Phytoplankton community structure and productivity in relation to the hydrological regime of the Gulf of Carpentaria, Australia, in summer

Abstract

To describe the influence of hydrology on the phytoplankton communites of the Gulf of Carpentaria, six phytoplankton parameters were measured on five transects: productivity, biomass, community structure, phospholipid-derived fatty acids, ratios of stable carbon isotopes and the relationship between photosynthesis and irradiance.

The mean (±s.e.) estimate of depth-integrated phytoplankton productivity was 914±185 mgC m^-2 day-' (n=9). In the shallow (<30 m) coastal waters, the productivity was 1430±400 mgC m^-2 day^-1 (n = 3). At the offshore stations (=30 m), it was 660 ±108 mgC m^-2 day^-1 (n = 6). At both the shallow and the offshore stations, primary productivity was distributed equally between the phytoplankton size fractions 0.6-2.0µm and > 10µm, with little in between.

The highest rates of growth occurred within a very narrow light regime: self-shading limited growth in deeper water (at light intensities less than 125 mol s^-1 m^-2), and growth was photoinhibited in shallow water (at light intensities greater than 150 mol s^-1 m^-2). The resulting biomass-specific primary productivity (mgC mChla^-1 day^-1) maximum did not coincide with the depth of either the chlorophyll a maximum or the highest nutrient concentration.

The natural carbon isotope ratio for the particulate matter showed that little land-derived carbon was exported beyond a narrow coastal fringe (about 10 to 20 km). The profiles of phospholipid fatty acids also showed that particulate organic matter was dominated by algal-derived compounds, which confirms that the bulk of the organic matter in the offshore Gulf of Carpentaria is of marine origin. Therefore, during the summer in this study, the stratified waters of the central gulf were both biologically and hydrologically independent of the coastal water and not influenced by terrestrial runoff. The phytoplankton production in the central gulf is maintained by nutrient supplies from internal sources, such as those that are remineralized in and resuspended from the sediment.