Phytoplankton blooms and production sources on the south-east Australian continental shelf

During a large-scale ecosystem study on the south-east Australian shelf in spring 1994, we opportunistically sampled a widespread phytoplankton bloom. Thalassiosira partheneia, a small centric diatom, was the primary species in the bloom, indicating that we had sampled the early stage of a typical s hort-lived spring bloom for the area. Pigment analysis indicated four disti nct communities that were coincident with the regional oceanography. The bl oom was strongest over the northern shelf where the East Australian Current overlaid uplifted nutrient-rich slope water, but absent at inner stations on the wide southern shelf, where slope water did not reach. The bloom was patchy over the southern outer shelf where slope water was present to the s urface and local conditions were influenced by topography. Pigment and stab le isotope data indicated that primary production in this area was almost e ntirely oceanic. A slight trend for seaward enrichment of sediment delta C- 13 is best explained by limited macroalgal growth in shallow waters. There was little fresh organic matter in the sediment even for stations under the bloom, suggesting that the bloom did not reach the seabed directly. The la ck of specific zooplankton grazing pigments suggests that zooplankton grazi ng was minimal.