Seasonal and spatial patterns of Penilia avirostris and three tunicate species in the southern Mid-Atlantic Bight

The cladoceran Penilia avirostris and three tunicate species, Oikopleura dioica, Dolioletta gegenbauri and Thalia democratica, form a mesozooplankton group which ingests a wide range of particles from pico- to micro- plankton, grows rapidly due to asexual reproduction, and thus can have major impacts on phytoplankton populations. These four zooplankton species were the most abundant tunicate and cladoceran species in a study where zooplankton were sampled biweekly at five stations across the inner continental shelf in the Mid-Atlantic Bight in 2005, 2006 and 2007. Vertical tows were taken at shallow stations and depth stratified vertical tows at stations >10 m. P. avirostris and O. dioica had highly predictable seasonal cycles with peak abundances in July and August. D. gegenbauri also was present during this time period if upwelling favorable winds were present, which implies cross shelf transport from source populations in slope waters and the Gulf Stream. T. democratica only appeared in pulses when southerly winds were increasing in strength. The co-occurrence P. avirostris and the tunicate species with abundant Synechococcus and heterotrophic nanoflagellates during highly stratified summer conditions provide potential connections to microbial food webs as well as grazing opportunities on event scale blooms of dinoflagellate and diatoms species present in the area.