Physical and biological community characteristics of planktonic veliger patches in the southern Mid-Atlantic Bight

Early life history stages are crucial to understanding dispersal and population connectivity of mollusks, however current knowledge on planktonic veliger larvae and their associations with water mass hydrography and the zooplankton community is limited. Planktonic veligers can numerically dominate the macrozooplankton community > ~ 105 mm in length, and because of their dense shells they can influence the acoustical and optical properties of the water. Practical limitations such as the manual processing of net samples severely limits data collection regarding oceanic veligers. Imaging systems have been increasingly utilized in zooplankton studies, as they can continuously sample larger geographical areas with increased precision. In this study, the In situ Ichthyoplankton imaging system (ISIIS) was deployed during April 2018 to resolve areas of high veliger density near the shelf-break in the southern Mid-Atlantic Bight, which is influenced by fronts and eddies associated with the warm Gulf Stream and colder inshore waters. The ISIIS was towed along cruise tracks transiting the shelf break and captured high-resolution images of the zooplankton community as well as hydrographic and optical data (temperature, salinity, chlorophyll-a fluorescence, absorption, and backscattering) throughout the upper 50 m of the water column. Veligers were imaged in high-densities, composing nearly 75% of organisms ≥ ~ 105 mm. Patches occurred at depths of approximately 10.6m (SD = 0.92) and typically correlated with higher densities of hydromedusae (4.05%, known predators of veligers). Results from this study advance our understanding of the biological and physical environmental influences on veliger abundances and distribution in shelf systems.