Open Ocean


Open Ocean, Ulwandle oluvulekileyo, Ulwandle oluvulelekile, Oop See

Connecting ocean basins, ecosystems, people, nations and cultures.

© Steve Benjamin

What is the Open Ocean?

The open ocean is the water column between the sea surface and the seabed and is often called the pelagic zone. Stretching from the ocean surface to thousands of kilometres below sea level, the open ocean spans all across the globe, connecting nations, peoples and cultures. 

This pelagic zone is home to moving species who have to feed, escape predators and find shelter in an open moving ecosystem, regulated by bursts of primary productivity, swirling ocean eddies and dynamic ocean currents.

The size of the pelagic zone increases as you move further from the coast and down the slopes off the continental shelf. Light, temperature, nutrients and oxygen availability decrease with increasing depth and divide the open ocean into four main zones:

The sunlit zone (0 to -200m): Receiving the most light, the topmost zone of the ocean is the realm of tiny photosynthesising algae responsible for half of the world’s oxygen production.  The phytoplankton in the sunlit zone, eaten by tiny animals or zooplankton, are the foundation of complex food webs and are important to the functioning of all marine ecosystems on earth. 

The sunlit zone is the most biologically diverse in the open ocean, and creatures range from tiny plankton, invisible to the naked eye, to the largest creatures on earth – whales, whale sharks and giant fish like the sunfish, marlin and tuna. 

Twilight zone (-200 to -1000m): This zone is less biologically diverse than the sunlit zone as not enough light reaches this zone for photosynthesis to occur. The twilight or mesopelagic zone receives just enough light that the difference between daytime and night-time is still apparent. Animals living in this zone rely on organic matter and waste sinking from the sunlit layer to provide a source of energy. Many animals also move from this zone into the sunlight zone at night and back to hide in the depths during daylight.

Midnight zone (-1000m to -4000m): The midnight zone is one of the least biologically diverse zones in the open ocean because of its lack of light, high water pressure, low temperatures, little oxygen and limited nutrients. Animals living in this zone must be adapted to living at these extremes. 

Abyssopelagic zone (-4000m to -6000m): Like the midnight zone, the abyssopelagic- the water column above the deep abyssal plains, is characterised by the absence of light, extremely high water pressure and low temperatures, oxygen, nutrients and diversity. However, this zone is home to a range of interesting animals adapted to living in the deepest part of the open ocean.

Who lives in the Open Ocean?

Strange creatures, some rarely seen by humans, are found in the expanse of the open ocean – Blue Whales, Giant squid, Oarfish, Sunfish and even large colonial sea squirts called Pyrosomes. 

With decreasing amounts of light, nutrients and oxygen at greater depths, the number of creatures able to live in the open ocean decreases with depth and different creatures characterises each of the different zones.   

In the sunlit zone, Phytoplankton are the primary producers that make food from sunlight and their bursts of productivity fuel the biological processes taking place in the open ocean. Phytoplankton use light to photosynthesize, turning carbon dioxide into oxygen. With no places for attachment, most algae and seaweeds are not able to survive in this zone, the exception being floating brown seaweeds known as Sargassum and the tiny phytoplankton. Feeding on phytoplankton are animal or zooplankton, fish larvae, and commercially important small forage fish like sardines, redeye and anchovy. While fast-moving visual predators (they use their eyes to detect prey) like dolphins, sharks and large predatory fish such as tuna, marlin and swordfish feed on the abundant schools of pelagic fish in the sunlit zone. 

The open ocean is the realm of long-distance wanderers and migratory species that travel long distances between different areas in different life-history phases. Large marine mammals like whales, travel long journeys from their Antarctic feeding grounds to South African waters in winter to mate and calve. Turtles feed offshore and travel the ocean currents but return to the beach where they hatched to nest. Seabirds travel thousands of kilometres from their nesting grounds in the Southern Ocean to feed on schools of pelagic fish in the offshore waters. Filter feeders like barnacles, mussels and worms move through the open ocean as larvae and as passengers attached to larger animals. Some of these filter feeders attach to turtles, whales, ships and sometimes debris like shipwrecks and floating rubbish, using them to travel across oceans.  

The largest migration on earth happens in the open ocean every single day. Some fish like lanternfish, use the twilight and midnight zones to hide from visual predators during the day and perform a daily vertical migration to the upper zone at night to feed and back down at dawn each morning. At greater depths, animals are adapted to living without light and at high pressures. Gulper eels, anglerfish, some shark species and squid can live at these depths. Many have evolved ways to live in the dark.  Some of these animals even glow in the dark. These animals are called bioluminescent and they use light organs to attract prey, mates and defend themselves. Other animals are completely transparent, to blend in with the darkness and avoid predators.

Why is the Open Ocean important?

The open ocean provides us with many benefits in terms of ecosystem services. The ocean is critical in regulating the temperature of the earth by absorbing heat and regulating air temperature. Phytoplankton in the water column also produces an enormous amount of oxygen while capturing (sequestering) and storing carbon dioxide.

Most of the open ocean is a shared space that is used by people around the world. In South Africa, boat-based fishers rely on the open ocean for livelihoods, food and recreation. The small pelagic fishery that harvests sardine and pilchards employs many people and provides high nutrient protein that is rich in healthy fats and oils. Fishing for large pelagic fish like tuna is also an important economic activity. The open ocean is also important for South Africa’s eco-tourism industry. Boat-based whale watching and shark diving tours all generate much-needed revenue for the tourism sector.

How do we care for the Open Ocean?

Most of the open ocean is shared by all nations in the world and actions to maintain healthy open ocean ecosystems require a collaborative effort by multiple governments. 

Reduce and manage pollution – Plastic pollution can affect the survival of birds, fish and mammals and has even been found to interfere with invertebrate survival. Our individual choices and actions can affect the amount of plastic pollution in our oceans. We should be mindful to reduce our plastic waste and recycle what we can. Responsible practices by industry and government to curb pollutants should be encouraged to maintain the health of open ocean ecosystems. 

Manage fisheries resources and avoid overfishing – In South Africa, many commercially important fish species, like sardines, anchovy, tuna and other predatory fish are found in open ocean pelagic waters. Overharvesting of these fish can have knock-on effects on dependent species like seabirds, other fish and sharks that rely on forage fish. Ensuring catches are sustainable for both target species and others that depend on pelagic fish is a key component of effective fisheries management. Innovation in fishing gear to reduce bycatch, the species accidentally caught while targeting other species, is an important part of managing longline fisheries. Individual governments and industries are responsible for the amount of fishing that takes place within South African waters. To ensure fish stocks of species that travel across international borders are maintained for future generations takes cooperation between nations. Regional Fisheries Management Organisations (RFMOs) work together to manage shared fish stocks. No-take zones of some Marine Protected Areas help conserve the seabed and entire water column providing some refuge areas to open ocean species. The Agulhas Front, Southwest Indian Seamount and Orange Shelf Edge Marine Protected Areas (MPAs) help protect turtles, mako sharks, blue sharks and other open ocean species. Parts of the zoned Southeast Atlantic seamount and the uThukela MPAs also protect open ocean species. Scientists need to better understand the connections between the seabed and pelagic parts of the ocean to advise on the vertical zonation of MPAs. 

Monitor and mitigate climate change – Climate change poses a significant threat to the health of oceans. Ocean warming (the increase in sea temperatures) and reduced oxygen are likely to disrupt the functioning of open ocean ecosystems. The distribution of species and the timing of life-history events (like breeding and spawning) are likely to shift. Physical oceanographic features like currents, eddies and sea levels have changed and influenced the frequency and intensity of upwelling events, productivity patterns and species distributions. Ocean acidification, where carbon dioxide changes ocean chemistry by making it more acidic, is a serious threat to the ecological functioning of the oceans. A more acidic ocean erodes and dissolves shells of some plankton, crustaceans and molluscs while affecting the ability of shallow reef-building corals to produce their skeletons.  Individuals can make small impactful changes to reduce their carbon footprints but international cooperation is needed to reduce carbon emissions and mitigate climate change at a global level. Keeping ocean ecosystems healthy helps us cope with climate change.

How do we learn more about the Open Ocean?

Open ocean scientists research many aspects of the ocean. Oceanographers study the physical aspects of the ocean like ocean currents, waves, eddies and gyres. Some study climate change and how higher ocean temperatures have changed how much oxygen is in the water and even how salty it is. Some researchers study where the millions of tiny animals found in the ocean live and their many connections in the food web. Many scientists are still finding new patterns in where animals like whales, dolphins, sharks and turtles migrate and study movement and connectivity patterns. Fisheries scientists use sound to determine the stock size of small pelagic fish and also survey for eggs and larvae. The annual sardine run is one of the most interesting events that take place in the dynamic open ocean with associated movements between sardines and their many fish, seabird and shark predators. All these researchers are all working to uncover some of the mysteries of our pelagic ecosystems.

Caroline Mbelwa

Doctoral candidate at the University of Cape Town and the South African Environmental Observation Network (SAEON)

Studied: Bachelor of Science (University of Cape Town), Bachelor of Science, Honours (University of Cape Town), Master of Science (University of Cape Town)

What is interesting about the ecosystem you are working in?

The physical oceanography of the open ocean is complex as it is influenced by different ocean currents; namely the cold Benguela Current (Atlantic Ocean) and warm Agulhas Current (Indian Ocean), both with distinct water properties which are suitable for a wide range of species. There are also many finer-scale features like eddies and gyres and fronts between different water masses. These oceanographic features drive biology and ecosystem processes and understanding these patterns and processes is intriguing!

What are the challenges with working in your ecosystem?

Challenges in the open Ocean include and are not limited to a shortage of physical and biological data as it is difficult to sample the open ocean due to high costs. Some regions have extreme conditions (too deep, too cold, stronger currents or winds etc.) thus making it difficult to sample in those areas. Ship-based work is very expensive and requires special technology that is often inaccessible to African researchers. We need national and global collaboration to understand the open ocean.

What are you working on and why is it important?

I am working on understanding the connectivity between the oceanographic features observed within the Marine Protected Area network of South Africa. This is important as we are able to have a better representation of the physical oceanography within marine reserves. The different water bodies and processes that drive biological diversity need to be represented in our MPA network.

Mzansea: revealing south Africa's marine ecosystems Logo

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