This work was the first large-scale biological oceanographic study to be undertaken in the coastal eastern Indian Ocean adjacent to Western Australia, and covered both northwest (Exmouth Peninsula to the Abrolhos Islands) and southwest (Cape Naturaliste to Cape Leeuwin) regions. The study area was dominated by the Leeuwin Current (LC), an anomalous eastern boundary current that transports tropical water poleward and prevents deep nutrients from reaching the surface by creating large-scale downwelling.

Leeuwin Current and offshore waters were consistently associated with low nitrate concentrations and low phytoplankton biomass and production (< 200 mgC m^-2 d^-1). However, the physical forcing of the LC was offset, during the summer months, by upwelling associated with wind-driven inshore countercurrents (Ningaloo and Capes Currents), which provided a mechanism to access high nutrient concentrations normally confined to the base of the LC. Production rates in these countercurrents were significantly higher than expected (~ 700 – 1300 mgC m^-2 d^-1) along this otherwise oligotrophic coast.

Phytoplankton biomass within the Leeuwin Current was largely confined to the base of the LC’s mixed layer, forming a deep chlorophyll maximum (DCM). Between 10 and 40 % of total water column production was attributable to the DCM. Coupling between nutrients at depth and the DCM indicate that the balance between light and nutrient availability is critical in controlling primary productivity in the LC. Variation in the depth (and therefore production) of the DCM was also related to changing oceanographic conditions along the length of the study area, including variation in the strength of the LC and the presence of offshore eddies.

Phytoplankton community composition was quite distinct between LC/offshore and shelf/countercurrent regions. Smaller sized phytoplankton (including cyanobacteria and prochlorophytes) dominated the Leeuwin Current waters, and were primarily dependent on regenerated forms of nitrogen at both the surface and DCM. In upwelling regions, larger phytoplankton (including diatoms) were more abundant, although production was still heavily reliant on regenerated forms of nutrients. Thus, both in the DCM and upwelling countercurrents, nitrogen recycling via heterotrophy appears to play a critical role in sustaining primary productivity.

Limited seasonal investigations off the Capes region of southwestern Australia showed that the winter production scenario can be very different than summer conditions, with strong Leeuwin Current flow that meanders onto the continental shelf and entrains seasonally nutrient-enriched shelf waters. However, production in the LC was still low (< 450 mgC m^-2 d^-1) due to light limitation resulting from both increased light attenuation and reduced surface irradiance characteristic of the winter months.

This investigation provides fundamental knowledge on physical-biological coupling off Western Australia, with implications for fisheries management in view of seasonal and inter-annual variability in the strength of both the Leeuwin Current and inshore countercurrents.


For further information on this work, or to request article re-prints, please e-mail: c.hanson@ecu.edu.au