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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.

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.

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.

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. © 2015 John Wiley

The Sacramento Deep Water Ship Channel (SDWSC) in the San Francisco Estuary, which is an active commercial port, is critical habitat for pelagic fish species including delta smelt (Hypomesus transpacificus), longfin smelt (Spirinchus thaleichthys), and Sacramento perch (Archoplites interruptus). Pelagic organism decline has been attributed to covarying factors such as manipulation of habitat, introduction of invasive species, decrease in food production, and contaminant exposure. Quantification of bioavailable toxicant loads in the SDWSC is limited despite previous surveys that have detected elevated contaminant concentrations in the sediments. Therefore, the focus of the present study was to characterize the bioavailability of the contaminants in the SDWSC from six sites along the channel. At each site, organochlorine pesticides (OCPs), pyrethroid insecticides, polyaromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs) were quantified in sediment, zooplankton, and suspended solids. In addition, Tenax extraction was used to measure the bioaccessible fraction of sediment-associated contaminants freely dissolved in the water. Bioaccessible contaminants in the sediment provided an uptake route for these stressors into invertebrates and fish with bioaccessible OCPs being found at all sites, particularly 4,4'-dichlorodiphenyldichloroethylene (DDE). Bifenthrin was the only pyrethroid detected in the chosen matrices and it was found at concentrations below levels of concern. Bioaccessible PAHs were found at all sites, with highest detections for phenanthrene and pyrene. No PCBs were detected in sediments, but were detected in both suspended solids and zooplankton. Contaminant concentrations overall were significantly higher in suspended solids, followed by zooplankton and sediments. The highest sediment concentrations of DDE, fluoranthene, pyrene, and dibenzo[a,h]anthracene exceeded sediment quality benchmarks indicating potential risk to sediment-dwelling species. Finally, elevated contaminant levels were found in both suspended solids and zooplankton, suggesting additional risk to pelagic species in the SDWSC., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Wildfire activity is increasing globally. The resulting smoke plumes can travel hundreds to thousands of kilometers, reflecting or scattering sunlight and depositing particles within ecosystems. Several key physical, chemical, and biological processes in lakes are controlled by factors affected by smoke. The spatial and temporal scales of lake exposure to smoke are extensive and under-recognized. We introduce the concept of the lake smoke-day, or the number of days any given lake is exposed to smoke in any given fire season, and quantify the total lake smoke-day exposure in North America from 2019 to 2021. Because smoke can be transported at continental to intercontinental scales, even regions that may not typically experience direct burning of landscapes by wildfire are at risk of smoke exposure. We found that 99.3% of North America was covered by smoke, affecting a total of 1,333,687 lakes ≥10 ha. An incredible 98.9% of lakes experienced at least 10 smoke-days a year, with 89.6% of lakes receiving over 30 lake smoke-days, and lakes in some regions experiencing up to 4 months of cumulative smoke-days. Herein we review the mechanisms through which smoke and ash can affect lakes by altering the amount and spectral composition of incoming solar radiation and depositing carbon, nutrients, or toxic compounds that could alter chemical conditions and impact biota. We develop a conceptual framework that synthesizes known and theoretical impacts of smoke on lakes to guide future research. Finally, we identify emerging research priorities that can help us better understand how lakes will be affected by smoke as wildfire activity increases due to climate change and other anthropogenic activities., (© 2024 Oak Ridge National Laboratory and The Author(s). Global Change Biology published by John Wiley & Sons Ltd.)

Humans increasingly dominate Earth's natural freshwater ecosystems, but biomass production of modified ecosystems is rarely studied. We estimate potential fish total standing stock in USA reservoirs is 3.4 billion (B) kg, and approximate annual secondary production is 4.5 B kg y -1 . We also observe varied and non-linear trends in reservoir fish biomass over time, thus previous assertions that reservoir fisheries decline over time are not universal. Reservoirs are globally relevant pools of freshwater fisheries, in part due to their immense limnetic footprint and spatial extent. This study further shows that reservoir ecosystems play major roles in food security and fisheries conservation. We encourage additional effort be expended to effectively manage reservoir environments for the good of humanity, biodiversity, and fish conservation., (© 2024. The Author(s).)

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., (© 2022. The Author(s).)

Increased nitrogen (N) deposition rates over the past century have affected both North American and European mountain lake ecosystems. Ecological sensitivity of mountain lakes to N deposition varies, however, because chemical and biological responses are modulated by local watershed and lake properties. We evaluated predictors of mountain lake sensitivity to atmospheric N deposition across North American and European mountain ranges and included as response variables dissolved inorganic N (DIN = NNH 4 +  + NNO 3 - ) concentrations and phytoplankton biomass. Predictors of these responses were evaluated at three different spatial scales (hemispheric, regional, subregional) using regression tree, random forest, and generalized additive model (GAM) analysis. Analyses agreed that Northern Hemisphere mountain lake DIN was related to N deposition rates and smaller scale spatial variability (e.g., regional variability between North American and European lakes, and subregional variability between mountain ranges). Analyses suggested that DIN, N deposition, and subregional variability were important for Northern Hemisphere mountain lake phytoplankton biomass. Together, these findings highlight the need for finer-scale, subregional analyses (by mountain range) of lake sensitivity to N deposition. Subregional analyses revealed differences in predictor variables of lake sensitivity. In addition to N deposition rates, lake and watershed features such as land cover, bedrock geology, maximum lake depth (Z max ), and elevation were common modulators of lake DIN. Subregional phytoplankton biomass was consistently positively related with total phosphorus (TP) in Europe, while North American locations showed variable relationships with N or P. This study reveals scale-dependent watershed and lake characteristics modulate mountain lake ecological responses to atmospheric N deposition and provides important context to inform empirically based management strategies., (Copyright © 2021. Published by Elsevier Ltd.)

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., (© 2021 John Wiley & Sons Ltd.)

Climate change and other anthropogenic stressors have led to long-term changes in the thermal structure, including surface temperatures, deepwater temperatures, and vertical thermal gradients, in many lakes around the world. Though many studies highlight warming of surface water temperatures in lakes worldwide, less is known about long-term trends in full vertical thermal structure and deepwater temperatures, which have been changing less consistently in both direction and magnitude. Here, we present a globally-expansive data set of summertime in-situ vertical temperature profiles from 153 lakes, with one time series beginning as early as 1894. We also compiled lake geographic, morphometric, and water quality variables that can influence vertical thermal structure through a variety of potential mechanisms in these lakes. These long-term time series of vertical temperature profiles and corresponding lake characteristics serve as valuable data to help understand changes and drivers of lake thermal structure in a time of rapid global and ecological change., (© 2021. The Author(s).)

The concentration of dissolved oxygen in aquatic systems helps to regulate biodiversity 1,2 , nutrient biogeochemistry 3 , greenhouse gas emissions 4 , and the quality of drinking water 5 . The long-term declines in dissolved oxygen concentrations in coastal and ocean waters have been linked to climate warming and human activity 6,7 , but little is known about the changes in dissolved oxygen concentrations in lakes. Although the solubility of dissolved oxygen decreases with increasing water temperatures, long-term lake trajectories are difficult to predict. Oxygen losses in warming lakes may be amplified by enhanced decomposition and stronger thermal stratification 8,9 or oxygen may increase as a result of enhanced primary production 10 . Here we analyse a combined total of 45,148 dissolved oxygen and temperature profiles and calculate trends for 393 temperate lakes that span 1941 to 2017. We find that a decline in dissolved oxygen is widespread in surface and deep-water habitats. The decline in surface waters is primarily associated with reduced solubility under warmer water temperatures, although dissolved oxygen in surface waters increased in a subset of highly productive warming lakes, probably owing to increasing production of phytoplankton. By contrast, the decline in deep waters is associated with stronger thermal stratification and loss of water clarity, but not with changes in gas solubility. Our results suggest that climate change and declining water clarity have altered the physical and chemical environment of lakes. Declines in dissolved oxygen in freshwater are 2.75 to 9.3 times greater than observed in the world's oceans 6,7 and could threaten essential lake ecosystem services 2,3,5,11 .

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.

Globally, lake surface water temperatures have warmed rapidly relative to air temperatures, but changes in deepwater temperatures and vertical thermal structure are still largely unknown. We have compiled the most comprehensive data set to date of long-term (1970-2009) summertime vertical temperature profiles in lakes across the world to examine trends and drivers of whole-lake vertical thermal structure. We found significant increases in surface water temperatures across lakes at an average rate of + 0.37 °C decade -1 , comparable to changes reported previously for other lakes, and similarly consistent trends of increasing water column stability (+ 0.08 kg m -3 decade -1 ). In contrast, however, deepwater temperature trends showed little change on average (+ 0.06 °C decade -1 ), but had high variability across lakes, with trends in individual lakes ranging from - 0.68 °C decade -1 to + 0.65 °C decade -1 . The variability in deepwater temperature trends was not explained by trends in either surface water temperatures or thermal stability within lakes, and only 8.4% was explained by lake thermal region or local lake characteristics in a random forest analysis. These findings suggest that external drivers beyond our tested lake characteristics are important in explaining long-term trends in thermal structure, such as local to regional climate patterns or additional external anthropogenic influences.

Competing Interests: Conflict of Interest Statement: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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 (T w -T a ) as a proxy for sensible heat flux (Q H ). If Q H is directed upward, corresponding to positive T w -T a , it can enhance CO 2 and CH 4 emissions from inland waters, thereby contributing to increased greenhouse gas concentrations in the atmosphere. The students found mostly negative T w -T a across small ponds, lakes, streams/rivers and the sea shore (i.e. downward Q H ), with T w -T a becoming increasingly negative with increasing T a . Further examination of T w -T a using high-frequency temperature data from inland waters across the globe confirmed that T w -T a is linearly related to T a . Using the longest available high-frequency temperature time series from Lake Erken, Sweden, we found a rapid increase in the occasions of negative T w -T a with increasing annual mean T a since 1989. From these results, we can expect that ongoing and projected global warming will result in increasingly negative T w -T a , thereby reducing CO 2 and CH 4 transfer velocities from inland waters into the atmosphere.

Winter conditions are rapidly changing in temperate ecosystems, particularly for those that experience periods of snow and ice cover. Relatively little is known of winter ecology in these systems, due to a historical research focus on summer 'growing seasons'. We executed the first global quantitative synthesis on under-ice lake ecology, including 36 abiotic and biotic variables from 42 research groups and 101 lakes, examining seasonal differences and connections as well as how seasonal differences vary with geophysical factors. Plankton were more abundant under ice than expected; mean winter values were 43.2% of summer values for chlorophyll a, 15.8% of summer phytoplankton biovolume and 25.3% of summer zooplankton density. Dissolved nitrogen concentrations were typically higher during winter, and these differences were exaggerated in smaller lakes. Lake size also influenced winter-summer patterns for dissolved organic carbon (DOC), with higher winter DOC in smaller lakes. At coarse levels of taxonomic aggregation, phytoplankton and zooplankton community composition showed few systematic differences between seasons, although literature suggests that seasonal differences are frequently lake-specific, species-specific, or occur at the level of functional group. Within the subset of lakes that had longer time series, winter influenced the subsequent summer for some nutrient variables and zooplankton biomass., (© 2016 The Authors. Ecology Letters published by CNRS and John Wiley & Sons Ltd.)

Objectives: This study was aimed at investigating the prognostic role of multiple lymph node basin drainage (MLBD) in patients with positive sentinel lymph node (SLN) biopsy., Background: MLBD is frequently observed in patients with trunk melanoma undergoing SLN. The prognostic value of MLBD in SLN-positive patients is still debated., Methods: Retrospective data from 312 trunk melanoma patients with positive SLN biopsy (1991-2012) at 6 Italian referral centres were gathered in a multicentre database. MLBD was defined at preoperative lymphoscintigraphy. Clinical and pathological data were analysed for their association with disease-free interval (DFI) and disease-specific (DSS) survival., Results: MLBD was identified in 34.6% of patients (108/312) and was significantly associated with >1 positive SLN (37 vs. 15.2%; p < 0.001) and with >1 positive lymph node (LN) after complete lymph node dissection (CLND) (50.9 vs. 34.8%; p = 0.033). No differences were observed according to drainage pattern in patients who had negative and positive non-SLN at CLND. MLBD was not associated with either DFI or DSS. Multivariate analyses showed that tumour thickness, ulceration, and number of metastatic LNs were associated with worse DFI and DSS, while regression confirmed its protective role in survival., Conclusion: In positive SLN patients, MLBD has no association with survival, which is mainly related to American Joint Committee on Cancer (AJCC) prognostic factors. Since the overall number of positive LNs drives the prognosis, the importance of a CLND in all the positive basins is confirmed., (© 2017 S. Karger AG, Basel.)