Benthic-pelagic coupling : rocky intertidal communities and nearshore oceanographic conditions across multiple scales

Doctoral Thesis


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University of Cape Town

In Chapter 1, I characterize geographic patterns in rocky intertidal communities across 6° of latitude along the west coast of South Africa and examine the spatial structure of functional group biomass in relation to wave action and upwelling intensity. Despite between-habitat differences in biomass, most functional groups showed similar regional trends in exposed and sheltered habitats, but weaker non-significant between-habitat associations were observed when considering differences due to site. Divese geographic trends were observed, with only specialized kelp-trapping limpets showing a smoothly decreasing trend with latitide. Abrupt changes in the abundance of several species were observed at about 32° or near 34.5° S. Correlations between functional groups were strongest in the low (exposed) and mid (sheltered) shores, and supported the possibility that species interactions, particularly (1) competitive dominance by filter feeders and gardening limpets and (2) habitat facilitation by filter feeders partially account for local differences in functional-group abundances. Nearshore oceanographic conditions were characterized using satellite-measured sea surface temperature (SST), verification by in situ loggers, and analysis of Offshore Ekman Transport (OET) indices. A clear discontinuity at about 32° S partitioned the coast into a northern region characterized by consistenly strong and spatially homogenous upwelling, and a southern region distinguished by significant mesoscale variation in seasonal upwelling intensity among sites, with clear upwelling centers alternating with "downstream" areas only weakly influenced by upwelling. Functional group relationships with SST differed between north and south regions. Local abundances were generally more variable in the south, where greater biomass of ephemeral and corticated macroalgae, as well specialized gardening and kelp-trapping limpets, were associated with upwelling centers. In contrast, high-shore filter-feeder and predator biomasses were significantly greater at downstream sites. These results suggest that oceanographic discontinuities around 32° S may frame community dynamics and that upwelling intensity influences community structure and generates regular spatial differences in interaction webs along the South West Cape. This study represents the critical first step to identifying spatial scales at which processes regulate communities, and provides a series of testable hypotheses that can be evaluated by experimental comparative approaches.

Includes bibliographical references (leaves 111-148).