Testing the intermittent upwelling hypothesis: Intercontinental comparisons of barnacle recruitment between South Africa and Australia

Abstract

Recent debates have arisen as to whether productivity and subsequent ecological processes increase linearly with increasing intensity of upwelling, or whether productivity responds more favourably to upwelling of intermediate magnitude, as predicted by the intermittent upwelling hypothesis (IUH). Most studies on the topic take place within eastern boundary systems, where the intensity and frequency of upwelling are high. Here, we test the generality of the IUH towards the other end of the upwelling spectrum, within two regions located at similar latitudes along western boundary currents of two continents. We measured barnacle recruitment and colonisation, which we expected to be linked positively to productivity, across eight rocky shores along the east coasts of South Africa and Australia selected to capture a range of upwelling regimes. Based on Bakun Upwelling Indices (BUI), the four South African sites experienced persistent to intermittent levels of upwelling, whilst the four sites along the east coast of Australia were predominantly downwelling sites with occasional upwelling events. Satellite chlorophyll a concentrations ([Chl-a]) also showed a marked difference between the two continents, with 2–3 times higher concentrations in South Africa than Australia. In situ sea temperature measurements revealed slightly different oceanographic patterns, which were nonetheless compatible with both BUI and [Chl-a] measurements. Barnacle recruitment was typically greater within South Africa and but was generally found to vary unimodally with mean BUI (i.e. being greater at sites that experienced moderate upwelling conditions) and to increase linearly with increasing upwelling frequency between January and April when barnacle larvae are known to be most abundant in the water column. Viewed in isolation, our data provide moderate support for the IUH. But when placed in a broader context, with our eight study locations representing just one end of the upwelling continuum, they provide strong evidence for the IUH.