Phytoplankton distribution in the Bismarck Sea north of Papua New Guinea: The effect of the Sepik River outflow

Chlorophyll and carotenoid pigments were measured at stations in the Bismarck Sea north of Papua New Guinea and along the equator between 143°E and 152°E as part of the Tropical River Ocean Processes In Coastal Settings (TROPICS) programme in August 1997 and January/February 2000. Algal class abundances were estimated from these pigments using the CHEMTAX programme. The algal classes likely to be present (based on microscopy, HPLC chromatograms, literature and unpublished data from the region) and estimated by CHEMTAX were diatoms, cryptophytes, chlorophytes, haptophytes, chrysophytes, Synechococcus and Prochlorococcus. The first period corresponded to the SE tradewind season, when there was a drought in Papua New Guinea and the discharge from the Sepik River was unusually low. Samples were collected on four N–S transects (approximately 150 km in length): one to the east of the Sepik River, one from the mouth of the Sepik River and two to the west of the Sepik River. Stations in the central Bismarck Sea and along the equator were characterised by a deep chlorophyll maximum (DCM), and haptophytes and Prochlorococcus were the major algal classes. Stations affected by the Sepik River plume were characterised by a more homogeneous distribution of chlorophyll with chlorophytes making a significant contribution, particularly at shallower depths. Synechococcus and chrysophytes were moderately abundant, while cryptophytes and diatoms were of minor importance. The second cruise occurred during the NW monsoon under conditions of high river flow, and samples were collected along a transect from the mouth of the Sepik River to Manus Island and in the central Bismarck Sea. Chlorophyll concentrations were higher than in 1997, particularly in near surface waters, while a DCM was generally not observed at any station in the Bismarck Sea—even at stations upstream (and upwind) and far removed from the mouth of the Sepik River. Prochlorococcus made a smaller, and chlorophytes a larger, contribution to chlorophyll than in 1997. Unlike the situation in 1997, there were large station to station variations in the vertical profiles of chlorophyll and algal class distributions which cannot be attributed directly to the plume from the Sepik River. Pigment ratios, fitted by the CHEMTAX programme, were generally in good agreement with literature values.