- Dona Paula, Goa, India.
- +91-0832- 2450327
|Abstract Submission No.||ABS-2022-06-0125|
|Title of Abstract||WIO-Benth A project to map benthic habitats and communities in the Western Indian Ocean|
|Authors||Sean Fennessy*, Bernadine Everett, Bernardino Malawene, Errol Wiles, Fiona MacKay, Jean-Jacques Bé, Julius Okondo, Martin Ohldieck, Mary Kishe, Mike Roberts, Patrick Vianello, Rui Mutombene|
|Organisation||Oceanographic Research Institute|
Durban, KZN, South Africa
|Abstract||The aim is to describe, map and model continental shelf and upper slope seabed habitats and their benthic communities in the western part of the WIO. The project is supported by WIOMSA, partnered by the EAF-Nansen Programme. While typically vulnerable habitats (corals, mangroves, seagrasses) are reasonably well known in the WIO, little is known about the remaining shelf and upper slope. The project is using existing survey data: acoustic, chart and satellite-derived data on seabed attributes; seawater parameters; and trawled benthic communities. WIO shelf gradients are shallow, and the shelf break, which often originates from submerged coastlines, ranges mainly between depths of 100-150m. The offshore distance of the break varies, with very narrow shelves predominating, apart from near large rivers where it can be up to 140 km offshore; these areas are characterized by depositional centres of finer sediments (mud). There are extensive coral reefs along the coasts of Kenya, Tanzania, Mozambique and Madagascar, but no true coral reefs off South Africa. Most reefs occur within 2 km of the coast and can act as a barrier to offshore sediment flow. However, the vast majority of the shelf and slope comprises unconsolidated sediments of terrigenous origin, though the reefs produce large volumes of carbonate and bioclastic sediment. Seabed habitat types were also mapped based on interpolation of benthic faunal communities through assignment of taxon habitat type preferences.Over the latitudes 2°S to 32°S, and over four depth strata (0-50, 50-100, 100-200, 200-500m), mean seawater surface temperature declined from the tropics, with a peak of around 30°C at 10°S to 13°S and a low of around 22°C at 28°S to 29°S. The latter indicates persistent upwelling in that area, while the former reveals the heating effect of the South Equatorial Current. Mean bottom temperatures, however, only showed this spatial trend for the 0-50 and 50-100 m strata; at 100-200 m depth, temperatures were consistently low (14° to 15°C) from 19°S southwards, and were consistently lower (10° to 12°C) at 200-500 m along the whole region. These patterns were apparent in both surface and bottom water in the austral summer, but in winter were less apparent in surface water, yet persisted in the bottom water. Very warm (ca. 28-30°C) water extended to depths of 80100 m from 2°S to 22°S, whereafter the influence of cooler, deeper water was apparent closer to the surface, particularly from 28oS southwards.Trawled faunal communities (based on frequency of occurrence of benthic taxa only) were dominated by fish families, although many invertebrates were only identified above family level. Depth was an over-riding driver of distribution, with influence too of latitude and seabed habitat. The most frequently encountered family was the Penaeidae (shrimp), followed by lizardfishes (Synodontidae), armoured gurnards (Peristediidae) and cuttlefishes (Sepiidae). On the shelf, penaeids and lizardfishes dominated while on upper slope habitats to 500m, armoured gurnards, cuttlefishes and coffinfishes (Chaunacidae) prevailed. Invertebrates dominated in depths >500 m, notably deep-sea shrimps (Aristeidae), pandalid shrimps (Pandalidae) and lobsters (Nephropidae). Penaeids dominated in shelf trawls in the tropical north (2°S -10°S) and central latitudes (10°S - 16°S), but were replaced by lizardfishes on unconsolidated habitats in the south (16°S - 32°S). On the slope, armoured gurnards dominated throughout, with skates (Rajidae) common in the north, greeneyes (Chlorophthalmidae) in the centre and cuttlefishes in the south.This is an ambitious attempt to map and broadly categorize seabed habitats and associated communities, on the shelf and upper slope of a very large area in the WIO, which will also assist decision-makers with regional marine spatial planning.|