The effects of beaver dams on lake plankton in the Maple River, Michigan.

Abstract

In a classic study of three 1st order streams in Michigan, Chandler (1937) found that plankton underwent a quantitative decrease as it moved downstream from the lake source. This study was important because it established that plankton contributions from lakes were minimal due to the retentive capability of aquatic macrophytes in the stream that acted as sinks for lake plankton. He found that when vegetation was removed greater amounts of plankton were exported downstream, and plankton decrease was no longer observed. Although he was careful to choose unpolluted streams, the three streams he studied in 1937 lacked beaver, a previously abundant stream inhabitant. Therefore, our understanding of plankton transport from a lake source is incomplete because it does not reflect the influence of this ecologically important herbivore, one that probably influences the nature of the materials transported downstream. Before European settlement in North America, beaver populations were found in great numbers in aquatic habitats from the Arctic to northern Mexico. But due to trapping for the fur trade in the 1600's -1800's beaver were almost extinct by the 1900's (Johnson and Chance 1974). Today, the beaver population in the northern United States and Canada is increasing rapidly because of the absence of predators and strict laws that regulate trapping. Therefore, the streams that Chandler studied (including the Maple River) are quite different today. Chandler described the East Branch of the Maple River at the outlet of Douglas Lake as possessing abundant vegetation and a few sites with small wood and leaf debris. Currently, this section of the Maple River has three functional beaver dams and the stream is full of woody debris from beaver activity. So it would be of interest to conduct a survey of planktonic movement downstream and to determine whether plankton follows a similar decreasing pattern from the lake source, as Chandler (1937) observed, or instead, if beaver dams act as a source of planktonic biomass.