Seasonal variations in abundance, biomass and grazing rates of microzooplankton in a tropical monsoonal estuary

Abstract

Seasonal abundance, composition and grazing rates of microzooplankton (20–200 µm) in the Zuari estuary were investigated to evaluate their importance in food web dynamics of a tropical monsoonal estuary. Average abundances of microzooplankton (organisms × 10⁴ l^-1) during the three seasons were 0.44 (southwest monsoon), 1.13 (post-monsoon) and 0.96 (pre-monsoon). Protozoan (ciliates, heterotrophic dinoflagellates and sarcodines) accounted for most (96 %) of the microzooplankton community, with micrometazoan (nauplii and copepodid stages of copepods, fish eggs, etc.). being far less abundant. Among protozoans, ciliates (loricates and aloricates) were most numerous (69 % of the total microzooplankton). Statistically significant (p < 0.001) co-variations of microzooplankton with other biological parameters such as chlorophyll a and bacterial biomass were observed. Salinity influenced microzooplankton distribution, with an optimum range of 15–20. Microzooplankton formed a large organic carbon pool, accounting for 24–40 % of the total carbon in the living matter. Seasonally averaged microzooplankton biomasses were 22.3, 36.1 and 24.6 mmol C m^-3, respectively, during the southwest monsoon, post-monsoon and pre-monsoon periods, and were largely supported by non-living particulate carbon (detritus) particularly during the non-monsoon seasons. Experimental studies revealed significant microzooplankton grazing on phytoplankton standing stock, mainly (>60 %) by the pico and nano fraction (<20 µm) for most of the year. Phytoplankton growth rates (day^-1) ranged between 0.69 and 1.24. Microzooplankton grazing was estimated to consume 30–82 % of the phytoplankton standing stock, and 58–97 % of the daily primary production. Results of the present study highlight the role of the microzooplankton as an important consumer of phytoplankton production