New insights into the Kimberley phytoplankton and their ecology

The Kimberley is a remote region where the eastern edge of the Indian Ocean interacts with the broadening continental shelf of Northern Australia to generate massive tides. During a 2010 research voyage the phytoplankton communities of the region were elucidated from a combination of light microscopy, remote sensing and size fractionated pigment analysis. In strong contrast to previous work from the NW Shelf and the Gulf of Carpentaria ≥ 80% of the phytoplankton at the shelf break (~ 200 m water depth) and further offshore were found to be < 2 μm (picoplankton) and dominated by Synechococcus. Streaks of Trichodesmium were visible but cell counts suggested they were only the 9 ^th most abundant taxa. Pigment analysis indicated coccolithophorids were consistently ~ 20% of the total phytoplankton biomass across the region of the cruise. Shelf scale blooms of coccolithophorids are periodically reported in the shallow seas of Northern Australia but only small blooms were observed in MODIS (Moderate Resolution Imaging Spectroradiometer) true colour images from the Kimberley region during the voyage. In shallower waters closer to shore the concentration of phytoplankton rose dramatically. There were concomitant changes in community composition including a decline in Prochlorococcus and pelagophytes and a rise in the diversity and abundance of medium to large diatoms. This distinctive, near shore, diatom community was spatially heterogeneous and largely composed of species previously reported as rare in northern Australia.