Phytoplankton dynamics in the central Great Barrier Reef—I. Seasonal changes in biomass and community structure and their relation to intrusive activity

Abstract

Phytoplankton dynamics in the central Great Barrier Reef and the relation of these dynamics to the seasonal upwelling of nutrient-enriched Coral Sea water onto the outer continental shelf were followed over an annual cycle. Event scale changes were sampled by frequent (2-day to 2-week) re-occupations of a cross-shelf transect during the latter half of the summer. Intrusive activity between November and April episodically injects nitrate-N enriched water into oligotrophic shelf waters. Persistent mid-shelf and intermittent outer shelf mid-water phytoplankton accumulations were observed from late-January to March. The 10, 10to2and<2μm phytoplankton size fractions were, respectively, dominated by diatoms, microflagellates + unarmored dinoflagellates and unicellular cyanobacteria + coccoid eukaryotes + very small flagellates. In the absence of intrusive activity, nanoplankton (10 to 2 μm) and picoplankton (<2μm) dominated biomass. These two size fractions dominated biomass on the outer shelf and in the Coral Sea throughout the year. Picoplankton frequently made up50% of the chlorophyll standing crop. Mid-shelf blooms, in which the highest chlorophyll levels (2μg 1 −1) were found, were largely due to increases of the 10μm fraction. In contrast to marked summer fluctuations of mid-shelf diatom populations, nano- and picoplankton standing crop varied little with time, cross-shelf location or depth. Low water column DIN levels and DIN:Chla ratios suggest phytoplankton populations were nitrogen-limited. In open waters of the central GBR, phytoplankton apparently convert most of the intruded nitrogen into particulate form.