Evaluation of foodweb shifts with the addition of two species of invasive planktivorous fish

Background/Question/Methods

Invasive species can cause massive changes in ecosystem structure and the impacts may lead to functional changes in nutrient processes, ecological integrity and human usage.  There remain important unanswered questions about the impacts of rapid successional invasions of a temperate lake system by planktivorous fish.  Top down cascading trophic dynamics may lead to compositional changes in phytoplankton and conditions that favor dominance by potentially toxic cyanobacteria. In this study, we compared seasonal shifts in diet composition of white perch (Morone americana) before and after the addition of a second invasive planktivorous fish species, alewife (Alosa psuedoharengus) in to Missisquoi Bay, Lake Champlain, in Vermont, USA.  The bay is shallow, eutrophic and lacks a large community of piscivorous predators. Specifically, we addressed whether there has been shift in white perch feeding preferences in the presence of alewife by comparing seasonal patterns from 2005 (no alewife) to 2010 (alewife present).  We also compared overlap between the two species.  Fish and ambient zooplankton samples were collected on a weekly basis in both years.  

Results/Conclusions

Our results show considerable plasticity in the diet of white perch, but a strong preference for large zooplankton taxa (Daphnia spp.) in early summer months, prior to the onset of cyanobacteria blooms.  This trend is present in both years but the presence of alewife is coincident with a slight decrease in the proportion of the white perch diet that consists of zooplankton.  Alewife consumed almost exclusively zooplankton, but the composition is dominated by smaller taxa such as copepods, so overlap with the white perch is limited.  The ambient zooplankton community has shifted between 2005 and 2010 to include a higher proportion of small-bodied zooplankton such as Bosmina spp..  High nutrient levels and the additive effect of selective predation on zooplankton by both species of fish may be leading to a trophic cascade by releasing grazing on algae in early summer months.  A related mesocosm study in process will explore the mechanism behind changes in the phytoplankton composition.  This study will add to the understanding of how invasive speces can change ecosystem structure and function.