A mixed-layer nutrient climatology of Leeuwin Current and Western Australian shelf waters: Seasonal nutrient dynamics and biomass

We present a seasonal climatology of the nutrient environment for waters off southwestern Australia with the intention of identifying spatial and seasonal characteristics of the nutrient environment and identify situations where the shelf may be susceptible to anthropogenic nutrient stress. The seasonal climatologies were generated from historical hydrographic data contained within the CSIRO Atlas of Regional Seas. The data presented here suggest the surface waters of the southwestern Australian shelf, the Leeuwin Current and offshore are all low in nitrogen (less than 0.5 μM) year round and that primary productivity is nitrogen limited. The shelf waters contain some dissolved phosphate, at relatively low levels (up to 0.25 μM) but diatom production may be limited by low levels of silicic acid (silicate) which are less than 2 μM year round. The Leeuwin Current is largely devoid of phosphate but contains reasonably high levels of silicate (up to 4 μM) and may be a silicate source to surrounding waters. A cross-shelf gradient in chlorophyll a biomass suggests that terrestrial nutrient sources make an important contribution to primary productivity. Offshore, a seasonal (wintertime) increase in chlorophyll a biomass coincides with a deepening of the mixed layer and is presumably supported by the mixing of deep water nutrients or chlorophyll from the deepwater maximum into the euphotic zone associated with this deepening. Further observations, particularly cross-shelf profiles from winter and profiles along the core of the Leeuwin Current, are required to fully separate the influence of the Leeuwin Current from other potential seasonal nutrient sources.