Functional complementarity in plankton communities along a gradient of acid stress

This study considered changes in the plankton communities of 50 Adirondack Mountain lakes, along an acidity gradient from pH 7.3 to 4.2. The objective was to test the hypothesis that changes in the biomass of certain species are compensated by opposite changes in the biomass of other species having similar ecosystem functions. This hypothesis was proposed by Frost et al. (1995); Linking Species and Ecosystems. Chapman and Hall, NY, pp. 224–239.) and referred to as ‘complementarityʹ. Results from the multi-lake survey, as well as data from two experimental studies, provide support for the hypothesis. With increasing acidity, there were declines in the number of plankton species, and pronounced changes in taxonomic composition. There were also reductions in food web complexity, measured as number of trophic links per species. The most acidic lakes displayed extreme dominance of a few taxa per trophic level, and characteristic acid-tolerant species such as Diaptomus leptopus (a calanoid copepod), Keratella taurocephala (a rotifer), and Chlamydomonas sp. (a chlorophyte alga). In contrast, the total biomass of producers and herbivores was unrelated to lakewater acidity. Acid lakes contained levels of phytoplankton biomass (chlorophyll a) consistent with measured levels of nutrients, and zooplankton biomass was consistent with its resource base (the phytoplankton). Experimental results indicated that complementary changes can occur rapidly at lower trophic levels when replacement species are already present in the community.