The Zooplankton Populations of the Horseshoe Chain of Lakes, Richmond, MN

The Horseshoe Chain of Lakes, located near Richmond, Minnesota, is an impoundment created by the construction of a dam on the Sauk River. Weekly zooplankton samples were taken in the two largest lakes, Horseshoe and Cedar Island, of the Chain. The sample period was from 21 June to 26 October 1984. Zooplankton were identified to species where possible, and enumerated for each sample., Twenty-eight protozoan species, 39 rotifer species, and 3 types of copepods were identified. Densities averaged 1521 and 2277 cells/l for the protozoans _of the two lakes. Biomass averaged 22.5 and 18. 8 μg/l. Rotifers were the most numerous group with average densities of 3482 and 4051/l. Mean biomass was 70.7 and 79.8 μg/l. The copepods had mean densities of 91 and 233/1 for Horseshoe and Cedar Island Lake respectively. Biomass averaged 278.5 and 939. 7 μg/l. The cladocerans had average densities of 75 and 101/1. Biomass averages of 512.8 μg/l and 465.2 μg/l were found.Mean diversity was calculated as 3.3 and 3.0 for the zooplankton community of the two lakes. Mean equitabilities of 0.64 and 0.56 were found.The Horseshoe Chain of Lakes is classified as a eutrophic lake. The zooplankton species, abundance, and biomass are of the type found in eutrophic situations. Diversity was higher than expected for a eutrophic lake. The high diversity may be accounted for by the effects of the Sauk River as it passes through the Chain of Lakes. The river provides the needed nutrients and food allowing for a great diversity of organisms.