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Studying shallow and deep marine food-webs in temperate South Africa

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A plankton pump is lowered to about 80 metres to sample deep-water microscopic primary producers -- the basal link of the deep-water food-web (Picture: Dylan Howell)

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A baited remote underwater video station consisting of a tripod, a camera with cable to the boat and a bait container to attract fish. The footage of the fish community is analysed to derive an index of abundance. This survey method is independent of SCUBA divers and can be employed to water depths of 100 metres (Picture: Steve Benjamin).

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The bulk of the benthic growth on the reefs is made up of invertebrate animals which can be collected by SCUBA divers and quantitatively assessed by standardised photographic methods (Picture: Steve Benjamin)

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The deep reefs, out of the reach of SCUBA divers, have to be sampled with more sophisticated equipment such as a remotely operated vehicle (ROV; top). A manipulator arm (bottom) is employed to collect deep-water reef invertebrates and bring them to the surface (Pictures: Albrecht Götz and Elodie Heyns)

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Some invertebrates, here a basket star (top) and a sea spider (bottom), are more than three times as large on the deeper reefs compared to their shallow-dwelling counterparts (Pictures: Bruce Donovan and Ryan Palmer)

Albrecht Götz, SAEON and Elodie Heyns, SAIAB

As opposed to terrestrial protected areas, marine protected areas (MPAs) are open systems and characterised by diffuse food-webs due to a higher occurrence of omnivore animals (opportunistic, general feeders that eat both plants and animal material as their primary food source).

As a result, the impacts of spatial closures such as MPAs are difficult to evaluate without information on the food-webs that they aim to protect. This knowledge is also crucial to improve traditional single-species fisheries management regulations such as bag and size limits because of the importance of protecting the appropriate food sources of targeted fish species throughout their life stages.

Baseline surveys

SAEON and the South African Institute for Aquatic Biodiversity (SAIAB) conducted research in the Tsitsikamma National Park (TNP) situated in the centre of the warm temperate biogeographic region of the South African coastline.

The TNP is one of the largest and oldest MPAs in South Africa. Consequently, reef communities within this MPA are thought to be close to pristine, rendering it an ideal location to undertake baseline surveys on food-web interactions for the region.

Due to the difficulties associated with the accessibility of sub-tidal ecosystems, especially deep-water reef communities (deeper than 30m and at the depth limit for SCUBA diving), there is a general lack of baseline information concerning the ecosystem functioning of these communities.

Furthermore, MPAs such as the TNP have been identified as core areas to conserve marine biodiversity. Because they destructively alter the habitats they sample, traditional deep-water survey methods such as dredges and bottom trawls are not suitable for surveying protected areas.

Exploring the deeper reefs

In recent years new technologies such as Remotely Operated Vehicles (ROVs) and Baited Remote Underwater Video (BRUV) have allowed scientists to begin exploring these deeper reefs with minimal impact on habitats.

In the current study, these remote underwater video systems are employed concomitantly to biomarker techniques (fatty acids and stable isotopes) and traditional low-impact sampling methods (plankton sampling, fishing, fish and invertebrate trapping, SCUBA diving surveys and collections).

This allows scientists to describe the shallow and deep reef communities, establish their food-web interactions and determine how these two systems differ in terms of ecosystem functioning.

It is expected that the use of several complementary state of the art technologies will critically improve our understanding of how conservation and fishery resources within the near-shore marine environment should be managed holistically.

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