Climate change disrupts ecological systems in many ways. Many documented responses depend on species' life histories, contributing to the view that climate change effects are important but difficult ...to characterize generally. However, systematic variation in metabolic effects of temperature across trophic levels suggests that warming may lead to predictable shifts in food web structure and productivity. We experimentally tested the effects of warming on food web structure and productivity under two resource supply scenarios. Consistent with predictions based on universal metabolic responses to temperature, we found that warming strengthened consumer control of primary production when resources were augmented. Warming shifted food web structure and reduced total biomass despite increases in primary productivity in a marine food web. In contrast, at lower resource levels, food web production was constrained at all temperatures. These results demonstrate that small temperature changes could dramatically shift food web dynamics and provide a general, species-independent mechanism for ecological response to environmental temperature change.
Elevated allochthonous inputs of organic matter are increasingly recognized as a driver of ecosystem change in lakes, particularly when concurrent with eutrophication. Evaluation of lakes in a ...nutrient-color paradigm (i.e., based on total phosphorus and true color) enables a more robust approach to research and management. To assess temporal and spatial patterns in nutrient-color status for U.S. lakes and associated food web attributes, we analyzed the U.S. Environmental Protection Agency’s National Lakes Assessment (NLA) data. With 1000+ lakes sampled in 2007 and 2012 in a stratified random sampling design, the NLA enables rigorous assessment of lake condition across the continental U.S. We demonstrate that many U.S. lakes are simultaneously experiencing eutrophication and brownification to produce an abundance of “murky” lakes. Overall, “blue” lakes decreased by ~ 18% (46% of lakes in 2007 to 28% in 2012) while “murky” lakes increased by almost 12% (24% of lakes in 2007 to 35.4% in 2012). No statistical differences were observed in the proportions of “green” or “brown” lakes. Regionally, murky lakes significantly increased in the Northern Appalachian, Southern Plains, and Xeric ecoregions. Murky lakes exhibited the highest epilimnetic chlorophyll a concentrations, cyanobacterial densities, and microcystin concentrations. Total zooplankton biomass was also highest in murky lakes, primarily due to increased rotifer and copepod biomass. However, zooplankton : phytoplankton biomass ratios were low, suggesting reduced energy transfer to higher trophic levels. These results emphasize that many lakes in the U.S. are simultaneously “greening” and “browning”, with potentially negative consequences for water quality and food web structure.
We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland ...waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments.
Nanomolar concentrations of steroid hormones such as 17β-estradiol can influence the reproductive development and sex ratios of invertebrate and vertebrate populations. Thus their release into ...surface and ground waters from wastewater facilities and agricultural applications of animal waste is of environmental concern. Many of these compounds are chromophoric and susceptible to photolytic degradation. High intensity UV-C radiation has been demonstrated to degrade some of these compounds in engineered systems. However, the degradation efficacy of natural solar radiation in shallow fresh waters is less understood. Here photolytic experiments with 17β-estradiol demonstrated modest photodegradation (~
26%) when exposed to simulated sunlight between 290 and 720 nm. Photodegradation significantly increased (~
40–50%) in the presence of 2.0–15.0 mg/l of dissolved organic carbon (DOC) derived from humic acids of the Suwannee River, GA. However, rates of photodegradation reached a threshold at approximately 5.0 mg/l DOC. Observed suppression of photolysis in the presence of a radical inhibitor (i.e. 2-propanol) indicated that a significant proportion of the degradation was due to radicals formed from the photolysis of DOC. Although photodegradation was greatest in full sunlight containing UV-B (290–320 nm), degradation was also detected with UV-A (320–400 nm) and visible light (400–720 nm) alone.
Fish foraging and energy flow between trophic levels are predicted to decline with freshwater browning due to decreased light availability. Studies investigating these predictions have focused on ...juvenile and adult fishes; however, the larval stage represents a “critical period” in development. We investigated the indirect (i.e., prey availability) versus direct (i.e., visual foraging) effects of browning on zooplankton–larval fish interactions by altering water colour with SuperHume (absorbance at 440 nm = 1.6–10.8 m
−1
). Phytoplankton and zooplankton densities were monitored across treatments for 1 month to simulate the time leading up to fish spawning in the spring. Larval largemouth bass (Micropterus salmoides) were then introduced to assess indirect effects on fish feeding, growth, and survival. Direct effects on largemouth bass and bluegill (Lepomis macrochirus) foraging on zooplankton were determined with separate short-term experiments. Browning did not directly alter the ability of larvae to capture prey. However, indirect effects on larval fish foraging, growth, and survival were observed as phytoplankton and zooplankton abundance decreased with increased browning. Our data suggest browning may reduce energy transfer to larval fish due to a reduction in prey but not visual foraging.
We examined seasonal and spatial patterns in dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) in the Chowan River watershed, North Carolina, a blackwater river which ...discharges into the second largest estuary in the United States, the Albemarle–Pamlico Estuarine System. From April 2008 to May 2010, DOC concentration did not significantly vary across seasons (range 7.69–30.39 mg L
−1
); however, CDOM molecular size and aromaticity increased throughout the spring, decreased during the summer and fall, and remained relatively low in the winter. Spectral slope ratios suggested microbial processing of CDOM in the spring and photodegradation of CDOM in the summer and fall. Spatially, DOC and CDOM concentrations were similar in the mainstem and at the mouths of two tributaries, Bennetts Creek and Wiccacon River, but were significantly higher upstream on the tributaries. DOC concentration was positively correlated with CDOM absorbance coefficients at 254 and 350 nm; however, these optical proxies explained only ~60 % of the variance. DOC and CDOM absorption loads to the Albemarle Sound ranged from 2.63 × 10
10
g year
−1
and 9.84 × 10
10
m
2
year
−1
, respectively, in a dry year and 7.9 × 10
10
g year
−1
and 2.2 × 10
11
m
2
year
−1
, respectively, in a wet year, which are comparable to non-blackwater rivers with larger watersheds. Blackwater rivers may therefore represent “hotspots” in coastal carbon chemistry, with seasonal variations in the quality and quantity of DOC and CDOM influencing estuarine food web dynamics and net ecosystem metabolism.
While there are many local and regional sampling efforts in lakes across the U.S., it is often difficult to compile this information into a cohesive framework to make national‐ or continental‐scale ...conclusions. To address this gap, the U.S. Environmental Protection Agency partners with States and Tribes to implement the National Lakes Assessment, which is a national‐scale, coordinated lake sampling program. The resulting data include colocated biological, chemical, habitat, and human use measures that are available to the scientific community. We provide a brief overview of the program, describe the data, and discuss some of the ways the results have been used. The availability of a consistent data set of this magnitude is an impressive and unique resource for the limnological community.
Lake trophic state is a key ecosystem property that integrates a lake's physical, chemical, and biological processes. Despite the importance of trophic state as a gauge of lake water quality, ...standardized and machine-readable observations are uncommon. Remote sensing presents an opportunity to detect and analyze lake trophic state with reproducible, robust methods across time and space. We used Landsat surface reflectance data to create the first compendium of annual lake trophic state for 55,662 lakes of at least 10 ha in area throughout the contiguous United States from 1984 through 2020. The dataset was constructed with FAIR data principles (Findable, Accessible, Interoperable, and Reproducible) in mind, where data are publicly available, relational keys from parent datasets are retained, and all data wrangling and modeling routines are scripted for future reuse. Together, this resource offers critical data to address basic and applied research questions about lake water quality at a suite of spatial and temporal scales.
We generated suspended sediment solutions using river sediments and river water at concentrations similar to those observed during 1.5 year floods (Q1.5) and a dam removal (∼325 mg L−1) on the Deep ...River, North Carolina. Suspended sediment solutions were exposed to simulated solar radiation, equivalent to one clear, summer day at the study site (35°N). Concentrations of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), total dissolved nitrogen (TDN), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), soluble reactive phosphorus (SRP) and total dissolved iron (Fed) were measured before and after exposure. Total dissolved carbon (TDC) budgets for each experiment were produced using DOC and DIC data. Sediment suspensions in the presence of simulated solar radiation were significant sources of dissolved C (119 ± 11 μmol C L−1 d−1; ± values indicate 1 standard error) and DON (1.7 ± 0.5 μmol N L−1 d−1) but not DIN or SRP. Extrapolations through the Deep River water column suggest that suspended sediments in the presence of light represent dissolved organic matter fluxes of 3.92 mmol C m−2 d−1 and 40 μmol N m−2 d−1. Additionally, sediment suspensions lowered river water Fed concentrations immediately (∼24%) and progressively (∼40–90%) in both light and dark treatments. Our research suggests suspended sediments in river ecosystems are potential sources of dissolved organic C and dissolved organic N while effectively removing Fed from the water column.
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