MIDNIGHT MARGINS
Slopes, Rand, Amathambeka, Imithangala yasolwandle, Skuinstes
Deep dark slopes at the edge of underwater continents with surprising patches of corals and sponges
What are Slopes?
Slopes are the sloping edges or margins where the continental shelf descends into the deep-sea abyssal plains. Although many slope habitats may appear uniform, there are occasional surprising patches of cold-water corals, sponges and other habitat-forming animals.
Globally, continental slopes are often considered to start at a depth of 200m but in many regions, the steeper area where the angle of the shelf changes (shelf edge) is considered to mark the start of the continental slope. In South Africa, the slope is considered to extend from beyond the shelf break to a depth of 3500m where the slope joins the abyssal plains. Both gentle and steep-sided slopes are found in the midnight zone, with the seabed communities of slopes rarely seen by South African scientists.
In contrast to the shallower continental shelf, slopes experience lower temperatures, complete darkness, less oxygen, reduced food availability and higher pressure.
No light reaches the depths of slopes and animals living here obtain food through bottom currents carrying nutrients from shelf ecosystems and from organic matter falling from the sunlit layer.
Who lives on Slopes?
Most slopes are sandy with apparent low diversity but in some areas, there are patches of deep-sea corals or other habitat -forming invertebrates, home to diverse invertebrate communities. In deep cold water coral habitats, suspension feeders like stony corals, bottlebrush soft corals, other soft corals and glass and other sponges form habitats and food for other animals. A higher abundance of animals generally inhabits shallower parts of slope ecosystems with biodiversity generally decreasing with depth.
There are challenges with working below 200m and scientists need special equipment to study the biodiversity of slope ecosystems. However, recent camera surveys on west coast slopes show fishes such as rattails and spiny eels (or rippletails), lantern fish and deep-water sharks and rays. Deep-water crabs and urchins have also been seen.
In South Africa, young South African scientists are studying deep-water corals. The study of deep-water corals is relevant, not only because they play an important role in fisheries, but because corals are sensitive to changes in ocean temperature and acidity. They provide clues to our past and future climate, particularly fossilised corals. They also represent an exciting research opportunity to discover and describe new species.
Why are Slopes important?
Slopes are important in maintaining fisheries and food resources by preserving habitats for commercially harvested species like hake, kingklip and crabs. Scientific exploration to discover more of South Africa’s slope ecosystems is underway, hopefully shedding more light on deep-sea ecosystems. Some habitats on slope ecosystems likely play a key role in carbon and climate regulation and this needs more research attention.
How do we care for Slopes?
Slopes are one of the deepest ecosystems in South Africa and like all ecosystems need management that accounts for their vulnerabilities. To care for slopes, we need to:
Manage slope fisheries – Fisheries are one of the most widespread pressures on slopes. Effective management of fisheries resources and their associated ecosystems is central to effective management of slopes. This is especially important for fisheries that make contact with the seabed, such as trawling for deep-sea fish (hake and kingklip) and crustaceans (deep water prawns, langoustines and crabs). Deep-sea fisheries are vulnerable to overexploitation and deep-sea habitats experience little natural disturbance so are sensitive to activities that can damage the seabed. The coral patches on slopes are often slow-growing and, therefore, slow to recover from damage. These ecosystems are very sensitive to damage as they provide habitat for other animals. Learn more about Vulnerable Marine Ecosystems and help identify, map and manage VMEs.
Increase research to guide management of slope ecosystems – Beyond the shelf edge, South Africa’s slopes are not exposed to many other pressures as yet but more research is needed to better understand these poorly researched ecosystems, detect change and ensure that existing fisheries benefits are maintained.
Protect representative slope habitats -Maintaining areas of slope ecosystems in good condition is essential for further research and maintaining fisheries resources. In South Africa, slope ecosystems are generally poorly protected. There is some representation of slopes protected in a number of offshore Marine Protected Areas (MPAs). These MPAs range from the cold Atlantic waters (Orange Shelf MPA, Southeast Atlantic Seamounts MPA) to the warmer Indian Ocean (Southwest Indian Seamount MPA, iSimangaliso MPA, Port Elizabeth Corals MPA, Amathole Offshore MPA and Protea Banks MPA)
How do we learn more about Slopes?
South African scientists are just beginning to explore and uncover some of the mysteries of slope ecosystems. To research slopes, like other deep ecosystems, scientists need to use special equipment. Scientists also have to invest quite a bit of time to access slopes which can take a long time to reach where the margin is far offshore such as off the West Coast. On the eastern margin, South Africa’s continental shelf is a very narrow shelf and shallow shelf breaks make the slope more accessible. On the south coast, the shelf extends 250m out to sea off the southernmost tip of Africa.
Leila Nefdt
MSc Student, Biological Sciences, University of Cape Town
Studied: Bachelor of Science (University of the Western Cape), Bachelor of Science, Honours ( University of the Western Cape )
What is interesting about the ecosystem you are working in?
My favourite thing about continental slopes is the mystery of it all, especially the deeper portion. Being a transitional zone between the continental shelf edge and the abyssal plains, it is a tricky region to sample, hence there is still so much to explore. I look forward to learning more about what is living along these “areas of change” and how these deep-sea creatures adapt to the physical changes encountered along the slopes.
What are the challenges with working in your ecosystem?
The deep-sea community in South Africa is really small, but active, however, research capacity is currently insufficient. There are many challenges in terms of tackling deep-sea sampling, including the expense of deep-sea research which requires ships that can stay offshore for long periods, limitations in technology (for example the only Remotely Operated Vehicle available for research in South Africa is limited to depths shallower than 300m) and challenging field conditions such as the super-strong Agulhas current. Funding is a major challenge when it comes to deep-sea sampling and research in South Africa, as it costs a great deal to cover the fuel and manpower needed to develop the tools, equipment and expertise to do this kind of work.
What are you working on and why is it important?
For my Masters, I am using underwater photographs of the seabed taken during the ACEP Deep Secrets Cruise in September-October 2016 (facilitated by the African Coelacanth Ecosystem Programme (ACEP) ) to 1) classify marine communities that live on the seabed along the continental shelf edge and slope off the southern margin of South Africa and to 2) set conservation targets for these ecosystems. In doing so, we aim to get a better understanding of the biodiversity patterns and environmental drivers of these communities. At the same time, we hope to increase offshore marine research capacity, our knowledge base and understanding of our offshore environment so that we can better inform management practices as well as protect these areas.
This is the first time that the continental shelf edge and slopes of southern Africa have been explored like this; providing new insights into South Africa’s poorly studied deep-sea ecosystems. Knowing that there will be more information to turn to and having a better understanding of our deep-sea ecosystems, will help us in moving forward with South Africa’s National Biodiversity Assessment (NBA) and improving our marine management as well as contributing toward South Africa’s emerging Marine Spatial Planning processes. This work is important because it allows us to know what kind of communities live on the seabed, where they are most vulnerable and how to protect them. This can guide efforts to expand marine protected areas.
MzanSea 