Phytoplankton and oceanography of the Coffs Harbour region, Eastern Australia

The East Australian Current (EAC) is the Western Boundary Current (WBC) of the South Pacific Gyre, transporting warm water from tropical to temperate latitudes along Eastern Australia. Due to climate change, the EAC is warming and strengthening, which is expected to impact on phytoplankton abundance, distribution and composition. This thesis aims at providing the first taxonomic phytoplankton time-series survey (May 2011 - September 2012) in the tropical-temperate transition zone of Eastern Australia (~30°S, Coffs Harbour). An interdisciplinary approach of oceanography, microscopy, phytoplankton pigment analysis through CHEMTAX and statistics was used. By applying this approach, the phytoplankton community was estimated and investigated under contrasting oceanographic conditions, along crossshelf gradients of environmental variables and throughout one annual cycle. The influence of the EAC on the shelf-scale and seasonal phytoplankton variability was examined in detail. Microscopy analysis revealed the abundance of 137 microphytoplankton taxa within 74 genera, including diatoms, dinoflagellates, silicoflagellates and Trichodesmium erythraeum. In addition, CHEMTAX determined the presence of pico- and nanophytoplankton, including cyanobacteria, cryptophytes, euglenophytes, haptophytes, pelagophytes and prasinophytes. Both microscopy and CHEMTAX revealed diatoms as being the most abundant phytoplankton taxon off Coffs Harbour. Shelf-scale phytoplankton abundance, distribution and composition were found to be driven by the highly variable oceanographic environment, and, on a seasonal scale, by the combination of the EAC and intrinsic seasonal cycles. Upwelling triggered two diatom blooms during December 2011 (Pseudo-nitzschia spp.) and September 2012 (Leptocylindrus danicus). This thesis has provided baseline information on interactions between oceanography and phytoplankton dynamics at ~30°S, Eastern Australia. The results from this survey are a key reference for future studies investigating changes in phytoplankton communities along the east Australian coast as a consequence of the strengthening EAC. Furthermore, the findings of this thesis find applicability in globally changing subtropical WBC systems. All of these systems are currently undergoing long-term changes, particularly in tropicalisation evidenced by poleward species shifts.