An ocean colour remote sensing study of the phytoplankton cycle off Western Australia

The concentration of phytoplankton in waters off the Western Australian coastline contrast with the coastal waters west of southern Africa and South America. The lack of favourable upwelling conditions results in the majority of the southeastern Indian Ocean surface waters being nutrient poor. Which is reflected in their low productivity. Several areas either on or in close proximity to the coastline are notably more productive. The associated forcing terms generating phytoplankton favourable conditions differ between areas. as do the seasons in which they occur. Measurements of chlorophyll a concentration. the major photosynthetic pigment contained in phytoplankton, may be directly related to oceanic bioproductivity. Using data collected by the Coastal Zone Color Seamier between 1979-86, this work aims to improve the understanding of the spatial and temporal changes that occurred in chlorophyll a abundance in the southeastern Indian Ocean. The highest seasonal mean concentrations occur in Summer (January-March) and Autumn (April-June); the former occurring in waters of the North West Shelf and the latter in close coastal areas of Western Australia south of North West. Cape. Concentrations observed in the offshore oceanic regions are mostly poor. Exceptions to this occur in proximity to the adjacent Indonesian islands and directly south of Albany (possibly due to northwards flow of subantarctic nutrient-rich waters). A considerable interannual variation was also noted, with the highest mean chlorophyll concentrations occurring in 1981. 1982 and 1983.The influence of the forcing terms on chlorophyll a appears to vary significantly among the waters of North West Shelf, Western and southern Western Australian coastline. This is most notable in the interseasonal variations. The changes observed interannually and their influence on chlorophyll a are not easily discernible. but there may be some connection with the La Nina/El Nino related changes in both currents and winds