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

Abstract

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 short-lived spring bloom for the area. Pigment analysis indicated four distinct communities that were coincident with the regional oceanography. The bloom 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 surface and local conditions were influenced by topography. Pigment and stable isotope data indicated that primary production in this area was almost entirely oceanic. A slight trend for seaward enrichment of sediment δ¹³C 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 lack of specific zooplankton grazing pigments suggests that zooplankton grazing was minimal.