Photosynthesis–irradiance parameters and community structure associated with coastal filaments and adjacent waters in the northern Arabian Sea

Comparisons were made among size-fractionated photosynthesis-irradiance (P-E) parameters, chlorophyll a size distributions, and accessory pigment composition of natural phytoplankton assemblages in filaments, coastal upwelling waters, and an oligotrophic region of the northern Arabian Sea during the Fall Intermonsoon in 1995. Differences between P-E parameters, PBmax and αB, were observed between filaments and adjacent waters and were associated with differences in phytoplankton community structure. In a southern filament and coastal upwelled waters, the majority of the estimated biomass (chlorophyll a) was present in the larger (2–20 and 20–200 μm) size fractions; dominant accessory pigments were 19′-butanoyloxyfucoxanthin and peridinin. In higher salinity waters, high percentages of chlorophyll a and lutein/zeaxanthin were observed in the smallest size-fraction (<2 μm). Whole water values of PBmax ranged from 1.77 to 2.31 (g C g chl a−1 h−1) when the majority of the biomass was in the largest fractions. Higher values (more than 4.48 g C g chl a−1 h−1) were determined in whole water samples for communities comprised primarily of small cells. A size dependence was also observed in the value of αB, 0.017 or greater (g C g chl a−1 h−1)/(μmol quanta m−2 s−1) for whole water samples at stations dominated by small cells and 0.013 when derived from stations dominated by large cells. The observed pattern of larger phytoplankton associated with upwelling and filament waters was consistent with previous investigations and was, for the most part, comparable to findings in the California Current system. Our results show that differences in taxonomic composition and photosynthetic characteristics were indeed present between filament waters and other distinct regions; these results suggest that taxonomic variations may be associated with size-related variations in P-E parameters. Our findings provide a unique data set describing filament biology in the northern Arabian Sea during the Fall Intermonsoon thus adding important details in efforts to model biogeochemical processes in this region.