BOSMINA-LONGIROSTRIS ANTENNULE MORPHOLOGY AS AN INDICATOR OF INTENSITY OF PLANKTIVORY BY FISHES

Analysis of cladoceran fossil remains from sediment cores from two sui tes of lakes-Tuesday, Paul, and Peter lakes in the Upper Peninsula of Michigan and the Herring River chain of lakes on Cape Cod, Massachuset ts-reveals a high incidence of the cornuta form of Bosmina longirostri s. This form is characterized by having relatively short, extremely do wn-curved or down- and out-curved antennules, as opposed to the normal Bosmina morphology of relatively long and slightly curved antennules. Cornuta form constitutes on average 36% of Bosmina remains accumulate d in sediments for the period 1944-1987 in minnow (Phoxinus eos, P. ne ogaeus) dominated Tuesday Lake. Two periods of low percent cornuta for m coincide with known fish manipulations that resulted in reduced plan ktivory by minnows and increased invertebrate predation. An earlier co incidence of low percent cornuta form with decreased planktivory is in ferred. In bass dominated Paul and Peter lakes, cornuta form Bosmina r emains occur in sediments from periods when minnows invaded these lake s. Bosmina remains, of which 20-50% are cornuta form, dominate surface sediments from the Herring River chain of lakes, which are connected to the Atlantic Ocean and frequented by alewife (Alosa pseudoharengus) , an obligate planktivore. Analysis of a sediment core from Gull Pond, the furthest from the sea, shows that cornuta form is common only aft er European settlement when modifications of channels connecting the l akes to the sea may have enhanced alewife migration inland. Cornuta fo rm Bosmina is correlated with high planktivory in seven lakes from two different geographic areas and over time spans of decades to centurie s. Cornuta form Bosmina may, therefore, be a useful indicator of high planktivory in both paleolimnological and neolimnological contexts.