Mesozooplankton distribution and grazing during the productive season in the Northwest Antarctic Peninsula (FRUELA cruises)

Mesozooplankton distribution and community structure in the Bellinghausen-Bransfield sector of the Antarctic Ocean were investigated during the FRUELA cruises (December 1995–February 1996). Total mesozooplankton biomass ranged between 0.015 and 1.43 g C m−2. Biomass was higher in the Southern boundary of the Antarctic Circumpolar Current (SbyACC) area and in coastal waters of the Antarctic Peninsula. Total mesozooplankton abundance ranged from 0.4×103 to 1.3×105 individuals m−2, of which 41.6–99.5% corresponded to copepods, mainly families Oithonidae, Oncaeidae, Pseudocalanidae, Calanidae and Metrididae. There was no evidence of coupling between mesoscale physical features and biomass or community structure. While coastal stations mainly at the Gerlache Strait remained in a highly productive state through the spring–summer, oceanic stations experienced a marked shift from a productive condition during FRUELA 95 to a low biomass, pteropod-dominated situation during FRUELA 96, possibly due to changing weather conditions. The median ingestion rates of herbivorous crustaceans during the FRUELA cruises were 0.7 mg Chl a m−2 day−1. Measured ingestion rates represented only 0.1% of the chlorophyll standing stock or 10% of the daily primary production. Thus, crustacean mesozooplankton had little control on the development of phytoplankton blooms in the area.