Sporadic upwelling on a downwelling coast: Phytoplankton responses to spatially variable nutrient dynamics off the Gascoyne region of Western Australia
Computing, Health and Science
Computing, Health and Science Faculty Office
The Gascoyne continental shelf extends from North West Cape (21.3°S) to Shark Bay (26.5°S), Western Australia, and is dominated by the Leeuwin Current (LC), an anomalous oligotrophic eastern boundary current that transports tropical water poleward and generates large-scale downwelling. In summer, shelf dynamics are influenced by wind-generated countercurrents (the Ningaloo and Capes Currents) that flow equatorward and are associated with localized upwelling. Here, we examined phytoplankton responses (biomass levels and distribution, primary production rates, species composition) to small-scale upwelling in this ecosystem which is generally dominated by a poleward current, using physical, chemical and biological oceanographic field data collected during a two-week research cruise through the Gascoyne region in the early austral summer of 2000. We found LC and offshore waters to be associated with low phytoplankton biomass (21.4±6.9 s.d. mg chl a m-2) and low primary production (110-530 mg C m-2 d-1); surface (<50 m) waters were nitrate-depleted (generally<0.1 μM), with a strong nutricline present at the base of the mixed layer. Upwelling associated with the Ningaloo Current (NC) sourced water from this nutricline, and in conjunction with mixing generated by seaward offshoots, resulted in nitrate levels of up to 2-6 μM within the euphotic zone. Biomass in these NC waters (35.9±11.6 mg chl a m-2) was significantly higher than in LC/offshore regions, with primary production in the range of 840-1310 mg C m-2 d -1. Capes Current (CC) water was also highly productive (990 mg C m-2 d-1), and with low silicate levels and a high proportion of centric diatoms, was typical of an aging upwelled water mass. Thus, the dominance of the oligotrophic LC along the Gascoyne region can be offset by these equatorward countercurrents, although we hypothesize that the biological impact of any upwelling on the inner shelf would be a function of: (a) the depth of the LC's nutrient-depleted mixed layer, (b) the strength and duration of upwelling-favorable winds (i.e. the intensity of upwelling), and (c) geographical location, primarily with respect to the width of the continental shelf and resultant proximity of upwelling flows to deep nutrient pools.