Currents and Marine Life

Introduction

Currents are powerful physical forces in the seas. They move water and heat around the globe, and help determine the chemical make-up of the water column. Currents also are a major factor in ocean ecosystems. Two types of current motion, upwelling and downwelling, strongly influence the distribution and abundance of marine life.

Upwelling

Currents play a huge role in marine productivity, through a process called upwelling. Sea life is concentrated in the sunlit waters near the surface, but most organic matter is far below, in deep waters and on the sea floor. When currents upwell, or flow up to the surface from beneath, they sweep vital nutrients back to where they're needed most.

Nowhere is the link between ocean circulation and productivity more evident than around Antarctica. There, strong currents pump nitrogen and phosphate up from the deep sea to fuel vast blooms of algae and other plants. These plankton are eaten by swarms of shrimp-like crustaceans called krill. Because of upwelling nutrients, krill are abundant enough to feed the largest animals on earth, baleen whales, as well as myriad penguins, seals, and seabirds. In fact, despite the harsh conditions, the biomass of Antarctic krill is thought to be greater than that of any other animal on Earth.

Downwelling

The importance of upwelling to surface organisms is matched by the need of sea bottom life for downwelling, or the sinking of surface water. Surface water can be forced downward by the pressure of the “pile” of water that forms where currents converge or wind drives the sea against a coastline. But for bottom dwellers, the sinking of water caused by density changes is especially noteworthy. The global conveyer belt takes oxygen-rich surface water and flushes it through the deep sea. Without this renewal, the dissolved oxygen in bottom sediments and waters would quickly be used up by the decay of organic matter. Anaerobic bacteria would take over decomposition, leading to a build up of hydrogen sulfide. Few benthic animals would survive such toxic conditions.

In the most extreme cases, a lack of downwelling may lead to mass extinctions. Paleontologists have suggested that 250 million years ago, deep circulation slowed nearly to a stop, and the ocean began to stagnate. Low oxygen, sulfide and methane-rich waters filled the ocean deeps and then spread onto the continental shelves, wiping out 95% of all marine species in the greatest extinction event in Earth history.

Instructions

In this activity, you will explore the differences between upwelling and downwelling. Study the graphics and photographs illustrating upwelling and downwelling, then answer the questions about each process. Maps of the world’s major surface and deep currents are included as resources to help you understand where and how upwelling and downwelling occur.

 


Critical Thinking Question

Currents are important in marine ecosystems because they redistribute water, heat, nutrients, and oxygen about the ocean. At the same time, currents inevitably sweep over and carry off living organisms. Discuss how current flows might affect ocean organisms and species.

Check Answer

Scientists seek to understand and explain how the natural world works. Many of the questions raised in this endeavor have no absolute answers.

Although many sea creatures are powerful, efficient swimmers, many others are ungainly or even immobile. For these animals, currents could offer a free ride. Corals and sponges, for instance, are attached to the bottom as adults, but when they reproduce, they release volumes of planktonic larvae into the water column. These tiny creatures are free to "go with the flow" and could be carried long distances. This could allow individuals to escape overpopulated areas with too much competition for resources, and allow the species to spread into and colonize new habitat.

Once the juveniles settle down, currents will continue to bring other plankton and organic debris their way, providing a steady supply of food.

Even the free-swimming animals can benefit from riding currents. Many marine vertebrates such as herring, eels, and turtles, hatch in rivers or close to the coast. Small and weak, the juveniles need currents to carry them to the open ocean feeding grounds where they grow and mature.

Because currents tend to concentrate food resources in limited areas, such as by upwelling, they make it easier for predators to find prey.

But a reliance on currents - for transportation and for maintaining water conditions - could be the downfall of individuals and populations. When currents shift, because of plate tectonics or climate change, ecosystems would be thrown into turmoil. The young 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.