Trophic Interactions: Changes in Phytoplankton Community Structure Coinciding with Alewife Introduction (Alosa pseudoharengus)

Abstract

Conesus Lake is a eutrophic lake (MILLS 1975) and the most western of the Finger Lakes of New York State, USA. Abundance of a top level predator, the walleye (Stizostedion vitreum vitreum), decreased from a high of 12,000 individuals in 1966 to 9,614 individuals in 1975 to 1,850 individuals by 1985 in Conesus Lake (ABRAHAM 1989). Coincidental with the decline of the walleye in Conesus Lake was the proliferation of an obligate planktivore, Alosa pseudoharengus (ABRAHAM 1988). During the late 1970s (probably 1978 or 79), the alewife was accidentally introduced and became established in the lake. The pre-alewife zooplankton community was dominated by Daphnia pulex, Conochilus unicornis and Cyclops bicuspidatus. The overwhelming dominance of D. pulex in 1972 was impressive. It was the dominant cladoceran on each of the 50 sampling days throughout the year (CHAMBERLAIN 1975). Abundance reached as high as 36 individuals/L in the summer and 13 individuals/L in December. The pre-alewife phytoplankton community was dominated by larger (>70 ?m, greatest axial linear dimension) colonial, filamentous and unicelluar algae or net phytoplankton (MILLS 1975). With excellent historical phytoplankton and zooplankton data available, an opportunity existed to examine the impact of alewife introduction on plankton community structure in a large-lake ecosystem (13.7 km2) over a 16-year period.