Spiny water flea invasion alters fish mercury bioaccumulation rates

Bythotrephes cederströemi are a predatory cladoceran zooplankter that have invaded numerous inland lakes in North America, many of which are stratified and support offshore fishes like Cisco ( Coregonus artedi ). While changes in zooplankton community composition following Bythotrephes invasion predict an increase in Cisco mercury concentrations (Hg), this phenomenon was not detected from a survey evaluating temporal changes in Cisco Hg across a broad range of lakes varying in the presence or absence of Bythotrephes . Here, we compare temporal changes in Cisco bioaccumulation slopes (i.e., slopes of relationships between Cisco Hg and trophic position) from lakes experiencing Bythotrephes invasion over the study period to those already invaded (as a reference) over similar time periods. Our results show that bioaccumulation slopes after Bythotrephes invasion either changed direction entirely (from positive to negative relationships) or decreased in elevation relative to those prior to invasion. No such pattern was observed in previously invaded reference lakes. Reductions in Cisco bioaccumulation slopes and/or intercepts following Bythotrephes invasion suggest that conversion efficiency (and therefore growth) of Cisco increased after invasion (i.e., less Hg accumulates in fish at an equivalent trophic position after vs. before invasion). Back-calculated Cisco growth rates and size-at-age from a second complimentary study were greater in the presence of Bythotrephes than without, further supporting the hypothesis that changes in Hg bioaccumulation are likely due to increased conversion efficiency among invaded populations. These findings highlight the potential importance of foraging energetics over and above shifts in trophic position in modifying fish contaminant concentrations.