We know more now about the benefits of Seagrass meadows, such as the one above, and can take steps to manage them.
Jim Steele/Planet In The Balance
Seagrass ecosystems enable a wondrous diversity of marine life. Seagrass feeds ancient (but currently threatened) animals like green turtles, manatees and dugongs, sea urchins, parrot fish and geese. Seagrass supports major fisheries of pollock and cod and they’re home to seahorses. The ecosystem serves as a nursery ground for hundreds of species of juvenile fish. Seagrass supports clams, scallops, shrimp and spiny lobsters. Seagrass meadows have also been shown to reduce disease that can infect people, coral or fish. So, recent losses of seagrass have generated great concern and motivated restoration efforts world-wide. However, they are not doomed to collapse. The good news is most of the human factors that have reduced seagrass meadows can be and are being managed. Furthermore, rising levels of carbon dioxide will benefit their growth and recovery.
Unlike seaweeds that are anchored to hard surfaces, seagrass thrives on muddy or sandy bottoms where their roots absorb the rich supply of nutrients stored in the sediments. However, storms and heavy waves easily disturb such habitat. So, seagrass prefers sheltered estuaries, coves and bays. Unfortunately, sheltered waters are also prime real estate for humans and their boats. Much seagrass habitat has been lost to dredging of boat harbors. The chains that anchor boats to their moorings can scour the sea floor as the boats shift with the tides and currents. Nets seeking tasty bottom fish and dragged across the seafloor have plowed up meadows. Fortunately, people are working to prevent such damage by restricting fishing zones or inventing seagrass friendly moorings.
The ancestors of today’s seagrasses were flowering land plants that returned to the ocean a million years ago. To photosynthesize, seagrass colonization was limited to shallow coastlines with clear water and adequate sunlight. Most species prefer water that’s only 3 to 9 feet deep. But to remain at the proper depths, seagrass had to be resilient. Ice ages caused sea levels to rise and fall 400 feet eliminating old habitat and creating new ones. None of today’s seagrass meadows existed 6,000 thousand years ago.
It would have been extremely difficult for seagrass ancestors to successfully invade the oceans under today’s atmospheric CO2 concentration and still photosynthesize. Carbon dioxide is quickly converted to unusable ions after entering the water. Under current concentrations, only 1% remains as vital CO2. However, a million years ago, atmospheric CO2 was more than 7 times greater (3000 ppm) than today (410 ppm).
The biggest evolutionary hurdle for seagrasses was surviving toxic sediments. Seagrass meadows accumulate organic matter as leaves and shoots are grown and shed. Unfortunately, as bacteria decompose that organic matter, they consume all the oxygen. Without oxygen different bacteria convert sulfur molecules into toxic sulfides that could kill the grass. So, seagrasses evolved channels that transported oxygen from their leaves to their roots, creating an “oxygen shield.” Many species evolved symbiotic relationships with specific bacteria and clams. The clams and bacteria benefitted from the grass’ oxygen. The bacteria sheltering in the clams then converted toxic sulfide into harmless chemicals. Seagrass success depends on generating more oxygen than bacterial decay can consume, and that battle explains many recent seagrass die-offs.
As human populations grew and settled along the coast, they altered seagrass ecosystems by clearing the land for lumber and agriculture, or by overgrazing. Increased soil erosion was carried to the sea creating murky ocean waters that reduced sunlight. Runoff of sewage and agricultural fertilizers added nutrients that promoted plankton blooms, which also reduced sunlight. With less light, there is less photosynthesis to generate oxygen. Without enough oxygen, toxic sulfides invade and kill the seagrass. The good news is such lost seagrass ecosystems are not happening everywhere. It is not a global crisis. The losses due to past ignorance of the ecosystem’s natural dynamics are now being repaired. Seagrass meadows with good water quality are thriving and people are now managing sediment runoff better and developing waste-water treatment to reduce nutrient pollution.
In the 1930s along the coast of Virginia, hurricanes and disease had completely denuded several seagrass meadows. Seventy years later the seagrass had yet to return. The problem was without flowering plants there were no local seeds. Seagrass seeds are heavy and quickly fall to the seafloor. Without a seed supply from nearby seagrass, a denuded meadow may take centuries to recover. So, people now harvest seeds from distant healthy patches and sow them where seagrass once thrived.
By maintaining good water quality and minimizing boat-related damage, seagrass meadows are on the mend. Seagrass dependent fish and scallops are recovering. Florida’s manatees have increased 6-fold and are no longer rated as endangered. But manatees need warm winter refuges. So, counter-intuitively, the biggest threat to Florida’s manatees is now the loss of power plants and the warm water discharge that has served as manatee winter sanctuaries.
(Jim Steele is Director emeritus of San Francisco State’s Sierra Nevada Field Campus and authored Landscapes and Cycles: An Environmentalist’s Journey to Climate Skepticism. Contact: email@example.com.)