Changes at several trophic levels in Otsego Lake from the 1930s to present are described relative to changes in the planktivorous fish community. Prior to the late 1980s, cisco (Coregonus artedi) was the dominant planktivore. The alewife (Alosa pseudoharengus) appeared in 1986 and subsequently became dominant. Two patterns emerged when we contrasted the pelagic flora and fauna under those two dominant planktivore populations. When cisco were the primary planktivores, cladoceran size and biomass was significantly greater in the epilimnion (primarily Daphnia pulex) than in the hypolimnion (primarily Bosmina coregoni). Phytoplankton were dominated by net and large nannoplankton (>0.03 mm), with cyanobacteria uncommon until late summer. From 1973-1988, mean summer chlorophyll a was 2.3+/-1.2 (S.D.) ug.L-1 (range 0.68-4.2). Mean summer Secchi transparency was 5.2+/-0.5 m (range 3.8-7.8). A real hypolimnetic oxygen deficits (AHOD; a measure of oxygen consumption during periods of stratification) were 0.076-0.080 mg.cm(-2).day(-1). Since alewife have dominated, Daphnia have become rare in the lake. Mean cladoceran size in the epilimnion and hypolimnion (essentially all Bosmina) is much more uniform and reduced from prior size distributions. Phytoplankton abundance has increased, as has the mean summer chlorophyll a (6.4+/-1.5 S.D ug.L-1, range 3-8.7). The dominant algae are now smaller nannoplankton (<0.03 mm) and cyanobacteria. Mean summer Secchi transparency decreased substantially to 3.2±0.3 m (range 2.5-4.0) and AHOD increased (range 0.090-0.102 mg.cm(-2).day(-1)). Whitefish (Coregonus clupeaformis) and cisco have declined, while lake trout (Salvelinus namycush)have increased. All changes have occurred despite significant efforts to reduce non-point source nutrient loading to the lake. While the establishment of alewives has enhanced the production of Otsego Lake's salmonid fishery to date, it has increased the challenge to those interested in managing the lake in an ecologically sustainable manner.
