Preventing Ecosystem Collapse: Seagrass

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. Recently, 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. Still, 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 remedied. 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 to harbor 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 are dragged across the seafloor but also plow up seagrass 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 100 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 6000 years ago. The Everglades’ Florida Bay formed 4000 years ago. Since then, seagrass meadows have flourished and disappeared periodically, but are now at their greatest extent.

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 less usable ions after entering the water. Under current concentrations, only 1% remains as vital CO2. However, a 100 million years ago, plants flourished under increased atmospheric CO2 that was up to 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 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 benefit from the grass’ added oxygen and help aerate the sediment further. Bacteria sheltering in clams then convert toxic sulfides into harmless chemicals. Seagrass success largely depends on generating more oxygen than bacterial decay can consume, and that battle explains many seagrass die-offs, such as recent die-offs in the Everglades’ Florida Bay.

As human populations grew and settled along the coast, they altered seagrass ecosystems by clearing the land for lumber and agriculture, and by overgrazing. Increased soil erosion was carried to the sea creating murky ocean waters 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 generate oxygen. Without enough oxygen, toxic sulfides can invade and kill the seagrass. The good news is such lost seagrass ecosystems are not happening everywhere, and many unaffected regions support prosperous seagrass ecosystems. 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 improved water quality are thriving and people are now managing sediment runoff better and developing waste-water treatment to reduce nutrient pollution. A 2010 die-off of seagrass in Australia’s Shark Bay, now a World Heritage site, generated scary headlines in scientific journals and the mass media drumming up fears of an existential crisis. The seagrass died during a “marine heat wave” supporting beliefs that only global warming could kill seagrass in a relatively pristine and protected ocean bay. However, the “marine heat wave” alleged to have killed the seagrass, was caused by a strong La Niña that caused warmer tropical waters to be transported (via the Leeuwin current) down the west coast of Australia. These periodic and natural warm water intrusions have been dubbed the “Ningaloo Niño”. The northerly winds that drove that warm water southward also suppresses the normally cold air arriving from the Southern Ocean region. The normal upwelling of colder deep waters is also suppressed. Once the strong La Nina conditions waned, the regional climate reversed causing several years of cold spells.

The greatest diversity of seagrass species thrives in the warmest waters. So normally, scientists would expect that organisms exposed to a constantly changing climate, induced by periodic warm Ningaloo Niño, would have adapted to those natural temperature fluctuations. Indeed, the immediate seagrass killer now appears not to have been warmer temperatures. Years of heavy grazing by non-native cattle and sheep made the watershed that drained into Shark Bay increasingly vulnerable to erosion. La Niña’s coincidentally increase rainfall during Australia’s monsoon season. Those heavy rains and eroding soil combined to produce a murky river discharge that flowed 10 miles out into the bay. The closer Shark Bay’s seagrass meadows were to the river delta, the greater the die-off. Seagrass meadows escaping those light?reducing waters were typically still thriving. Hopefully the wrong analysis that blamed global warming, will not lead to bad remedies and misguide any efforts to protect Shark Bay from further lethal river discharge. Lastly, the legacy of seagrass reproduction created one other problem. 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. Without flowering grasses there are no local seeds to initiate recovery. Seagrass seeds are heavy and quickly fall to the seafloor, so many seagrasses spread slowly. Without a very nearby seed supply, a denuded meadow may take centuries to recover. The good news is people are now harvesting seeds from distant healthy patches and sowing them where seagrass once thrived. By maintaining good water quality and minimizing boat-related damage, seagrass meadows are on the mend. Dependent fish and scallops are slowly 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 manatees living in Florida’s seagrass ecosystem is the loss of power plants and the warm water discharge that has served as a manatee winter sanctuary.

12/31/2020 modified online version to be printed in BattleBorn Media newspapers 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: naturalclimatechange@earthlink.net

Preventing Ecosystem Collapse: Pt 2 Caribbean Coral

Media headlines have been promoting unrealistic fears of ecosystem collapse due to climate change. Such fears get supported when the International Union for the Conservation of Nature (IUCN) designates some ecosystems as endangered, such as Caribbean Coral Reefs. But reefs are resilient and the human factors threatening individual reefs can be remedied.

 

The Caribbean reef ecosystem consists of thousands of individual reefs spanning from the east coast of Mexico and Central America to Florida and the Bahamas and south to the coast of Venezuela. Fifteen thousand years ago these reefs did not exist because sea level had fallen by 400 feet during the ice age. Modern reefs became established 8,000 years ago by colonizing newly flooded coastlines.

 

Caribbean Coral Reef Boundary

Based on one IUCN criterion the Caribbean reef ecosystem was designated “Least Concern” due to the widespread occurrence of individual reefs. In contrast, the loss of 59% of total coral cover between 1971 and 2006 prompted the IUCN to designate the reef system as “Endangered”.

 

Coral cover naturally fluctuates with seaweeds (macro-algae). Coral are killed by hurricanes, disease or bleaching,  which allows seaweeds too colonize the vacated space. The seaweed is gradually reduced by algae-eating animals which allows coral to return to their former dominance. Coral usually recover within 15 to 20 years, but recently their recovery has been extremely limited thus reducing coral cover. Unlike the demonized sea urchins that threaten Alaska’s kelp forest, algae-eating urchins are vitally important in maintaining the balance between seaweeds and Caribbean coral. The recent lack of coral recovery is largely attributed to a new disease that devastated urchin populations in the 1980s and minimized the urchins’ consumption of seaweeds.

 

Caribbean corals had been decimated in the 1980s by the novel White Band disease. However, that disease only affected two coral species from the genus Acropora - staghorn and elkhorn coral. Those species are now considered endangered. Acropora’s evolutionary strategy was to quickly colonize vacated shores produced by natural disturbances like hurricanes. These coral species thus dedicated their energy to rapid growth to out compete the seaweeds. That adaptation allowed staghorn and elkhorn coral to rapidly colonize flooded coasts as sea level recovered from the last Ice Age and dominate modern Caribbean reefs. But that strategy required diverting energy from building stronger reefs or resisting disease.  

 

Because Acropora require shallow habitat they’re vulnerable to storm damage. So, they evolved a reproductive strategy that produced new colonies by cloning new coral from storm damaged fragments. However, cloning reduces genetic diversity which also made them more vulnerable to new diseases.

 

Mortality from bleaching also reduced coral cover. Bleaching from unusually warm temperatures during summer 2005 and the 1998 El Nino is often highlighted. Surprisingly, fatal cold weather bleaching is rarely mentioned. Yet in January 2010 along the Florida Keys, cold weather killed 11.5 percent of the coral, which was 20 times worse than the 2005 warm weather mortality. Understanding why both warm and cold weather causes bleaching provides insight into how coral have successfully adapted to ever changing climates over the past 220 million years.

 

Shallow water corals depend on photosynthesizing symbiotic algae (aka symbionts) that provide over 90% of the coral’s energy. However, those corals will remove one symbiont species and acquire a new symbiont that is better adapted to the changing weather conditions. During the winter, colder temperatures and less light reduce photosynthesis. So, coral increase their density of symbiotic algae to counteract reduced productivity. But if it is too cold, the symbionts keep their energy supply for themselves. As a result, coral remove the “freeloaders” causing bleaching. A more productive cold?tolerant symbiont must then be acquired, or the coral die.

Coral polyp with inner symbiotic algae 

 

In contrast during the summer, more light and higher temperatures produce so much energy, coral reduce their number of symbionts. Because photosynthesis also produces potentially harmful chemical by-products, coral remove symbionts to reduce the production of harmful chemicals. That too causes coral to bleach, and unless a better adapted symbiont is  acquired, the coral will die. Despite that mortality risk, research now shows by switching their symbionts, coral can quickly adapt to warmer or cooler climates and enhance the species survival.

 

Studying fossil reefs, scientists determined that Caribbean corals had been declining decades before widespread bleaching and disease outbreaks occurred. Growing human populations cleared the land for farms, sugarcane and banana plantations. Resulting soil runoff reduced water clarity required for efficient photosynthesis. Increased sewage also reduced clarity and introduced pathogens. Those stressors made coral more susceptible to subsequent bleaching and disease. Soil runoff also added nutrients that tipped the ecological balance to favor seaweed growth, while overfishing removed seaweed?eating fish that once restricted seaweed dominance.

 

We can, and are controlling soil runoff and treating sewage. Fishing regulations are restoring the ecosystem that had balanced seaweeds and coral. And with those protections, naturally resilient coral will steadily recover.