Could dreissenid veligers be the lost biomass of invaded lakes?

Abstract

The ecological effects of invading adult zebra and quagga mussels (Dreissena polymorpha and Dreissena bugensis, respectively) are well-documented worldwide, but the ecology and fate of their veliger larvae are poorly understood. We addressed potential errors encountered when estimating veliger biomass. Faced with uncertainty surrounding an established length–mass relationship for veligers, we developed a new dry mass relationship that works for both fresh and formalin-preserved animals. We used this new equation to estimate that veliger dry biomass averaged 74 ± 0.6% of the values obtained from the established equation. Therefore, researchers who assumed the established equation yielded wet mass and applied correction factors of 10 to 50% would have underestimated biomass. Use of coarse-mesh nets also leads to underestimates of biomass. We found that 153-µm-mesh nets captured only 3 to 27% of individuals, or 10 to 66% of veliger biomass captured in 64-μm-mesh nets. We also examined temporal trends, seasonality, and relative contribution of veliger larvae to total zooplankton biomass at several Laurentian Great Lake long-term monitoring sites in Ontario, Canada. Based on the new equation, veliger biomass generally increased from 2008 to 2014, with May to October means of 1 to 27 mg/m3 in Lake Ontario, Georgian Bay, and Lake Erie, or up to 39% of total zooplankton biomass. Veligers were present for most of the sampling season, but abundance peaked in August and September. In contrast, crustacean biomass has declined markedly in Lake Ontario since the 1990s. Given their dominance across many sites, accurate depiction of veliger dry mass may partially offset the observed losses in crustacean biomass to the pelagic food web following P control and the dreissenid invasion of the Great Lakes.