Aquatic food webs

Big fish eat little fish; that's how the food cycle works. Of course, there’s more to it than that.

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.

Unusually warm temperatures dominate three areas of the North Pacific: the Bering Sea, Gulf of Alaska and an area off Southern California. The darker the red, the further above average the sea surface temperature. NOAA researchers are tracking the temperatures and their implications for marine life.
Unusual North Pacific warmth jostles marine food chain
Warm ocean temperatures favor some species but not others. For instance, sardines and albacore tuna often thrive in warmer conditions. Pacific Coast salmon and steelhead rely on cold-water nutrients

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.

Juvenile coho salmon in the Cedar River.
Food for thought: Stream food web helps salmon growth
The experiment revealed that adding adult salmon tissue increased the productivity of invertebrates, with positive effects that rippled throughout the food web to benefit juvenile coho salmon by increasing their growth and body size.

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 they 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.

Satellite image of chlorophyll, a measure of phytoplankton biomass, in the world’s oceans. The white stars represent the locations of known zooplankton time series, as featured in COPEPOD’s searchable directory of more than 300 marine ecological time series and surveys.
"COPEPOD" database helps world's researchers study ocean plankton
Much like a doctor will check a few key indicators (temperature, blood chemistry, etc.) to assess the overall health of a patient, ocean scientists are looking at plankton to assess the overall health of ocean ecosystems.

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.


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)offsite link and Project FLOW (MI Sea Grant)offsite link

Last Modified: April 2015