The plankton ecology of south Western Australia: Temporal and spatial patterns

Plankton are a crucial component of the annual global carbon flux and knowledge of how they respond to environmental variation provides the basis for predictive biogeochemical modelling. Long term data from the Rottnest Island station indicate the oceanic environment in the SW is becoming significantly warmer, saltier and with more nitrate and phosphate in the euphotic zone. Our capacity to assess inter annual variability is limited but remotely sensed ocean colour data indicate a strong increase in phytoplankton biomass between 29 – 31°S and 113.5 - 114.5 E associated with weak or positive ENSO events. This increased biomass is most evident in summer to autumn when the coastal phytoplankton become increasingly diatom dominated. The causal mechanism is proposed to be the variability in sea level pressure resulting in a shift in wind patterns and an upwelling response by the Capes Current. The data from 6 research cruises (1995 – 2007) off the south and west coast of Australia provide some additional insights into factors that influence plankton distributions. The vast majority of the phytoplankton are less than 5 microns and poorly resolved by the light microscope. A combination of light microscopy, flow cytometry and HPLC pigment analysis shows Synecchoccus (small cyanobacteria) tend to be significantly more abundant offshore and near the surface during spring to summer. The Haptophytes were also found offshore but at greater depths and primarily during summer. In contrast, Chlorophytes and Prasinophytes are found nearer shore during summer and autumn. Zooplankton and larval fish biomass and species also show spatial patterns related to physical features that are associated with seasonal cycles and the Leeuwin Current.