Assessment of the potential impact of the mysid shrimp, Neomysis mercedis, on Daphnia

Abstract

Neomysis mercedis, an important invertebrate predator in some lakes and estuaries in North America, occurred at high densities (>2 mysids m -3 ) in Lake Washington in the early 1960s, a period when Daphnia were scarce in the lake. Because Neomysis feed selectively on Daphnia, it was hypothesized that mysid predation contributed significantly to the scarcity of Daphnia. To evaluate this hypothesis, mysid abundance was monitored in the lake (July 1989-February 1992), and whole lake predation impacts on Daphnia were estimated. Mysid abundance varied from 319 million, ∼0.1 mysids m -3 (February 1992) to 4276 million, ∼1.7 mysids m -3 (June 1991), and mysid biomass ranged from 1153 (October 1991) to 4700 kg dry weight (November 1990). A peak in Daphnia consumption was noted during autumn, when total mysid biomass was high. In late autumn/winter mysid consumption month -1 accounted for 28-95% of Daphnia biomass and 13-38% of production. Mysid densities in this study were 18x and 5x lower than in 1962 and 1975, respectively. By extrapolation, at a density of 3.5 mysids m -3 observed in the early 1960s, total Neomysis consumption demand month -1 exceeded 100% of Daphnia biomass (late autumn-early spring), and 100% of Daphnia production (late autumn-winter). Estimates of the total mysid consumption demand in this study are thus in accord with the hypothesis that Neomysis could control Daphnia abundance in Lake Washington.