Browsing by Author "Porter, Sean"

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    Changes in shallow‑reef community composition along environmental gradients on the East African coast
    (2017) Porter, Sean; Branch, George
    Abiotic determinants of community composition on shallow reefs are seldom examined at a scale transcending biogeographic regions, and knowledge for the western Indian Ocean is sparse. To address this deficit, turnover in reef community composition was quantified, using gradient forest analyses, along gradients of nine abiotic variables collected in situ or via remote sensing at 54 sites across and within three bioregions: Tropical Western Indian Ocean (Tropical WIO), Delagoa and Natal, spanning 4800 km of east African coastline (5.2–31.1°S). All oceanographic variables differed significantly among bioregions. At a scale across bioregions, wave height declined northwards, while temperature increased. Values for suspended inorganic sediments, chlorophyll-a concentration [chl-a], turbidity and nutrients were all lowest in Delagoa, in association with the scarcity of rivers there. The most novel and unexpected finding was that the northwards decline in wave action was more strongly associated with changes in community composition across bioregions than any other variable. Temperature was the second-most important variable at that scale. Within bioregions, temperature and [chl-a] had strongest relationships with community composition in the Tropical WIO. By contrast, in Delagoa, reef profile, susceptibility to sand inundation and reef heterogeneity were most strongly associated with community composition; and in Natal, depth, turbidity and suspended sediment were foremost. Variables of greatest importance within bioregions distilled to (1) those influencing food/energy supply ([chl-a], turbidity, depth and suspended sediments), which were of particular importance in the Tropical WIO and Natal bioregions, and (2) those altering sand inundation and small-scale current dynamics of reefs (profile, heterogeneity and depth), which were leading factors in the Delagoa Bioregion.
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    Iterative mapping of marine ecosystems for spatial status assessment, prioritization, and decision support
    (2023-03-31) Harris, Linda R.; Karenyi, Natasha; Porter, Sean; Kirkman, Stephen; Pfaff, Maya; van Niekerk, Lara; Atkinson, Lara J.; Bernard, Anthony; Cawthra, Hayley; de Wet, Willem; Filander, Zoleka; Green, Andrew; Herbert, David; Holness, Stephen; Lamberth, Stephen; Livingstone, Tamsyn; Lück-Vogel, Melanie; Mackay, Fiona; Makwela, Mapula; Palmer, Ryan; van Zyl, Wilhem
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    Sand-mediated divergence between shallow reef communities on horizontal and vertical substrata in the western Indian Ocean
    (2017) Porter, Sean; Branch, George
    Distinctions are rarely made between vertical and horizontal surfaces when assessing reef community composition yet physical differences are expected because of hydrodynamic differences and sediment accumulation on flat surfaces. As sand often diminishes biotic cover, we hypothesised that vertical surfaces will support a greater biomass but have lower diversity due to domination by a few species. To test this, we quantified sessile communities on vertical and horizontal surfaces at three sites in the Delagoa Bioregion on the east coast of South Africa. Community composition consistently differed: vertical communities were dominated by various filter feeders, especially the ascidian Pyura stolonifera, whereas those on horizontal reef comprised a mixture of filter feeders and various algae. The total number of species and all diversity metrics were significantly greater for horizontal reef surfaces. Contrastingly, Simpson’s dominance and biomass were significantly greater for vertical reef surfaces. Percentage cover of sand explained much of the variation in community composition whereas depth did not. Small scale topographic differences in substratum orientation associated with differences in sand inundation will therefore influence both D and E diversity. Coastal developments and activities that alter sand movements and delivery to the coastal zone are therefore likely to have a profound influence on the maintenance and diversity of shallow subtidal communities.