Phytoplankton biomass, production and grazing mortality in Exmouth Gulf, a shallow embayment on the arid, tropical coast of Western Australia

Phytoplankton biomass, production and grazing mortality were measured in Exmouth Gulf, a shallow embayment on the arid, tropical coast of Western Australia. In the Gulf, chlorophyll a concentrations were typically 0.2-0.3 mg m(-3) and phytoplankton production rates were mostly below 25 mg of C m(-3) d(-1). The low phytoplankton biomass and production in the Gulf are seemingly related to the aridity of the region and hence the small terrestrial runoff of nutrients. In dilution experiments, which complemented C-14 incorporation experiments, the proportion of potential primary production grazed ranged from 79 to 155% in the fluorometric analysis of chlorophyll a and from 73 to 191% in the HPLC analysis of chlorophyll a. There may be some excess phytoplankton production on the relatively well flushed, western side of the Gulf. On the eastern side of the Gulf, however, the estimated grazer biomass ranged between 4.6-8.8 mg of C m(-3), not much less than the estimated phytoplankton biomass (6-15 mg of C m(-3)), and this grazer population appeared to consume more organic matter than the phytoplankton population could produce. The disproportionally large grazer biomass and the relatively high grazing mortality of phytoplankton may be due to the supply of additional organic matter from benthic macroalgal communities and/or mangrove and salt flat systems present on the eastern side of the Gulf. (C) 1998 Elsevier Science B.V.