Seasonal succession of zooplankton in coastal waters of the Argentine Sea (Southwest Atlantic Ocean): prevalence of classical or microbial food webs

The demographic characteristics of marine zooplankton make it especially suitable for examining the variability of marine ecosystems. The zooplankton annual succession was studied at a permanent coastal station in the Argentine Sea (38 degrees 28S, 57 degrees 41W) in relation to physical conditions and phytoplankton size fractions. Small copepods (<1 mm total length), mainly represented by Oithona nana (Cyclopoida) and adults and copepodites of Calanoida, numerically dominated the metazooplankton throughout the year. In summer, small copepods also exceeded large copepods in biomass. Larvaceans (mostly <1 mm total length) were the second most important metazooplankton group, with strong dominance of Oikopleura dioica. The zooplankton succession exhibited two main periods throughout the year: (1) a cold winterspring period characterized by a dominant classical herbivore food web in which the large copepod Calanoides carinatus and lamellibranch larvae were associated with the lowest temperatures and highest Chl-a and microphytoplankton, and (2) a warm summer period dominated by a microbial food web in which microbial filter-feeders such as Oithona nana, Paracalanus spp., Oikopleura dioica and Penilia avirostris predominated and the highest density of picophytoplankton and lowest concentrations of Chl-a were recorded. The implications of the present findings for the growth and survival of fish larvae distributed in the study area are discussed.