Seagrasses: important coastal marine plants at risk

This month, SeaWeb is focusing attention on seagrasses, their importance, and the organizations working to preserve them. Sea grasses are not to be confused with seaweeds (algae); seagrasses are actually "flowering" plants much like we find on land - male and female, pollen and seeds. They grow in marine shallows and estuaries where their root structures take in nutrients (which differs from the roots of algae which serve primarily to secure the plant to the bottom).

Seagrasses play a key role in the ocean's ability to sequester carbon via photosynthesis. However, that capability is being impacted by man-made activities. Our current population of approximately 7 billion puts a tremendous demand on agriculture to produce enough food. To do that has required the introduction of powerful, nitrogen-based fertilizers. When these fertilizers make there way to the oceans, with they often do, eutrophication occurs. This is when the excess nitrogen causes an increase in phytoplankton and macro algae. This in turn, blocks sunlight and hampers photosynthesis in the seagrasses. The seagrass beds are weakened and ultimately an entire marine ecosystem can shift from seagrass-based to algal-based.

Coastal urban development can also impact seagrass beds as building and land reclamation can occur in prime seagrass areas. Worldwide, we are currently losing seagrass populations at a rate of 7% on average each year. Adding to that decline are the impacts from climate change. As we begin to see more and more periods of hot and wet weather, the increased rains and floods mean more freshwater flowing into local shorelines, thereby altering the salinity levels that are crucial for the healthy growth of seagrasses.

Mankind is not the only beneficiary of seagrasses. Juvenile sealife, from lemon sharks to scores of fish, find a safe haven within seagrass beds, while sea turtles and dugongs or manatees depend on the plants as a food source.

There are several organizations working to conserve and protect seagrasses. Seagrass-Watch is one such group that monitors and assesses the health of seagrasses in Australia and initiates replanting programs in an attempt to regain seagrass beds that have been lost to flooding. The World Seagrass Association, in conjunction with Conservation International, has produced a learning syllabus for marine management groups and government agencies. While designed to be the foundation for professional training and education, the syllabus does provide the layperson with a good sense of the importance of seagrasses and the threats that these plants are facing. You can click here to download the syllabus and give it a read.

Here's an excerpt:

The Ecological Role of Seagrass Beds Seagrass beds are important because:

• They provide food for macrograzers (e.g. sea turtles) and micrograzers (e.g. sea urchins)
• They provide refuges and nurseries for larvae and juveniles of many fish species (e.g. groupers, snappers, barracudas, yellow grunts)
• They reduce coastal erosion, filter water, and trap sediment
• They produce oxygen and take up carbon dioxide

Food and shelter

• Juvenile fish and prawns mature in seagrass beds
• Lobsters, crabs, and seahorses also live in seagrasses, as do important molluscs such as clams and conch
• Marine herbivores, like dugongs, rely on seagrass as their primary food source, a healthy dugong can eat up to 40kg wet weight of sea grass per day
• Seagrasses are also an important part of green sea turtle diets, turtles eat around 2kg wet weight of sea grass each day
• Coastal waterfowl, such as ducks, geese, and swans eat seagrass too

Sediment and water clarity

• Seagrass leaves trap sediment particles, improving water clarity
• Seagrass roots also stabilize sediments, keeping the water clear
• Seagrass root structure keeps coastal erosion under control

Oxygen and carbon dioxide

• Seagrass takes up carbon dioxide during the process of photosynthesis and converts it to food for the plant
• As a result of this process, seagrasses release oxygen that other marine creatures use
• Oxygen is the major gas that seagrasses pump into sediments, keeping them aerated

There are many forms of marine life and complex ecosystems further out at sea that get a lot of deserved attention and concern. But right along our coastlines, right under our noses, are critical ecosystems that revolve around seagrasses which require our attention as they benefit man as well as the oceans themselves.

Read about seagrasses at SeaWeb.
Read about Seagrass-Watch.
Read about the World Seagrass Association.
Download the Seagrass Syllabus.