The community of microscopic animals and microbes that make up the ocean’s twilight zone upcycle nutrients to ensure their survival in the dark. According to a study conducted by researchers at the University of Hawai’i at Mnoa, zooplankton—small, free-floating animals—primarily rely on microzooplankton—an even smaller group of organisms—to consolidate the few waste products in the water and transform them into food of higher quality. The review was distributed in Limnology and Oceanography.
“Zooplankton have, on occasion, been named ‘watchmen’ of the remote ocean in light of the fact that their taking care of conduct figures out which materials arrive at the sea’s profound inside,” said Connor Shea, lead creator of the review and doctoral understudy in the UH Mnoa School of Sea and Geology and Innovation (SOEST) Branch of Oceanography. “As a result, knowing where these animals get their food helps us understand how the ocean processes and stores carbon dioxide from the atmosphere of the Earth.”
The layer that marks the transition between the ocean’s well-lit surface and its dark interior is known as the twilight zone and lies between 200 and 1,000 meters below the surface. Because it is too dark for plants to grow in this area, the communities that live there are almost entirely dependent on the materials that are made in the water above them.
“Zooplankton have been dubbed the ‘gatekeepers’ of the deep sea because their feeding habits determine which materials reach the ocean’s deep interior. So understanding where these animals get their food is crucial because it helps us understand how the ocean processes and stores carbon dioxide from the Earth’s atmosphere.”
Connor Shea, lead author of the study and doctoral student in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST) Department of Oceanography.
Microzooplankton, which are about the size of a human red blood cell, recycle old organic material that settles from above, most of which is excrement from animals that live at shallower depths. This is how the microzooplankton get their food. In an environment that would otherwise be a food desert for larger organisms, this process concentrates nutrients.
One of the most common euphausiid (krill) species in the North Pacific Ocean is T. inspinata. Credit: Karen Stamieszkin
Food web structure
Shea and his co-creators worked together with a huge, interdisciplinary group of researchers through the Products program to examine the elements of the zooplankton local area in the subarctic Upper East Pacific Sea, in the Bay of The Frozen North. The team looked at nitrogen isotopes in zooplankton samples they had collected. The investigation permitted the scientists to make derivations about how the food web is organized.
Shea stated, “We discovered how the community of zooplankton living in the twilight zone of the Northeast Pacific makes a living, despite inhabiting a notably unproductive region where the supply of food from the surface is exceedingly low.” The twilight zone of the Northeast Pacific is located in the Pacific Ocean.
The microzooplankton, which are often less than a sesame seed in size, are eaten by the zooplankton, which are more nutritious than the debris they consume. The food web in this environment is highly efficient and organized around recycling relatively low-quality food in this way.
Copepods of the family Neocalanus rule the zooplankton local area in the Subarctic Upper East Pacific Sea for a large part of the midyear. Credit: Karen Stamieszkin
“Although we know that microzooplankton exist below the well-lit surface ocean, they are not very abundant, so it has been difficult to determine whether they were an active component of the community in the past,” Shea said. Therefore, it was exciting to learn that they played a significant role in this deep-sea food web.”
The ocean naturally has a lower capacity to absorb some of the carbon dioxide produced by humans in areas of the ocean where the plankton community uses organic matter more effectively, such as the study area. Consequently, understanding how zooplankton communities process carbon, which to them represents food and energy, enables us to comprehend the ocean’s role in absorbing atmospheric carbon dioxide,” Shea added.
More information: Connor H. Shea et al, Small particles and heterotrophic protists support the mesopelagic zooplankton food web in the subarctic northeast Pacific Ocean, Limnology and Oceanography (2023). DOI: 10.1002/lno.12397
