Bentho-pelagic mismatches in intertidal sandflats in relation to seasonal upwelling: the role of endobenthic ecosystem engineering

Abstract

Allochthonous transfers are ubiquitous features of marine ecosystems. While these transfers typically subsidise recipient ecosystems and increase donor control, mismatches in allochthony and biotic recipients can also occur. However, driving processes are not well understood, particularly in the context of biotic determinants in coastal ecosystems. Using a 2 yr dataset, we show that allochthonous inputs related to seasonal offshore upwelling on the west coast of South Africa do not lead to strengthening of bottom-up regulation in intertidal lagoonal sandflat ecosystems. Despite an increase in mean phytoplankton biomass by 46.4% at benthic study sites during summer upwelling seasons, benthic primary producer (microphytobenthos) biomass did not increase proportionally, and neither did consumer (macro- and meiofauna) population metrics. Unexpectedly, levels of these response variables were greater during winter downwelling periods than upwelling phases. Correlation analyses indicated that the bentho-pelagic mismatch documented in relation to upwelling may be related to coincidental increases in densities of endobenthic burrowing sandprawnsKraussillichirus kraussi, which are key ecosystem engineers, principally due to rapid sediment reworking and destabilisation. The predominantly negative correlations recorded between benthic response variables and sandprawn density align with our past experimental work, suggesting that the suppressive effect of sandprawns on benthic primary producers and consumers evident in small-scale experiments also manifests over large temporal scales, thereby overriding potential subsidising effects of upwelling. Our findings indicate that in sandflat ecosystems, endobenthic ecosystem engineers that are rapid benthic destabilisers may be important in determining responses to marine allochthonous inputs and the degree of donor control in recipient ecosystems.