Eelgrass (Zostera marina) is a submerged flowering plant that forms extensive underwater meadows in shallow, coastal marine environments. Though often mistaken for seaweed, eelgrass is a true plant with roots, stems, flowers, and seeds. These vital meadows are found across the Northern Hemisphere, inhabiting bays, estuaries, and calm inlets from temperate to subpolar regions.
The Importance of Eelgrass
Eelgrass beds are among the most productive ecosystems globally. They serve as crucial habitats and nursery grounds, providing shelter for numerous marine species, including many commercially important fish and invertebrates. Through photosynthesis, these meadows release significant amounts of oxygen into the water, supporting aquatic life. Their dense root systems also stabilize sediments, preventing coastal erosion and improving water clarity by trapping suspended particles. Additionally, eelgrass plays a significant role in mitigating climate change by sequestering carbon dioxide in its biomass and the underlying sediments, a process known as “blue carbon” storage.
Animals That Graze on Live Eelgrass
A diverse array of marine animals directly consumes living eelgrass, forming a direct link in the food web. Among invertebrates, certain species actively graze on eelgrass blades. For instance, some sea urchins, snails, and crustaceans, including certain amphipods, consume the plant material. While many small invertebrates primarily graze on algae growing on eelgrass blades, some amphipods directly consume large quantities of eelgrass leaves and even seeds.
Fish species that directly graze on eelgrass are less common but include certain parrotfish and pinfish in some regions. These fish use specialized mouthparts to feed on the plant material. However, many fish species found within eelgrass beds primarily feed on the invertebrates and epiphytes associated with the plants rather than the eelgrass itself.
Birds, particularly waterfowl, are significant grazers of eelgrass. Brant geese are well-known for their strong reliance on eelgrass, often making it their primary food source, especially during migration and overwintering periods. Other ducks, such as Widgeons and certain scaup species, also consume eelgrass blades or rhizomes.
Large marine mammals also contribute to direct eelgrass consumption. Manatees and dugongs, often referred to as “sea cows,” are iconic grazers that consume substantial amounts of eelgrass. Certain sea turtle species, like the green sea turtle, also feed on eelgrass, particularly as adults.
Animals That Consume Decaying Eelgrass
Not all consumption of eelgrass involves direct grazing of living plants; a significant portion enters the detrital food web. When eelgrass blades die or are shed, they undergo decomposition by microorganisms such as bacteria and fungi. This process breaks down the plant material into nutrient-rich detritus, which then becomes a food source for a different group of organisms known as detritivores.
Detritivores are animals that feed on this decaying organic matter, playing a crucial role in nutrient cycling within eelgrass ecosystems. Examples include various species of worms, amphipods, and certain crabs, like shore crabs. These benthic invertebrates sift through the sediments and consume the decomposing plant fragments. Other detritivores, such as sea cucumbers and some shrimp species, also contribute to this important recycling process. This pathway ensures that the energy and nutrients stored in dead eelgrass are returned to the ecosystem, supporting a wide range of marine life.
Eelgrass and the Broader Food Web
Beyond direct consumption, eelgrass meadows underpin a much wider and complex marine food web. These dense underwater structures provide shelter and foraging areas for countless species that may not directly consume the plant itself. Juvenile fish, including commercially important species like Pacific herring and salmon, seek refuge within the blades, protected from predators during their vulnerable early life stages.
Many invertebrates, such as crabs, shrimp, and various mollusks, inhabit eelgrass beds, finding both protection and abundant food sources in the form of epiphytes or smaller organisms living among the blades. Predatory fish and birds then hunt within these rich environments, feeding on the smaller fish and invertebrates that thrive there. The energy initially captured by eelgrass, whether through direct grazing or the detrital pathway, flows upward through these interconnected trophic levels, supporting the health and productivity of the entire coastal ecosystem.
Protecting Eelgrass Ecosystems
Given their extensive ecological contributions, conserving eelgrass beds is a pressing concern for marine health. These meadows face numerous threats, primarily from human activities. Pollution, including nutrient runoff from land, can lead to excessive algal growth that shades out eelgrass, hindering its photosynthesis. Physical disturbances such as dredging, coastal development, and boat propeller damage directly destroy eelgrass habitats.
Climate change also poses a significant risk, with rising water temperatures and altered ocean chemistry impacting eelgrass survival and distribution. The introduction of invasive species can further disrupt these delicate ecosystems, competing with or directly harming eelgrass. Protecting these vital underwater meadows involves reducing pollution, managing coastal development responsibly, and implementing restoration projects to ensure their continued role in supporting marine biodiversity and coastal resilience.