Recently, ecologists are discovering that anthropogenic sources of stress (e.g., pollutants) may not have direct lethal effects on species, but might alter the strength and direction of trophic interactions when stress is examined in the context of the entire ecological community. Road deicers have recently been identified as a rising chemical pollutant to freshwaters in the United States resulting in salinization, or salt loading. Continued salinization of both built and natural environments is likely to have a detrimental impact on survival, growth and development of resident larval amphibians via impairments to their physical environment. Since larval amphibians are often cited as playing an important role in pond food webs, any interaction impacted by salt stress (e.g., increasing vulnerability to predation) may have indirect effects on amphibians. We examined the impacts of salt stress on lentic food web dynamics in two microcosm experiments. In May – July 2007, we assessed gray treefrog (Hyla versicolor) tadpole survival, growth and periphyton feeding rates under a sodium chloride (NaCl) gradient (60-945 mg/L Cl-). Gray treefrog egg masses were obtained from the Patuxent Wildlife Research Center, tadpoles added to the microcosms at the free-feeding stage, and periphyton sampled every 10 days. From December 2007 – January 2008 we also examined the effects of NaCl on phytoplankton and zooplankton using the same NaCl gradient described above. Microcosms ponds were sampled over 4 weeks for phytoplankton biomass and zooplankton density.
Results/Conclusions
We found that NaCl stress had no direct impact on tadpole survival or growth and no direct effect on periphyton growth. However, NaCl did have negative indirect effects on periphyton growth in the presence of gray treefrog tadpoles, suggesting that when under salt stress, tadpoles exert amplified grazing pressure on periphyton resources. Salt loading had positive effects on phytoplankton whereby chlorophyll-a concentration was greatest in microcosms containing moderate and high NaCl concentrations. The high NaCl treatment had a negative impact on the zooplankton community by reducing survival. Plausible explanations include indirect effects on zooplankton due to a shift in the phytoplankton community (i.e., shifts in their primary food source) or direct lethality to zooplankton. Overall, our results suggest that low levels of environmental toxicants that are elevated in the built environment can lead to complex patterns in pond food webs. Differential susceptibility among trophic levels to salt stress in our system might very well lead to altered trophic interactions, even the relative strength of bottom-up versus top-down control.
