The response of microzooplankton (20–200 μm) to coastal upwelling and summer stratification in the southeastern Arabian Sea

Abstract

During the late summer monsoon (LSM), coastal upwelling and river runoff have increased the nutrient concentration in the inshore region of the southeastern Arabian Sea. This has resulted in elevated chlorophyll- a (av. 40±18 mg m −2), phytoplankton abundance (av. 474±116×10 6 ind. m −2) and primary production (av. 917±616 mg C m −2 d −1). Diatoms were the major component of the phytoplankton community (av. 60±8% of the total abundance) during this period, followed by dinoflagellates (18±12%). However, the inshore locations of 10°N and 13°N transects behaved differently, with an exceptional abundance of phytoflagellates (>10 10 ind. m −2). The microzooplankton (MZP) community was comprised of heterotrophic dinoflagellates (av. 60%), ciliates (av. 30%) and copepod nauplii (av. 5%); these showed marked variation in abundance between stations. The highest abundance (av. 283×10 4 ind. m −2) was at the inshore location of 10°N transect, where phytoflagellates were abundant. Contrasting to the LSM, intense surface layer stratification and depletion of nitrate (with a nitracline at 60 m depth), observed in the southeastern Arabian Sea during the spring intermonsoon (SIM) period, caused low phytoplankton abundance (av. 141±86×10 6 ind. m −2), chlorophyll- a (av. 19±11 mg m −2) and primary production (av. 146±68 mg C m −2 d −1). High temperature (>29 °C) and nitrate-depleted surface waters favoured the proliferation of Trichodesmium erythraeum at most of the locations. Total abundance of MZP during the SIM was markedly low (av. 20×10 4±13×10 4 ind. m −2), as was the species richness and diversity (0.36±0.02 and 0.57±0.15, respectively) compared to the LSM (1.17±0.31 and 2.72±0.34, respectively). During the SIM, the MZP community that occurs in the southeastern Arabian Sea is important, since smaller individuals are widespread and form the majority of phytoplankton community. The present study points also to the fact that the MZP could play an important role, even in nutrient-enriched environment, if smaller phytoplankton are abundant.