Trophic-Level Responses to Nutrient Treatment of Meromictic and Glacially Influenced Coghill Lake

Abstract

Prior to a 1990–1994 run decline, Coghill Lake consistently produced the highest single-system sockeye salmon Oncorhynchus nerka catches in Prince William Sound. Although limnological and juvenile fisheries data are not available before the run declined, we hypothesized that sockeye salmon fry produced from 3 consecutive years of high escapements (1980–1982) might have overgrazed the zooplankton forage base and reduced the lake’s sockeye salmon rearing capacity. In 1993 the Exxon Valdez Oil Spill Trustee Council approved and helped fund a nutrient enrichment stock restoration project for Coghill Lake to mitigate the oil-damaged fishery resources. During 4 years of nutrient enrichment (1993–1996), the seasonal mean phosphorus concentration increased 22%, algal biomass of mainly genera edible by zooplankton increased 220%, and the standing stock (density) of the prevalent zooplankton (Cyclops) increased 117% compared to pre-enrichment years (1988– 1992). The sockeye salmon population averaged 263,604 smolts before enrichment compared to 940,411 during treatment. This increased smolt production is attributed to increases in primary and secondary production because the number of smolts produced per spawner also increased during enrichment. The trophic-level responses to nutrient enrichment in meromictic, glacially influenced Coghill Lake were similar to those observed for other clearwater holomictic and meromictic lakes treated in Alaska.