Phytohydrography, Gulf Stream rings, and herbivore habitat contrasts

The Gulf Stream constitutes a major ecological discontinuity separating two distinct phytohydrographic provinces: the Slope Water and the Northern Sargasso Sea. Differences between phytoplankton assemblages are maximal above well-established seasonal thermoclines. Due to systematic differences in the composition, quantity, distribution, and variability of their phytoplankton assemblages, the upper euphotic zones of the Slope Water and the Northern Sargasso Sea represent very different herbivore habitats. Although the Northern Sargasso Sea has been thought to be a relatively monotonous biogeographic province, within its geographic boundaries Gulf Stream cold core rings constitute largescale floral non-homogeneities. The rings observed differed markedly in species composition from both the Slope Water and the Northern Sargasso Sea. Although species groups isolated by correspondence analysis were not strictly confined to one or another phytohydrographic province they represented ecotypes differing in their apparent 'responsiveness' to variation in nutrient concentration.