Net phytoplankton in a Gulf Stream warm-core ring: species composition, relative abundance, and the chlorophyll maximum layer

Abstract

Gulf Stream warm-core ring (WCR) 82B and surrounding waers (Sargasso Sea, Slope Water) were sampled during three cruises from April to August 1982, to assess changes in the species composition and relative abundance of the large, rare netphytoplankton. Analysis of net samples indicated that the major changes in dinoflagellate species composition in the ring occurred between the April and June cruises, while the major changes in diatom species composition occurred between the June and August cruises. Centric diatom species composition and relative abundance were stable over cruise-length time periods (3 weeks) in April and June but varied greatly in the 1.5 month interval between cruises. Between April and June, the dominant centric diatom species in the ring changed dramatically, although the overall species assemblages (presence or absence of diatom and dinoflagellate taxa) remained similar. The opposite occurred between the June and August cruises: the overall species assemblages showed large changes while the abundant centric diatoms did not. Based on the species composition and relative abundance of centric diatoms,the chlorophyll maximum layers exhibited a greater similarity to surface layers at the same station than to chlorophyll maximum layers at other locations. This is consistent with the hypothesis that the chlorophyll maximum forms as a result of cells sinking from the surface, rather than each layer being composed of distinct, separate species assemblages. The chlorophyll maximum in WCR 82B represented a phytoplankton numerical abundance maximum in 70% of the profiles examined.