The influence of mixing on primary productivity: A unique application of classical critical depth theory

Mixing and primary productivity was examined in upwelling influenced nearshore waters off south western Eyre Peninsula (SWEP) in the eastern Great Australian Bight (EGAB), the economically and ecologically important shelf region off southern Australia that forms part of the Southern and Indian oceans. Mixing/stratification in the region was highly temporally variable with a unique upwelling circulation in summer/autumn (November–April), and downwelling through winter/spring (May–September). Highest productivity was associated with upwelled/stratified water (up to 2958 mg C m−2 d−1), with low productivity during periods of downwelling and mixing (∼300–550 mg C m−2 d−1), yet no major variations in macro-nutrient concentrations were detected between upwelling and downwelling events (silica > 1 μmol L−1, nitrate/nitrite > 0.4 μmol L−1, phosphate > 0.1 μmol L−1). We hypothesise that upwelling enriches the region with micro-nutrients. High productivity off SWEP appears to be driven by a shallowing of mixed layer depth due to the injection of upwelled waters above Zcr. Low productivity follows the suppression of enrichment during downwelling/mixing events, and is exacerbated in winter/spring by low irradiances and short daylengths.