Human disturbance and the paleolimnological record of change in the zooplankton community of Lake Harriet, Minnesota [Eutrophication]

Distributions of pollen and diatom microfossils and sediment chemistry from a 75-cm Pb-dated sediment core were used to reconstruct the impact of human disturbance on the zooplankton of Lake Harriet, Minnesota over the last 700 yr. During presettlement times Bosmina remains were most abundant but small numbers of Daphnia pulex postabdominal claws were present. Coincident with the colonization of the lake watershed by European farmers, as indicated by increases in Ambrosia pollen, Bosmina carapaces and antennules increased in size. This suggests increases in levels of invertebrate predation. Daphnia galeata appears at the same time storm sewer construction caused a major increase in lake trophic status as indicated by diatom fossils. The appearance of D. galeata indicates the establishment of a two-tiered Daphnia community with D. galeata in the epiliminoin and D. pulex in the hypolimnion. When northern pike (Esox lucius) were introduced into the lake in the mid-1950's, their predation caused a change in the dominant zooplanktivore, which in turn led to a decline in both Daphnia species. The changes recorded in the Lake Harriet core suggest that fossil zooplankton are more sensitive indicators of predation than of lake trophic status.