Paleolimnological analysis of whole-lake experiments: an overview of results from Experimental Lakes Area Lake 227

Abstract

High resolution analysis of laminated sediments from experimentally fertilized Lake 227 was used to compare sediment geochemistry and fossil abundance (siliceous algae, pigments, cladoceran remains) with 20 yr (1969–1989) of continuous historical records. Members of all fossil groups were correlated to the biomass of their respective producer populations (r = 0.52–0.66, P < 0.05, n = 20). Correlations were greatest when fossil abundance was expressed per unit organic matter and least when calculated as the accumulation rate. Comparison of groups showed that fossils of soft-bodied zooplankton (copepods, rotifers) and pigments from dinoflagellates were completely unreliable. The most informative fossils were chitinous remains of zooplankton (e.g., Bosmina), remains of siliceous algae, and pigments from chlorophytes and cyanobacteria. Ecosystem-level paleolimnology showed that fertilization was the most significant eutrophication event in the last 400 yr and that it impacted both pelagic and littoral communities. However, fossil zooplankton and pigment analyses indicated that food-web interactions (zooplanktivory, herbivory) also regulated plankton abundance and composition. Additionally, fossil analyses showed that some natural eutrophication occurred prior to 1969 and that plankton communities have continued to vary since 1975, despite constant rates of fertilization with nitrogen and phosphorus.