by Meenakshi Advani Rai, ACIW Mumbai
The Ocean Food Chain
Currents are powerful physical forces in the seas responsible for the functioning and sustenance of the ocean food chain.
Currents redistribute water, heat, nutrients, and oxygen throughout the ocean and directly impact marine ecosystems by inevitably carrying off living organisms that support marine life. With the right nutrients and flow at a suitably shallow depth, where there is plenty of sunlight, such a current itself can become a suitable habitat for photosynthesizing tiny algae called phytoplankton – small plant organisms and primary producers at the start of the food chain of the invisible forest of the ocean. As the population of drifting phytoplankton grows, the water becomes a suitable habitat for zooplankton, which feed on the phytoplankton.
Zooplankton are tiny marine invertebrates, creatures free to “go with the flow,” carried for both short and long distances. Currents become habitats for forage fish that feed on them, 
and in turn a habitat for large predatory fish, and over time they become a moving habitat for multiple types of marine life.
Sponges and coral release volumes of planktonic larvae into the water column when they reproduce. Even the free-swimming animals can benefit from riding currents. Small and weak, the juveniles need currents to carry them to the open ocean feeding grounds, where they grow and mature. Many marine vertebrates such as herring, eels and turtles, hatch in rivers or close to the coast. Kelp forests provide food and shelter for thousands of fish, invertebrates and marine mammal species.
Some Features of Currents
Ocean currents depend on climate, including temperature, the gravitational pull of the sun and moon, warm and cold currents, and wind. We have surface currents and deep water currents. These currents travel thousands of kilometres, in the process establishing a global conveyer belt that brings about different climatic conditions across the world. Ocean currents move vertically or horizontally, also influenced by the land masses that border the ocean, the topography or the shape of the ocean basin.
Marine topographies include coastal and oceanic land forms, coastal estuaries and shorelines to continental shelves and coral reefs. The submerged surface has mountainous features, undersea volcanoes, ocean trenches, submarine canyons, oceanic plateaus and abyssal plains.
How climate is determined by ocean currents
Horizontal currents carry cool or warm water with them over extended distances. It is the displaced water that affects the air by warming or cooling it, thereby transferring the same effect to the land surface over which it blows.
Warm and cool ocean currents
Cold currents are large masses of cold water that move towards the equator, from a level of high altitude to lower levels, while warm currents move further away from the equator, at higher temperatures.
The impact of inconsistencies in currents on marine life
When currents shift, because of plate tectonics or climate change, ecosystems can be thrown into turmoil. Young organisms could be carried into areas where they could not survive, and adults could be bathed in water whose temperature, salinity, or chemistry they cannot tolerate.
Ocean System Degradation
Ninety percent of waste is from city roads and gutters, which go into lakes, streams and rivers, ending up in the ocean.
Biological magnification
Pesticides or heavy metals move up the food chain, as they are incorporated into the diet of aquatic organisms such as zooplankton, which in turn are eaten by fish, which then may be eaten by bigger fish, large birds, animals or humans.
When plastics break down in the sea they become microplastics.
Plastics in the ocean kill or harm more than 300,000 marine animals every year. Fish mistake smaller plastic particles for food and feed on them in enormous quantities. Some starve after doing so, mistakenly believing they have eaten enough because their stomachs are full.
Greenhouse gases increase sea surface temperatures and rising sea level, causing extreme weather events and the loss of coastal protection.This has created irreversible damage such as coral bleaching, fish migration disruption, drowning wetlands, ocean acidification – a disastrous positive feedback loop.
Today, the Arctic is warming twice as fast as anywhere on earth.
The sea ice is declining by more than 10% every 10 years, creating warmer air temperatures and disrupting the ocean circulation. The production of algae – the foundation of the Arctic food web – depends on the presence of sea ice.
As sea ice diminishes, algae diminishes, which has ripple effects on species from Arctic cod to seals, whales, bears, walruses, penguins and other megafauna. Sea ice is a critical habitat for Antarctic krill, the food source for many mammals in the Southern Ocean. Sea ice has diminished and Antarctic krill populations have declined, resulting in declines in the species dependent on the krill.
*Wild fish are used to feed farmed fish, reducing the natural food chain requirement for larger fish. When crowded, tons of fish create a lot of waste and pollute the ocean.
*Overfishing is closely tied to bycatch – the capture of unwanted sea life while fishing for a different species, causing the loss of billions of fish, along with hundreds of thousands of sea turtles and cetaceans.
Overfishing has targeted the largest fish at the top of the food chain
Approximately 90% of fish stocks of large predatory fish are gone. According to the United Nations, 17% of fish stocks worldwide are overexploited; 52% are fully exploited and 7% are depleted.
Marine life depends on climate and its ocean currents
*When the Atlantic Conveyer current works, the dead plankton sink to the bottom, replaced at the surface, with nutrient-rich water that encourages production.
Winds and currents mix, bringing nutrients back up to the sunlit surface waters, without which the phytoplankton would eventually completely run out of nutrients, affecting the entire ocean food chain.
Where Do You Begin?
- Know more about the seafood on your plate.
- Say good-bye to microbeads and single-use plastics.
- Help with cleaning up and keeping coastal communities clean.
- Support marine organizations.
References:
National Oceanic and Atmospheric Administration
All photos courtesy of Unsplash
