Ecophysiology of intertidal corals along the east coast of South Africa : coping in marginal habitats.

Abstract

Coral survival in a time of climate change will depend largely on the ability to tolerate, acclimatise and adapt to changes in their natural environment. High-latitude intertidal corals along the east coast of South Africa withstand extreme temperature fluctuations, and endure highly marginal conditions for coral survival and growth. Intertidal corals have largely been ignored throughout years of coral research, yet they provide scope for understanding adaptation and acclimatisation in marginal conditions. With focus on two scleractinian species occurring in rock pools, Pocillopora verrucosa and Anomastrea irregularis, the aim was to compare population size distributions and physiological responses (zooxanthellae density, chlorophyll a and lipid content) to season and latitude. Heterotrophy as a coping mechanism was also investigated. Five sites along the KwaZulu-Natal (KZN) coastline were sampled to determine latitudinal differences, from Sodwana Bay (27°S) to Munster (31°S), the southernmost point of coral distribution in this study. Rock pools experience large temperature fluctuations over short time periods, emphasising the acute thermal stress endured by corals. At spring tide temperatures can fluctuate by more than 10°C over one tidal cycle. Temperatures in rock pools along the coast are a function of rock pool size and depth rather than latitude, with short-term fluctuations not seeming harmful to corals. Coral population structure provides insight into the life history, juvenile input and mortality of these intertidal corals. High variability in size distributions between sites suggests that different disturbances are acting on intertidal coral populations and localised conditions are likely structuring these communities, nevertheless, populations see relatively stable and likely to persist in these habitats. Zooxanthellae density and lipid content decreased from summer to spring in P. verrucosa, whereas A. irregularis showed minimal seasonal patterns. Chlorophyll a was highest in winter in both species, possibly in response to low temperatures and light. Differences in δ¹³C and δ¹⁵N between coral tissue and zooxanthellae suggest that heterotrophy contributes more to the metabolism of A. irregularis than P. verrucosa. Isotope signatures also reveal that corals at Sodwana Bay are either feeding less or comprise a different diet to those at other sites. Lipid utilisation and heterotrophy may comprise important energy sources in coping with marginal conditions of the intertidal zone. Rock pools may thus constitute and important habitat for coral survival, growth and reproduction, allowing the southward extension of their range, which will be imperative during times of local and global environmental change.