:P. In experiment 2, grazer identity had strong effects on dissolved water nutrient concentrations, N recycling (measured with a 15N tracer), and periphyton stoichiometry. In accordance with stoichiometry theory, catfish increased N concentrations and recycling rates leading to higher periphyton N:P, while tadpoles had greater effects on P availability leading to lower periphyton N:P. Our experiments elucidate the importance of both the density and identity of grazers in controlling periphyton biomass and stoichiometry through consumptive and nutrient-mediated effects, and support the power of ecological stoichiometry theory to predict feedbacks on producer stroichiometry arising from consumer stoichiometry through nutrient recycling. [ABSTRACT FROM AUTHOR]

Animals can be important in nutrient cycling in particular ecosystems, but few studies have examined bow this importance varies along environmental gradients. In this study we quantified the nutrient cycling role of an abundant detritivorous fish species, the gizzard shad (Dorosoma cepedianum), in reservoir ecosystems along a gradient of ecosystem productivity. Gizzard shad feed mostly on sediment detritus and excrete sediment-derived nutrients into the water column, thereby mediating a cross-habitat translocation of nutrients to phytoplankton. We quantified nitrogen and phosphorus cycling (excretion) rates of gizzard shad, as well as nutrient demand by phytoplankton, in seven lakes over a four-year period (16 lake-years). The lakes span a gradient of watershed land use (the relative amounts of land used for agriculture vs. forest) and productivity. As the watersheds of these lakes became increasingly dominated by agricultural land, primary production rates, lake trophic state indicators (total phosphorus and chlorophyll concentrations), and nutrient flux through gizzard shad populations all increased. Nutrient cycling by gizzard shad supported a substantial proportion of primary production in these ecosystems, and this proportion increased as watershed agriculture (and ecosystem productivity) increased, In the four productive lakes with agricultural watersheds (>78% agricultural land), gizzard shad supported on average 51% of phytoplankton primary production (range 27%–67%). In contrast, in the three relatively unproductive lakes in forested or mixed-land-use watersheds (>47% forest, <52% agricultural land), gizzard shad supported 18% of primary production (range 14–23%). Thus, along a gradient of forested to agricultural landscapes, both watershed nutrient inputs and nutrient translocation by gizzard shad increase, but our data indicate that the importance of nutrient translocation by gizzard shad increases more rapidly. Our results therefore support the hypothesis that watersheds and gizzard shad jointly regulate primary production in reservoir ecosystems. [ABSTRACT FROM AUTHOR]

We report a reversal in the sign of the herbivore-phytoplankton interaction between the zebra mussel (Dreissena polymorpha) and Microcystis aeruginosa, a harmful planktonic cyanobacterium. A pair of large-scale manipulations of mussel density in the same lake in consecutive years showed that when phosphorus concentrations were very low (mean total phosphorus [TP] ∼ 3 µg L-1), the effect of Dreissena on the biomass of M. aeruginosa was monotonically negative across the full range of sustainable mussel densities. When the enclosures were fertilized with phosphorus (mean TP ∼ 9 µg L-1), there was a monotonically positive effect of Dreissena on M. aeruginosa across the same mussel gradient. These contrasting results indicate that D. polymorpha feeds on M. aeruginosa, as shown in previous laboratory feeding experiments, but that the positive effects of D. polymorpha on M. aeruginosa can be larger than the negative effects of consumption. A sign reversal in the interaction between these two species is congruent with highly variable patterns in the response of M. aeruginosa to D. polymorpha invasion across lake and river systems in North America. [ABSTRACT FROM AUTHOR]

Presents a study that investigated how watershed land use relates to phytoplankton primary production and photosynthetic parameters in 12 reservoirs in Ohio and examined spatial variation in these parameters. Materials and methods; Seasonal variation in photosynthetic parameters at inflow and outflow sites; Relationships between watershed land use and primary production, chlorophyll and total phosphorus.

As the lowest point in the surrounding landscape, lakes act as sensors in the landscape to provide insights into the response of both terrestrial and aquatic ecosystems to climate change. Here a novel suite of climate forcing optical indices (CFOI) from lakes across North America is found to respond to changes in air temperature, precipitation, and solar radiation at timescales ranging from a single storm event to seasonal changes to longer-term interdecadal trends with regression r² values ranging from 0.73 to 0.89. These indices are based on two optical metrics of dissolved organic carbon (DOC) quality: DOC specific absorbance (a*320) and spectral slope (S275-295), where the ratio a*320 to S275-295 gives a composite climate forcing index. These indices of DOC quality are more responsive to climate forcing than is DOC concentration. A similar relationship between the component indices a*320 and S275-295 is observed across a wide range of lake types. A conceptual model is used to examine the similarities and differences in DOC-related mechanisms and ecological consequences due to increased temperature vs. precipitation. While both warmer and wetter conditions increase thermal stratification, these two types of climate forcing will have opposite effects on water transparency as well as many ecological consequences, including oxygen depletion, the balance between autotrophy and heterotrophy, and depth distributions of phytoplankton and zooplankton. [ABSTRACT FROM AUTHOR]

We asked if intraspecific variation within a population of the toxic, bloom-forming phytoplankter, Microcystis aeruginosa, leads to differential vulnerability to grazing by the invasive, filter-feeding zebra mussel (Dreissena polymorpha). We performed two series of laboratory feeding experiments in which D. polymorpha were presented with several different sympatric M. aeruginosa clones in a two-species mixture with the high-quality, nutritious alga, Ankistrodesmus falcatus. Mean selectivity across M. aeruginosa clones ranged from near zero (i.e., no feeding on M. aeruginosa) to near one (equal feeding on the two algal species), evidence of maximal variation in grazing vulnerability across twenty clones of M. aeruginosa--a species assumed to be largely 'inedible' to grazers--from the same population. This range of vulnerability is essentially equal to that typically measured across all phytoplankton species. Large intraspecific variation in grazing vulnerability, and its ecological consequences, could influence the promotion or control of noxious blooms of toxigenic cyanobacteria. [ABSTRACT FROM AUTHOR]

We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments. [ABSTRACT FROM AUTHOR]