Food Webs Are Difficult to Study

Nestled in the upper San Francisco Estuary, where rivers meet ocean waters, is California’s largest tidal wetland: Suisun Marsh (pronounced “suh-soon”) (Figure 1). If you were to dive beneath the water’s surface there, you would be amazed by all the activity you find. Over 50 fish species and countless invertebrates inhabit the marsh [1].

The combination of fishes, invertebrates, and their foods makes up what scientists call a food web. Food webs help scientists describe what animals are eating in a place. Food webs organize what animals are eating by splitting them up into trophic levels. Trophic levels indicate what a plant or animal eats and what it gets eaten by. The number of trophic levels in a place depends on the number and diversity of species that live there.

In a typical marsh, there are four to five trophic levels. The first level includes organisms that make their own food. These are called primary producers. Primary producers in a marsh are typically plants and phytoplankton. They get energy from the sun to produce their own food with the nutrients from the soil, water, and air. The animals that eat the primary producers are the primary consumers (usually tiny organisms such as bacteria). Secondary consumers (such as mysid shrimp) eat primary producers and primary consumers. The next level up is tertiary consumers. Tertiary consumers eat lower-level consumers. A fish that eats mysid shrimp is a tertiary consumer.

How do we figure out what a food web looks like? How do we know what these animals eat and what trophic level they are in? Scientists identify food in animal stomachs to understand food webs. However, because so many animals are small, it is hard to identify the exact foods in their stomachs. This has made it hard for scientists to get a complete picture of a food web. Luckily, a new tool called “isotope analysis” gives scientists a way to map food webs more completely.

Using Isotopes to Study Food Webs

Isotopes are different versions of the same element. To understand what isotopes and elements are, we first need to define what atoms are. Everything around you is made up of tiny building blocks called atoms. Atoms are made up of even tinier pieces called protons, neutrons, and electrons (Figure 2A). Different atoms have different numbers of protons, neutrons, and electrons.

We organize atoms by the number of protons they have and define them as elements. For example, nitrogen is an element whose atoms have seven protons. When atoms of the same element have different numbers of neutrons, however, they are called isotopes of that element. For example, the element nitrogen has two isotopes. Isotopes with more neutrons are heavier than isotopes with less neutrons (Figure 2B).

To know what trophic level an organism is in, scientists use nitrogen isotope analysis. The difference in isotope weights is the key to nitrogen isotope analysis. Organisms process heavy nitrogen isotopes more slowly than light nitrogen isotopes. This means that when they eat stuff with nitrogen in it, they will keep more of the heavy isotopes which are slow and hang around in the gut. Meanwhile the light nitrogen isotopes rush through and leave the body.

Phytoplankton (primary producers) have less heavy nitrogen isotopes than zooplankton (primary consumers). Mysid shrimp (secondary consumers) have less heavy nitrogen isotopes than a striped bass (tertiary consumer) (Figure 2C). This process is very predictable, so looking at nitrogen isotopes allows scientists to figure out how many trophic levels exist in a food web.

To know what animals are eating, scientists use carbon isotope analysis. Carbon is an element whose atoms have six protons. Heavy carbon isotopes have seven neutrons while light carbon isotopes have six. Some primary producers take up more heavy carbon when they photosynthesize sunlight, while others keep more light carbon isotopes.

Once scientists understand which primary producers keep more light carbon and which keep more heavy carbon, they can connect them to animals with similar types of carbon isotopes. So, a plant that has more heavy carbon will pass on more heavy carbon to a primary consumer than a light-carbon plant does for its own primary consumer (Figure 3). By understanding isotopes, scientists can use nitrogen to figure out how many levels there are in a food web and carbon to understand the connections between those levels. Pretty neat stuff!

The Suisun Marsh Food Web

In autumn 2011, scientists from the University of California, Davis, wanted to understand the food web of Suisun Marsh in the San Francisco Estuary. To untangle the food web of Suisun Marsh (Figure 1), they collected samples of algae and plants (the suspected primary producers). They also collected consumers such as zooplankton, clams, shrimps, and fish—anything they suspected could be in the food web. Then, samples were taken to the lab to look at the nitrogen and carbon isotopes each species had.

The results from the nitrogen isotopes showed scientists that Suisun Marsh’s aquatic food web had four trophic levels: primary producers, primary consumers, secondary consumers, and tertiary consumers. Nitrogen isotope analysis confirmed that plants and algae were the primary producers in the marsh. Nitrogen showed only one primary consumer in the marsh – small pileworms. The pileworms had relatively little heavy nitrogen. Secondary consumers had more heavy nitrogen. The secondary consumers were amphipods and mysid shrimp. Striped bass had even more heavy nitrogen, so it was a tertiary consumer (Figure 3).

The results showed large gaps in the food web. Curiously, no primary consumers other than pileworms were found. This likely means we missed other small primary consumers in the marsh (bacteria, for example) because we did not sample them. There were other gaps as well. We know of other fish and invertebrates in the marsh’s waters that we did not sample. For example, even though we did not sample Sacramento splittail, we know from other studies that they eat clams. Since it is so hard to study aquatic environments, sometimes it takes putting together information from multiple studies to get a complete answer to our questions (Figure 3).

The data the scientists got from carbon isotopes showed how the different trophic levels were connected. In Suisun Marsh, more heavy carbon belonged to phytobenthos and submersed aquatic vegetation. Phytobenthos are algae that live underwater on the marsh mud. Submersed aquatic vegetation are plants that grow entirely under water (Figure 3). Brazilian waterweed is an example of submerged aquatic vegetation.

More light carbon was found in terrestrial vegetation, emergent aquatic vegetation, and phytoplankton. Plants that grow only on land are called terrestrial vegetation. Bushes, trees, and grass are examples of terrestrial vegetation. Emergent aquatic vegetation are plants that start growing below water and emerge into the air as they grow. Tules are a member of this group. Phytoplankton are microscopic algae that float around in the water and turn nutrients and carbon dioxide into food.

Once the scientists understood the different carbon types of the primary producers, they looked at the consumers in the food web to see which producers they were getting their carbon from. Following the path of light and heavy carbon isotopes, scientists could trace what the consumers were eating. Many invertebrates (such as zooplankton and clams) ate a lot of phytoplankton, decaying emergent vegetation, and decaying terrestrial vegetation. Other invertebrates (such as amphipods) had more of the heavy carbon isotopes, which connected them to submersed aquatic vegetation and phytobenthos.

Higher-level consumers used all of the carbon sources. Most secondary consumers had more light carbon isotopes. This was true for Black Sea jellyfish, yellowfin gobies, and prickly sculpin. Shrimp, on the other hand, had more of the heavy carbon isotopes. Striped bass, one of the most important fishes in the marsh, fell practically in the middle of the carbon spectrum. This means that striped bass ate a large range of other consumers (Figure 3) [2].

Future Research

Untangling food webs is tricky, but using isotope analysis gets us closer to understanding the mysteries of who eats whom. The knowledge gained from these studies helps scientists predict how climate change and development projects near Suisun Marsh may impact species in the San Francisco Estuary. If these changes wiped out any one species in the food web, everything could change. Predicting how wetland species will react to changes is crucial for management and conservation of these species. Much more work needs to be done to deepen our understanding of how all the plants and animals in this important area are connected so that we can better protect them.

Glossary

Invertebrates: ↑ Invertebrates are animals without a backbone.

Trophic Level: ↑ Describes the overall categories of a food web. Primary producer, primary consumer, secondary consumer, and tertiary consumer are all different trophic levels.

Primary Producer: ↑ Organisms that produce their own food. Plants are primary producers.

Primary Consumer: ↑ Organisms that only eat primary producers. Zooplankton can be primary consumers.

Secondary Consumer: ↑ Organisms that eat primary consumers and primary producers. Amphipods are an example of a secondary consumer because they eat bacteria and aquatic vegetation.

Tertiary Consumer: ↑ Organisms that eat secondary and primary consumers. Striped bass are tertiary consumers that eat amphipods (secondary consumers).

Atom: ↑ A tiny building block of our universe, made up of electrons, protons, and neutrons.

Element: ↑ The type of atom, based upon the number of protons the atom has. For example, the element carbon has 6 protons in each of its atoms.

Isotope: ↑ An isotope of an element depends on the number of neutrons it has. Different isotopes of the same element have different behaviors based upon the number of neutrons they have.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.