Macrocystis (Laminariales, Phaeophyceae) in South Africa: distribution, morphology, and potential susceptibility to warming events

Abstract

Macrocystis pyrifera (Laminariales), widely known as “giant kelp,” is dominant on temperate coastlines of the eastern Pacific and the Southern Hemisphere. It is recorded in South Africa, but with very little information available on its distribution and ecology. Here, we document the known distribution and provide an account of the basic population structure and morphology of three populations of South African M. pyrifera. It is confined to sheltered sites inside forests of the dominant kelp (typically Ecklonia maxima), along only ca. 200 km of coastline on the cool temperate southern west coast. It is absent from most of the Benguela upwelling system further north on the west coast and also from the adjacent warm temperate conditions to the east. This “angustifolia” ecomorph in South Africa generally only grows to 2–3 (maximum 10) m in upright length and has much higher upright density than in global regions where the large “pyrifera” ecomorph is dominant. A series of controlled thermal stress experiments was also carried out to investigate the upper limit of thermal tolerance of photosynthesis of South African M. pyrifera, in comparison with the locally dominant kelp E. maxima. Thermal stress experiments found mean oxygen production by M. pyrifera to be greatest at 15 °C, but declining above 17.5 °C. In comparison, mean oxygen production by E. maxima was greatest at 17.5 °C, but was significantly reduced at 22.5 °C. It is likely that temperature tolerance determines the eastward limit of the species in South Africa, but other factors, probably including high wave action, limit its north-westerly spread. These findings highlight the potential vulnerability of the species in South Africa to documented anomalous warming events such as marine heat waves.