Aquatic Food Webs

Big fish eat little fish; that's how the food cycle works. Of course, there’s more to it than that. A whirlwind spiral up an aquatic food chain goes like this: Phytoplankton feed the zooplankton that feed the small fish and crustaceans that feed the larger fish that feed the even bigger fish that feed us.

Phytoplankton is the base of several aquatic food webs.
Source: NOAA Alaska Fisheries Science Center MESA Project

Taking it a little more slowly and stopping at each trophic level (feeding level), we start with the primary producers. Microscopic phytoplankton floating in the upper layers of the ocean use the sun’s energy to photosynthesize carbohydrates. These carbohydrates can be eaten for energy, and these plants - mostly diatoms and algae - are the foundation of the majority of the ocean’s biological community. In areas of the ocean where there is not light, some producers can even create energy by using the process of chemosynthesis instead of photosynthesis.

Zooplankton—animal planktonic forms—drift through the water grazing on the phytoplankton. These "grazers" include copepods and larval stages of fish and benthic, or bottom-dwelling, animals that make up the second trophic level. Copepods and other plankton, both animal and plant, nourish filter-feeding organisms that strain their food directly from the water such as bivalves, tube worms, and sponges. This third trophic level also includes other organisms which feed on plankton such as amphipods, larval forms of fish and crustaceans, jellies, and many types of small fish.

Schools of larger fish create the next trophic level. They feast on the smaller fish, wasting as much as they consume. The uneaten fish parts and waste sink to the bottom, where it may be eaten by bottom-dwelling carnivores or decomposed by bacteria and ultimately returned to nutrients usable by plants. At higher trophic levels, these large fish are food for even higher level predators called top predators. Top predators can be birds, reptiles, mammals, or even larger fish and many are opportunistic feeders. This means that they may eat anywhere within the food chain and sometimes they even eat each other.

In reality, many different food chains interact to form complex food webs. This complexity may help to ensure survival in nature. If one organism in a chain becomes scarce, another may be able to assume its role. However, some changes in one part of the food web may have effects at various trophic levels, or any of the feeding levels that energy passes through as it continues through the ecosystem. Humans play an important role as one of the top predators in these food webs. It is our responsibility to ensure that our fisheries are sustainable and that we are not polluting the ocean with toxins that bio-accumulate in food chains.

Educational Connection

Education plays an important role in the health of our aquatic food webs. Whether students live inland or on the coasts, their actions affect the health of one of our major food sources. This collection contains a variety of multimedia, lesson plans, data, activities, and information to help students better understand the interconnectedness of food webs and the role of humans in that web.

Adapted from: Marine Food Web Fact Sheet (RI Sea Grant) and Project FLOW (MI Sea Grant)
Last Modified: 6/15/2011

Features

Fish Predators Decline in Stellwagen Bank National Marine Sanctuary

August 2010 (UNH Gulf of Maine Cod Project)
The numbers of top-level predators in Stellwagen Bank National Marine Sanctuary, such as halibut and swordfish, decreased significantly from population levels 100 years ago. The National Marine Sanctuaries Conservation Series report describes fish populations in the sanctuary that are resilient, but have suffered declines in numbers and species diversity over time... Read More

Scientists Study Sea Trout in Food Web

March 2011 (MS/AL Sea Grant)
Many of us enjoy fishing as a pastime or a livelihood, but have you considered the origins of the fish that you're catching? Kevin Dillon, Ph.D., and Richard Fulford, Ph.D., at The University of Southern Mississippi's Gulf Coast Research Laboratory have. In fact, they've made a detailed study of it... Read More

Decades of Research Find 'Unprecedented' Change in Lake Michigan

September 2010 (NOAA)
The complex network of predators and prey that inhabit Lake Michigan has changed so drastically in recent decades that future trends for the food web are murky. Several studies show that these trends are driven by non-native mussels that invaded Lake Michigan beginning in the late 1980s... Read More