Spiny water flea (Bythotrephes longimanus) impacts on zooplankton communities of Voyageurs National Park

Background/Question/Methods

Bythotrephes longimanus, an invasive cladoceran from Northern Europe, is rapidly spreading through the northern Great Lakes region along a temperature-dependent latitudinal band, and has become established in the large interconnected lakes of Voyageurs National Park (VOYA) in Northern Minnesota. As a voracious predator, Bythotrephes consume up to 40 smaller invertebrates per day per individual, and has been shown to be capable of changing zooplankton abundances and community compositions in invaded lakes. Here, we conducted long-term zooplankton surveys of 5 large VOYA lakes (Rainy, Namakan, Kabetogama, Sandpoint and Crane) in order to investigate if zooplankton communities have changed after Bythotrephes invasion. Additionally, we conducted a spatial contrast test between those lakes with high Bythotrephes densities (Rainy, Namakan and Kabetogama) and those with significantly lower densities (Sandpoint and Crane) using another data set.

Results/Conclusions

We found that zooplankton community composition was drastically altered in post-invasion samples compared to pre-invasion temporal contrast ones, as well as in the high density lake samples compared to low density spatial contrast ones. Dominance shifted towards copepods and larger bodied or gelatinous cladoceran species, while overall zooplankton biomass was reduced by 40-60%. Most cladoceran species showed severely decreased populations and changing seasonal abundances, suggesting a gradual and accumulative effect of Bythotrephes. Importantly, Bythotrephes also seems to have important non-consumptive effects on native competitor species like Leptodora kindtii and predaceous copedpods. Foodweb implications for the affected ecosystems and VOYA fisheries are evident.