Abundance and dynamics of microplankton in the central tropical Indian Ocean

Abstract

In the central part of the Indian Ocean, primary production and quantitative distribution of microheterotrophs were studied in Aug.-Sept. Primary production in areas of divergence and in waters of the Trade Wind Current was high (0.3-1.5 g C m-2 d-1 [day]). At some stations, at the water surface it was up to 15-20 mg C m-3 d-1. In areas of convergence it decreased (0.1-0.2 g C m-2 d-1). Deep maxima of accumulation of active phytoplankton were recorded at the upper boundary of the thermocline. Total number of bacteria in the euphotic zone varied from 6 to 15 .times. 104 ml-1, biomass from 10-30 m-3 (wet weight) production and respiration from 5-15 mg m-3 d-1, and from 3-10 mg O2 1-1, respectively. Below 200 m the above parameters decreased 1 order of magnitude or more. The amounts of olive cells found at depths of 200-2000 m varied from 60-400 ml1-. Observations suggest that olive cells represent cysts of microflagellates inhabiting deep waters. Species composition, total number, biomass, patterns of vertical distribution and diurnal dynamics of planktonic protozoans (zooflagellates, ciliates) are described. Numbers of nauplii and Radiolaria as components of microzooplankton are given. Functional roles of the main groups of microplankton in pelagic communities of the tropical oceanic waters investigated are evaluated via energy-flow schemes.