The Centre For Water Research

Title: Sporadic upwelling on a downwelling coast: Phytoplankton responses to spatially variable nutrient dynamics off the Gascoyne region of Western Australia

Authored by: Hanson, C.E., Pattiaratchi, C.B. and Waite, A.M.

Abstract:

The Gascoyne continental shelf extends from North West Cape (21.3ºS) to Shark Bay(26.5 ºS), Western Australia, and is dominated bythe Leeuwin Current (LC), an anomalous oligotrophic eastern boundarycurrent that transports tropical water poleward and generates large-scale downwelling. In summer, shelf dynamics are influenced by windgenerated 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 am -2) and low primaryproduction (110â€"530 mg Cm 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 byseaward offshoots, resulted in nitrate levels of up to 2â€"6 μM within the euphotic zone. Biomass in these NC waters (35.9+11.6mg chl am -2) was significantlyhigher than in LC/offshore regions, with primary production in the range of 840â€"1310 mg Cm -2 d -1. Capes Current (CC) water was also highlyproductive (990mg Cm -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 intensityof upwelling), and (c) geographical location, primarilywith respect to the width of the continental shelf and resultant proximityof upwelling flows to deep nutrient pools.

Reference: Hanson, C.E., Pattiaratchi, C.B. and Waite, A.M., 2005, Sporadic upwelling on a downwelling coast: Phytoplankton responses to spatially variable nutrient dynamics off the Gascoyne region of Western Australia, Continental Shelf Research, , 25: 1561-1582

Keywords: Primaryproduction ; Coastal upwelling; Oligotrophic; Eastern Indian Ocean; Leeuwin Current; Ningaloo Current; Western Australia; Gascoyne 21ºS to 30ºS and 111ºE to 115ºE