Whole-lake experiments: The annual record of fossil pigments and zooplankton

The annual record of fossil pigments and zooplankton was compared with detailed contemporaneous records from two manipulated lakes from 1940 to 1986. Annually resolved sedimentary records accurately monitored known changes in plankton communities, identified periods of trophic change, and proved a powerful tool for examining long-term, complex interactions. Both Paul and Peter Lakes underwent the same three complete changes in their fish assemblages (trout, cyprinid, bass), and Peter Lake received repeated inputs of lime. Alterations in fish community composition produced long-lived changes in zooplankton communities that cascaded to the microbial level of the food web. Liming, in concert with trophic changes, caused distinctive phytoplankton dynamics in Peter Lake. Paleolimnological data recorded all major plankton dynamics known from coeval limnological data. Specifically, the sediment record showed transitions in cladoceran size structure and species composition, changes in water clarity resulting from both food web and chemical manipulations that affected vertical zonation of primary producers, and changes in absolute abundance of all algal divisions except dinoflagellates. Undegraded Chl a indicated deep blooms and, in conjunction with Chl c, fucoxanthin and P-carotene, indicated metalimnetic chrysophytes. Transient (2-3 yr) nonselective increases in sedimentation corresponded with increases in grazing rates. Isorenieratene indicated overlap of photic and anoxic zones, revealing changes in transparency and conditions for pigment preservation. A historical perspective of the intrinsic variability of lakes is necessary to evaluate the effects of manipulative or natural disturbances (Likens 1983; Schindler 1987). Whole-lake experiments demonstrate that nutrient loading ultimately delimits potential productivity and much of the unexplained variation may be due to dynamic food web effects (Schindler 1987; Carpenter and Kitchell 1988). Cascading trophic interactions are strongest in lakes in which size-selective planktivory by fish influences the abundance of large-bodied zooplankton (Carpenter et al. 1985, 1987). Both chemical and trophic perturbations may induce fluctuations in ecosystem structure that develop Acknowledgments This work was supported by NSF grants BSR 8604996, BSR 86-0627 1, and BSR 85-21832 and the Sigma Xi Grants-in-Aid-of-Research program. We ihank Arthur Hasler and coworkers for conducting the original whole-lake manipulations, Sharon Barta for zooplankton microfossil analyses, Martin Berg and Marianne Douglas for assistance with coring, and John P. Smol for preliminary algal microfossil analysis and helpful comments. over years to decades (Schindler 1987; Carpenter 1988). These temporal dynamics are difficult to quantify in studies lasting only a few years. Paleolimnology rapidly yields long-term data that are often otherwise unattainable. The sedimentary record integrates over both time and space, synthesizing ecosystem responses to manipulation. Zooplankton fossils monitor populations, size structure, and the effects of planktivory (Kerfoot 1974, 198 1; Mitchell and Kitchell 1980). Carotenoids can record the dynamics of algal communities (Ziillig 198 1; Sanger 1988). Unfortunately, unambiguous interpretation of sedimentary signals is difficult (Binford et al. 1983; Swain 1985). These difficulties can be resolved through paleolimnological studies of manipulated lakes where results have been documented for many years. However, direct comparisons of coeval limnological and paleolimnological records are rare (Haworth 1980). This paper compares the annual record of fossil pigments and pelagic zooplankton remains with contemporaneous limnologi-