Copepod grazing and their impact on phytoplankton standing stock and production in a tropical coastal water during the different seasons

The grazing rate of copepods on the total and size-fractionated phytoplankton biomass in a coastal environment (off Kochi, southwest coast of India) were measured during pre-monsoon (PRM), peak southwest monsoon (PKSWM), late southwest monsoon (LSWM) and post-southwest monsoon (PSWM). The phytoplankton standing stock (chlorophyll a-Chl. a) and growth rate (GR) were less during the PRM (Chl. a 0.58 mg m(-3); GR 0.23 +/- 0.02) and PSWM (Chl. a 0.89 mg m(-3); GR 0.30 +/- 0.05) compared to PKSWM (Chl. a 6.67 mg m(-3); GR 0.43 +/- 0.02) and LSWM (Chl. a 4.09 mg m(-3); GR 0.40 +/- 0.04). The microplankton contribution to the total Chl. a was significant during the PKSWM (41.83%) and LSWM (45.72%). Copepod density was lesser during the PRM (1354 No m(-3)) and PSWM (1606 No m(-3)) than during PKSWM and LSWM (4571 and 3432 No m(-3), respectively). Seasonal changes in phytoplankton biomass, phytoplankton size structure, and copepod community were closely related to the hydrographical transformations in the study domain. Dominant calanoid copepods in the study region ingested 8.4 to 14.2% of their daily ration from phytoplankton during the PRM and PSWM, which increased to > 50% during the PKSWM and LSWM. The cyclopoid Oithona similis was abundant during the PKSWM, ingesting only 21% of their daily ration from phytoplankton. Temporal variation in the phytoplankton biomass and copepod species composition caused differences in community level topdown control. The copepod community ingestion on phytoplankton was high during the LSWM (18,583 mu g C m(-3)d(-1)), followed by PKSWM (9050 mu g C m(-3)d(-1)), PSWM (1813 mu g C m(-3)d(-1)), and PRM (946 mu g C m(-3)d(-1)). During the low Chl. a period (PRM and PSWM), dominant calanoid copepods showed a positive selectivity for the micro-and nanophytoplankton size fractions, whereas during the high Chl. a period (PKSWM and LSWM), they showed a positive selection for nano-phytoplankton fractions. Irrespective of the seasons, dominant calanoid copepods showed a negative selection of pico-phytoplankton fraction. The cyclopoid O. similis and Poecilostomatoid Corycaeus danae showed a positive selection of nanoand pico-phytoplankton fractions rather than micro-fraction. The grazing pressure of copepod community ingestion onmicro-fraction was less (0.56% of the phytoplankton biomass and 1.06% of the phytoplankton production) during the PKSWM. This study provides, for the first time, clear findings on the seasonal variation in the topdown control of phytoplankton by copepods in a tropical coastal water ecosystem and discusses its implications on phytoplankton blooming, plankton food web, and biogeochemistry.