MOUNTAINS UNDER THE SEA
Seamounts and Ridges, Iintaba eziphantsi kolwandle, Izintaba namagquma angaphansi kolwandle, Seeberge en heuwels
Productive peaks where migratory animals feed and rest
What are Seamounts and Ridges?
Seamounts, ridges and plateaus are the mountains, mountain ranges and elevated parts of the deep-sea floor. These high points are set apart from the surrounding seabed by their topography and the high diversity of animals associated with their often rocky and steep sides. Like oases in the desert, they provide stopping places for migratory animals and are spawning and breeding sites for fish.
Seamounts are mountains below the sea and were formed millions of years ago by volcanic activity. These mountains do not reach the surface, like islands, but may span multiple depth zones rising from the deep to the sunlit or twilight zone. Ocean currents well up around seamounts, delivering nutrients and plankton. As important habitats for open ocean animals, fish and invertebrates, seamounts have high biodiversity and often higher abundance and biomass of animals than in the surrounding area. Animal communities on the seabed of seamounts often form complex three-dimensional habitats that provide homes for many species.
Ridges are generally considered as mountain ranges beneath the sea and be very large or smaller features. Large ridges may distinguish ocean basins like the mid-Atlantic ridge, which separates the African continental plate from the South American Continental plate. South Africa has one small ridge feature in its mainland territory; the Port Elizabeth Ridge also referred to as the Kingklip Ridge. This ridge lies off the coast of Gqeberha (formerly known as Port Elizabeth) and is protected within the Port Elizabeth Corals Marine Protected Area. This 40 km long narrow ridge creates a unique and productive seascape environment and has high biological diversity.
Plateaus are large flatter pieces of seafloor raised above the abyssal plain. South Africa has only one plateau in its ocean territory, the Agulhas Plateau, a feature that formed more than 90 million years ago. The Agulhas Plateau is a key remnant of the ancient breakup of the supercontinent Gondwana. It formed as the moving plates passed over a volcanic hotspot, similar to how the Hawaiian Island chain formed. The Agulhas Plateau developed near what is known as the “Triple junction” where Gondwanaland broke up into Africa, Antarctica and South America. It rises 2 500 metres above the surrounding seafloor which descends to depths of below 5000 metres The plateau covers an area of about 300 000 km2. In South Africa, part of the Agulhas Plateau is protected within the Agulhas Front Marine Protected Area.
Who lives on Seamounts and Ridges?
South African scientists have yet to study the seabed biodiversity of the seamounts within our territory. Elsewhere, seamounts are characterised by amazing communities of fixed or sessile animals attached to the seabed, rich fish communities and open ocean wanderers. With limited light, invertebrate suspension feeders, predators and migratory species characterise Seamounts, Ridges and Plateaus.
Invertebrate filter feeders and suspension feeders like sponges, soft corals, sea fans and stony corals live on the hard substrate of their rocky slopes. Many of these filter-feeding animals provide habitats and hiding places for animals like mussels, crabs, lobsters and brittle stars. Sea pens, able to anchor in sand and mud, and even on rock (as per recent discoveries) inhabit parts of these features. Although they are less common, burrowing tubeworms, sea cucumbers and other deposit feeders live on the soft sediment areas of seamounts and plateaus.
The diverse invertebrate communities attract dense aggregations of fish. Commercially exploited fish like Orange Roughy occur or spend parts of their life history associated with seamounts. The ridge off Gherberha is part of an important breeding area for kingklip, another important commercial fish species. Sharks in turn are attracted by the abundance of fish life at these productive hotspots. Other long-distance wanderers like turtles and seabirds use seamounts as feeding and resting places on their long travels due to the abundance of food found at these special sites.
The Kingklip Chorus
Kingklip (Genypterus capensis) are species of cusk eels and are a popular and very valuable seafood in South Africa. They stay close to the seabed and love hiding in holes or even burrows. They are caught as bycatch in the hake trawl and longline fisheries
© Robyn Adams
Like many other cusk eels – Kingklip are able to sing. They use drumming muscles to call to each other across the ocean. Males and females each have their own calls and sound different to one another, much like altos and tenors.
In South Africa, scientists have found large numbers of Kingklip aggregating at the Port Elizabeth ridge. Scientists suspect that the ridge is an important place for spawning, when fish release eggs and sperm into the water. This making the PE ridge a special place for the conservation of this fish
Why are Seamounts and Ridges important?
Seamounts, Ridges and Plateaus provide homes to amazing deep-water animal communities. They are incredibly productive ecosystems that attract whales, turtles, sharks, seabirds and many other threatened species. Healthy seamounts, ridges and plateaus can help maintain fish stocks for future generations. The diversity associated with steep rocky features sustain fish populations like kingklip and are important as spaces for breeding and spawning.
Seamounts may also help animals adapt to climate change. As temperatures change, animals will be able to travel to a depth zone that suits them best. This could help maintain communities that may not be able to survive in other areas with fewer options. New work is underway to help understand how seamounts help store carbon to contribute to a stable climate.
Kingklip
How do we care for Seamounts and Ridges ?
South African seamounts are relatively remote and experience limited pressure. A few long liners fish for large pelagic (open ocean) fishes in the vicinity of seamounts. Elsewhere, seamounts also experience pressure from trawling, mining and petroleum extraction.
Manage activities that will impact the seabed- The animals living on seamounts, ridges and plateaus are all sensitive to seabed damage as they are slow growing and slow to recover after damage. The animals like cold-water corals, soft corals and sponges form habitats (see Vulnerable Marine Ecosystems) and are critical to maintaining biodiversity. Mining, petroleum activities and bottom trawling should all be regulated and managed in these sensitive areas. Prospecting and seismic surveys are a disturbance to animals and may cause physical damage to muddy ecosystems. Drilling activities pose pollution risks that can harm animals. Petroleum operations may introduce alien species which impacts biodiversity. Noise pollution can change feeding and communication behaviours of certain species. Representative areas should be safeguarded in protected areas. The regulation of these activities is done at the government level.
Increase research to guide management of seamount, ridges and plateau ecosystems – no research has been done on South Africa’s seamount ecosystems. We need to map and understand these ecosystems, provide baselines to detect change and provide advice for effective management. Facilitate international collaboration and local research on seamount ecosystems.
Protect representative slope habitats- Seamounts and especially their Vulnerable Marine Ecosystems should be safeguarded in Marine Protected Areas. Some of our Seamount ecosystem types are protected in the South East Atlantic Seamounts MPA and the South West Indian Seamount MPA. The Kingklip or Port Elizabeth Ridge is protected in the Port Elizabeth Corals MPA and parts of the Agulhas plateau is protected in the Agulhas Front MPA.
How do we learn more about Seamounts and Ridges ?
Mari-Lise Franken
MSc candidate (UCT) working on Vulnerable Marine Ecosystems
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?
Seamounts are one of the most fascinating ecosystems because of their concentration of life, their ecology and the many species they support. Rising up from the depths they are hotspots of high biodiversity on the more uniform slopes or abyss of the deep sea. They have swirling currents and attract open ocean animals to feed in their productive waters. Seamounts are often colonised by habitat-forming animals like deep sea sponges, sea fans, back corals and deep water stony corals. These long lived fragile animal forests are considered to be indicators of potential Vulnerable Marine Ecosystems that are sensitive to the impacts of deep sea fisheries.
What are the challenges with working in your ecosystem?
South Africa still needs to develop the technological and human expertise to research seamount ecosystems with limited opportunities to participate in this type of research. Seamounts are far offshore and remote, so it is very expensive to get to these habitats in addition to the equipment and technical expertise needed to map and survey seamounts. Recent planned seamount expeditions were cancelled due to the impacts of Covid 19 and South Africa needs to develop new research partnerships and international collaborations to start planning seamount research.
What are you working on and why is it important?
I am researching Vulnerable Marine Ecosystems or VMEs. These are animal communities that are easily disturbed by human activities, and are slow to recover, or which may never recover if impacted. Cold-water coral reefs, octocoral gardens and deep-sea sponge reefs or fields are widely recognized as VMEs but features such as seamounts, offshore banks and submarine canyons are considered very likely to host VMEs. My research is important because of the role that these animal forests play in marine ecosystems and their sensitivity to human impact. Mapping areas of high densities of these animals and predicting where they are likely to occur is essential to effective management needed to maintain the benefits that animal forests deliver to humans and other species. They store carbon, help regulate energy flow, provide homes for eggs, young fishes and adults and are some of the beautiful ecosystems you can imagine. Fisheries need to manage VMEs to achieve eco-certification which helps ensure that fisheries are sustainable and also provides additional economic benefits through access to international and more valuable markets.
Acknowledgements
The ACEP Deep Forests (NRF Grant 110765) and the UKRI funded One Ocean Hub are acknowledged for helping build the capacity and knowledge base for better management of seamounts and for supporting science engagement and ocean literacy to share this information
MzanSea 