Spatial patterns in phytoplankton growth and microzooplankton grazing in the Arabian Sea during monsoon forcing

Spatial patterns in the rates of phytoplankton growth and microzooplankton grazing were investigated in the Arabian Sea during the Southwest Monsoon (August–September) and early Northeast Monsoon (December) seasons in 1995 using the seawater dilution technique. Nutrient-enhanced growth rates (μn) averaged 1.2 d-1 in the upper euphotic zone for both cruises and were similar between higher and lower nutrient stations, the former (>1.0 μM NO3) being characteristic of the upwelling-influenced western coastal portion of the study region and the latter (<0.5 μM NO3) being typical of the central basin. Growth rates without added nutrients (μ0) were also comparable between cruises but strongly related to ambient nutrient conditions, averaging 1.1 d-1 (91% of μn) at the higher nutrient stations and 0.5 d-1 (44% of μn) at the lower nutrient stations. The rate estimates for phytoplankton losses to microzooplankton grazing (m) averaged 0.6 d-1 for the upper euphotic zone and did not vary systematically between low- and high- nutrient stations. As a consequence, μ0 and m were largely in balance for the more oligotrophic stations, while the eutrophic stations showed a growth differential over grazing of about 0.6 d-1. These experimental results are consistent with observed differences in community structure, namely the dominance of picoplankton in oligotrophic offshore regions and the increased importance of the large diatom-mesozooplankton grazing pathway in the richer coastal areas. Overall, the spatial patterns, if not magnitudes, of the community responses to Southwest and Northeast Monsoon forcing were remarkably similar in this study, allowing for a relatively simple interpretation of the influence of enhanced nutrient supply on the rates and fates of phytoplankton production.