A comparison of phytoplankton populations of the Arabian Sea during the Spring Intermonsoon and Southwest Monsoon of 1995 as described by HPLC-analyzed pigments
We used diagnostic pigments to estimate the relative abundances of different algal groups in the Arabian Sea during the Spring Intermonsoon (cruise TN045) and late Southwest Monsoon (cruise TN050) of 1995. Northern and southern transect lines were occupied during each cruise. These transects started near the coast of the Arabian Peninsula at 22.38°N and 18.50°N, respectively, and extended >1200 km offshore. The pigment concentrations at the offshore stations (>1000 km from the coast) were low during both cruises, and the composition of the phytoplankton resembled that found in oligotrophic, open-ocean waters. The pigment compositions at the inshore stations (<1000 km from the coast) were markedly different during both cruises. During TN045, the inshore waters of the northern transect were dominated by a mixture of diatoms and haptophytes. Large particles (>18 μm) accounted for up to 23% of the total chlorophyll a-related pigments (TCHLA, chlorophyllide a+monovinyl chlorophyll a+divinyl chlorophyll a). Pigment biomass (=500 ng TCHLA L -1) was homogeneously distributed in the upper water column at these stations. By contrast, pigment biomass at the inshore stations of the southern transect was concentrated in a pronounced deep chlorophyll maximum. The phytoplankton were clearly partitioned in the water column, with photosynthetic prokaryotes and eukaryotes dominating in the upper mixed layer and below, respectively. The high levels of divinyl chlorophyll a observed (up to 300 ng l -1) represent the highest concentrations ever recorded for this prochlorophyte marker. The size-fractionated pigment analyses revealed that 85% of the TCHLA passed through 2 μm filters, indicating dominance by the picophytoplankton. During TN050, the inshore portion of the northern transect displayed a clear example of a diatom-to- Phaeocystis successional event. Diatoms dominated at coastal upwelling stations and were progressively replaced by Phaeocystis at offshore stations. At the inshore stations of the southern transect, diatoms dominated the phytoplankton community, and a large fraction of the TCHLA (21-48%) was retained by 18 μm filters. A filament with high diatom biomass was observed 700 km offshore, documenting that horizontal advection is an important mechanism for removing phytoplankton from coastal waters in this region. However, lower than expected TCHLA concentrations were measured during this period relative to those observed in earlier years in the CZCS imagery. Several hypotheses are discussed to explain this enigma, including sampling biases, losses by grazing and sinking, phytoplankton dilution via upwelling, and Si limitation of diatom growth.