W.G. Sprules | USGS Science

USGS2010

The Lake Ontario zooplankton community before (1987-1991) and after (2001-2005) invasion-induced ecosystem change

We assessed changes in Lake Ontario zooplankton biomass, production, and community composition before (1987–1991) and after (2001–2005) invasion-induced ecosystem changes. The ecosystem changes were associated with establishment of invasive dreissenid mussels and invasive predatory cladocerans ( Bythotrephes and Cercopagis ). Whole-lake total epilimnetic plus metalimnetic zooplankton production declined by approximately half from 42.45 (g dry wt∙m −2 ∙ year −1 ) during 1987–1991 to 21.91 (g dry wt∙m −2 ∙ year −1 ) in 2003 and averaged 21.01 (g dry wt∙m −2 ∙ year −1 ) during 2001–2005. Analysis of two independent data sets indicates that the mean biomass and biomass proportion of cyclopoid copepods declined while the same measures increased for the invasive predatory cladocerans. Changes in means and proportions of all other zooplankton groups were not consistent between the data sets. Cyclopoid copepod biomass and production declined by factors ranging from 3.6 to 5.7. Invasive predatory cladoceran biomass averaged from 5.0% to 8.0% of the total zooplankton biomass. The zooplankton community was otherwise resilient to the invasion-induced disruption as zooplankton species richness and diversity were unaffected. Zooplankton production was likely reduced by declines in primary productivity but may have declined further due to increased predation by alewives and invasive predatory cladocerans. Shifts in zooplankton community structure were consistent with increased predation pressure on cyclopoid copepods by alewives and invasive predatory cladocerans. Predicted declines in the proportion of small cladocerans were not evident. This study represents the first direct comparison of changes in Lake Ontario zooplankton production before and after the invasion-induced disruption and will be important to food web-scale investigations of invasion effects.

Journal of Great Lakes Research↗

USGS2009

Shifts in the diet of Lake Ontario alewife in response to ecosystem change

In the 1990s, the Lake Ontario ecosystem was dramatically altered due to continued invasions of exotic species including dreissenid mussels and predatory cladocerans. We describe the diet and biomass of prey in the stomachs of adult (≥ 109 mm TL) and sub-adult (< 109 mm TL) alewife ( Alosa pseudoharengus ) in 2004 and 2005 across seasons and depths and compare our results to data from 1972 to 1988. During 2004 and 2005, adult alewife consumed primarily zooplankton prey at bottom depth zones < 70 m and primarily Mysis at bottom depth zones > 70 m. Mysis dominated the diets of adult alewife in all seasons except during the summer of 2004 when zooplankton dominated. Mysis dominated the diets of sub-adult alewife during early and late spring and zooplankton dominated the diets in summer and fall. Bythotrephes and Cercopagis were observed in the diets of both sub-adult and adult alewife. Diporeia was observed only rarely in adult alewife diets. The biomass of prey in alewife stomachs varied seasonally and increased with bottom depth for adult alewife. Alewife diets in 2004–2005 differed from those in 1972 and 1988 with an increase in the prevalence of Mysis , and a decline in the prevalence of zooplankton. The biomass of prey in adult alewife stomachs declined in 2004 and 2005 compared to 1972 and 1988, at bottom depth zones < 70 m but not at bottom depth zones > 70 m suggesting reduced food availability closer to shore. We hypothesize that consumption levels at the shallower depth zones, as indicated by very low biomass of prey in alewife stomachs, may not be sufficient to sustain alewife growth. The increased prevalence of Mysis and common occurrence of predatory cladocerans in the diet of alewife means that alewife have shifted to a higher trophic position.

Journal of Great Lakes Research↗