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Freshwater ecosystems face ever increasing environmental and ecological challenges. Recognition of those challenges is not new—over 30 years ago, a series of workshops convened to develop a research agenda (The Freshwater Imperative) to address those challenges. It was recognized that a critical component of solving the problems that freshwater ecosystems face would require greater interdisciplinary collaboration. Here, we discuss the context for the "Revisiting the Freshwater Imperative" workshop and describe how a small‐workshop model can be used effectively to seed longer term collaborative partnerships in support of meeting the challenges freshwater ecosystems face. Some key recommendations include: (1) engage with participants prior to the workshop to maximize efficient use of time and prime divergence‐convergence thinking; (2) promote a bottom‐up participant driven structure; (2) develop an agenda with enough flexibility to accommodate participant driven changes (i.e., a "live" agenda); (3) utilize best practices for facilitating team science; and (4) provide coordinating structure for post‐workshop working group activities. [ABSTRACT FROM AUTHOR]

Processes regulating the rate of oxygen depletion determine whether hypoxia occurs and the extent to which greenhouse gases accumulate in seasonally ice‐covered lakes. Here, we investigate the oxygen budget of four arctic lakes using high‐frequency data during two winters in three shallow lakes (9–13 m maximal depth) and four winters in 24 m deep main basin of Toolik Lake. Incubation experiments measured sediment metabolism. Volume‐averaged oxygen depletion measured in situ was independent of water temperature and duration of the ice‐covered period. Average rates were between 0.2 and 0.39 g O2 m−2 d−1 in the shallow lakes and between 0.03 and 0.14 g O2 m−2 d−1 in Toolik Lake, with higher rates in smaller lakes with their larger sediment area to volume ratio. Rates decreased to ~ 20%–50% of initial values in late winter in the shallow lakes but less or not at all in Toolik. The lack of a decline in Toolik Lake points to continued oxygen transport to the sediment–water interface where oxygen consumption occurs. In all lakes, lower in situ oxygen depletion than in incubation measurements points toward increasing anoxia in the lower water column depressing loss rates. In Toolik, oxygen loss during early winter was less in years with minimal snow cover. Penetrative convection occurred, which could mix downwards oxygen produced by photosynthesis or excluded during ice formation. Estimates of these terms exceeded photosynthesis measured in sediment incubations. Modeling under ice‐oxygen dynamics requires consideration of optical properties and biological and transport processes that modify oxygen concentrations and distributions. [ABSTRACT FROM AUTHOR]

The Winter Limnology Network is a project supported by the U.S. National Science Foundation that aims to develop a network of researchers trained in conducting limnological research on frozen lakes. This project addresses the lack of training in areas such as ice safety, operating equipment in subzero temperatures, and collecting samples from under snow and ice. The network uses a team science approach and welcomes participation from researchers interested in paired winter-summer sampling. To build capacity for winter research, the network organized a comprehensive winter limnology training workshop for early career limnologists. The workshop included ice safety training, hands-on field research, and discussions on team science and science communication. The Winter Limnology Network plans to offer this training again in the future and encourages researchers to join their efforts. [Extracted from the article]

Drivers of phytoplankton and zooplankton dynamics vary spatially and temporally in estuaries due to variation in hydrodynamic exchange and residence time, complicating efforts to understand controls on food web productivity. We conducted approximately monthly (2012–2019; n = 74) longitudinal sampling at 10 fixed stations along a freshwater tidal terminal channel in the San Francisco Estuary, California, characterized by seaward to landward gradients in water residence time, turbidity, nutrient concentrations, and plankton community composition. We used multivariate autoregressive state space (MARSS) models to quantify environmental (abiotic) and biotic controls on phytoplankton and mesozooplankton biomass. The importance of specific abiotic drivers (e.g., water temperature, turbidity, nutrients) and trophic interactions differed significantly among hydrodynamic exchange zones with different mean residence times. Abiotic drivers explained more variation in phytoplankton and zooplankton dynamics than a model including only trophic interactions, but individual phytoplankton–zooplankton interactions explained more variation than individual abiotic drivers. Interactions between zooplankton and phytoplankton were strongest in landward reaches with the longest residence times and the highest zooplankton biomass. Interactions between cryptophytes and both copepods and cladocerans were stronger than interactions between bacillariophytes (diatoms) and zooplankton taxa, despite contributing less biovolume in all but the most landward reaches. Our results demonstrate that trophic interactions and their relative strengths vary in a hydrodynamic context, contributing to food web heterogeneity within estuaries at spatial scales smaller than the freshwater to marine transition. [ABSTRACT FROM AUTHOR]

Estuaries are among the most productive of aquatic ecosystems. Yet the collective understanding of patterns and drivers of primary production in estuaries is incomplete, in part due to complex hydrodynamics and multiple controlling factors that vary at a range of temporal and spatial scales. A whole-ecosystem experiment was conducted in a deep, pelagically dominated terminal channel of the Sacramento-San Joaquin Delta (California, USA) that seasonally appears to become nitrogen limited, to test whether adding calcium nitrate would stimulate primary productivity or increase phytoplankton density. Production did not respond consistently to fertilization, in part because nitrate and phytoplankton were dispersed away from the manipulated area within 1–3 days. Temporal and spatial patterns of gross primary production were more strongly related to stratification and light availability (i.e., turbidity) than nitrogen, highlighting the role of hydrodynamics in regulating system production. Similarly, chlorophyll was positively related not only to stratification but also to nitrogen—with a positive interaction—suggesting stratification may trigger nutrient limitation. The average rate of primary production (4.3 g O2 m−2 d−1), metabolic N demand (0.023 mg N L−1 d−1), and ambient dissolved inorganic nitrogen concentration (0.03 mg N L−1) indicate that nitrogen can become limiting in time and space, especially during episodic stratification events when phytoplankton are isolated within the photic zone, or farther upstream where water clarity increases, dispersive flux decreases, and stratification is stronger and more frequent. The role of hydrodynamics in organizing habitat connectivity and regulating physical and chemical processes at multiple temporal and spatial scales is critical for determining resource availability and evaluating biogeochemical processes in estuaries. [ABSTRACT FROM AUTHOR]

Ponds are often identified by their small size and shallow depths, but the lack of a universal evidence-based definition hampers science and weakens legal protection. Here, we compile existing pond definitions, compare ecosystem metrics (e.g., metabolism, nutrient concentrations, and gas fluxes) among ponds, wetlands, and lakes, and propose an evidence-based pond definition. Compiled definitions often mentioned surface area and depth, but were largely qualitative and variable. Government legislation rarely defined ponds, despite commonly using the term. Ponds, as defined in published studies, varied in origin and hydroperiod and were often distinct from lakes and wetlands in water chemistry. We also compared how ecosystem metrics related to three variables often seen in waterbody definitions: waterbody size, maximum depth, and emergent vegetation cover. Most ecosystem metrics (e.g., water chemistry, gas fluxes, and metabolism) exhibited nonlinear relationships with these variables, with average threshold changes at 3.7 ± 1.8 ha (median: 1.5 ha) in surface area, 5.8 ± 2.5 m (median: 5.2 m) in depth, and 13.4 ± 6.3% (median: 8.2%) emergent vegetation cover. We use this evidence and prior definitions to define ponds as waterbodies that are small (< 5 ha), shallow (< 5 m), with < 30% emergent vegetation and we highlight areas for further study near these boundaries. This definition will inform the science, policy, and management of globally abundant and ecologically significant pond ecosystems. [ABSTRACT FROM AUTHOR]

Our changing climate is having effects on freshwater ecosystems in all seasons, especially winter. High latitude lakes, wetlands, and rivers are experiencing shorter periods of ice cover, and lower latitudes systems that used to freeze are experiencing open water conditions throughout the winter. A 2019 AGU Chapman conference convened aquatic scientists to examine these changes and address the implications of changing winters to aquatic life, chemistry, and physics. Several studies demonstrate decreased ice cover duration than in the past. The removal of an ice "lid" from lakes and rivers impacts the exchange of gases with the atmosphere and the predominant types of metabolism occurring in the waters below, with the potential for more photosynthesis and an increase in oxic versus anoxic metabolism when the lid is removed. Multiple studies indicated an increase in the interannual variability of winters, especially in terms of ice‐cover duration and ice quality. Increased variability may simply be an outcome of a more variable winter climate or small differences in environmental conditions such as temperature that can have strong effects on gas exchange, light transmission, and turbulence when ice forms. A question that merits further consideration is whether and how winters of shorter duration and severity will change the dynamics of freshwater systems. Are there memory or legacy effects that carry over to the next season or year? There is much work to be done to understand how changing winters will impact the biogeochemical behavior of lakes and rivers in the coming decades. Plain Language Summary: Winters at mid‐high latitudes are changing rapidly with important implications for freshwater ecosystems. An AGU Chapman conference, convened in 2019, brought together scientists to outline what we know about the changes that are occurring and what questions need further study. This special issue is a collection of the papers that developed from that workshop. These studies demonstrate that lakes and rivers at mid‐high latitudes are experiencing less ice cover duration and greater variability in year‐to‐year ice cover. Changing ice and snow cover impacts processes occurring under ice in winter by altering the light regime, mixing, biota present, and the availability of oxygen. There are many important research questions regarding winter dynamics that still need to be addressed but one of the most important ones has to do with how changing winters might influence the biota in other seasons. Key Points: Several of the papers point to interannual variability in winter and seasonal ice cover along with decreased ice cover durationAvailable winter datasets have grown supporting conceptual developments, but increased efforts focused on ecological responses are neededFuture research should examine connections between seasons, deploy submersed sensors year‐round, and use remote sensing approaches [ABSTRACT FROM AUTHOR]

The impacts of wildfire smoke on lake habitats remains unclear. We determined the metabolic response to smoke in the epi‐pelagic and two littoral habitats in Castle Lake, California. We compared light regime, gross primary production, ecosystem respiration, and net ecosystem production in years with and without smoke. During the smoke period incident ultraviolet‐B (UV‐B) radiation and photosynthetically active radiation (PAR) decreased by 53% and 28%, respectively, while the water column extinction coefficient of UV‐B and PAR increased by 20% and 18% respectively. Epi‐pelagic productivity increased during smoke cover because of decreased solar inputs. PAR values remained sufficient to saturate productivity, suggesting observed differences were primarily the result of changes in UV‐B. Littoral‐benthic productivity did not change, possibly reflecting adaptation to high‐intensity UV‐B light in these habitats. Our results highlight the importance of understanding how prolonged wildfire smoke alters the amount of energy produced from specific habitats in lakes. Plain Language Summary: Large wildfires are increasing worldwide, but the impacts of wildfire smoke on lakes are unknown. We determined how photosynthetic activity and organism's respiration respond to smoke in the offshore and nearshore habitats in a mountain lake. Smoke limited the damage to algae caused by ultraviolet‐B radiation, resulting in increased offshore photosynthesis. Photosynthesis in the nearshore habitat did not change during smoke cover, possibly because organisms in shallow habitats have adapted to the higher intensity of damaging ultraviolet‐B radiation. Our results highlight the importance of understanding how prolonged smoke conditions can alter lakes' energy flows and food webs. Key Points: Wildfire smoke reduced ultraviolet‐B and photosynthetically active radiationLake habitats respond differently to changes in light from smoke, pelagic primary production increased with no change to littoral metabolismSmoke alters energy flows with implications for food web structure in lakes [ABSTRACT FROM AUTHOR]

Ecosystem metabolism is a key ecological attribute and easy to describe, but quantifying metabolism in estuaries is challenging. Properly scaling measurements through time and space requires consideration of hydrodynamics and mixing water from heterogeneous sources, making any estimation uncertain. Here, we compared three methods for modeling ecosystem metabolism in a portion of the Sacramento‐San Joaquin Delta. Metabolism estimates based on laboratory incubations, continuous in situ buoys, and an oxygen isotope approach all indicated the system was net heterotrophic, and calculated rates were comparable in magnitude when averaged over the 2‐month study. Daily metabolic rates based on in situ buoys were the most variable, likely due to horizontal and vertical advection and poor portrayal of the dissolved oxygen budget. After temporally averaging in situ buoy estimates or smoothing the dissolved oxygen time series for tidal effects, rates were more comparable to the other methods, which may be necessary to account for tidal advection and unbalanced contributions from subhabitats within the metabolic footprint. Incubation‐based rates represent the finest temporal and spatial scale and only account for pelagic processes, which may explain why incubation‐based rates were lower than the other two methods. The oxygen isotope method provided temporally and spatially integrated rates that were bracketed by the other two methods and may be a valuable tool in systems matching the model requirements. Because uncertainty arises in each method from a number of assumptions and scaling calculations, the resolution of metabolic rates in estuaries is likely coarser and more variable than in other aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Warming winters will reduce ice cover and change under‐ice conditions in temperate mountain lakes, where snow contributes most of winter cover on lakes. Snow‐dominated mountain lakes are abundant and highly susceptible to climate warming, yet we lack an understanding of how climate variation and local attributes influence winter processes. We investigated climatic and intrinsic controls on ice phenology, water temperature, and bottom‐water dissolved oxygen (DO) in 15 morphologically diverse lakes in the Sierra Nevada and Klamath Mountains of California, USA, using high‐frequency measurements from multiple (2–5) winters. We found that ice phenology was determined by winter climate variables (snowfall and air temperature) that influence ice‐off timing, whereas ice‐on timing was relatively invariant among years. Lake size and morphology mediated the effect of climate on lake temperature and DO dynamics in early and late winter. Rates of hypolimnetic DO decline were highest in small, shallow lakes, and were unrelated to water temperature. Temperature and oxygen dynamics were more variable in small lakes because heavy snowfall caused ice submergence, mixing, and DO replenishment that affected the entire water column. As the persistence of snow declines in temperate mountain regions, autumn, and spring climatic conditions are expected to gain importance in regulating lake ice phenology. Water temperature and DO will likely increase in most lakes during winter as snowpack declines, but morphological attributes such as lake size will determine the sensitivity of ice phenology and under‐ice processes to climate change. Plain Language Summary: Mountain lakes in temperate regions are at risk of losing ice cover as winters become warmer and snowfall decreases. Small mountain lakes are numerous and provide important habitats for aquatic species, yet we currently lack an understanding of what controls ice duration and under‐ice conditions in these snow‐dominated systems because most are remote and inaccessible. We used hourly measurements within a diverse set of California mountain lakes across multiple winters to explore how climate variables and lake or watershed characteristics influence the duration and timing of ice cover, as well as under‐ice temperature and oxygen concentrations. We found that winter snowfall and air temperature controlled the timing of ice breakup and the length of ice duration. Lake size and shape controlled oxygen and temperature immediately after ice formation, but late winter conditions depended on winter climate variables that dictated ice cover duration. Oxygen depletion rates and the prevalence of low oxygen conditions were greater in small, shallow lakes than in large, deep lakes. As winters warm, most mountain lakes will have shorter ice cover, warmer water temperatures, and increased oxygen concentrations, but small and large lakes will respond differently to the loss of snow and ice cover. Key Points: Winter air temperature and snowfall drive ice‐off timing and ice phenology of temperate mountain lakesLake size and morphology mediate how climate affects water temperature and dissolved oxygen dynamics during early and late winterRates of hypolimnetic oxygen decline are highest in shallow lakes and do not vary with water temperature [ABSTRACT FROM AUTHOR]

Wildfire smoke often covers areas larger than the burned area, yet the impacts of smoke on nearby aquatic ecosystems are understudied. In the summer of 2018, wildfire smoke covered Castle Lake (California, USA) for 55 days. We quantified the influence of smoke on the lake by comparing the physics, chemistry, productivity, and animal ecology in the prior four years (2014–2017) to the smoke year (2018). Smoke reduced incident ultraviolet-B (UV-B) radiation by 31% and photosynthetically active radiation (PAR) by 11%. Similarly, underwater UV-B and PAR decreased by 65 and 44%, respectively, and lake heat content decreased by 7%. While the nutrient limitation of primary production did not change, shallow production in the offshore habitat increased by 109%, likely due to a release from photoinhibition. In contrast, deep-water, primary production decreased and the deep-water peak in chlorophyll a did not develop, likely due to reduced PAR. Despite the structural changes in primary production, light, and temperature, we observed little significant change in zooplankton biomass, community composition, or migration pattern. Trout were absent from the littoral-benthic habitat during the smoke period. The duration and intensity of smoke influences light regimes, heat content, and productivity, with differing responses to consumers. [ABSTRACT FROM AUTHOR]

The extent to which terrestrial organic matter supports aquatic consumers remains uncertain because factors regulating resource flows are poorly understood. We sampled 12 lakes throughout the Sierra Nevada (California, USA) spanning large gradients in elevation and size to evaluate how watershed attributes and lake morphometry influence resource flows to lake carbon pools and zooplankton. We found that the size and composition of carbon pools in lakes were often more strongly determined by watershed or lake features rather than by elevational position. Using three different tracers of resource origin (δ13C, Δ14C, C:N ratio), we found terrestrial contributions to most lake resource pools (dissolved organic carbon, particulate organic matter (POM), sediments) and pelagic consumers (zooplankton) were more strongly related to local‐scale watershed features such as vegetation cover or watershed area: lake area rather than to elevation. Landscape patterns in multiple tracers indicated consistent contribution of within‐lake C sources to bulk resource pools across elevations (POM, sediments, zooplankton). δ13C‐enrichment of lake C pools and overlap with δ13C of terrestrial resources can arise due to reduced fractionation of 13C by phytoplankton under CO2 limitation, therefore we recommend careful consideration of potential environmental drivers when interpreting among‐lake patterns in δ13C. Our findings emphasize the importance of local‐scale variation in mediating terrestrial contributions to lake food webs. Key Points: Lake and landscape features are better predictors of the composition of lake resource pools than elevationAllochthony gradients in mountain systems can vary substantially among different lake resource poolsAquatic producers are the dominant contribution to lake carbon pools across a large gradient in landscape vegetation cover [ABSTRACT FROM AUTHOR]

Scientific Significance Statement: Metabolic stoichiometry predicts that dissolved oxygen (O2) and carbon dioxide (CO2) in aquatic ecosystems should covary inversely; however, field observations often diverge from theoretical expectations. Here, we propose a suite of metrics describing this O2 and CO2 decoupling and introduce a conceptual framework for interpreting these metrics within aquatic ecosystems. Within this framework, we interpret cross‐system patterns of high‐frequency O2 and CO2 measurements in 11 northern lakes and extract emergent insights into the metabolic behavior and the simultaneous roles of chemical and physical forcing in shaping ecosystem processes. This approach leverages the power of high‐frequency paired O2–CO2 measurements, and yields a novel, integrative aquatic system typology which can also be applicable more broadly to streams and rivers, wetlands and marine systems. [ABSTRACT FROM AUTHOR]

Mountain lakes experience extreme interannual climate variation as well as rapidly warming air temperatures, making them ideal systems to understand lake‐climate responses. Snowpack and water temperature are highly correlated in mountain lakes, but we lack a complete understanding of underlying mechanisms. Motivated by predicted declines in snowfall with future temperature increases, we investigated how surface heat fluxes and lake warming responded to variation in snowpack, ice‐off, and summer weather patterns in a high elevation lake in the Sierra Nevada, California. Ice‐off timing determined the phenology of lake exposure to solar radiation, and was the dominant mechanism linking snowpack to lake temperature. The relative importance of heat loss fluxes (longwave radiation, latent and sensible heat exchange) varied among wet and dry years. Declines in snowpack and ice cover in mountain systems will reduce variability in lake thermal responses and increase the responsiveness of lake warming to atmospheric forcing. [ABSTRACT FROM AUTHOR]

This study evaluated how the availability of nutrients and organic carbon interact to influence the associations between autotrophic and heterotrophic micro‐organisms during lake biofilm development. Considering that decomposers are often better competitors for nutrients than producers in aquatic environments, we hypothesized that heterotrophs would outcompete autotrophs for available nutrients unless heterotrophs were limited by organic carbon provided by autotrophs.To test our hypothesis, we evaluated autotrophic (algae) and heterotrophic (fungi, bacteria) biomass in response to a factorial enrichment of nutrients (nitrogen and phosphorus in combination) and glucose using nutrient‐diffusing substrates with either inorganic or organic discs in a subalpine lake. In the field, nutrient‐diffusing substrates were exposed to either natural sunlight or placed under a darkened experimental canopy to evaluate the response of heterotrophs to nutrients and carbon subsidies in the presence or absence of algae. We expected that heterotrophs would be limited by organic carbon on inorganic substrates in the absence of autotrophic production (i.e., dark treatments), and that organic substrates would provide a carbon subsidy for heterotrophic metabolism.Fungi were stimulated by nutrient enrichment on inorganic substrates in the presence of algae (light treatment), but not in the dark (without algae). The response of fungi to algal presence on inorganic substrates was similar in magnitude to the response of fungi to nutrients and glucose substrates incubated in the dark. In contrast to our expectations, elevated algal biomass did not stimulate heterotrophic bacteria in the presence of elevated nutrient levels on inorganic substrates, possibly owing to antagonistic interactions between bacteria and fungi.The positive effect of nutrients on algal biomass was significantly reduced in favour of heterotrophs when nutrients were combined with glucose, suggesting that heterotrophs were able to outcompete algae for available nutrients in the absence of carbon limitation.Synthesis. These results expand our understanding of how the availability of limiting resources governs the outcomes of complex interactions among micro‐organisms in aquatic biofilms, and suggests that background levels of organic carbon regulate producer and decomposer responses to nutrient availability during biofilm development. [ABSTRACT FROM AUTHOR]

Our objectives were to determine how temperatures in mountain lakes respond to changes in climate and to characterize how their responses are mediated by landscape or lake morphometric factors. Our analysis combines the use of high‐frequency climate and lake temperature data from 1983 to 2016 in a high‐elevation catchment in the Sierra Nevada of California with summer water temperature data from a set of 18 additional lakes scattered throughout the range. Average annual air temperatures warmed at 0.63°C decade−1, but variation in lake temperature was driven primarily by amount of precipitation as snow. By regulating the duration of ice cover and volume of inflowing spring snowmelt, variation in snowpack size accounted for 93% of variation in summer epilimnetic temperatures. The effect of snow on lake temperatures was mediated by variation in elevation and lake depth at landscape scales, creating a predictable mosaic of lake sensitivities to climate warming. [ABSTRACT FROM AUTHOR]

The integration of lakes into watershed-scale energy budgets remains a major goal of aquatic ecosystem ecology. However, this enterprise has focused almost exclusively on temperate and boreal systems and on zooplankton as representatives of system-wide energy flow. We investigated the proportion of consumer biomass derived from terrestrial sources, allochthony, in three classes of high-elevation lakes—alpine, large and small montane—of varying geometry and watershed ecosystem development, and across five taxa, including macrobenthic invertebrates and fish. We used stable isotopes of carbon (13C:12C), nitrogen (15N:14N), and hydrogen (2H:1H) to fit a modular Bayesian mixing model, which estimated proportional assimilation of phytoplankton, algal periphyton, and terrestrial organic matter by each consumer. The stable isotope analysis was supplemented with a comparison of fatty acid profiles between consumers and producers, and with a Daphnia magna rearing study involving aquatic and terrestrial nutrient sources. The most probable estimate of allochthony across consumer taxa was 41% in small montane lakes (< 0.1 ha) with high terrestrial C loading. For large montane (3-11 ha) and alpine lakes (0.8-3 ha), with substantially less terrestrial influence, allochthony was just 4 and 3%, respectively. Allochthony was also lower on average for benthic grazers than for pelagic consumers. Our results corroborate previous findings that lake size, depth, and light penetration are dominant physical controls on allochthony, but add that it sharply declines at high elevation due to changes in terrestrial primary production near or above tree line. [ABSTRACT FROM AUTHOR]

Abstract: Few coupled lake‐watershed studies examine long‐term effects of climate on the ecosystem function of lakes in a hydrological context. We use 32 years of hydrological and biogeochemical data from a high‐elevation site in the Sierra Nevada of California to characterize variation in snowmelt in relation to climate variability and explore the impact on factors affecting phytoplankton biomass. The magnitude of accumulated winter snow, quantified through basinwide estimates of snow water equivalent (SWE), was the most important climate factor controlling variation in the timing and rate of spring snowmelt. Variations in SWE and snowmelt led to significant differences in lake flushing rate, water temperature, and nitrate concentrations across years. On average in dry years, snowmelt started 25 days earlier and proceeded 7 mm/day slower, and the lake began the ice‐free season with nitrate concentrations ~2 μM higher and water temperatures 9°C warmer than in wet years. Flushing rates in wet years were 2.5 times larger than those in dry years. Consequently, particulate organic matter concentrations, a proxy for phytoplankton biomass, were 5–6 μM higher in dry years. There was a temporal trend of increase in particulate organic matter across dry years that corresponded to lake warming independent of variation in SWE. These results suggest that phytoplankton biomass is increasing as a result of both interannual variability in precipitation and long‐term warming trends. Our study underscores the need to account for local‐scale catchment variability that may affect the accumulation of winter snowpack when predicting climate responses in lakes. [ABSTRACT FROM AUTHOR]

The goals of our study were to (1) quantify production of CO2 during winter ice‐cover in arctic lakes, (2) develop methodologies which would enable prediction of CO2 production from readily measured variables, and (3) improve understanding of under‐ice circulation as it influences the distribution of dissolved gases under the ice. To that end, we combined in situ measurements with profile data. CO2 production averaged 20 mg C m−2 d−1 in a 3 m deep lake and ∼ 45 mg C m−2 d−1 in four larger lakes, similar to experimental observations at temperatures below 4°C. CO2 production was predicted by the initial rate of loss of oxygen near the sediments at ice‐on and by the full water column loss of oxygen throughout the winter. The time series data also showed the lake‐size and time dependent contribution of sediment respiration to under‐ice circulation and the decreased near‐bottom flows enabling anoxia and CH4 accumulation. [ABSTRACT FROM AUTHOR]

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The extent to which snowmelt flowing into ice-covered lakes spreads horizontally and mixes vertically influences retention of solutes derived from the landscape. To quantify these transport processes and retention, we combine time series temperature and specific conductance measurements in Toolik Lake (Alaska) and its major inflow, with measurements of discharge and meteorology, and profiles of specific conductance, temperature, fluorescence, chlorophyll a, and dissolved organic carbon (DOC) in spring of 3 yr. During early snowmelt, the concentration of DOC in the stream was 750 μM, twice that in the lake. During slow melt (discharge ( Q) < 4 m3 s−1), the incoming solute-rich intrusion spread lakewide below the ice. During melt with Q > 6 m3 s−1, the incoming water partially flushed the inlet basin and the more dilute water flowed over the original intrusion with a preferential flowpath to the outlet. Penetrative convection was restricted by the increased density gradients from the incoming plume and initially constrained to shallow mixing zones associated with the step changes in density. As ice thickness decreased to less than 1 m, heating caused density instabilities at the base of the intrusions that mixed solutes ∼ 10 m vertically, contributing to retention. Near-surface layers enriched with DOC persisted for ∼ 10 d during a rapid melt and for over 3 weeks when the melt was slow. Retention, of order 10-20%, also depended on the rapidity of melt and magnitude of discharge. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Fisheries & Aquatic Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Environmental drivers such as climate change are responsible for extreme events that are critically altering freshwater resources across the planet. In the continental US, these events range from increases in the frequency and duration of droughts and wildfires in the West, to increasing precipitation and floods that are turning lakes and reservoirs brown in the East. Such events transform and transport organic carbon in ways that affect the exposure of ecosystems to ultraviolet ( UV) radiation and visible light, with important implications for ecosystem services. Organic matter dissolved in storm runoff or released as black carbon in smoke selectively reduces UV radiation exposure. In contrast, droughts generally increase water transparency, so that UV radiation and visible light penetrate to greater depths. These shifts in water transparency alter the potential for solar disinfection of waterborne parasites, the production of carcinogenic disinfection byproducts in drinking water, and the vertical distribution of zooplankton that are a critical link in aquatic food webs. [ABSTRACT FROM AUTHOR]

We investigated the effects of a wildfire on stream physical, chemical and biological characteristics in a Mediterranean climate, comparing stream community structure and consumer resource use in burned versus unburned catchments in Santa Barbara County, CA, U.S.A., Canopy cover was lower and water temperature was higher in streams draining basins where the riparian vegetation burned than in streams in unburned basins or burned basins where riparian vegetation remained intact. Stream flow and suspended sediment concentrations during large post-fire storms and wet season nutrient levels were higher in burned than unburned catchments, with increased sedimentation after flood peaks., A year after fires, algal levels were highest in streams where riparian vegetation burned and lowest in streams in burned basins where the riparian canopy remained intact. In contrast, streams in burned basins had lower particulate organic matter, detritivore and predator levels than unburned basins, regardless of whether riparian vegetation burned. Where present, southern California steelhead trout ( Oncorhynchus mykiss) were extirpated from burned basins., Algivore densities were high in streams with burned riparian vegetation for two post-fire years before declining to unburned stream levels. Shredder densities rebounded in streams in burned basins with intact riparian vegetation, but remained low for 4 years where riparian vegetation burned. Predatory invertebrate densities increased at sites where trout were eliminated by wildfire., Hydrogen stable isotope analysis indicated that the diets of most invertebrate taxa in streams with burned riparian vegetation a year after fires were comprised of a higher proportion of algal material than riparian detritus relative to invertebrates in streams with intact riparian vegetation., Wildfire impacts on stream food webs are determined, in part, by fire severity in the riparian zone. Streams with burned riparian canopies supported algal-based food webs and streams with intact riparian canopies sustained detrital-based food webs. Fire affected basal resources (nutrients, light, allochthonous inputs) with bottom-up effects on primary producers and consumers, but top-down effects were decoupled at the trophic link between invertebrate predators and primary consumers. [ABSTRACT FROM AUTHOR]

Lake ecosystems and the services that they provide to people are profoundly influenced by dissolved organic matter derived from terrestrial plant tissues. These terrestrial dissolved organic matter (tDOM) inputs to lakes have changed substantially in recent decades, and will likely continue to change. In this paper, we first briefly review the substantial literature describing tDOM effects on lakes and ongoing changes in tDOM inputs. We then identify and provide examples of four major challenges which limit predictions about the implications of tDOM change for lakes, as follows: First, it is currently difficult to forecast future tDOM inputs for particular lakes or lake regions. Second, tDOM influences ecosystems via complex, interacting, physical-chemical-biological effects and our holistic understanding of those effects is still rudimentary. Third, non-linearities and thresholds in relationships between tDOM inputs and ecosystem processes have not been well described. Fourth, much understanding of tDOM effects is built on comparative studies across space that may not capture likely responses through time. We conclude by identifying research approaches that may be important for overcoming those challenges in order to provide policy- and management-relevant predictions about the implications of changing tDOM inputs for lakes. [ABSTRACT FROM AUTHOR]

We characterized the effect of an extreme rain event on the biogeochemistry and ecosystem metabolism of an oligotrophic Sierra Nevada (California) lake. During a 10 hour period of an autumn season rainfall event, lake discharge increased from <1.0 L s-1 to over 3000 L s-1, reaching a rate one order of magnitude higher than found during peak snow-tmelt. Large quantities of terrestrial particulate and dissolved organic matter were washed into the lake. An entire season of autochthonous dissolved organic carbon (DOC) was flushed and replaced by allochthonous DOC as light attenuation increased by >300%. The resulting truncation of the photic zone, reduction of water column chlorophyll- a, and increase in particulate and dissolved organic matter available to microbes resulted in a 47% reduction in whole lake gross primary production and 30% increase in respiration relative to average autumn values. As a consequence, the lake went from being slightly autotrophic to strongly heterotrophic. If rain events increase in frequency, as many climate change models predict, increased terrestrial inputs to Sierran lakes may result in more frequent periods of reduced primary production, increased periods of hypoxia and anoxia, and an ecosystem shift toward net heterotrophy during the ice-free season. [ABSTRACT FROM AUTHOR]

To examine the influence of landscape characteristics and landscape position on aquatic biogeochemistry, we sampled a total of 76 lakes within 14 different lake-chains spanning the latitudinal extent of the high-elevation Sierra Nevada (California). We measured water chemistry, dissolved organic matter (DOM), nutrients, and biotic variables in study catchments that encompassed representative ranges of area (3-22 km), elevation (2,200-3,700 m.a.s.l), elevation change (50-700 m), and average slope (13°-26°). Hierarchical models were used to account for variability in biogeochemistry because they explicitly maintain the framework of lakes within individual lake-chains while accounting for variation among lake-chains. Unconditional means models, where lake-chain was a random effect, revealed significant differences among lake-chains for nearly all biogeochemical variables. Models explained 42-95% of this variability, with the majority of the variation (70%) explained by the among lake-chain component. To explore the amount of additional variation explained by lake landscape position, we added lake network number (LNN) to models. LNN explained a significant amount of additional variation (7% average) in 8 of 23 biogeochemical parameters. However, it explained more variability within individual lake-chains (75%), where among lake-chain differences did not obscure patterns. Patterns of increase with LNN were found for dissolved organic carbon and nitrogen, fluorescence of DOM, alkalinity, and bacterioplankton abundance, whereas nitrate and nitrogen to phosphorus nutrient ratios decreased. LNN explained variation because it served as a proxy for underlying catchment characteristics that changed consistently along downstream flow paths. To characterize the amount of variation explained by catchment characteristics alone, we fit a third model that included lake-chain as a random effect and landscape or lake morphometry attributes as fixed effects. Catchment characteristics explained about as much additional variation (6%) as LNN, but for substantially more biogeochemical parameters (18 out of 23). The catchment characteristics most predictive of biogeochemistry were land-cover factors delineating alpine and subalpine zones (elevation, slope, or proportions of rock or shrub cover). In general, catchment characteristics were stronger predictors of biogeochemistry than characteristics of lake morphometry, emphasizing the relative importance of landscape processes in snowmelt-dominated lake ecosystems. [ABSTRACT FROM AUTHOR]

We characterized spatial and temporal variability in net ecosystem production (NEP), community respiration (CR), and gross primary production (GPP) over an ice-free season in an oligotrophic high-elevation lake using high-frequency measurements of dissolved oxygen. We combined the use of free-water and incubation chamber measurements to compare pelagic and benthic habitats and estimate their relative contributions to whole-lake metabolism. Despite a brief period of predominant heterotrophy after snowmelt, both free-water and incubation chamber measurements confirmed autotrophy of the epilimnion in all habitats throughout the ice-free season. In contrast, benthic incubation chambers showed the benthos to be consistently heterotrophic. Although temperature was the strongest seasonal driver of benthic metabolism, bacterioplankton density and indexes of organic matter quality explained the most variability in pelagic metabolism. Driven largely by benthic metabolism, free-water measurements of GPP and CR were twice as high in littoral than pelagic habitats. However, rates of water column primary production overlying the littoral benthos were high enough to overcome net benthic heterotrophy, and seasonal mean NEP in littoral habitats remained positive and not significantly different from pelagic habitats. Benthic rates averaged about 25% of whole-lake metabolism. Pelagic metabolism measurements were affected by littoral rates about half the time, with the degree of isolation between the two a function of advection and water column stability. These results emphasize the importance of characterizing spatial and temporal variability in metabolism within the context of physical dynamics and challenge the notion that benthic metabolism will necessarily be larger than pelagic metabolism in oligotrophic lakes. [ABSTRACT FROM AUTHOR]

Many marine invertebrates with complex life cycles produce planktonic larvae that experience environmental conditions different from those encountered by adults. Factors such as temperature and food, known to impact the larval period, can also affect larval size and consequently the size of newly settled juveniles. After documenting natural variation in the size of cyprids (the final larval stage) of the barnacle Balanus glandula, we experimentally manipulated temperature and food given to larvae to produce cyprids of differing sizes but within the size range of cyprids found in the field. In a set of trials in which larvae of B. glandula were raised on full or reduced rations in the laboratory and subsequently outplanted into the field as newly metamorphosed juveniles, we explored the effects of larval nutrition and size on juvenile performance. Larvae that received full rations throughout their feeding period produced larger cyprids (with more lipid and protein). These larger cyprids grew faster as juveniles and sometimes survived better in the field than juveniles from larvae that had their food ration reduced in the last feeding instar. For naturally settling barnacles brought into the laboratory within 2 days of settlement and fed, we found that initial juvenile size was a good predictor of juvenile size even after 2 weeks of growth. By manipulating food given to juveniles that were derived from larvae fed either full or reduced rations, we found that larval nutritional effects persisted in juveniles for 2-3 times the period that larvae experienced altered food rations. [ABSTRACT FROM AUTHOR]

Abstract Dike material was used as fill to construct high, mid, and low intertidal elevations in a subsided marsh located in the South Slough National Estuarine Research Reserve, Oregon. Marsh surface elevation change (including fill consolidation and compression of the original marsh soils), vertical accretion, tidal channel development, emergent vegetation colonization, and fish use were monitored over 3 years. Significant marsh surface elevation loss was detected at all elevations, with fill consolidation accounting for 70% of the loss at the highest elevation. Vertical accretion averaged 0.19 cm/yr in the sparsely vegetated Kunz Marsh compared with 0.70 cm/yr at the densely vegetated reference sites. Tidal channel development was influenced as much by marsh surface gradient as by marsh surface elevation. Vegetation colonization was directly correlated with elevation, whereas density and species richness of fish was inversely correlated with elevation. Manipulating the marsh surface to mid-marsh elevations favors rapid vegetation colonization and facilitates vertical accretion-dominated tidal channel development. Low marsh elevations result in initially slower developing vegetation colonization and channel development but are more beneficial to fish during the early stages of marsh recovery. High marsh elevations appear to sacrifice tidal channel development and associated fish access for rapid vegetation colonization. [ABSTRACT FROM AUTHOR]

Citizen science projects have a long history in ecological studies. The research usefulness of such projects is dependent on applying simple and standardized methods. Here, we conducted a citizen science project that involved more than 3500 Swedish high school students to examine the temperature difference between surface water and the overlying air (Tw-Ta) as a proxy for sensible heat flux (QH). If QH is directed upward, corresponding to positive Tw-Ta, it can enhance CO2 and CH4 emissions from inland waters, thereby contributing to increased greenhouse gas concentrations in the atmosphere. The students found mostly negative Tw-Ta across small ponds, lakes, streams/rivers and the sea shore (i.e. downward QH), with Tw-Ta becoming increasingly negative with increasing Ta. Further examination of Tw-Ta using high-frequency temperature data from inland waters across the globe confirmed that Tw-Ta is linearly related to Ta. Using the longest available high-frequency temperature time series from Lake Erken, Sweden, we found a rapid increase in the occasions of negative Tw-Ta with increasing annual mean Ta since 1989. From these results, we can expect that ongoing and projected global warming will result in increasingly negative Tw-Ta, thereby reducing CO2 and CH4 transfer velocities from inland waters into the atmosphere. [ABSTRACT FROM AUTHOR]

A series of vertical profiles of dissolved oxygen (DO) collected periodically over two consecutive ice-free seasons in an oligotrophic high-elevation lake (Emerald Lake, California) were used to investigate volumetric and areal rates of gross primary production (GPP), community respiration (CR), and net ecosystem production (NEP). Diel patterns in DO did not weaken with depth in this lake, where the entire 10-m water column was within the euphotic zone and where a deep chlorophyll a (Chl a) maximum was common during periods of thermal stratification. During stratification, both GPP and CR increased with depth, and heterotrophy (NEP < 0) tended to occur below the thermocline in association with higher Chl a and particulate matter concentrations. With the onset of autumn mixing each year, vertical gradients i metabolism weakened or disappeared and the entire water column was autotrophic. Net autotrophy over the growing season was confirmed using three methods of estimating whole-lake metabolism. During periods of stratification, flux across the thermocline, where eddy diffusivities were near molecular, was small (4% of total epilimnetic fluxes), while within the hypolimnion, where stratification was weaker and eddy diffusivities larger, fluxes between strata were more substantial (12% of total fluxes). For this lake and other small lakes with low wind speeds and Lake numbers near 10, mixing due to turbulence should be included in computations of metabolism within the hypolimnion. However, single-station measurements from within the epilimnion provide a reasonable estimate of seasonal metabolism, especially in the autumn when the lake is mixing on a diel basis. [ABSTRACT FROM AUTHOR]

We examined bacterioplankton community composition (BCC; using polymerase chain reaction-based phylogenetic fingerprinting) and the composition of dissolved organic matter (DOM; using scanning fluorometry) within lake chains in high-elevation catchments of the Sierra Nevada, California. We compared temporal BCC dynamics in two connected cirque lakes and their inlets and found that lake : inlet community similarity of both lakes declined with increasing residence time because of seasonal snowpack melting. However, for over 3 yr the headwater lake remained distinct from its inlet even during peak snowmelt (residence time <5 d), whereas the downstream lake was consistently similar to both its inlet and the upstream lake. We subsequently sampled 17 lake chains (4-8 lakes each) throughout the Sierra Nevada longitudinally to examine how landscape position and catchment characteristics influenced the similarity in BCC and DOM among lakes and their connecting streams. Headwater lakes were significantly less similar to their inlet streams than were downstream lakes. BCC was heterogeneous throughout each catchment, but headwater inlet BCC was consistently distinct from all downstream samples. In contrast, DOM exhibited a trend of increasing homogeneity as water moved downstream: upstream and downstream waters were distinct but without clear headwater outliers. The similarity in BCC and DOM between any two lakes within a chain declined with geographic distance. Our results illustrate the contrasting roles of environmental selection and dispersal in structuring bacterioplankton communities, supporting both strong within-lake environmental selection for communities distinct from terrigenous inputs and a weaker effect of geographic dispersal differentiating communities among lakes linked by advective flow. [ABSTRACT FROM AUTHOR]