PHYTOPLANKTON COMMUNITY STRUCTURE AND PRODUCTIVITY IN RELATION TO THEHYDROLOGICAL REGIME OF THE GULF OF CARPENTARIA, AUSTRALIA, IN SUMMER

To describe the influence of hydrology on the phytoplankton communites of the Gulf of Carpentaria, six phytoplankton parameters were measure d on five transects: productivity, biomass, community structure, phosp holipid-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-1 (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 (greater-than-or-equal-to 30 m), it was 660 +/- 108 mgC M-2 d ay-1 (n = 6). At both the shallow and the offshore stations, primary p roductivity was distributed equally between the phytoplankton size fra ctions 0.6-2.0 mum and > 10 mum, with little in between. The highest r ates of growth occurred within a very narrow light regime: self-shadin g limited growth in deeper water (at light intensities less than 125 m ol 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-spec ific primary productivity (mgC mgChla-1 day-1) maximum did not coincid e with the depth of either the chlorophyll a maximum or the highest nu trient concentration. The natural carbon isotope ratio for the particu late matter showed that little land-derived carbon was exported beyond a narrow coastal fringe (about 10 to 20 km). The profiles of phosphol ipid fatty acids also showed that particulate organic matter was domin ated by algal-derived compounds, which confirms that the bulk of the o rganic 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 ph ytoplankton production in the central gulf is maintained by nutrient s upplies from internal sources, such as those that are remineralized in and resuspended from the sediment.