Hot-spots of primary productivity: An Alternative interpretation to Conventional upwelling models

Abstract

The eastern Great Australian Bight (EGAB) forms part of the Southern and Indian Oceans and is an area of high ecological and economic importance. Although it supports a commercial fishery, quantitative estimates of the primary productivity underlying this industry are open to debate. Estimates range from <100 mg C m −2 day −1 to > 500 mg C m −2 day −1. Part of this variation may be due to the unique upwelling circulation of shelf waters in summer/autumn (November–April), which shares some similarities with highly productive eastern boundary current upwelling systems, but differs due to the influence of a northern boundary current, the Flinders current, and a wide continental shelf. This study examines spatial variations in primary productivity in the EGAB during the upwelling seasons of 2005 and 2006. Daily integral productivity calculated using the vertically generalised production model (VGPM) showed a high degree of spatial variation. Productivity was low (<800 mg C m −2 day −1) in offshore central and western regions of the EGAB. High productivities (1600–3900 mg C m −2 day −1) were restricted to hotspots in the east that were influenced by the upwelled water mass. There was a strong correlation between the depth of the euphotic zone and the depth of the mixed layer that suggested that ∼50% of the euphotic zone lay below the mixed layer depth. As a result, high rates of primary productivity did not require upwelled water to reach the surface. A significant proportion of total productivity in the euphotic zone (57% in 2005 and 65% in 2006) occurred in the upwelled water mass below the surface mixed layer. This result has implications for daily integral productivities modelled with the VGPM, which uses surface measures of phytoplankton biomass to calculate productivity. Macro-nutrient concentrations could not be used to explain the difference in the low and high productivities (silica > 1 μmol L −1, nitrate/nitrite > 0.4 μmol L −1, phosphate > 0.1 μmol L −1). Mixing patterns or micro-nutrient concentrations are possible explanations for spatial variations in primary productivity in the EGAB. On a global scale, daily rates of primary productivity of the EGAB lie between the highly productive eastern boundary current upwelling systems, and less productive coastal regions of western and south eastern Australia, and the oligotrophic ocean. However, daily productivity rates in the upwelling hotspots of the EGAB rival productivities in Benguela and Humboldt currents.