The three-dimensional structure of seagrass meadows creates many different habitats both on and between leaves, and between shoots on and in the seabed. Seagrass meadows thus contribute positively to biodiversity in coastal areas. Some organisms use the meadows as feeding areas while others use them as a place to hide or as spawning or nursery areas.
While tropical seagrass meadows provide feeding areas for large and exotic animals such as sea turtles and manatees; in Denmark, swans and geese are basically the only large animals that directly feed in eelgrass meadows. Yet, eelgrass meadows in Denmark also houses extensive food webs and indirectly act as a food source – even for fish. Shellfish and snails feed on bacteria and micro- and macro-algae on the surface of eelgrass leaves, and they are eaten by small fish, which in turn are eaten by larger fish.
Eelgrass is also affected by the other organisms in and around the meadows. It is crucial, for instance, that both phytoplankton and coatings of bacteria and algae on the leaves are kept in check by grazing so that light can reach the leaves. In a well-functioning seagrass ecosystem, snails and shellfish graze on the coated leaves, zooplankton and filtrators graze on the planktonic algae, small fish feed on the grazers, and predatory fish feed on the small fish. This is called ‘top-down control’ – as opposed to regulation by nutrient supply, which is called ‘bottom-up control’. But overfishing can upset the balance between primary producers, grazers, small fish and predatory fish. Fewer predatory fish give small fish and jellyfish glory days. They feed on the zooplankton and small animals, which can then no longer suppress phytoplankton and leaf coatings. The result is increased overshadowing of the eelgrass. Burrowing invertebrates such as sandworm can also affect eelgrass by overturning the seabed, thereby burying or uprooting the seeds and germinating seedlings and making it difficult for the new plants to establish.