Analysis of spatiotemporal variation in dissolved organic carbon concentrations for streams with cropland-dominated watersheds.

It remains a challenge to understand how dissolved organic carbon (DOC) is cycled from farmlands to rivers due to the complex interaction between farming practices, the baseflow hydrology of predominantly flat lowlands, and seasonal environmental influences such as snowpack. To address this, field DOC concentrations were measured monthly throughout the year at sub-basin scales across the Chippewa River Watershed, which falls within the Corn Belt of the Midwestern United States. These DOC dynamics in stream water from croplands were benchmarked against the data sampled from hilly forested areas in the Connecticut River Watershed. The Soil Water Assessment Tool (SWAT) simulation was applied to provide potential predictive variables associated with daily baseflow. Our study outlines a framework using the combination of primary field data, hydrological modeling, and knowledge-based reclassification of Land Use/Land Cover (LULC) data to analyze the viability of modeling the spatial and temporal variations of cropland stream DOC concentrations. Calibration of the SWAT model resulted in the overall daily Nash-Sutcliffe model efficiency coefficient (NSE) of 0.67 and the corresponding R2 = 0.89. Our main results show: 1) baseflow DOC concentrations from croplands were substantially higher throughout the year relative to other landcover areas, especially for spring runoff/snowmelt scenarios, 2) an empirical analysis explained ~82 % of the spatial gradient of annual mean observed DOC concentrations, and 3) with the addition of hydrological simulated variables, a linear model explained ~81 % of monthly and 54 % of daily variations of observed DOC concentrations for cropland sub-basins. Our study identified key factors regulating the spatiotemporal DOC concentrations in cropland streamflow; the contribution here promotes to strengthen future analytical models that link watershed characteristics to carbon cycling processes in a large freshwater ecosystem.

Pesticides alter ecosystem respiration via phytoplankton abundance and community structure: Effects on the carbon cycle?

Freshwater systems are critical to life on earth, yet they are threatened by the increasing rate of synthetic chemical pollution. Current predictions of the effects of synthetic chemicals on freshwater ecosystems are hampered by the sheer number of chemical contaminants entering aquatic systems, the diversity of organisms inhabiting these systems, the myriad possible direct and indirect effects resulting from these combinations, and uncertainties concerning how contaminants might alter ecosystem metabolism via changes in biodiversity. To address these knowledge gaps, we conducted a mesocosm experiment that elucidated the responses of ponds composed of phytoplankton and zooplankton to standardized concentrations of 12 pesticides, nested within four pesticide classes, and two pesticide types. We show that the effects of the pesticides on algae were consistent within herbicides and insecticides and that responses of over 70 phytoplankton species and genera were consistent within broad taxonomic groups. Insecticides generated top-down effects on phytoplankton community composition and abundance, which were associated with persistent increases in ecosystem respiration. Insecticides had direct toxic effects on cladocerans, which led to competitive release of copepods. These changes in the zooplankton community led to a decrease in green algae and a modest increase in diatoms. Herbicides did not change phytoplankton composition but reduced total phytoplankton abundance. This reduction in phytoplankton led to short-term decreases in ecosystem respiration. Given that ponds release atmospheric carbon and that worldwide pesticide pollution continues to increase exponentially, scientists and policy makers should pay more attention to the ways pesticides alter the carbon cycle in ponds via changes in communities, as demonstrated by our results. Our results show that these predictions can be simplified by grouping pesticides into types and species into functional groups. Adopting this approach provides an opportunity to improve the efficiency of risk assessment and mitigation responses to global change.

Critical Review of Polyphosphate and Polyphosphate Accumulating Organisms for Agricultural Water Quality Management.

Despite ongoing management efforts, phosphorus (P) loading from agricultural landscapes continues to impair water quality. Wastewater treatment research has enhanced our knowledge of microbial mechanisms influencing P cycling, especially regarding microbes known as polyphosphate accumulating organisms (PAOs) that store P as polyphosphate (polyP) under oxic conditions and release P under anoxic conditions. However, there is limited application of PAO research to reduce agricultural P loading and improve water quality. Herein, we conducted a meta-analysis to identify articles in Web of Science on polyP and its use by PAOs across five disciplines (i.e., wastewater treatment, terrestrial, freshwater, marine, and agriculture). We also summarized research that provides preliminary support for PAO-mediated P cycling in natural habitats. Terrestrial, freshwater, marine, and agriculture disciplines had fewer polyP and PAO articles compared to wastewater treatment, with agriculture consistently having the least. Most meta-analysis articles did not overlap disciplines. We found preliminary support for PAOs in natural habitats and identified several knowledge gaps and research opportunities. There is an urgent need for interdisciplinary research linking PAOs, polyP, and oxygen availability with existing knowledge of P forms and cycling mechanisms in natural and agricultural environments to improve agricultural P management strategies and achieve water quality goals.

Consistent effects of pesticides on community structure and ecosystem function in freshwater systems.

Predicting ecological effects of contaminants remains challenging because of the sheer number of chemicals and their ambiguous role in biodiversity-ecosystem function relationships. We evaluate responses of experimental pond ecosystems to standardized concentrations of 12 pesticides, nested in four pesticide classes and two pesticide types. We show consistent effects of herbicides and insecticides on ecosystem function, and slightly less consistent effects on community composition. Effects of pesticides on ecosystem function are mediated by alterations in the abundance and community composition of functional groups. Through bottom-up effects, herbicides reduce respiration and primary productivity by decreasing the abundance of phytoplankton. The effects of insecticides on respiration and primary productivity of phytoplankton are driven by top-down effects on zooplankton composition and abundance, but not richness. By demonstrating consistent effects of pesticides on communities and ecosystem functions and linking pesticide-induced changes in functional groups of organisms to ecosystem functions, the study suggests that ecological risk assessment of registered chemicals could be simplified to synthetic chemical classes or types and groups of organisms with similar functions and chemical toxicities.

Effects of pesticides on exposure and susceptibility to parasites can be generalised to pesticide class and type in aquatic communities.

Pesticide pollution can alter parasite transmission, but scientists are unaware if effects of pesticides on parasite exposure and host susceptibility (i.e. infection risk given exposure) can be generalised within a community context. Using replicated temperate pond communities, we evaluate effects of 12 pesticides, nested in four pesticide classes (chloroacetanilides, triazines, carbamates organophosphates) and two pesticide types (herbicides, insecticides) applied at standardised environmental concentrations on larval amphibian exposure and susceptibility to trematode parasites. Most of the variation in exposure and susceptibility occurred at the level of pesticide class and type, not individual compounds. The organophosphate class of insecticides increased snail abundance (first intermediate host) and thus trematode exposure by increasing mortality of snail predators (top-down mechanism). While a similar pattern in snail abundance and trematode exposure was observed with triazine herbicides, this effect was driven by increases in snail resources (periphytic algae, bottom-up mechanism). Additionally, herbicides indirectly increased host susceptibility and trematode infections by (1) increasing time spent in susceptible early developmental stages and (2) suppressing tadpole immunity. Understanding generalisable effects associated with contaminant class and type on transmission is critical in reducing complexities in predicting disease dynamics in at-risk host populations.

Lake Bacterial Assemblage Composition Is Sensitive to Biological Disturbance Caused by an Invasive Filter Feeder.

One approach to improve forecasts of how global change will affect ecosystem processes is to better understand how anthropogenic disturbances alter bacterial assemblages that drive biogeochemical cycles. Species invasions are important contributors to global change, but their impacts on bacterial community ecology are rarely investigated. Here, we studied direct impacts of invasive dreissenid mussels (IDMs), one of many invasive filter feeders, on freshwater lake bacterioplankton. We demonstrated that direct effects of IDMs reduced bacterial abundance and altered assemblage composition by preferentially removing larger and particle-associated bacteria. While this increased the relative abundances of many free-living bacterial taxa, some were susceptible to filter feeding, in line with efficient removal of phytoplankton cells of <2 µm. This selective removal of particle-associated and larger bacteria by IDMs altered inferred bacterial functional group representation, defined by carbon and energy source utilization. Specifically, we inferred an increased relative abundance of chemoorganoheterotrophs predicted to be capable of rhodopsin-dependent energy generation. In contrast to the few previous studies that have focused on the longer-term combined direct and indirect effects of IDMs on bacterioplankton, our study showed that IDMs act directly as a biological disturbance to which freshwater bacterial assemblages are sensitive. The negative impacts on particle-associated bacteria, which have been shown to be more active than free-living bacteria, and the inferred shifts in functional group representation raise the possibility that IDMs may directly alter bacterially mediated ecosystem functions. IMPORTANCE Freshwater bacteria play fundamental roles in global elemental cycling and are an intrinsic part of local food webs. Human activities are altering freshwater environments, and much has been learned regarding the sensitivity of bacterial assemblages to a variety of these disturbances. Yet, relatively few studies have focused on how species invasion, which is one of the most important aspects of anthropogenic global change, affects freshwater bacterial assemblages. This study focuses on the impact of invasive dreissenid mussels (IDMs), a globally distributed group of invasive species with large impacts on freshwater phyto- and zooplankton assemblages. We show that IDMs have direct effects on lake bacterioplankton abundance, taxonomic composition, and inferred bacterial functional group representation.

Agrochemicals increase trematode infections in a declining amphibian species.

Global amphibian declines have often been attributed to disease, but ignorance of the relative importance and mode of action of potential drivers of infection has made it difficult to develop effective remediation. In a field study, here we show that the widely used herbicide, atrazine, was the best predictor (out of more than 240 plausible candidates) of the abundance of larval trematodes (parasitic flatworms) in the declining northern leopard frog Rana pipiens. The effects of atrazine were consistent across trematode taxa. The combination of atrazine and phosphate--principal agrochemicals in global corn and sorghum production--accounted for 74% of the variation in the abundance of these often debilitating larval trematodes (atrazine alone accounted for 51%). Analysis of field data supported a causal mechanism whereby both agrochemicals increase exposure and susceptibility to larval trematodes by augmenting snail intermediate hosts and suppressing amphibian immunity. A mesocosm experiment demonstrated that, relative to control tanks, atrazine tanks had immunosuppressed tadpoles, had significantly more attached algae and snails, and had tadpoles with elevated trematode loads, further supporting a causal relationship between atrazine and elevated trematode infections in amphibians. These results raise concerns about the role of atrazine and phosphate in amphibian declines, and illustrate the value of quantifying the relative importance of several possible drivers of disease risk while determining the mechanisms by which they facilitate disease emergence.

Evaluation of primary production in Lake Erie by multiple proxies.

Direct measurements of rates of primary production in Lake Erie are few and uncertainties surround rate measurements based on radiocarbon and the light-dark bottle incubation methods. For these reasons, we conducted a series of simultaneous primary productivity measurements in Lake Erie in July and August of 2003, based on incubation with [14C]-NaHCO3, the light-dark bottle method, and incubation with (18)O enriched water. Significant differences in the rates of primary production obtained by incubations with [(18)O]-H2O (0.19-34.60 mmol-O2 m(-3) h(-1)), [14C]-NaHCO3 (0.03-90.50 mmol-C m(-3) h(-1)), and light-dark bottles (0.06-60.78 mmol-O2 m(-3) h(-1)) were evident in six out of nine comparisons. Within the epilimnion, [(18)O]-H2O rates of primary production were significantly different from rates based on [14C]-NaHCO3 and light-dark bottles in all four comparisons and lower rates were obtained in three out of four comparisons. Eutrophic conditions in Sandusky Bay, Lake Erie were evident from the high primary production rates of 20.50-34.60 mmol-O2 m(-3) h(-1) ([(18)O]-H2O), 34.39-90.50 mmol-C m(-3) h(-1) ([14C]-NaHCO3), and 46.66-60.78 mmol-O2 m(-3) h(-1) (light-dark bottle). The photosynthetic quotient (PQ), or ratio of O2 production to CO2 consumption during photosynthesis, averaged 0.64+/-0.33 and 1.93+/-1.93, respectively, based on a comparison of [(18)O]-H2O to [14C]-NaHCO3 rates or light-dark bottle to [14C]-NaHCO3 production rates, respectively, demonstrating that photosynthesis in Lake Erie communities primarily follows expected stochiometric trends. The average of the ratio of production rates based on incubation with [(18)O]-H2O relative to those obtained by the light-dark incubation method was 0.66+/-0.33, indicating a tendency for the [(18)O]-H2O method to provide slightly lower estimates of production in Lake Erie. Lower estimates of primary production based on [(18)O]-H2O incubation relative to the other two approaches is most likely a consequence of consumption of labeled O2 within the cell or dilution of label by the release of O2 from supersaturated cells. This latter effect may be particularly characteristic of eutrophic environments.