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.

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Factors that regulate phytoplankton dynamics in shallow, productive lakes are poorly understood. Water-column phytoplankton chlorophyll (avg 105 pg liter-') determined on 39 occasions over a 22-month period in shallow (mean depth, 1.7 m) Lake Apopka, Florida, correlated best with average daily windspeed compared with other environmental variables. High phytoplankton biomass reflects windinduced resuspension of a meroplanktonic algal maximum (MAM) that exists on the aphotic lake bottom in a layer -5 cm thick; this assemblage is dominated by microplankton-sized diatoms (>60% of total biomass) that can occur in cellular resting stages. Experiments demonstrate that the MAM can grow and photosynthesize at rates similar to surface populations when exposed to moderate irradiances. Direct inoculation of this meroplanktonic assemblage into surface waters during periods of high wind can account for a doubling in surface phytoplankton and diatom biomass as well as an increase in algal community size. A conceptual model outlines the influence that resuspension of the MAM has on ecosystem structure and function-in the lake.It is often assumed that phytoplankton dynamics are regulated by biogeochemical cycles of suspended materials in lakes, despite the fact that the water column in many lakes may interact readily with underlying sediments (e.g. Dokulil et al. 1979) especially in shallow lakes that are susceptible to changes in light fields due to bottom mixing (Carper and Bachmann 1983). In these globally common systems, mixing events can produce large increases in phytoplankton production and shifts in community composition (Padisak et al. 1988). However, it is unclear whether changes in phytoplankton biomass are associated with pulses

The phototropic picoplankton communities of Lakes Huron and Michigan were studied from 1986 through 1988. Abundances in the surface-mixed layer ranged from 10 000 to 220 000 cells∙mL−1 with a seasonal maximum during the period of thermal stratification. During thermal stratification, maximum abundances were generally found within the metalimnion/hypolimnion at depths corresponding to the 0.6–6.0% isolumes. The picoplankton community was dominated by single phycoerythrin-containing (PE) Synechococcus (59%) with lesser amounts of chlorophyll fluorescing cells (21%), PE colonial Synechococcus-like cells (11%), other PE colonial Chroococcales (6%), and other cells (3%). Single PE Synechococcus was abundant throughout the year whereas chlorophyll-fluorescing and colonial cyanobacteria were more abundant during the periods of spring isothermal mixing and summer stratification, respectively. Picoplankton accounted for an average of 10% (range 0.5–50%) of phototrophic biomass. Phototrophic organisms that passed 1-, 3-, and 10-μm screens were responsible for an average of 17% (range 6–43%), 40% (21–65%), and 70% (52–90%) of primary production. Maximum contributions of < 1, < 3, and < 10 μm size fractions occurred during the period of thermal stratification. Primary production by phototrophic picoplankton was found to equal production in the < 1 μm size fraction.

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The importance of the zooplankton-protozoan trophic coupling was determined experimentally by measured changes in protozoan growth rates with increasing zooplankton biomass. In five of six experiments conducted in Lake Michigan, a significant inverse relationship between protozoan growth and zooplankton biomass was observed (avg r2 = 70%), Zooplankton clearance rates on protozoan assemblages (range, 1.0-6.2 ml (pg dry wt)-I d '1 were comparable to those previously measured for phytoplankton which suggested that protozoa are important prey for zooplankton. Clearance rates on individual protozoan taxa [O-15.6 ml (Fg dry wt)-I d-l] were size-dependent. Rates were greatest for taxa ~20 grn in size (mainly nanoflagellates and small ciliates). In contrast to findings for phytoplankton, no evidence emerged for grazer resistance nor growth enhancement by planktonic protozoa in response to grazing. The high flux rates for macrozooplankton on heterotrophic nanoflagellates observed in all experiments (0.2-6.0 pg C liter-' d-l) provided evidence that a large fraction of picoplankton C may be directly transferred to higher trophic levels via a picoplankton-flagellate-zooplankton coupling.

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