A review of ecosystem services from edge-of-field practices in tile-drained agricultural systems in the United States Corn Belt Region.

Edge-of-field management practices that reduce nutrient pollution from tile drainage while contributing habitat and other ecosystem services are needed to enhance agricultural systems in the US Corn Belt Region. In this review, we identified edge-of-field and catchment scale agricultural conservation practices for intercepting and treating tile drainage. The reviewed conservation practices were (1) controlled drainage, also known as drainage water management (USDA-NRCS Code 554); (2) drainage water recycling (USDA-NRCS Code 447); (3) denitrifying bioreactors (USDA-NRCS Code 605); (4) saturated buffers (USDA-NRCS Code 604); and (5) constructed or restored wetlands designed for water quality improvement (USDA-NRCS Code 656) herein referred to as water quality wetlands. We examined 119 studies that had information on one or more of the following ecosystem services: water retention, water quality improvement (e.g., nitrate, phosphate, sediment, or pesticide retention), wetland habitat (for birds, aquatic organisms, and pollinators), crop yield improvement, and other benefits (e.g., recreation, education, aesthetic appreciation, greenhouse gas retention). We found the five edge-of-field practices were all effective at removing nitrate with varying degrees of other potential benefits and disservices (e.g., greenhouse gas production). Drainage water recycling and water quality wetlands have the potential to provide the most co-benefits as they provide surface water systems for capturing surface flows in addition to tile drainage while also potentially providing habitat and recreation opportunities. However, the following research needs are identified: 1) the disservices and benefits associated with drainage water recycling have not been adequately evaluated; 2) surface flow dynamics are understudied across all reviewed management practices; 3) a complete accounting of phosphorus species and flow pathways for all management practices is needed; 4) field evaluations of the habitat benefit of all management practices are needed; and 5) greenhouse gas dynamics are understudied across all management practices. While all management practices are expected to reduce nitrate loads, addressing these knowledge gaps will help inform holistic management decisions for diverse stakeholders across the US Corn Belt.

Contaminant Exposure and Transport from Three Potential Reuse Waters within a Single Watershed.

Global demand for safe and sustainable water supplies necessitates a better understanding of contaminant exposures in potential reuse waters. In this study, we compared exposures and load contributions to surface water from the discharge of three reuse waters (wastewater effluent, urban stormwater, and agricultural runoff). Results document substantial and varying organic-chemical contribution to surface water from effluent discharges (e.g., disinfection byproducts [DBP], prescription pharmaceuticals, industrial/household chemicals), urban stormwater (e.g., polycyclic aromatic hydrocarbons, pesticides, nonprescription pharmaceuticals), and agricultural runoff (e.g., pesticides). Excluding DBPs, episodic storm-event organic concentrations and loads from urban stormwater were comparable to and often exceeded those of daily wastewater-effluent discharges. We also assessed if wastewater-effluent irrigation to corn resulted in measurable effects on organic-chemical concentrations in rain-induced agricultural runoff and harvested feedstock. Overall, the target-organic load of 491 g from wastewater-effluent irrigation to the study corn field during the 2019 growing season did not produce substantial dissolved organic-contaminant contributions in subsequent rain-induced runoff events. Out of the 140 detected organics in source wastewater-effluent irrigation, only imidacloprid and estrone had concentrations that resulted in observable differences between rain-induced agricultural runoff from the effluent-irrigated and nonirrigated corn fields. Analyses of pharmaceuticals and per-/polyfluoroalkyl substances in at-harvest corn-plant samples detected two prescription antibiotics, norfloxacin and ciprofloxacin, at concentrations of 36 and 70 ng/g, respectively, in effluent-irrigated corn-plant samples; no contaminants were detected in noneffluent irrigated corn-plant samples.

Potential of water quality wetlands to mitigate habitat losses from agricultural drainage modernization.

Given widespread biodiversity declines, a growing global human population, and demands to improve water quality, there is an immediate need to explore land management solutions that support multiple ecosystem services. Agricultural water quality wetlands designed to provide both water quality benefits and wetland and grassland habitat are an emerging restoration solution that may reverse habitat declines in intensive agricultural areas. Installation of water quality wetlands in the Upper Midwest, USA, when considered alongside the repair and modification of aging agricultural tile drainage infrastructure, is a likely scenario that may mitigate nutrient pollution exported from agricultural systems and improve crop yields. The capacity of water quality wetlands to provide habitat within the wetland pool and the surrounding grassland is not well-studied, particularly with respect to potential habitat changes resulting from drainage infrastructure upgrades. For the current study, we produced spatially explicit models of 37 catchments distributed throughout an important region for agriculture and biodiversity, the Des Moines Lobe of Iowa. Four scenarios were considered - with and without improved drainage and with and without water quality wetlands - to estimate the net potential habitat implications of these scenarios for amphibians, grassland birds, and wild bees. Model results indicate that drainage modification alone will likely result in moderate direct losses of suitable amphibian habitat and large declines in overall habitat quality. However, inclusion of water quality wetlands at the catchment scale may mitigate these amphibian habitat losses while also increasing grassland bird and pollinator habitat. The impacts of water quality wetlands and drainage modernization on waterfowl in the region require additional study.

Seasonal Distribution of Cyanobacteria in Three Urban Eutrophic Lakes Results from an Epidemic-like Response to Environmental Conditions.

Cyanobacterial communities of three co-located eutrophic sandpit lakes were surveyed during 2016 and 2017 over season and depth using high-throughput DNA sequencing of the 16S rRNA gene. All three lakes were stratified except during April 2017 when the lakes were recovering from a strong mixing event. 16S rRNA gene V4 sequences were parsed into operational taxonomic units (OTUs) at 99% sequence identity. After rarefaction of 139 samples to 25,000 sequences per sample, a combined total of 921,529 partial 16S rRNA gene sequences were identified as cyanobacteria. They were binned into 19,588 unique cyanobacterial OTUs. Of these OTUs, 11,303 were Cyanobium. Filamentous Planktothrix contributed 1537 and colonial Microcystis contributed 265. The remaining 6482 OTUs were considered unclassified. For Planktothrix and Microcystis one OTU accounted for greater than 95% of the total sequences for each genus. However, in both cases the non-dominant OTUs clustered with the dominant OTUs by date, lake, and depth. All Planktothrix OTUs and a single Cyanobium OTU were detected below the oxycline. All other Cyanobium and Microcystis OTUs were detected above the oxycline. The distribution of Cyanobium OTUs between lakes and seasons can be explained by an epidemic-like response where individual OTUs clonally rise from a diverse hypolimnion population when conditions are appropriate. The importance of using 99% identity over the more commonly used 97% is discussed with respect to cyanobacterial community structure. The approach described here can provide another valuable tool for assessing cyanobacterial populations and provide greater insight into the controls of cyanobacterial blooms.

Enhanced streamflow prediction with SWAT using support vector regression for spatial calibration: A case study in the Illinois River watershed, U.S.

Accurate streamflow prediction plays a pivotal role in hydraulic project design, nonpoint source pollution estimation, and water resources planning and management. However, the highly non-linear relationship between rainfall and runoff makes prediction difficult with desirable accuracy. To improve the accuracy of monthly streamflow prediction, a seasonal Support Vector Regression (SVR) model coupled to the Soil and Water Assessment Tool (SWAT) model was developed for 13 subwatersheds in the Illinois River watershed (IRW), U.S. Terrain, precipitation, soil, land use and land cover, and monthly streamflow data were used to build the SWAT model. SWAT Streamflow output and the upstream drainage area were used as two input variables into SVR to build the hybrid SWAT-SVR model. The Calibration Uncertainty Procedure (SWAT-CUP) and Sequential Uncertainty Fitting-2 (SUFI-2) algorithms were applied to compare the model performance against SWAT-SVR. The spatial calibration and leave-one-out sampling methods were used to calibrate and validate the hybrid SWAT-SVR model. The results showed that the SWAT-SVR model had less deviation and better performance than SWAT-CUP simulations. SWAT-SVR predicted streamflow more accurately during the wet season than the dry season. The model worked well when it was applied to simulate medium flows with discharge between 5 m3 s-1 and 30 m3 s-1, and its applicable spatial scale fell between 500 to 3000 km2. The overall performance of the model on yearly time series is "Satisfactory". This new SWAT-SVR model has not only the ability to capture intrinsic non-linear behaviors between rainfall and runoff while considering the mechanism of runoff generation but also can serve as a reliable regional tool for an ungauged or limited data watershed that has similar hydrologic characteristics with the IRW.

Temperature Decrease along Hyporheic Pathlines in a Large River Riparian Zone.

Hyporheic zones contribute to lower temperatures in many rivers, creating a longitudinal heterogeneous array of thermal refuges. In this study, we had the unique opportunity to show temperature reduction along actual hyporheic zone pathlines in a large river system that contribute to the maintenance of refuges through discharge into off-channel habitats. Temperature was monitored in a dense network of wells that were located along pathlines in small islands, from a calibrated ground-water flow model. Temperature along one 600-m pathline was reduced about 7 °C. Among three islands that were adjacent to the river, the northern two showed exponential decrease in temperature with distance, with fitted thermal Péclet numbers of 2.7 and 6.5, while the southern island showed no significant decrease. We suggest this is due to the higher infiltration rate in the wet season in this larger, more mature island, which suppresses hyporheic flow in the wet season. Stable isotope sampling showed that values of δ2H were higher in areas where we observed lower temperatures. The overall relationship of δ2H versus temperature was significant with a slope of -0.329. This implies that lower temperatures are associated with water that has had contact with deeper groundwater or that lower temperatures have been affected by local rainfall infiltration, or water that has entered the hyporheic zone in winter. These findings are important because they allow estimation of the temperature benefit that may be achieved in similar geomorphic settings, providing implications for riparian restoration.

Review of Watershed-Scale Water Quality and Nonpoint Source Pollution Models.

Watershed-scale nonpoint source (NPS) pollution models have become important tools to understand, evaluate, and predict the negative impacts of NPS pollution on water quality. Today, there are many NPS models available for users. However, different types of models possess different form and structure as well as complexity of computation. It is difficult for users to select an appropriate model for a specific application without a clear understanding of the limitations or strengths for each model or tool. This review evaluates 14 more commonly used watershed-scale NPS pollution models to explain how and when the application of these different models are appropriate for a given effort. The models that are assessed have a wide range of capacities that include simple models used as rapid screening tools (e.g., Long-Term Hydrologic Impact Assessment (L-THIA) and Nonpoint Source Pollution and Erosion Comparison Tool (N-SPECT/OpenNSPECT)), medium-complexity models that require detail data input and limited calibration (e.g., Generalized Watershed Loading Function (GWLF), Loading Simulation Program C (LSPC), Source Loading and Management Model (SLAMM), and Watershed Analysis Risk Management Frame (WARMF)), complex models that provide sophisticated simulation for NPS pollution processes with intensive data and rigorous calibration (e.g., Agricultural Nonpoint Source pollution model (AGNPS/AnnAGNPS), Soil and Water Assessment Tool (SWAT), Stormwater Management Model (SWMM), and Hydrologic Simulation Program Fortran (HSPF)), and modeling systems that integrate various sub-models and tools, and contain the highest complexity to solve all phases of hydrologic, hydraulic, and chemical dynamic processes (e.g., Automated Geospatial Watershed Assessment Tool (AGWA), Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) and Watershed Modeling System (WMS)). This assessment includes model intended use, components or capabilities, suitable land-use type, input parameter type, spatial and temporal scale, simulated pollutants, strengths and limitations, and software availability. Understanding the strengths and weaknesses of each watershed-scale NPS model will lead to better model selection for suitability and help to avoid misinterpretation or misapplication in practice. The article further explains the crucial criteria for model selection, including spatial and temporal considerations, calibration and validation, uncertainty analysis, and future research direction of NPS pollution models. The goal of this work is to provide accurate and concise insight for watershed managers and planners to select the best-suited model to reduce the harm of NPS pollution to watershed ecosystems.

Urban Stormwater: An Overlooked Pathway of Extensive Mixed Contaminants to Surface and Groundwaters in the United States.

Increasing global reliance on stormwater control measures to reduce discharge to surface water, increase groundwater recharge, and minimize contaminant delivery to receiving waterbodies necessitates improved understanding of stormwater-contaminant profiles. A multiagency study of organic and inorganic chemicals in urban stormwater from 50 runoff events at 21 sites across the United States demonstrated that stormwater transports substantial mixtures of polycyclic aromatic hydrocarbons, bioactive contaminants (pesticides and pharmaceuticals), and other organic chemicals known or suspected to pose environmental health concern. Numerous organic-chemical detections per site (median number of chemicals detected = 73), individual concentrations exceeding 10 000 ng/L, and cumulative concentrations up to 263 000 ng/L suggested concern for potential environmental effects during runoff events. Organic concentrations, loads, and yields were positively correlated with impervious surfaces and highly developed urban catchments. Episodic storm-event organic concentrations and loads were comparable to and often exceeded those of daily wastewater plant discharges. Inorganic chemical concentrations were generally dilute in concentration and did not exceed chronic aquatic life criteria. Methylmercury was measured in 90% of samples with concentrations that ranged from 0.05 to 1.0 ng/L.

Quantifying the effects of surface conveyance of treated wastewater effluent on groundwater, surface water, and nutrient dynamics in a large river floodplain.

Restoration and reconnection of floodplain systems provide multiple societal and ecosystem benefits, while providing municipalities the opportunity to attempt alternative approaches to maintain infrastructure protection and function. In some restored floodplains, treated wastewater effluent discharge is redirected over land instead of directly into rivers to allow natural flow and infiltration, to facilitate restoration designs such as levee setback, and to provide additional freshwater to floodplain ecosystems. However, indirect discharge of treated effluent over land may pose risks to surface and groundwater when pollutants like excess nutrients enter the floodplain and undergo transformation. We investigated the consequences for groundwater and surface water quality when effluent was redirected as open water channels over a floodplain surface. In this study, seasonal floodplain nutrient concentrations in groundwater and surface water were observed for more than 5 years as a floodplain and wastewater treatment plant underwent a major restoration project that included river-floodplain reconnection with levee setback and redirection of effluent discharge from a river channel to open flow across the restored floodplain. Nutrient loading to the surrounding floodplain groundwater and surface water was observed, but based on measures of hydrological connectivity, groundwater flow paths, and biogeochemistry, nutrients from the effluent moved within the floodplain with minimal effect to the surrounding floodplain water quality. We did not find evidence of substantial additional processing that could replace advanced nutrient treatment in this system, however we did observe evidence of diverse nutrient processes that may support enhanced retention if treatment channels were designed to enhance these processes. We suggest that indirect discharge of high quality treated effluent in a restored floodplain is a viable alternative to direct discharge into a river when groundwater flow directs that discharge to habitats where minimal nutrient sensitivity is expected.

Using SWAT to Evaluate Streamflow and Lake Sediment Loading in the Xinjiang River Basin with Limited Data.

Soil erosion and lake sediment loading are primary concerns of watershed managers around the world. In the Xinjiang River Basin of China, severe soil erosion occurs primarily during monsoon periods, resulting in sediment flow into Poyang Lake and subsequently causing lake water quality deterioration. Here, we identified high-risk soil erosion areas and conditions that drive sediment yield in a watershed system with limited available data to guide localized soil erosion control measures intended to support reduced sediment load into Poyang Lake. We used the Soil and Water Assessment Tool (SWAT) model to simulate monthly and annual sediment yield based on a calibrated SWAT streamflow model, identified where sediment originated, and determined what geographic factors drove the loading within the watershed. We applied monthly and daily streamflow discharge (1985-2009) and monthly suspended sediment load data (1985-2001) to Meigang station to conduct parameter sensitivity analysis, calibration, validation, and uncertainty analysis of the model. The coefficient of determination (R 2), Nash-Sutcliffe efficiency (NSE), percent bias (PBIAS), and RMSE -observation's standard deviation ratio (RSR) values of the monthly sediment load were 0.63, 0.62, 3.8%, and 0.61 during calibration, respectively. Spatially, the annual sediment yield rate ranged from 3 ton ha-1year-1 on riparian lowlands of the Xinjiang main channel to 33 ton ha-1year-1 on mountain highlands, with a basin-wide mean of 19 ton ha-1year-1. The study showed that 99.9% of the total land area suffered soil loss (greater than 5 ton ha-1year-1). More sediment originated from the southern mountain highlands than from the northern mountain highlands of the Xinjiang river channel. These results suggest that specific land use types and geographic conditions can be identified as hotspots of sediment source with relatively scarce data; in this case, orchards, barren lands, and mountain highlands with slopes greater than 25° were the primary sediment source areas. This study developed a reliable, physically-based streamflow model and illustrates critical source areas and conditions that influence sediment yield.

Floodplain restoration increases hyporheic flow in the Yakima River Watershed, Washington.

Hyporheic exchange between a river channel and its floodplain region assists in mediating processes such as nutrient removal and temperature regulation. Floodplain restoration in the form of levee setbacks are often carried out to improve the hyporheic exchange. In this study Light Detection and Ranging (LiDAR) data were used along with the head data from observation wells and stage data from rivers to setup and calibrate a groundwater model for 458 km2 of area within Gap to Gap reach of the Yakima River, WA. This area has witnessed several efforts of floodplain restoration in the form of levee setbacks. The groundwater model was used to quantify hyporheic flow emerging from the Yakima River in steady and transient states during pre-restoration (using LiDAR data of 2008) and post-restoration period (after levee setback using LiDAR data of 2013). The comparison of results from the model runs during pre and post-restoration periods showed that the length of the pathlines increased after levee setback for both steady and transient state model simulations. The largest increase of about 62 m was noticed in the month of September 2014 (pre: 398 m and post: 460 m). The study also showed that the direction of the flow changed following levee setback, expanding the area for hyporheic flux exchange between surface and groundwater. The model run during transient state also suggested that pathlines were longer during drier months compared to wet months. Overall, the study showed that levee setbacks improved the hyporheic connection between surface and groundwater in the Yakima floodplain which demonstrates that levee setback can provide a valuable hydrologic tool to restore ecosystem processes in previously leveed rivers.

Consumer Aggregations Drive Nutrient Dynamics and Ecosystem Metabolism in Nutrient-Limited Systems.

Differences in animal distributions and metabolic demands can influence energy and nutrient flow in an ecosystem. Through taxa-specific nutrient consumption, storage, and remineralization, animals may influence energy and nutrient pathways in an ecosystem. Here we show these taxa-specific traits can drive biogeochemical cycles of nutrients and alter ecosystem primary production and metabolism, using riverine systems that support heterogeneous freshwater mussel aggregations. Freshwater unionid mussels occur as distinct, spatially heterogeneous, dense aggregations in rivers. They may influence rates of production and respiration because their activities are spatially concentrated within given stream reaches. Previous work indicates that mussels influence nutrient limitation patterns, algal species composition, and producer and primary consumer biomass. Here, we integrate measures of organismal rates, stoichiometry, community-scaled rates, and ecosystem rates, to determine the relative source-sink nutrient dynamics of mussel aggregations and their influence on net ecosystem processes. We studied areas with and without mussel aggregations in three nitrogen-limited rivers in southeastern Oklahoma, USA. We measured respiration and excretion rates of mussels and collected a subset of samples for tissue chemistry and for thin sectioning of the shell to determine growth rates at each site. This allowed us to assess nutrient remineralization and nutrient sequestration by mussels. These rates were scaled to the community. We also measured stream metabolism at three sites with and without mussels. We demonstrated that mussel species have distinct stoichiometric traits, vary in their respiration rates, and that mussel aggregations influence nutrient cycling and productivity. Across all mussel aggregations, we found that mussels excreted more nitrogen than they sequestered into tissue and excreted more phosphorus than they sequestered except at one site. Furthermore, gross primary productivity was significantly greater at reaches with mussels. Collectively, our results indicate that mussels have ecosystem-level impacts on nutrient availability and production in nutrient-limited rivers. Within these streams, mussels are affecting the movement of nutrients and altering nutrient spiralling.

Vertically challenged: How disease suppresses Daphnia vertical migration behavior.

Parasitic infections are increasingly recognized as influential forces in the migratory behaviors of hosts ranging from butterflies to whales. In aquatic zooplankton, diel vertical migrations (DVMs) are among the most recurrent behaviors with implications for predator-prey interactions, nutrient cycling, and energy flow, yet how parasitism affects such migrations remains an open question. Here, we tested the effects of sporangia cluster disease (SCD) on DVM of the large-bodied Daphnia pulicaria, which is often considered a key component of lake food webs. By collecting depth-specific zooplankton samples across diel cycles, between years, and among lakes, we show that infection is associated with strong inhibition of host DVM; while all Daphnia tended to occur deeper during the day, uninfected Daphnia and especially gravid individuals migrated to shallower waters at night. In contrast, infected hosts-which could comprise 40% of the population-were more likely to remain deep regardless of time of day. Among infected hosts, the intensity of SCD (sporangia count per host) predicted the degree of DVM inhibition. These observations-coupled with lab-based assays showing that infected hosts exhibited fewer swimming movements and persisted at lower depths than uninfected conspecifics-suggest that parasite-induced inhibition of DVM is a "sickness behavior" resulting from increasing morbidity and energy depletion as the infection intensifies toward host death. Considering the importance of large-bodied Daphnia as regulators of water clarity and prey for fishes, parasite-induced alterations of host migratory behavior have broad potential to affect the redistribution of energy and nutrients within lake ecosystems.

Aggregated filter-feeding consumers alter nutrient limitation: consequences for ecosystem and community dynamics.

Nutrient cycling is a key process linking organisms in ecosystems. This is especially apparent in stream environments in which nutrients are taken up readily and cycled through the system in a downstream trajectory. Ecological stoichiometry predicts that biogeochemical cycles of different elements are interdependent because the organisms that drive these cycles require fixed ratios of nutrients. There is growing recognition that animals play an important role in biogeochemical cycling across ecosystems. In particular, dense aggregations of consumers can create biogeochemical hotspots in aquatic ecosystems via nutrient translocation. We predicted that filter-feeding freshwater mussels, which occur as speciose, high-biomass aggregates, would create biogeochemical hotspots in streams by altering nutrient limitation and algal dynamics. In a field study, we manipulated nitrogen and phosphorus using nutrient-diffusing substrates in areas with high and low mussel abundance, recorded algal growth and community composition, and determined in situ mussel excretion stoichiometry at 18 sites in three rivers (Kiamichi, Little, and Mountain Fork Rivers, south-central United States). Our results indicate that mussels greatly influence ecosystem processes by modifying the nutrients that limit primary productivity. Sites without mussels were N-limited with -26% higher relative abundances of N-fixing blue-green algae, while sites with high mussel densities were co-limited (N and P) and dominated by diatoms. These results corroborated the results of our excretion experiments; our path analysis indicated that mussel excretion has a strong influence on stream water column N:P. Due to the high N:P of mussel excretion, strict N-limitation was alleviated, and the system switched to being co-limited by both N and P. This shows that translocation of nutrients by mussel aggregations is important to nutrient dynamics and algal species composition in these rivers. Our study highlights the importance of consumers and this imperiled faunal group on nutrient cycling and community dynamics in aquatic ecosystems.

Ecological engineering practices for the reduction of excess nitrogen in human-influenced landscapes: a guide for watershed managers.

Excess nitrogen (N) in freshwater systems, estuaries, and coastal areas has well-documented deleterious effects on ecosystems. Ecological engineering practices (EEPs) may be effective at decreasing nonpoint source N leaching to surface and groundwater. However, few studies have synthesized current knowledge about the functioning principles, performance, and cost of common EEPs used to mitigate N pollution at the watershed scale. Our review describes seven EEPs known to decrease N to help watershed managers select the most effective techniques from among the following approaches: advanced-treatment septic systems, low-impact development (LID) structures, permeable reactive barriers, treatment wetlands, riparian buffers, artificial lakes and reservoirs, and stream restoration. Our results show a broad range of N-removal effectiveness but suggest that all techniques could be optimized for N removal by promoting and sustaining conditions conducive to biological transformations (e.g., denitrification). Generally, N-removal efficiency is particularly affected by hydraulic residence time, organic carbon availability, and establishment of anaerobic conditions. There remains a critical need for systematic empirical studies documenting N-removal efficiency among EEPs and potential environmental and economic tradeoffs associated with the widespread use of these techniques. Under current trajectories of N inputs, land use, and climate change, ecological engineering alone may be insufficient to manage N in many watersheds, suggesting that N-pollution source prevention remains a critical need. Improved understanding of N-removal effectiveness and modeling efforts will be critical in building decision support tools to help guide the selection and application of best EEPs for N management.

Festering food: chytridiomycete pathogen reduces quality of Daphnia host as a food resource.

When parasitic infections are severe or highly prevalent among prey, a significant component of the predator's diet may consist of parasitized hosts. However, despite the ubiquity of parasites in most food webs, comparisons of the nutritional quality of prey as a function of infection status are largely absent. We measured the nutritional consequences of chytridiomycete infections in Daphnia, which achieve high prevalence in lake ecosystems (>80%), and tested the hypothesis that Daphnia pulicaria infected with Polycaryum laeve are diminished in food quality relative to uninfected hosts. Compared with uninfected adults, infected individuals were smaller, contained less nitrogen and phosphorus, and were lower in several important fatty acids. Infected zooplankton had significantly shorter carapace lengths (8%) and lower mass (8-20%) than uninfected individuals. Parasitized animals contained significantly less phosphorus (16-18% less by dry mass) and nitrogen (4-6% less) than did healthy individuals. Infected individuals also contained 26-34% less saturated fatty acid and 31-42% less docosahexaenoic acid, an essential fatty acid that is typically low in cladocera, but critical to fish growth. Our results suggest that naturally occurring levels of chytrid infections in D. pulicaria populations reduce the quality of food available to secondary consumers, including planktivorous fishes, with potentially important effects for lake food webs.

Effects of species diversity on community biomass production change over the course of succession.

Over the past decade an increasing number of studies have experimentally manipulated the number of species in a community and examined how this alters the aggregate production of species biomass. Many of these studies have shown that the effects of richness on biomass change through time, but we have limited understanding of the mechanisms that produce these dynamic trends. Here we report the results of an experiment in which we manipulated the richness of freshwater algae in laboratory microcosms. We used two experimental designs (additive and substitutive) that make different assumptions about how patches are initially colonized, and then tracked the development of community biomass from the point of initial colonization through a period of 6-12 generations of the focal species. We found that the effect of initial species richness on biomass production qualitatively shifted twice over the course of the experiment. The first shift occurred as species transitioned from density-independent to dependent phases of population growth. At this time, intraspecific competition caused monocultures to approach their respective carrying capacities more slowly than polycultures. As a consequence, species tended to over-yield for a brief time, generating a positive, but transient effect of diversity on community biomass. The second shift occurred as communities approached carrying capacity. At this time, strong interspecific interactions caused biomass to be dominated by the competitively superior species in polycultures. As this species had the lowest carrying capacity, a negative effect of diversity on biomass resulted in late succession. Although these two shifts produced dynamics that appeared complex, we show that the patterns can be fit to a simple Lotka-Volterra model of competition. Our results suggest that the effects of algal diversity on primary production change in a predictable sequence through successional time.

Dining on disease: how interactions between infection and environment affect predation risk.

Despite growing interest in ecological interactions between predators and pathogens, few studies have experimentally examined the consequences of infection for host predation risk or how environmental conditions affect this relationship. Here we combined mesocosm experiments, in situ foraging data, and broad-scale lake surveys to evaluate (1) the effects of chytrid infection (Polycaryum laeve) on susceptibility of Daphnia to fish predators and (2) how environmental characteristics moderate the strength of this interaction. In mesocosms, bluegill preferred infected Daphnia 2-5 times over uninfected individuals. Among infected Daphnia, infection intensity was a positive predictor of predation risk, whereas carapace size and fecundity increased predation on uninfected individuals. Wild-caught yellow perch and bluegill from in situ foraging trials exhibited strong selectivity for infected Daphnia (3-10 times over uninfected individuals). In mesocosms containing water high in dissolved organic carbon (DOC), however, selective predation on infected Daphnia was eliminated. Correspondingly, lakes that supported chytrid infections had higher DOC levels and lower light penetration. Our results emphasize the strength of interactions between parasitism and predation while highlighting the moderating influence of water color. P. laeve increases the conspicuousness and predation risk of Daphnia; as a result, infected Daphnia occur predominantly in environments with characteristics that conceal their elevated visibility.

Examination of physical and regulatory variables leading to small dam removal in Wisconsin.

The decision to remove or repair a dam depends on multiple variables, many of which encompass both physical and social factors. In Wisconsin, the Department of Natural Resources is mandated to inspect small dams every ten years. A safety inspection often acts as a trigger event to a dam removal or repair decision. Although the issues surrounding a dam removal decision are often couched as ecological, these decisions are influenced by their social and regulatory context. In this work, we examine descriptive variables of Wisconsin dams that were inspected and consequently removed or maintained between 1985 and 1990. We hypothesize that geographic location, height of dam, size of impoundment, age of dam, and type of ownership determine the likelihood of a safety inspection, and the subsequent likelihood of removal. Using a logistic model, we find that publicly owned dams had the greatest probability of inspection after 1985. Of these dams, older dams and those with smaller impoundments were most likely to be removed. We were unable to build a strong predictive model for dam removal with our suite of variables, suggesting that a community's decision to remove or maintain a dam is complex and heterogeneous.