SPATIAL AND TEMPORAL DISTRIBUTION OF TROPICAL PHYTOPLANKTON SPECIES AND BIOMASS IN THE GULF OF CARPENTARIA, AUSTRALIA

The biomass and species composition of tropical phytoplankton in Albat ross Bay, Gulf of Carpentaria, northern Australia, were examined month ly for 6 yr (1986 to 1992). Chlorophyll a (chl a) concentrations were highest (2 to 5.7 mu g l(-1)) in the wet season at inshore sites, usua lly coinciding with low salinities (30 to 33 ppt) and high temperature s (29 to 32 degrees C). At the offshore sites chi a concentrations wer e lower (0.2 to 2 mu g l(-1)) and did not vary seasonally. Nitrate and phosphate concentrations were generally low (0 to 3.68 mu M and 0.09 to 3 mu M for nitrate and phosphate respectively), whereas silicate wa s present in concentrations in the range 0.19 to 13 mu M. The phytopla nkton community was dominated by diatoms, particularly at the inshore sites, as determined by a combination of microscopic and high-performa nce liquid chromatography (HPLC) pigment analyses. At the offshore sit es the proportion of green flagellates increased. The cyanobacterium g enus Trichodesmium and the diatom genera Chaetoceros, Rhizosolenia, Ba cteriastrum and Thalassionema dominated the phytoplankton caught in 37 mu m mesh nets; however, in contrast to many other coastal areas stud ied worldwide there was no distinct species succession of the diatoms and only Trichodesmium showed seasonal changes in abundance. This refl ects a stable phytoplankton community in waters without pulses of phys ical and chemical disturbances. These results are discussed in the con text of the commercial prawn fishery in the Gulf of Carpentaria and th e possible effect of phytoplankton on prawn larval growth and survival .