Seagrasses are a small group of around 60 flowering plants that mainly live in shallow waters from the tropics to the arctic. Although seagrasses are often confused with seaweeds, they are two very different groups; while seaweeds are relatively simple organisms that evolved from single-celled algae, seagrasses evolved from complex land plants and have leaves, stems, flowers, and roots. In fact, seagrasses are the only flowering plants that live their entire lives underwater. In a healthy ecosystem these grasses form vast meadows or beds, which are one of the most productive ecosystems in the world.
Seagrasses are important on many different levels as habitat, as part of the food chain, and as water quality control. We even rely on them too: many commercially important species, including Maryland’s blue crabs, depend on seagrass beds for shelter and nurseries.
By providing food and shelter for a wide variety of marine animals, seagrasses drastically increase local biodiversity. Many invertebrates, such as clams, worms, crabs, sea urchins, sea cucumbers, and sea stars live in or on top of the soft sediment of seagrass beds. Seagrasses are also an important source of food for many different herbivores such as some species of sea turtles, sea urchins, and herbivorous fish.
Often, seagrass meadows are used as a nursery for juvenile animals. The structure and complexity of the beds create a shelter from predators, keeping young animals safe from harm. The location of seagrass beds also deters predators because they are less likely to forage in shallow waters. Additionally, the dense meadows absorb wave energy by providing resistance against the moving water. The tall fronds act as a refuge from strong currents, providing a safe, calm space for tiny juveniles to mature.
Just like trees and grasses on land help hold soil in place, the complex roots systems of seagrass beds help stabilize the sea floor. This stabilization, along with the reduction of wave energy, helps protect eroding coastlines from wave action due to currents and acts as a buffer against major storms.
Seagrass beds are also important for determining water quality and the overall health of an ecosystem. Seagrass traps fine sediment and particles that are suspended. The leaves slow water flow, which allows more sediment to settle to the bottom. As a result, the clarity of the water increases, which allows more light to penetrate. This light promotes growth of seagrasses, which improves oxygen levels in the water. When seagrass make their food through photosynthesis, they produce oxygen that is vital to the health of other marine organisms. Just a square meter of seagrass can release 10 liters of oxygen in a single day!
Seagrasses recycle a lot of nutrients through decomposition and reabsorption. When seagrass decomposes, nutrients are released and reabsorbed by surrounding seagrasses and phytoplankton. Seagrass beds also filter out nutrients that come into the water through industrial discharge and agricultural and stormwater runoff. This filtration protects other habitats that are much more sensitive to this kind of nutrient loading. These beds also play an incredibly important role as carbon sinks. Carbon sinks are natural environments that suck carbon dioxide from the atmosphere and store it for a long period of time.
It is extremely important to protect and preserve these habitats because of the important roles they play and because we are losing seagrass at an alarming rate. Existing seagrass beds are declining at a rate of around 1.5% per year. Their losses have a major effect on marine biodiversity and on humans. Watermen whose livelihoods rely on a big harvest need seagrass beds to keep their catch coming back year after year. A number of threatened species, such as green sea turtles and some species of seahorses, also rely heavily on sea grasses and would go extinct without them. Seagrasses are vulnerable to excessive nutrient loading, diseases, temperature rise, and human activity such as dredging, overfishing, and trawling.
However, recent restoration plans have been put into action. For example, the Virginia Institute of Marine Science planted 7.65 million seagrass seeds in in the Chesapeake Bay and off the coast of Virginia in 2014. In our Coastal Bays, there has been a slight increase in coverage, but we are still 10,000 acres short of our peak in 2001. Seagrasses here are suffering greatly and most of the bays have an F grade in this category. That being said, there are always ways you can help. Since seagrasses are affected by excessive nutrients, it is important to be mindful of the amount of fertilizer used on your lawn. In addition, knowing where seagrass beds are located and avoiding them while boating will prevent scarring and can help coverage increase.