Factors influencing the standing crop of phytoplankton on the Australian Northwest Shelf seaward of the 40 m isobath

Abstract

An investigation was made to identify the physical factors that influence the distribution in space and time of the phytoplankton standing crop on the Australian Northwest Shelf (18°–22°S, 114°–121°E). During the 18 months of the investigation, 8 cruises were made to the study area. On each of these, there were two oceanographic transects across the shelf, from approximately 40 m, to waters deeper than 1000 m. At various points along each transect, profiles were obtained of temperature, salinity, nitrate-N, oxygen saturation, and in vivo chlorophyll fluorescence. Photosynthetically active radiation was monitored from dawn till sunset. In addition to these “seasonal transects”, special transects were made when tidal amplitude was at a maximum (spring tide) and minimum (neap tide). The standing crop of photoplankton was of the order of 20–40 mg of chlorophyll a per m 2, which is much more than that of the oligotrophic Central Indian Ocean Gyre, and much less than in eutrophic parts of the Arabian Sea and coastal waters of Somalia. The nutrient source for the Northwest Shelf water column is not runoff from the hinterland, nor upwelling, but slope water washing up onto the shelf in summer when the (southerly) Leeuwin Current is no longer flowing. The phytoplankton concentration that results from this enrichment is located at the base of the pycnocline, immediately above the bottom mixed layer, and its depth rises and falls with the rise and fall of the pycnocline. Because incident solar illumination in this area is so high, both in summer and in winter, there is a significant amount of phytoplankton (0.5 μg l −1) underneath the pycnocline over a considerable part of the shelf (∼40–100m); tidal mixing makes this readily available to benthic filter feeders. No surface phytoplankton concentrations or surface temperature fronts were encountered seaward of the 40 m isobath, presumably because surface waters are warmed so much by the sun that their buoyancy can withstand tidal mixing from below.