Macrocystis (Laminariales, Phaeophyceae) in South Africa: distribution, morphology, and potential susceptibility to warm
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23RD INTERNATIONAL SEAWEED SYMPOSIUM, JEJU
Macrocystis (Laminariales, Phaeophyceae) in South Africa: distribution, morphology, and potential susceptibility to warming events Michael J. Fleischman 1
&
John J. Bolton 1
&
Mark D. Rothman 1,2
Received: 28 June 2019 / Revised and accepted: 28 October 2019 # Springer Nature B.V. 2019
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. Keywords Macrocystis pyrifera . South Africa . Distribution . Morphometrics . Temperature tolerance . Marine heat waves
Introduction Macrocystis pyrifera (Linnaeus) C. Agardh is the largest of the brown algae, a member of the Laminariales (kelps), known in some regions as “giant kelp” (Coyer et al. 2001; Demes et al. 2009; Macaya and Zuccarello 2010). It has
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10811-019-01980-w) contains supplementary material, which is available to authorized users. * Michael J. Fleischman [email protected] 1
Department of Biological Sciences, University of Cape Town (UCT), Private Bag, Rondebosch 7701, South Africa
2
Department of Environment Forestry and Fisheries (DEFF), Private bag X2, Vlaeberg, Cape Town 8018, South Africa
been recorded to reach 60 m in total length (Schiel and Foster 2015). Like other Laminarian kelps, M. pyrifera displ
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