Climate-change models predict more frequent and intense summer droughts for many areas, including the midwestern United States. Precipitation quantity and intensity in turn drive the rates and ratios ...at which nitrogen (N) and phosphorus (P) are exported from watersheds into lakes, but these rates and ratios are also modulated by watershed land use. This led us to ask the question, is the effect of precipitation on phytoplankton nutrient limitation dependent on watershed land use? Across 42 lakes, we found that phytoplankton in lakes in agricultural landscapes were usually P limited but shifted to strong N limitation under increased drought intensity, and that droughts promoted N-fixing cyanobacteria. In contrast, phytoplankton in lakes with forested watersheds were consistently N limited, regardless of drought status. This climate-land use interaction suggests that droughts may increase the incidence of N limitation in agriculturally impacted lakes. N limitation would likely impair valuable ecosystem services such as drinking water, fisheries, and recreation by promoting the occurrence and severity of cyanobacterial blooms.
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.
Storm events disproportionately mobilize dissolved phosphorus (P) compared to nitrogen (N), contributing to reduction in load N:P. In agricultural watersheds, conservation tillage may lead to even ...further declines in load N:P due to dissolved P accumulation in the top soil layers. Due to an increase in this management activity, we were interested in the impacts of conservation tillage on N and P loads during storm events. Using a 20 year data set of nutrient loads to a hypereutrophic reservoir, we observed disproportionately increasing P loads relative to base flow during storm events, whereas N loads were proportional to discharge. We also observed a change in that relationship, i.e., even greater P load relative to base flow with more conservation tillage in the watershed. This suggests conservation tillage may contribute to significantly reduced N:P loads during storms with potential implications for the water quality of receiving water bodies.
Stream water quality can be greatly influenced by changes in agricultural practices, but studies of long‐term dynamics are scarce. Here we describe trends over 21 yr (1994–2014) in nutrients and ...suspended sediments in three streams in a Midwestern US agricultural watershed. During this time, the watershed experienced substantial changes in agricultural practices, most importantly a pronounced shift from conventional to conservation tillage. In the 1990s and early 2000s, NH4, soluble reactive P, and suspended sediment concentrations (standardized for discharge and season) each declined significantly (>4–12% per year) in at least two of the three streams (P < 0.01), whereas NO3 changed relatively little. However, since the early 2000s, declines in NH4 and sediment concentrations have slowed, soluble reactive P concentrations have not declined and may actually have increased, and NO3 concentrations have declined sharply. The more recent lack of decline in soluble reactive P coincides with a plateau in the prevalence of conservation tillage and may be because of increased soil P stratification due to long‐term reduced tillage. The more recent decline in NO3 may be due to improved efficiency of N fertilizer use, increased soil denitrification, and/or declines in atmospheric N deposition. Our study shows that stream concentrations of N, P, and sediment can respond in contrasting ways to changes in agriculture, and that temporal trends can moderate, accelerate, or reverse over decadal timescales. Management strategies must consider contrasting temporal responses of water quality indicators and may need to be adaptively adjusted at scales of years to decades.
Core Ideas
Stream water quality in a Midwestern watershed responds to agricultural management.
Conservation tillage and nutrient management appear to be major drivers of change.
Nitrogen, phosphorus, and sediment show contrasting dynamics over decadal timescales.
Management plans must anticipate temporally variable trends over multiple decades.
As the lowest point in the surrounding landscape, lakes act as sensors in the landscape to provide insights into the response of both terrestrial and aquatic ecosystems to climate change. Here a ...novel suite of climate forcing optical indices (CFOI) from lakes across North America is found to respond to changes in air temperature, precipitation, and solar radiation at timescales ranging from a single storm event to seasonal changes to longer‐term interdecadal trends with regression r2 values ranging from 0.73 to 0.89. These indices are based on two optical metrics of dissolved organic carbon (DOC) quality: DOC specific absorbance (a*320) and spectral slope (S275–295), where the ratio a*320 to S275–295 gives a composite climate forcing index. These indices of DOC quality are more responsive to climate forcing than is DOC concentration. A similar relationship between the component indices a*320 and S275–295 is observed across a wide range of lake types. A conceptual model is used to examine the similarities and differences in DOC‐related mechanisms and ecological consequences due to increased temperature vs. precipitation. While both warmer and wetter conditions increase thermal stratification, these two types of climate forcing will have opposite effects on water transparency as well as many ecological consequences, including oxygen depletion, the balance between autotrophy and heterotrophy, and depth distributions of phytoplankton and zooplankton.
Accurate estimation of constituent loads is important for studies of ecosystem mass balance or total maximum daily loads. In response, there has been an effort to develop methods to increase both ...accuracy and precision of constituent load estimates. The relationship between constituent concentration and stream discharge is often complicated, potentially leading to high uncertainty in load estimates for certain constituents, especially at longer-term (annual) scales. We used the
loadflex
R package to compare uncertainty in annual load estimates from concentration vs. discharge relationships in constituents of interest in agricultural systems, including ammonium as nitrogen (NH
4
-N), nitrate as nitrogen (NO
3
-N), soluble reactive phosphorus (SRP), and suspended sediments (SS). We predicted that uncertainty would be greatest in NO
3
-N and SS due to complex relationships between constituent concentration and discharge. We also predicted lower uncertainty with a composite method compared to regression or interpolation methods. Contrary to predictions, we observed the lowest uncertainty in annual NO
3
-N load estimates (relative error 1.5–23%); however, uncertainty was greatest in SS load estimates, consistent with predictions (relative error 19–96%). For all constituents, we also generally observed reductions in uncertainty by up to 34% using the composite method compared to regression and interpolation approaches, as predicted. These results highlight differences in uncertainty among different constituents and will aid in model selection for future studies requiring accurate and precise estimates of constituent load.
Sediment organic carbon (C) burial and CO2 fluxes in inland waters are quantitatively important in regional and global carbon budgets. Estimates of C fluxes from inland waters are typically based on ...limited temporal resolution despite potential large variations with season and weather events. Further, most freshwater C budget studies have focused on natural soft‐water lakes, while reservoirs and hard‐water systems are globally numerous. Our study quantifies C fluxes in two hard‐water, human constructed reservoirs (Ohio, USA) of contrasting watershed land use (agriculture vs. forest) using high‐resolution mass balance budgets. We show that during a dry summer, C retention and export via the dam were reduced compared to a wet summer. Both reservoirs were net CO2 sources during a wet summer, but CO2 sinks during a dry summer. Despite weather‐related summer differences, annual C fluxes within each reservoir were similar between years. Both reservoirs appear to be net autotrophic despite often being CO2 sources based on budgets. This is likely because CO2 fluxes in our hard‐water reservoirs were more strongly associated with DIC than DOC. Using our C fluxes and statewide watershed land use, we determined the regional importance of Ohio reservoirs in OC burial and CO2 emissions. We estimate that Ohio reservoirs bury up to 4 times more OC, but emit <25% of CO2, than predicted based on their area and recent global mean estimates in lentic ecosystems. Our results provide evidence that moderately old (~50 years), temperate hard‐water reservoirs are important OC sinks but contribute little to CO2 emissions.
Key PointsTemperate reservoirs are large sediment carbon sinksTemperate reservoirs are small carbon dioxide sourcesWeather events important for carbon budget interpretation
Lakes and reservoirs bury large quantities of organic carbon (C) and nutrients (nitrogen, N; phosphorus, P) in their sediments, especially when expressed relative to the small area they occupy. ...Global estimates of C and nutrient burial rates in reservoirs require a quantitative understanding of the wide variation in the rates and ratios at which C, N and P are sequestered in sediments. We examined how catchment and reservoir characteristics relate to sediment organic C, N and P concentrations, stoichiometric burial ratios (C : P, C : N, N : P) and burial rates in 13 small reservoirs across a catchment land use gradient in the Midwestern United States. Sediment P concentrations were positively correlated with urban catchment land use and negatively correlated with agricultural catchment land use. Stoichiometric burial ratios varied with catchment land use. Both N : P and C : P were positively correlated with agricultural land use, while these ratios were negatively correlated with urban land use and forested land use. In general, rates of C, N and P burial per unit lake area were not related to land use in the catchment, but were all positively correlated with catchment area to lake area ratios. Results from our study reservoirs suggest that reservoir burial rates are more tightly coupled with morphometric catchment characteristics than with land use. Our results suggest that small reservoirs are regionally and globally significant for biogeochemical processing. However, regional variation requires that much more comprehensive sampling is needed for accurate estimates of global element burial rates.
Animals transform and translocate nutrients at ecologically relevant rates, contributing to eutrophication in aquatic ecosystems by mobilizing otherwise unavailable nutrients. Yet we know little ...about how animal-mediated nutrient cycling compares with external abiotic nutrient sources over long periods (years–decades) and at multiple timescales. To address this, we conducted a 19-year study in a eutrophic reservoir examining nitrogen (N) and phosphorus (P) inputs from watershed streams versus excretion by an abundant fish (gizzard shad, Dorosoma cepedianum) at weekly, monthly and seasonal timescales. Over the entire time period, watershed N and P loading was 33- and 3-fold greater than fish N and P excretion, respectively. However, fish N excretion exceeded watershed nutrient loading in 36% of weeks and 43% of months, and fish P excretion in 68% of weeks and 58% of months during the growing season. Fish excretion had lower temporal variability in both supply rate and N:P ratio than watershed loading. Fish excretion also supplied nutrients at a much lower molar N:P ratio than the watershed (mean of daily N:P supply ratios were 15 and 723, respectively). In eutrophic lakes with high fish biomass, fish excretion can strongly influence algal biomass and community composition. Eutrophication management efforts should consider removal of benthivorous fish, like gizzard shad, in addition to other watershed management practices to improve water quality. Future climate change will modulate the interplay between fish- and watershed-mediated nutrient dynamics by altering the geographic distribution of detritivorous fish and the frequency and severity of storm and drought events.
1. Catchments export nutrients to aquatic ecosystems at rates and ratios that are strongly influenced by land use practices, and within aquatic ecosystems nutrients can be processed, retained, lost ...to the atmosphere, or exported downstream. The stoichiometry of carbon and nutrients can influence ecosystem services such as water quality, nutrient limitation, biodiversity, eutrophication and the sequestration of nutrients and carbon in sediments. However, we know little about how nutrient stoichiometry varies along the pathway from terrestrial landscapes through aquatic systems. 2. We studied the stoichiometry of nitrogen and phosphorus exported by three catchments of contrasting land use (forest versus agriculture) and in the water column and sediments of downstream reservoirs. We also related stoichiometry to phytoplankton nutrient limitation and the abundance of heterocystous cyanobacteria. 3. The total N : P of stream exports varied greatly among catchments and was 18, 54 and 140 (molar) in the forested, mixed-use and agricultural catchment, respectively. Total N : P in the mixed layers of the lakes was less variable but ordered similarly: 35, 52 132 in the forested, mixed-use and agricultural lake, respectively. In contrast, there was little variation among systems in the C : N and C : P ratios of catchment exports or in reservoir seston. 4. Phytoplankton in the forested lake were consistently N limited, those in the agricultural lake were consistently P limited, and those in the mixed-use lake shifted seasonally from P- to N limitation, reflecting N : P supply ratios. Total phytoplankton and cyanobacteria biomass were highest in the agricultural lake, but heterocystous (potentially N fixing) cyanobacteria were most abundant in the forested lake, corresponding to low N : P ratios. 5. Despite large differences in catchment export and water column N : P ratios, the N : P of sediment burial (integrated over several decades) was very low and remarkably similar (4.3-7.3) across reservoirs. N and P budgets constructed for the agricultural reservoir suggested that denitrification could be a major loss of N, and may help explain the relatively low N : P of buried sediment. 6. Our results show congruence between the catchment export N : P, reservoir N : P, phytoplankton N versus P limitation and the dominance of heterocystous cyanobacteria. However, the N : P stoichiometry of sediments retained in the lakes was relatively insensitive to catchment stoichiometry, suggesting that a common set of biogeochemical processes constrains sediment N : P across lakes of contrasting catchment land use.
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