Food web effects and the disappearance of the spring clear water phase in Onondaga Lake following nutrient loading reductions

Abstract

The annual spring clear water phase (May–June) in Onondaga Lake, New York, unexpectedly disappeared in 2003 following several years of phosphorus and ammonia loading reductions at the Metropolitan Syracuse Wastewater Treatment Facility (Metro). Mean chlorophyll a concentration during May–June was higher from 2003 to 2007 than from 1990 to 2002, with mean Secchi disk depths <2 m. Large zooplankton (Daphnia sp.) were abundant during April–June before 2003 but were rare from 2003 to 2007, while abundance of small zooplankton (Bosmina longirostris) increased. Alewife (Alosa pseudoharengus) catches from electrofishing surveys dramatically increased in 2003, and hydroacoustic surveys estimated the alewife population to be between 1600 and 2300 fish/ha in spring 2005–2007. The alewife population in 2005 was dominated by a strong 2002 year class. Increasing biomass of the 2002 year class coincided with the timing of the shift from large to small zooplankton in late summer of 2002. This indicates that the strong 2002 alewife year class initiated a classic trophic cascade in Onondaga Lake, causing the decline and continuing low abundance of Daphnia sp. and the disappearance of the spring clear water phase. The increase in alewife may have been associated with decreasing ammonia concentrations following improvement to Metro. Unionized ammonia has been below levels considered toxic to nonsalmonid fish species since 1999, and the ammonia concentration continues to decrease in the lake. Thus, reductions in nutrient loading can lead to unanticipated food web effects causing decreases rather the expected increases in water clarity in the spring–early summer period.