Zooplankton are the tiny animals that float around on the surface of the ocean and feed on the microscopic plants that make up the phytoplankton, or on each other. They are so small that they cannot easily be seen and so for most people they don’t even exist. But what would happen if they didn’t?
We have clues from the fossil record. At the end of the Permian there was a huge extinction event that took out an estimated 95% of all living species at the time. Again, at the end of the Cretaceous, there was another massive extinction event which took out not only all the dinosaurs but many other species as well, including the majority of large marine species. The causes of these extinction events are still debatable although there is strong evidence for a meteor strike at the end of the Cretaceous. What does seem evident is that whole ecosystems collapsed because the food chains collapsed. When the plants and animals at the bottom of the food chain died in their millions or even billions, animals further up the food chain died out for lack of food. Even if they survived the initial catastrophe, they starved in the years of dearth following.
This is what would happen to the marine ecosystems if something were to happen to the zooplankton. Even now, this can be seen in the phenomenon known as El Nino, when the currents off South America change. Normally there are areas of upwelling off Peru where nutrient-rich cold waters rise to the surface, which leads to plankton blooms which feed the fish. When an El Nino event occurs, the currents change, the upwelling does not occur, the plankton blooms fail and the fish either starve or move elsewhere. Other animals such as penguins and seals are also affected and many chicks and pups unlucky enough to be born in such years will starve.
Half the whales of the world are baleen whales and they are totally dependent upon phyto and zooplankton for their food. These whales are migratory, going to warm tropical waters to raise their babies. Then they travel great distances to areas of upwelling in colder waters such as the Subantarctic Convergence where plankton thrive on rising nutrients. It takes millions of planktonic organisms in each mouthful to sustain a humpback whale. To sustain an entire population of even one species of baleen whales takes billions of plankton every year. Each whale has to eat enough to sustain itself not only for the six months of feeding but a further six months of travel, breeding and child rearing in nutrient and food-poor tropical waters.
Are phytoplankton enough to sustain the whales? Apparently not. What they need are krill; millions and millions of tiny crustaceans that resemble shrimp. These animals are the basis of the entire Antarctic food chain, supporting the fish, the sharks, penguins, seals and whales. Remove these tiny animals and potentially the entire ecosystem would collapse, with dire consequences to all these life forms.
So are the zooplankton of the world’s oceans in any danger? It doesn’t look like we are going to be hit by a meteor any time soon but there is a danger closer to home: ourselves. As we deplete the oceans of the larger fish, we are fishing lower and lower on the food chain. Now large factory ships are beginning to take krill. Some of it goes for fertilizers and a lot of it ends up in pet food. A few people are making a large profit on these tiny animals. But if we overfish the zooplankton communities, we risk the collapse of the oceans ecosystems and then we will find out just how important zooplankton are.