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Climate change is generating complex responses in both natural and human ecosystems that vary in their geographic distribution, magnitude, and timing across the global landscape. One of the major issues that scientists and policy makers now confront is how to assess such massive changes over multiple scales of space and time. Lakes and reservoirs comprise a geographically distributed network of the lowest points in the surrounding landscape that make them important sentinels of climate change. Their physical, chemical, and biological responses to climate provide a variety of information‐rich signals. Their sediments archive and integrate these signals, enabling paleolimnologists to document changes over years to millennia. Lakes are also hot spots of carbon cycling in the landscape and as such are important regulators of climate change, processing terrestrial and atmospheric as well as aquatic carbon. We provide an overview of this concept of lakes and reservoirs as sentinels, integrators, and regulators of climate change, as well as of the need for scaling and modeling these responses in the context of global climate change. We conclude by providing a brief look to the future and the creation of globally networked sensors in lakes and reservoirs around the world.

Recent climate-change research largely confirms the impacts on US ecosystems identified in the 2009 National Climate Assessment and provides greater mechanistic understanding and geographic specificity for those impacts. Pervasive climate-change impacts on ecosystems are those that affect productivity of ecosystems or their ability to process chemical elements. Loss of sea ice, rapid warming, and higher organic inputs affect marine and lake productivity, while combined impacts of wildfire and insect outbreaks decrease forest productivity, mostly in the arid and semi-arid West. Forests in wetter regions are more productive owing to warming. Shifts in species ranges are so extensive that by 2100 they may alter biome composition across 5-20% of US land area. Accelerated losses of nutrients from terrestrial ecosystems to receiving waters are caused by both winter warming and intensification of the hydrologic cycle. Ecosystem feedbacks, especially those associated with release of carbon dioxide and methane release from wetlands and thawing permafrost soils, magnify the rate of climate change.

Increases in terrestrially-derived dissolved organic matter (DOM) have led to the browning of inland waters across regions of northeastern North America and Europe. Short-term experimental and comparative studies highlight the important ecological consequences of browning. These range from transparency-induced increases in thermal stratification and oxygen (O 2 ) depletion to changes in pelagic food web structure and alteration of the important role of inland waters in the global carbon cycle. However, multi-decadal studies that document the net ecological consequences of long-term browning are lacking. Here we show that browning over a 27 year period in two lakes of differing transparency resulted in fundamental changes in vertical habitat gradients and food web structure and that these responses were stronger in the more transparent lake. Surface water temperatures increased by 2–3 °C in both lakes in the absence of any changes in air temperature. Water transparency to ultraviolet (UV) radiation showed a fivefold decrease in the more transparent lake. The primary zooplankton grazers decreased and in the more transparent lake were largely replaced by a two trophic level zooplankton community. These findings provide new insights into the net effects of the complex and contrasting mechanisms that underlie the ecosystem consequences of browning.

Abstract Freshwater ecosystems, including lakes, streams, and wetlands, are responsive to climate change and other natural and anthropogenic stresses. These ecosystems are frequently hydrologically and ecologically connected with one another and their surrounding landscapes, thereby integrating changes throughout their watersheds. The responses of any given freshwater ecosystem to climate change depend on the magnitude of climate forcing, interactions with other anthropogenic and natural changes, and the characteristics of the ecosystem itself. Therefore, the magnitude and manner in which freshwater ecosystems respond to climate change are difficult to predict a priori. We present a conceptual model to elucidate how freshwater ecosystems are altered by climate change. We identify eleven indicators that describe the response of freshwater ecosystems to climate change, discuss their potential value and limitations, and describe supporting measurements. Indicators are organized in three interrelated categories: hydrologic, water quality, and ecosystem structure and function. The indicators are supported by data sets with a wide range of temporal and spatial coverage, and they inform important scientific and management needs. Together, these indicators improve the understanding and management of the effects of climate change on freshwater ecosystems.

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.

Outgassing of carbon dioxide (CO2) from freshwater ecosystems comprises 12 %–25 % of the total carbon flux from soils and bedrock. This CO2 is largely derived from both biodegradation and photodegradation of terrestrial dissolved organic carbon (DOC) entering lakes from wetlands and soils in the watersheds of lakes. In spite of the significance of these two processes in regulating rates of CO2 outgassing, their relative importance remains poorly understood in lake ecosystems. In this study, we used groundwater from the watersheds of one subtropical and three temperate lakes of differing trophic status to simulate the effects of increases in terrestrial DOC from storm events. We assessed the relative importance of biodegradation and photodegradation in oxidizing DOC to CO2. We measured changes in DOC concentration, colored dissolved organic carbon (specific ultraviolet absorbance – SUVA320; spectral slope ratio – Sr), dissolved oxygen, and dissolved inorganic carbon (DIC) in short-term experiments from May–August 2016. In all lakes, photodegradation led to larger changes in DOC and DIC concentrations and optical characteristics than biodegradation. A descriptive discriminant analysis showed that, in brown-water lakes, photodegradation led to the largest declines in DOC concentration. In these brown-water systems, ∼ 30 % of the DOC was processed by sunlight, and a minimum of 1 % was photomineralized. In addition to documenting the importance of photodegradation in lakes, these results also highlight how lakes in the future may respond to changes in DOC inputs.

Mosquitoes have increased in their abundance and geographic distribution in northeastern North America, coinciding with an increase in extreme precipitation events and up to a doubling of dissolved organic matter (DOM) concentrations in some inland waters. Increases in DOM can reduce exposure of mosquito larvae to solar ultraviolet (UV) radiation. Although mosquito larvae are most common in shaded habitats, almost nothing is known about their susceptibility to damage by solar UV radiation, or the ability of DOM to create a refuge from damaging UV in their shallow-water habitats. We hypothesize that 1) exposure to solar UV radiation is lethal to mosquito larvae, 2) larvae lack photo-enzymatic repair to fix UV-damaged DNA, and 3) DOM shades larvae from lethal solar UV radiation. We tested these hypotheses with experiments that manipulated UV radiation, the photo-repair radiation necessary for photo-enzymatic DNA repair, and DOM. Exposure to solar UV radiation significantly decreased larval survivorship, while DOM significantly increased it. There was no evidence of photo-enzymatic DNA repair. Our findings confirm that solar UV radiation decreases habitat suitability for mosquito larvae, but DOM provides a refuge from UV. This highlights the need for vector control managers to prioritize high DOM and shaded habitats in their efforts to reduce mosquito populations.

Ultraviolet radiation has detrimental and beneficial effects in freshwater ecosystems. Ultraviolet radiation (UV)—the shortest-wavelength, highest-energy solar radiation that passes through the atmosphere—is a potent force affecting life on Earth. Exposure to UV, which reaches Earth's surface at wavelengths between 290 and 400 nm, can damage DNA and impair an organism's ability to reproduce, sense its environment, and resist disease. Ozone concentrated in the stratosphere shields living organisms from the most damaging wavelengths of UV, known as UV-B (280 to 320 nm). In the 1980s and 1990s, observations that this protective layer was becoming depleted, and that a seasonal ozone “hole” over the Antarctic was expanding, generated serious concerns about the potential negative effects of increased UV on aquatic ecosystems ( 1 ). More recently, however, it has become clear that even the shortest-wavelength, highest-energy UV reaching Earth has beneficial as well as detrimental effects on individual organisms and thus on natural ecosystems ( 2 ).

[This corrects the article DOI: 10.1371/journal.pone.0240261.].

Inputs of terrestrially derived dissolved organic matter (DOM) are increasing in alpine lakes due to multiple drivers such as climate change, tree line advancement, and insect epidemics. A 21 d microcosm experiment investigated three potential mechanisms by which increased inputs of terrestrial DOM subsidies might affect phytoplankton density, growth, and species assemblage: (1) directly, by providing nutrients enhancing growth of select phytoplankton species (nutrient stimulation hypothesis); (2) indirectly, through trophic forcing of zooplankton uniformly increasing the total biomass of all zooplankton that selectively graze on phytoplankton (trophic intensity hypothesis); and (3) indirectly, through trophic forcing of zooplankton by favoring zooplankton species that selectively graze on phytoplankton (trophic shift hypothesis). We manipulated DOM (terrestrial DOM additions vs. unmanipulated control), zooplankton (presence vs. absence), and incubation depth (epilimnion vs. hypolimnion) in a full 3 × 3 factorial design. Phytoplankton density and growth increased substantially and species assemblage shifted to near dominance by Asterionella formosa in the presence of DOM. Zooplankton biomass and growth increased with the addition of DOM, yet the species assemblage remained stable across treatments, and contributed to selective grazing effects on phytoplankton. Our data support the nutrient stimulation and trophic intensity hypotheses. While DOM effects have been classically attributed to stimulation by addition of fixed carbon, our experiments indicate that nutrient stimulation is also important. Additionally, the indirect DOM effect of trophic forcing can occur in the absence of selective effects of DOM on zooplankton.