The potential link between lake productivity and the invasive zooplankter Cercopagis pengoi in Owasco Lake (New York, USA)

The fishhook water flea (Cercopagis pengoi Ostroumov, 1891) is an invasive zooplankter that can decrease the abundance and diversity of cladocerans and rotifers, which theoretically could release phytoplankton from grazing pressure and increase algal primary productivity. In the last decade, C. pengoi established and primary productivity increased concurrently in Owasco Lake (New York, USA). We studied plankton density, primary productivity, and standard limnological conditions in Owasco Lake during summer 2007 (1) to document summer densities of invertebrate predators, (2) to investigate correlations between C. pengoi and the abiotic environment, and (3) to examine the relationships among C. pengoi, native zooplankton, and productivity. Although the maximum abundance of C. pengoi observed (245 ind./m 3 ) far exceeded that of any native invertebrate predator, at most locations and dates unimodal density peaks between 35-60 ind./m 3 were typical and comparable to Leptodora kindtii (Focke, 1844), the most common native planktivore. Abiotic conditions were suitable for growth and reproduction throughout the sampling period, but were not linearly correlated with C. pengoi abundance. We observed reciprocal trends between predacious cladoceran density (both C. pengoi and L. kindtii) and herbivorous cladoceran density, and between herbivorous cladoceran density and Chlorophyll a concentration. Although these trends are only corollary, they support the possibility that C. pengoi may affect the trophy of Owasco Lake by reducing grazing zooplankton beyond the level of the native planktivores. Further study is needed to quantify the relative contributions of C. pengoi and L. kindtii to seasonal changes in the herbivorous cladoceran assemblage.