Identifying the plants that are at the base of an animal’s food web can be difficult. If the animal can be observed directly feeding on plants (for example, elk consuming grasses or moose consuming willow), then it is relatively straightforward. But for predators such as eagles, halibut, otters and bears, it is more difficult to get at the origin of the food web.
For instance, the otter may be consuming crabs and clams, but what are the crabs and clams eating? It is usually impossible to determine the source of plant material by examining stomach contents of predators. However, plants carry unique characteristics that are incorporated into animal tissue by the animals that consume them. The old adage “You are what you eat” is true. Scientists have developed a method for tracing food webs from plants to predators, which relies on the unique isotopic composition of carbon and nitrogen in different plant groups, which is carried over into animal tissue. The isotopic composition, or signature, of an animal can tell us two things: 1) what plant groups are providing most of the production supporting the animal, and 2) how many steps, or trophic levels, are between the plant and the animal.
We have conducted a food web study of the plants supporting the production of littleneck clams and mussels at several locations throughout Kache-mak Bay. These bivalves are primarily filter-feeders and acquire their food by filtering food particles from the seawater they process. Our data indicate that littleneck clams in particular selectively assimilate the smaller diatoms that occur on the sediment surface, rather than the larger microalgae that occupy the water column. These small diatoms are suspended into the water column just above the sediment surface, and are available to the clam’s siphon. Although the small benthic diatoms represent a small proportion of the microalgae available in the water column, it is clear that clams selectively ingest and assimilate them. Other bivalves, including mussels and butter clams, also have an isotopic signature that indicates a heavy reliance on benthic diatoms, rather than water column phytoplankton. Predators, which consume these shellfish, also have a food web based on benthic diatom production.
These results suggest that aspects of the ecosystem, which support high rates of primary production by benthic diatoms, are important for the maintenance of shellfish production in Kachemak Bay. Diatoms living on intertidal flats and in subtidal sediments are particularly susceptible to changes in water quality and the availability of light. Changes in water quality, coastal land use, water depths, bay circulation and/or temperature could result in changes in benthic diatom productivity, and alterations to the Kachemak Bay food web.
Carolyn Currin is a research microbiologist located at the National Oceanic and Atmospheric Administration’s Center for Coastal Fisheries and Habitat Research in Beaufort, N.C. There she is the leader of the Coastal and Estuarine Ecology Team. Funding for the research in Kachemak Bay came from NOAA’s National Ocean Service. If you have questions about Kachemak Bay, contact reserve staff at 235-4799 or visit the Web site at www.kbayrr.org.
Sponsored by the Kachemak Bay Research Reserve
Artwork provided by Catie Bursch
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