A comparative study on microzooplankton communities in two tropical monsoonal estuaries

Abstract We studied on microzooplankton (MZP) communities from two major rivers, the Mandovi and the Zuari estuarine system of Goa on central west coast India. Despite both estuaries having a common point of discharge (eastern part of the Arabian Sea) and origin (in the Western Ghats), the Zuari estuary retains its marine condition; in contrast, the Mandovi estuary becomes a nearly freshwater condition for most parts of the year. The essential components of the marine food web, the MZP showed substantial seasonal variation driven by hydrographical parameters. Canonical correspondence analysis (CCA) depicts that prevailing differential environmental variables such as salinity, chlorophyll a, and dissolved oxygen influence the MZP assemblages in the tropical estuarine systems, which lead to the dominance of heterotrophic dinoflagellates (~80%) in the Zuari estuary for most parts of the year and dominance of loricate ciliates (~67%) in the Mandovi estuary. During monsoon, the seasonal intrusion of low oxygen water from the coastal region directs the dominance of heterotrophic dinoflagellates (22 to 80%) in these estuarine systems, particularly in the Zuari estuary. Whereas, prevailing high saline waters during the pre-monsoon (23 to 62%) and the post-monsoon (34 to 55%) supported the loricate ciliates dominated MZP community. The higher abundance of MZP in the upstream during the pre-monsoon and euhaline region of the downstream compared to the upstream (oligohaline) during monsoon and post-monsoon seems to be supported by microbial food (pico and nano-plankton), which warrants further investigation.