Deployment of an imaging system to investigate fine-scale spatial distribution of early life stages of the ctenophore Mnemiopsis leidyi in Chesapeake Bay

A high-resolution digital imaging system, the zooplankton visualization and imaging system (ZOOVIS) was deployed in the mid-region of Chesapeake Bay to investigate the fine-scale distribution of small individuals of the lobate ctenophore Mnemiopsis leidyi. Counts of individuals, mostly ,5 mm, were enumerated from each frame of ZOOVIS and merged with data from a conductivity-temperaturedepth to provide corresponding values of depth, temperature and salinity. Mnemiopsis leidyi rarely occurred below the pycnocline depth but commonly occurred in the upper water column. Where the water column was strongly stratified, M. leidyi tended to be more common near the pycnocline, but it was more abundant near the bottom at locations where the water column was well mixed. Horizontally, M. leidyi was more abundant on the shoulders of the middle Bay than in the deeper main channel. In this survey, M. leidyi occurred more frequently in relatively warm and low salinity water. Furthermore, small M. leidyi were patchily distributed and the patch size was ,700 m in horizontal extent. A zero-inflated, general additive model successfully simulated the spatial distribution of M. leidyi, with temperature and salinity as predictors. The ZOOVIS system proved to be a valuable tool to study fine-scale distributions of small and delicate gelatinous zooplankton, even in the relatively turbid conditions that prevailed in this estuarine system.