Blooms of the pelagic tunicate, Dolioletta gegenbauri: Are they associated with Gulf Stream frontal eddies?

Abstract

Satellite-directed sampling was used to determine whether blooms of D. gegenbauri are associated with warm filaments of Gulf Stream frontal eddies. Radio-transmitting drogues were used to mark the center of the bloom so that physical and biological covariables could be measured inside and outside of bloom waters. The bloom was not in the warm filament of a frontal eddy, but was 60-70 km northwest of the temperature front between outer-shelf water and the Gulf Stream.sbd.in upwelled water probably originating from the eddy''s cold core. This cold-core remnant (CCR) water was stranded between 2 middle-shelf fronts. The doliolid bloom resulted from the asexual production of gonozooids by the oozooid stage. This occurred primarily in the nearshore temperature and salinity front and in or beneath the pycnocline between CCR and overriding outer-shelf surface water. Several of the doliolid populations were estimated to be capable of clearing 40-120% of their resident water volume each day.sbd.removing particles of less than 50 .mu.m equivalent spherical diameter. Their removal of small particles is thought to be one of the primary reasons for poor copepod recruitment and low net zooplankton concentrations in the midst of doliolid blooms. The phytoplankton community was co-dominated by dinoflagellates and diatoms, indicating the strong influence of the Gulf Stream in these mid-shelf waters. Dominant diatoms were Thalassiosira subtilis and Rhizosolenia sp., both typical of Gulf Stream upwelling in the Georgia Bight. The net zooplankton occurred in low concentrations. There were no species typical of coastal or Gulf Stream surface water. Those samples not dominated by D. gegenbauri were co-dominated by Oithona sp., Oncaea sp., Euterpina sp., and ostracods. Based on the frequency and duration of Gulf Stream frontal eddies, dolioid blooms may persist for 7-9 days. Blooms are most common in the winter and spring, due in part to the density regulated mixing characteristics of coastal and upwelled Gulf Stream water. Blooms of D. gegenbauri may form when upwelled, cold-core remnant water is advected onto the middle shelf and is stranded between 2 fronts. Doliolids are adapted to respond quickly to the event time scale of physical and phytoplankton dynamics.