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Spotlight on coral bleaching responses in Sodwana Bay

By Ameil Harikishun, Department of Zoology and Entomology, Rhodes University and Albrecht Gӧtz, SAEON Elwandle Node
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Seven Mile Reef, showing a mixture of soft and hard corals typical for high-latitude reef communities (Picture: Geoff Spiby)

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The high-latitude reefs of South Africa constitute the southern limit of the geographic range of Western Indian Ocean coral reefs. These reefs are located within the iSimangaliso Wetland Park, a world heritage site that incorporates the St. Lucia and Maputoland Marine Protected Areas.

South African coral reefs are not formed by typical biogenic accretion, but rather form a thin veneer on Pleistocene sandstone. Although not accretive, the coral reefs of South Africa are rich in biodiversity for a high-latitude community.

South African coral reefs are grouped into a northern, southern and central reef complex. Given that the southern complex has sanctuary status (no usage) and the northern complex is isolated from tourist activities, the central complex reefs constitute the recreational diving hub of South Africa.

Coral bleaching in the Western Indian Ocean

The largest mass coral bleaching event in the Western Indian Ocean occurred in 1998, with reefs in the northern region (especially off Kenya, Maldives and Seychelles) having experienced extensive loss of coral cover. The severity of this mass bleaching event decreased in a southerly direction and bleaching was minimal on the high-latitude reefs of South Africa.

It was therefore suggested that South African reefs can be considered as potential refugia (areas in which a population of organisms can survive through a period of unfavourable conditions) from climate change. However, in 2005 a warm-water anomalous event resulted in moderate, but for the region unprecedented bleaching on South African reefs.

Considering that bleaching events have only recently been observed on the high-latitude reefs of South Africa, there is limited knowledge on how environmental stress induces bleaching on these reefs and their potential resistance, adaptation and community change. The warm-water anomalous event of 2005 demonstrated the necessity of a long-term coral bleaching monitoring programme for the region, which has since been running continuously.

In this programme, led by Kerry Sink from the South African National Biodiversity Institute and later joined by Ameil Harikishun, a SAEON Elwandle Node MSc student, the bleaching response is measured using a random visual sampling method in which corals are scored according to categories of bleaching severity. Bleach surveys have been conducted across 17 sites on the central reef complex of Sodwana Bay, with over 18 000 individual coral colonies from over 30 taxa sampled.

Linking coral bleaching to environmental variability

Sodwana Bay experiences small-scale, local upwelling events caused by canyons found on the associated continental shelf. The effect of these small-scale upwelling events on the adaptability and acclimatisation of Sodwana Bay corals is unknown.

It has been proposed that local, small-scale upwelling events during the summer months act as a possible mechanism for reducing bleaching in Sodwana Bay through mediation of higher sea temperatures. Reduced risk of bleaching is suggested to occur in one of two ways - the mixing of deep cooler waters with warm surface waters reduces the magnitude and duration of thermal stress on corals; or indirectly through frequent thermal fluctuations that result in corals developing thermal tolerance over time.

In contrast, it has been hypothesised that acclimatisation to upwelling has lowered thermal thresholds, making corals more susceptible to bleaching during warm-water anomalous events. Multiple fixed temperature loggers have been placed at several sites in Sodwana Bay, the first in 2002, to produce a thermal backdrop in which coral bleaching can be interpreted.

An in-depth analysis of the fine-scale temperature data is currently underway. This analysis will determine warm and cold water events (from local, small-scale upwelling events) in respective years and will offer insights into anomalous events that may have influenced bleaching in particular years.

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Dive team for the SeaKeys Coral Monitoring Programme (divers from left to right: Ameil Harikishun1, 2 ,3, Brent Chiazarri4, Georgina Jones5, Albe Bosman6, Erwan Sola4, Mari-Lise Franken1,8, Kerry-Lee Etsebeth4, Jennifer Olbers5, Kerry Sink1)

1South African National Biodiversity Institute, 2SAEON, 3Rhodes University, 4University of KwaZulu-Natal, 5Southern Underwater Research Group, 6iZiko Museums of South Africa, 7Ezemvelo KZN Wildlife, 8University of the Western Cape

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