Phytoplankton and mesozooplankton distribution and composition during transects of the Azores Subtropical Front

Two transects of the Azores Current centered on 22°W were carried out, in August 1998 and April 1999, to investigate relations between hydrography, the composition and distribution of particulate material as potential food for zooplankton, and the mesozooplankton communities in the surface 200 m. Small flagellates and picoplankton generally dominated the phytoplankton biomass during both cruises. However, large diatoms, in particular Rhizosolenia stolterfothii, were significant contributors to total phytoplankton biomass south of the Azores Front in April. Mesozooplankton size fractionated biomass was higher in April than August, reflecting higher abundance of a number of genera, e.g., Calocalanus, Calanus, Clausocalanus, Ctenocalanus, Oithona, Corycaeus, and Oncaea. The proportion of mesozooplankton biomass in the size fractions 200–500, 500–1000 and 1000–2000 μm was relatively uniform on both transects. The mesozooplankton communities were dominated by calanoid copepods, and the majority of significant species were most abundant to the north of the Azores Current. Neither mesozooplankton biomass or species composition was related to the concentrations of particulate matter. The results are discussed in relation to the food requirements of the mesozooplankton. The dominance of the phytoplankton standing stock by small cells suggests that other food sources in addition to phytoplankton must be important in the diet of mesozooplankton in the region of the Azores Front.