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Tiny Animals of Critical Importance

By Jean-Michel Cousteau and Holly Lohuis

While night diving with the Ocean Futures Society expedition team in the Channel Islands National Marine Sanctuary, Céline Cousteau finds herself in a snowstorm of wandering plankton. Photo: Carrie Vonderhaar, Ocean Futures Society

There is an inner galaxy of alien looking organisms on our own planet. Just night dive in open water with a bright light and witness for yourself the water column coming alive with all different forms of gelatinous plants and animals: plankton. It is an unknown world to most of us, a world of wandering, drifting plants and animals. They provide the base of the complex and extremely productive oceanic food web, on which we all depend in so many important ways.  

The word plankton comes from the Greek word planktos, meaning wandering and drifting. These plants and animals make up a huge amount of the biomass in our oceans, yet their significance in influencing key planetary functions is only now being appreciated. While these mysterious organisms provide opportunities to study the link between their abundance and the health of our oceans, there are many things we do know about the importance of diverse and abundant plankton populations.

Drifters and grazers

Phytoplankton (plant plankton) thrive in nutrient-rich waters and are generally found in sunlit waters. Like terrestrial plants, phytoplankton contain chlorophyll and require sunlight to live and grow. Through the process of photosynthesis, phytoplankton take in carbon dioxide and release oxygen. In fact, as much as 50% of our oxygen comes from phytoplankton. They are one of the world’s most important producers of oxygen and provide important food for the first order of consumers in the aquatic food web, zooplankton. 

Many zooplankton, or animal plankton, are grazers, filtering the planktonic soup of phytoplankton and other floating organic material.  These animals participate in the largest migration on the planet, which happens under the cover of darkness every single day. At night trillions of tiny plankton move from the deep scattered layer well over thousand feet of depth (300+m) towards the surface to feed. Come early morning hours, they then dive down to escape hungry mouths from above. So even though plankton are considered wanders, living a life adrift in the open ocean, they actually make an amazing vertical movement every single day.  

The richness of our seas is directly related to abundance and diversity of drifting plants and animals. They not only provide the base of a rich web of life in the oceans, they are even preyed upon by the largest animals ever to exist, blue whales.  Blue whales filter their favourite food, krill, and copepods through their large baleen plates. During the peak of their feeding summer months, one blue whale can eat up to 8,000lbs (3600kg) of krill per day. 

Trophic cascade

Here is where it gets interesting. One of the more exciting scientific findings in the past half century is the discovery of wide-spread trophic cascades. A trophic cascade is an ecological process that starts at the top of the food chain and tumbles all the way down to the bottom and can have powerful indirect interactions that actually change entire ecosystems for the good or bad, depending on what is removed or brought back into the system.  A famous example is in Yellowstone National Park since wolves were reintroduced in 1995 after being locally extinct for over seventy years. Wolves not only helped control their prey population (deer and elk) from overpopulating and overgrazing, but by keeping their prey populations in check, the presence of wolves helped all levels of a complex web of life in Yellowstone to benefit. Since wolves returned to Yellowstone, the elk and deer are stronger, the aspens and willows are healthier, the grasses taller and the beaver population is thriving.

The same trophic cascade effects have also been well documented in the ocean with the comeback of the great whales, including humpback whales, fin whales, and blue whales. These giant whales eat some of the smallest animals in the ocean, different baitfish and krill. Some countries argued that killing whales is good for us; fewer whales mean more seafood for us to eat. But what scientists actually documented was that as whale populations declined around the world, so did their favourite prey, small schooling fish and krill. What was discovered is that whales not only eat these small prey animals, they also keep them alive. In fact, whales help sustain the entire living system of the ocean.

Some of the large baleen whales feed on krill at the darker depths and rise to the surface where they release vast amounts of fecal matter. These plumes are rich in iron and nitrogen—nutrients that are generally scarce in the surface waters. And these nutrients fertilize the plant plankton in the only place where plant plankton can survive, the sunlight waters close to the surface. Fertilizing the surface waters is not the only thing these whales do by plunging up and down through the water column; they also kick the phytoplankton back up to the surface, giving it more time to reproduce before it sinks into the abyss. 


But today most of the large baleen whale populations are still greatly reduced in numbers compared to just 150 years ago. Only less than 7 percent of ocean wilderness is protected today and marine scientists are recognizing the importance of protecting complete marine ecosystems in keeping all of us healthy and our oceans thriving. With new knowledge of trophic cascades, we can now begin to focus on ocean conservation and recovery efforts on an ecosystem-wide approach. We now know this is essential in maintaining the structure, function, and biodiversity of most natural oceanic ecosystems. 

We owe much to the ocean. My own personal connection to the ocean started at a young age when my father strapped on my back his newly invented SCUBA equipment and allowed me to breath underwater for the very first time. That was back in 1945, 75 years ago. Back then, the understanding of ecosystems’ interconnection and trophic cascades was not well documented in the scientific literature. Since then, we have learned a tremendous amount about our interconnection and dependence on healthy, productive oceans. There is no doubt, we still have much to learn, appreciate and protect. 

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