Seagrasses form expansive underwater meadows globally, acting as a foundational element in marine ecosystems. These flowering plants provide complex habitats, supporting diverse marine life. They are considered ecosystem engineers, actively modifying their surroundings to create unique and productive underwater landscapes.
Major Marine Mammal and Reptile Grazers
Large marine herbivores primarily consume seagrass, acting as important grazers within these underwater meadows. Manatees, often called “sea cows,” are substantial aquatic mammals that rely heavily on seagrass for their diet. These mammals can weigh between 1,000 and 3,000 pounds and consume approximately 15% of their body weight in seagrass daily.
Dugongs, closely related to manatees, are the only strictly marine herbivorous mammals, with their diet consisting almost entirely of seagrass. An adult dugong can eat between 28 and 40 kilograms of seagrass per day, leaving noticeable grazing trails as they excavate entire plants, including the roots.
Green sea turtles are another prominent reptile grazer of seagrass, with sea vegetation forming a substantial part of their diet. These turtles, which can weigh up to 500 pounds, consume approximately two kilograms of seagrass daily. Their feeding method involves cropping the leafy growth while leaving the roots intact, a process that helps maintain the health of the seagrass beds, similar to how mowing a lawn promotes growth.
Fish, Invertebrate, and Bird Consumers
Beyond large marine mammals and reptiles, a diverse group of fish, invertebrates, and birds also feed on seagrass. Some fish species, such as parrotfish and surgeonfish, directly consume seagrass. While many fish use seagrass meadows for shelter and as nursery grounds, certain vegetarian species integrate seagrass into their primary diet.
Invertebrates also graze on seagrass, including sea urchins, conchs, and some crabs. Sea urchins scrape away at epiphytes on seagrass leaves and can also graze directly on the blades. Smaller invertebrates like amphipods and isopods often feed on these epiphytic algae, helping to keep the seagrass blades clean and promoting their growth.
Certain bird species, particularly migratory waterfowl, also forage on seagrass. Brant geese heavily rely on seagrass, consuming shoots and roots. Other species like Eurasian and American wigeons feed on seagrass, diving for leaves, shoots, and seeds in coastal wetlands and estuaries. Their grazing activities contribute to the dynamics of seagrass ecosystems.
The Ecological Importance of Seagrass Grazing
Grazing by various animals plays an important role in maintaining the health and structure of seagrass meadows. This consumption prevents seagrass overgrowth, which can lead to unhealthy beds. By removing older blades, grazers stimulate new growth and improve meadow productivity, similar to pruning terrestrial plants.
Grazing also influences nutrient cycling. As animals consume seagrass, they process plant material and release nutrients back into the water and sediment, making them available for other organisms. This supports the food web and helps maintain the biodiversity of these underwater habitats.
Selective feeding habits can also shape the species composition and genetic diversity of seagrass beds. By consuming certain species or plant parts, grazers can create conditions that favor the growth of others, contributing to a more diverse and resilient ecosystem.
Threats to Seagrass Ecosystems and Their Inhabitants
Seagrass ecosystems and the animals that depend on them face numerous challenges. Pollution, particularly from nutrient runoff from agriculture and urban areas, leads to algal blooms that reduce the light reaching seagrass, hindering photosynthesis and growth. Chemical pollutants, including pesticides, herbicides, and heavy metals, can directly harm seagrass and accumulate in marine organisms, disrupting physiological processes. Plastic pollution, from large debris to microplastics, also impacts seagrass by altering its structure and potentially transferring toxins into the food chain.
Climate change presents a major threat, with rising sea temperatures, ocean acidification, and altered sea levels affecting seagrass growth and distribution. Increased water temperatures can stress seagrass, potentially leading to range shifts and making them more vulnerable to disease.
Coastal development, including dredging and infrastructure construction, directly destroys seagrass habitats and degrades water quality. Direct physical impacts from human activities, such as boat propellers and anchors, also cause significant damage by cutting and uprooting plants. These propeller scars can take years to recover, and continued damage may prevent recovery. The combination of these threats contributes to a global decline in seagrass meadows, with almost 30% of known seagrass areas lost due to human activities.