Patterns and potential environmental drivers of mesophotic communities of the warm temperate shelf of the Amathole Region, South Africa

Adams, Luther A

Patterns and potential environmental drivers of mesophotic communities of the warm temperate shelf of the Amathole Region, South Africa

Master Thesis

2021

Abstract

Foundational biodiversity research has seen a recent shift from the collection of epibenthic data using destructive methods to less destructive methods that include visual surveys- using underwater camera platforms to explore the seabed. South African mesophotic ecosystems are under-sampled compared to both their shallower and deeper counterparts. The Amathole offshore region, considered as a transition zone between the Agulhas and Natal ecoregions, is a historically unexplored region of the South African coastline. This thesis aimed to define and describe the benthic communities and identify the processes driving their distribution on the temperate shelf in the Amathole offshore region, using a Remotely Operated Vehicle (ROV). This study piloted the application of the Australian developed CATAMI classification scheme to annotate images collected by ROV in South Africa. Data were collected on the ACEP: Imida Frontiers project on board the RV Phakisa during January and May 2017 off the Kei River, Amathole offshore region. This survey combined 14 sites comprising 215 images from remotely operated vehicle imagery and nine environmental variables from 30 to 100 m water depth. Multivariate analyses (multidimensional scaling and a cluster dendrogram) of image data produced nine benthic communities. Similarly, multivariate analyses (principal component analyses, distancebased linear model and distance-based redundancy analysis and constrained binary divisive clustering analysis) of the environmental data revealed that substratum type and correlates of depth to be the main variables likely responsible for the observed biodiversity patterns. Additionally, the LINKTREE analysis revealed a depth break at 74 m which established the boundary between the upper and lower mesophotic zone in this region. Rhodolith bed communities were discovered in the upper mesophotic and are a welcomed novel ecosystem type for South African benthic ecologists. The upper mesophotic zone was also characterised by communities of dense brittle star aggregations and reefs dominated by macroalgae. The lower mesophotic zone was characterised by animal forests consisting of communities dominated by sponge gardens with diverse growth forms and dense stands of canopy forming gorgonians. This thesis provides recommendations for future research and guidelines for future ROV field sampling. It also highlights the need for greater sampling effort by ROV on unconsolidated substratum and at depths greater than 74 m. The use of morphospecies in image classification to define macrobenthic communities on an unexplored continental shelf was effective despite limited knowledge of species. Similarly, the environmental variables that structure these temperate shelf communities were identified. Information from this study contributed to the foundational biodiversity information needed to inform marine spatial planning and spatial management efforts for the newly proclaimed Amathole Offshore Marine Protected Area and the greater Amathole offshore region.

Keywords

Biological Sciences

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