Benthic Pelagic Coupling by an Invasive Grazer: Dreissenid Mussel Impacts on Phytoplankton Biomass and the CHL:TP Relationship

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Dreissenid invasion of North American and European freshwaters have transformed benthic and pelagic food webs of these ecosystems. Here, we use meta analysis and a combination of statistical approaches (classical regression, hierarchical modeling) to examine the impacts of these benthic filter feeders on phytoplankton biomass (Chl), total phosphorus (TP), and the Chl:TP relationship across gradients of lake size and trophic status. Overall, dreissenids reduced Chl by 40 to 45%, with the magnitude of impact dependant on ecosystem size in stratified, but not mixed, lakes. Impacts on Chl were prolonged, with no indication of diminishing within 10 years of invasion. In contrast, impacts on TP were much smaller (15% reduction) and only significant in stratified lakes. Across broad trophic gradients (i.e. among lakes) classical regression (ANCOVA) indicated a significant decline in the Chl:TP relationship after invasion, but no change in slope. In contrast, hierarchical modeling, which included individual lake responses, indicated a decline in the slope of the Chl:TP relationship within 24 of the 27 lakes in the study. Our results indicate that 1) Chl:TP regression equations developed prior to invasion will systematically overestimate Chl, 2) at broad scales (among lakes), both top-down and bottomup mechanisms control phytoplankton biomass in invaded lakes, and 3) within lakes top-down grazing mechanisms dominate in controlling annual variations in Chl in invaded lakes.


Presented at the Joint Meeting with ASLO & NABS, Santa Fe, NM.