How Seagrass Grows And Why It’s Important

A mature seagrass bed in the Florida Keys. Photo by Cora Berchem, Save the Manatee Club.
A mature seagrass bed in the Florida Keys. Photo by Cora Berchem, Save the Manatee Club.

By Tiare “TJ” Fridrich, Manatee Biologist

Seagrasses are flowering plants that grow in marine environments. They are the only true plant that can live and reproduce entirely underwater. Like land plants, seagrass leaves contain chloroplasts—membrane-bound, subcellular structures that conduct photosynthesis—and use photosynthesis to convert light energy from the sun into chemical energy for growth. Seagrasses have a significant light requirement for their growth. It is for this reason that seagrasses are only found in the shallow, calm, and clear waters of coastal marine environments.

During the process of photosynthesis, chloroplasts absorb carbon dioxide (CO2) and water (H2O). Within the chloroplast, the water molecules lose electrons, becoming oxygen (O2), which escapes the cell, and the CO2 gains electrons, forming sugars, which are then stored in the plant’s leaves, stems, and roots. In the marine environment, the carbon dioxide that plants use for photosynthesis is dissolved in the ocean water; during photosynthesis, seagrasses absorb this dissolved CO2 from the water, which is a form of carbon sequestration. Carbon sequestration is the process of capturing and storing carbon dioxide, a process that prevents it from escaping into the atmosphere. In its gaseous form, CO2 is a greenhouse gas that traps heat within Earth’s atmosphere and radiates it back towards the Earth’s surface. The ocean can help prevent increased warming by absorbing CO2 from the atmosphere wherever air meets the water, absorbing an estimated 25% of the world’s CO2 emissions from human activities. Seagrasses play a very important role in storing this carbon.

Beyond storing carbon in their tissues from photosynthesis, seagrasses can store carbon through trapping sediment (insoluble or non-dissolvable particles) in their leaves. Seagrasses grow and spread in two ways: (1) sexually, by producing pollen and seeds; and (2) asexually, by creating a connected underground network of roots called rhizomes, which can send up new shoots. By cloning themselves asexually, seagrasses can form dense underwater seagrass meadows. Within these meadows, the wide blades of the seagrass leaves reduce the flow of water along the ocean floor, slowing down wave action and allowing sediments to settle down to the bottom. These sediments trap organic matter like decaying leaves, algae, and organisms—all of which are sources of organic carbon—and prevent them from being reabsorbed by the atmosphere. Once the sediment has settled, the seagrass roots and rhizomes in the seagrass meadow act to hold down these sediments, preventing them from being stirred up by the movement of waves and storms.

While seagrass sediments harbor lots of carbon, they are often oxygen-deficient. Oxygen is critical to the process of decomposition, which is the breakdown of carbon-rich organic matter. This lack of oxygen further prevents the organic matter in sediment from breaking down, which is another way that seagrass meadows help to sequester carbon.

Unfortunately, seagrasses around the world are under threat from climate change, sea level rise, human development, and nutrient pollution. Seagrasses have evolved to grow within specific light, salinity, and temperature ranges and may not be able to keep up with the rapidly changing ocean conditions now and in the future. Coastal development has also led to an increase in freshwater runoff and nutrient pollution from fertilizers and septic tanks, which feed algal blooms. Algal blooms, which form on the surface of the water, can block light to seagrass beds and reduce their growth. Oftentimes, the algal blooms also contain toxins that are harmful, and when the alga eventually dies, it will sink to the bottom and smother seagrasses, killing them. Seagrasses are also harmed inadvertently by boats when a propeller, motor, or hull comes into contact with the seagrass bed, cutting the leaves or ripping out the roots. This physical damage, known as a seagrass scar, can take years to recover, and oftentimes, the seagrass beds need assistance to be fully restored.

Florida has lost over fifty thousand acres of seagrass over the last ten years due to algal blooms and urban development. As stewards of the marine environment, we can help protect our seagrass beds by not fertilizing our lawns and taking care to lift our engines when boating near or over seagrass beds. More broadly, reducing our contributions to climate change is critical to the continued survival of seagrass beds around the world.

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