Microzooplankton grazing and protozooplankton community structure in the South Atlantic and in the Atlantic sector of the Southern Ocean

Abstract

Microzooplankton grazing and protozooplankton community structure in the South Atlantic and Atlantic sector of the Southern Ocean were investigated during late austral summer (January/February) 1993. Grazing rates and numerical abundances of protozooplankton were estimated in the surface waters and at the subsurface chlorophyll maximum (SCM) by employing the dilution technique and epifluorescent microscopy. Protozooplankton abundance co-varied with chlorophyll concentrations at both depths. Nanoheteroflagellates (< 20 μm) dominated numerically at all stations while the > 20 μm component was dominated by ciliates, comprising aloricate ciliates and tintinnids. Instantaneous growth rates of alga along the transect ranged between 0.24 and 1.86 ay −1 in surface waters and between 0.06 and l.87 day −1 at the SCM. Instantaneous grazing rates of microzooplankton on phytoplankton varied from 0 to 0.33 day −1 in the surface waters and between 0 and 0.58 day −1 at the SCM. This level of grazing corresponds to a daily loss of 0–23% of the initial standing stock (0–46% of potential production) in the surface waters and between 0 and 44% (0–60% of potential production) of the initial standing stock at the SCM. Analysis of variance and multiple range tests indicate that both the initial standing stock and potential production removed were not significantly different between depths ( F=0.84; F=0.29: P<0.05). Indirect evidence presented, suggests that microzooplankton are preferentially grazing on the nano- and picophytoplankton size fractions. The spatial distribution of phytoplankton size classes in the different zones of the Southern Ocean has important ecological implications for the oceanic carbon flux. South of the Antarctic Polar Front (APF), where larger netphytoplankton dominates chlorophyll concentration, the bulk of the photosynthetically fixed carbon appears to be channelled into the meso/macrozooplankton component or is lost by sedimentation. However, north of the APF, the contribution of the smaller fractions to total chlorophyll increases, suggesting a relative increase in the amount of carbon channelled into the smaller grazing fractions.