Physical oceanographic controls on phytoplankton distribution in the Banda Sea and Western Australian region

Marine phytoplankton rely on nutrients and light to survive and grow. By controlling the supply of nutrients and by moving phytoplankton around the upper ocean, ocean physics play a critical role in influencing the distribution of phytoplankton. The physical processes that influence phytoplankton distribution act over a wide range of temporal and spatial scales. Using in-situ, and remotely-sensed measurements, this thesis investigates the relationship between ocean physics and phytoplankton distributions in the Banda Sea and off the coast of Western Australia, over a range of scales including synoptic mesoscale eddies, seasonal monsoons, climatological features and ENSO impacts. In the Banda Sea, the distribution of chlorophyll-a is used as an indicator to describe the biophysical response of the Banda Sea to the seasonal winds of the southeast monsoon. The in-situ and remotely sensed data support the idea that phytoplankton respond to upwelling driven by the southeast monsoon (June - September). In-situ physical data from the BIOP98 cruise suggest an upwelling signal, and analysis of phytoplankton pigments indicates a community dominated by diatoms to the east and a more oligotrophic community to the west; further supporting the idea of an upwelling response. Satellite observations reveal a strong seasonal signal and an east-west gradient in chlorophyll-a, indicating monsoon driven upwelling on the eastern side of the basin. Remotely sensed observations of chlorophyll-a during the 1997-1998 El Nino suggest that the distribution of chlorophyll-a in the Banda Sea is modulated by ENSO, most likely through changes to the thermocline depth and wind regime. South of the Indonesian Archipelago, the waters off central Western Australia (WA) are unique in terms of their biological and physical oceanographic character. Unusually for an eastern boundary current, the Leeuwin flows poleward and contributes to prevailing downwelling conditions along the WA coast. Significant spatial and temporal variability in phytoplankton distribution exists and a key regional feature of the phytoplankton variability is a seasonal peak in chlorophyll-a in the winter May-July period on the central WA shelf. To test the hypothesis that this seasonal cycle is related to nutrient inputs, this thesis attempts to address several potential sources: (1) upwelling from off-shelf; (2) rainfall and associated runoff; (3) Leeuwin Current transport; ( 4) shall owing and/or mixing of the nutricline; and (5) benthic supply. Nutrient inputs from Leeuwin Current transport and coastal runoff appear to be most important, although sparse data prevent a definite conclusion. Phytoplankton distribution off central WA also displays persistent mesoscale features. Both satellite and in-situ estimates of surface chlorophyll-a suggest low phytoplankton concentrations in cyclonic (\upwelling\") eddies and high concentrations in anti-cyclonic (\"downwelling\") eddies. The anti-cyclonic eddies entrain regionally high chlorophyll-a shelf waters and then export this material offshore as· these eddies propagate westward. The formation of these anti-cyclonic eddies occurs at two preferred zones along the central WA shelf near 29°S and between 31° and 33°S and their formation and movement offshore controls the mixed layer distribution of phytoplankton in the open ocean off the central WA coast during the May-October period."