Seagrass – Watts Up With That?

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Seagrass – Watts Up With That?

by Jim Steele

modified online version printed in BattleBorn Media newspapers

Seagrass ecosystems enable a wondrous variety of marine life. Seagrass feeds ancient (but currently threatened) animals such as green turtles, manatees and dugongs, sea urchins, parrot fish and geese.

Seagrass supports the main fisheries for pollock and cod and is home to seahorses. The ecosystem serves as a breeding ground for hundreds of young fish species. Seagrass supports clams, scallops, prawns, and lobsters. Seagrass meadows have also recently been shown to reduce diseases that can infect humans, corals or fish. The recent loss of seaweed has caused great concern around the world and motivated restoration efforts.

Yet they are not doomed to collapse. The good news is that most of the human factors that have reduced seagrass beds can and will be remedied. In addition, rising levels of carbon dioxide will benefit their growth and recovery.

Unlike algae, which are anchored on hard surfaces, seaweed thrives on muddy or sandy bottoms, where its roots absorb the rich supply of nutrients stored in the sediments. However, storms and strong waves easily disrupt this habitat. Seagrass therefore prefers protected estuaries, bays and bays. Unfortunately, sheltered waters are also a prime property for people to house their boats. Much seagrass habitat has been lost through the dredging of boat harbors.

The chains that anchor boats to their moorings can chafe the ocean floor as the boats move with the tides and currents. Nets that look for tasty bottom fish are pulled across the seabed, but they also plow seagrass beds. Fortunately, people are working to prevent such damage by restricting fishing zones or inventing seagrass-friendly berths.

The ancestors of today's seagrasses were flowering land plants that returned to the ocean a million years ago. For photosynthesis, seagrass colonization was limited to shallow coasts with clear water and sufficient sunlight. Most species prefer water that is only 3 to 9 feet deep. But to stay at the correct depth, seaweed had to be resilient. Ice ages caused sea levels to rise and fall 400 feet, eliminating old habitats and creating new ones.

None of today's seagrass meadows existed 6,000,000 years ago. The Florida Bay of the Everglades was founded 4,000 years ago. Since then, seagrass meadows have been blooming and disappearing at regular intervals, but are now the largest.

It would have been extremely difficult for seagrass ancestors to successfully penetrate the oceans under today's atmospheric CO2 concentration and still carry out photosynthesis. Carbon dioxide is quickly converted into less usable ions after entering the water. At the current concentrations only 1% remains as vital CO2. A million years ago, however, plants bloomed under increased atmospheric CO2, which was up to seven times higher (3000 ppm) than it is today (410 ppm).

The greatest evolutionary hurdle for seagrass has been the survival of toxic sediments. Seagrass meadows accumulate organic matter as leaves and shoots grow and peel off. Unfortunately, when bacteria break down organic matter, they use up all of the oxygen. Without oxygen, various bacteria convert sulfur molecules into toxic sulfides, which could kill the grass. Seagrasses developed channels that transported oxygen from their leaves to their roots and thus formed “oxygen protection”.

Many species developed symbiotic relationships with specific bacteria and mussels. The mussels benefit from the addition of oxygen in the grass and help aerate the sediment further. Bacteria that protect themselves in mussels then convert toxic sulfides into harmless chemicals. The success of seagrass depends largely on producing more oxygen than bacterial decay can use up. This struggle explains many seaweed diebacks, such as the recent one in the Florida Bay of the Everglades.

As the human population grew and settled along the coast, they changed the seagrass ecosystems by clearing and overgrazing the land for wood and agriculture. Increased soil erosion was carried into the sea, creating cloudy sea water that reduced sunlight. Sewage runoff and agricultural fertilizers added nutrients that promoted plankton blooms, which also reduced sunlight.

With less light, there is less photosynthesis to produce oxygen. Without enough oxygen, toxic sulfides can penetrate the seaweed and kill it. The good news is that such lost seagrass ecosystems are not common and many unaffected regions support prosperous seagrass ecosystems. It's not a global crisis. The losses due to ignorance of the natural dynamics of the ecosystem in the past are now being corrected. Seagrass meadows are thriving with improved water quality and people are now better managing sediment runoff and developing wastewater treatment to reduce nutrient pollution.

The 2010 death of seaweed in Shark Bay, Australia, now a World Heritage Site, made frightening headlines in scientific journals and in the mass media and raised fears of an existential crisis. The seaweed died during a "marine heat wave" that supported the belief that only global warming could kill seaweed in a relatively pristine and protected ocean bay. However, the “marine heat wave” that allegedly killed the seaweed was caused by a strong La Niña that resulted in warmer tropical waters being transported (via the Leeuwin Current) along the west coast of Australia.

These periodic and natural interventions in warm water were known as "Ningaloo Niño". The north winds that drove the warm water south also suppress the normally cold air that comes from the Southern Ocean region. The normal swelling of colder deep waters is also suppressed. As soon as the severe La Nina conditions subsided, the regional climate reversed, causing cold spells for several years.

The greatest variety of seagrass species thrive in the warmest waters. Normally, scientists would expect that organisms exposed to the ever-changing climate caused by intermittently warm Ningaloo Niño have adapted to these natural temperature fluctuations. In fact, the immediate seaweed killer now doesn't seem to have been warmer temperatures. Years of vigorous grazing by non-native cattle and sheep have made the watershed that drained into Shark Bay increasingly prone to erosion.

La Niña's random increase in rainfall during the monsoon season in Australia. These heavy rains and eroding soil combined resulted in a murky river runoff that flowed 10 miles out into the bay. The closer the seagrass meadows of Shark Bay are to the river delta, the greater the death rate. Seagrass meadows that escaped these light-reducing waters usually still bloomed. Hopefully, the wrong analysis blaming global warming will not lead to bad remedial action and misdirect any efforts to protect Shark Bay from further deadly runoffs.

Finally, the legacy of seagrass reproduction has created another problem. By the 1930s along the Virginia coast, hurricanes and disease had completely bared several seagrass beds. Seventy years later the seaweed hadn't returned.

Without flowering grasses, there are no local seeds to initiate recovery. Seagrass seeds are heavy and quickly fall to the sea floor, so many seaweeds are slow to spread. Without a seed supply very close by, it can take centuries for a bared meadow to recover. The good news is that people are now harvesting seeds from distant, healthy spots and sowing seeds where seaweed once thrived.

By maintaining good water quality and minimizing boat damage, seagrass meadows are on the mend. Dependent fish and scallops are slowly recovering. Florida's manatees have increased sixfold and are no longer considered endangered. But manatees need warm winter huts.

Contrary to your intuition, the greatest threat to manatees living in Florida's seagrass ecosystem is the loss of power plants and the hot water drainage that has served as a manatee winter sanctuary.

Jim Steele is Director Emeritus of San Francisco State's Sierra Nevada Field Campus and author of Landscapes and Cycles: An Environmentalist's Journey to Climate Skepticism

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