FUNCTIONAL COMPLEMENTARITY IN PLANKTON COMMUNITIES ALONG A GRADIENT OF ACID STRESS

This study considered changes in the plankton communities of 50 Adiron dack 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 c ertain species are compensated by opposite changes in the biomass of o ther species having similar ecosystem functions. This hypothesis was p roposed by Frost et al. (1995; Linking Species and Ecosystems. Chapman and Hall, NY, pp. 224-239.) and referred to as 'complementarity'. Res ults 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 m ost acidic lakes displayed extreme dominance of a few taxa per trophic level, and characteristic acid-tolerant species such as Diaptomus lep topus (a calanoid copepod), Keratella taurocephala (a rotifer), and Ch lamydomonas sp. (a chlorophyte alga). In contrast, the total biomass o f producers and herbivores was unrelated to lakewater acidity. Acid la kes contained levels of phytoplankton biomass (chlorophyll a) consiste nt with measured levels of nutrients, and zooplankton biomass was cons istent with its resource base (the phytoplankton). Experimental result s indicated that complementary changes can occur rapidly at lower trop hic levels when replacement species are already present in the communi ty. (C) 1998 Elsevier Science Ltd. All rights reserved.