Preventing Ecosystem Collapse 3: Seagrass

by Jim Steele

modified online version that will be printed in BattleBorn Media newspapers

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 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 thousand 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 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.

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

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January 1, 2021 6:27 am

Sea grasses in the Chesapeake Bay are also under stress. The biggest issues are water quality (algae blooms from nutrients coming in from the rivers that feed the Bay) and the rip rap installed along the shorelines to prevent erosion which eliminates the gentle slopes from water’s edge to the deeper water.

Our house is on a point on the lower western side on a creek coming in and we have no grass (I’m guilty of rip rapping my entire shoreline). The good news is there is a strong interest in getting grasses to return including introducing oysters back to filter the water and with not disturbing the grass beds when they finally take hold. The nutrient issue is much harder to tackle. The Bay watershed is huge and there is a constant battle getting funding for projects to improve the water quality draining into the rivers feeding it.

Reply to  rbabcock
January 1, 2021 11:13 am

‘Way back when I worked at Curtis Bay, Maryland, the water was so polluted that Blue Claw Crabs swarmed around the alkaline outfall from one of the plants producing crystalline molecular sieves. Several years of “Save the Bay” and the fish were returning so strongly that water inlet strainers at the Domino sugar evaporators were overflowing with excess fish.

Reply to  Enginer01
January 1, 2021 6:54 pm

A few years ago I read the report published in the journal Scientific Reports by the U.S. Geological Survey on the pollution in the Potomac and Susquehanna Rivers. The concern was the high levels of estrogens from birth control pills in the water getting into the fish. Reportedly significant numbers of male fish were found to be “growing eggs”. Another “endocrine disrupter” found at the time was BPA, which led to another big study and the reducing of BPA in plastics, etc..

The big comment at the time, on line was, “maybe there was a link between the feminized fish and the claim of ever increasing numbers of beta males in American society”. 😁

Reply to  rbabcock
January 1, 2021 5:36 pm

Sounds like you are in my old stomping grounds from high school and college – St. Mary’s County Maryland. I used to live on the St. Inigoes Creek at the mouth where it enters the St. Mary’s River at the Potomic. Did most of my undergraduate work at Saint Mary’s College of Maryland. It is a beautiful area, but the politics are terrible. Couldn’t live there now on a bet.

Last edited 1 year ago by OweninGA
Reply to  rbabcock
January 1, 2021 5:38 pm

Sounds like you are in my old stomping grounds from high school and college – St. Mary’s County Maryland. I used to live on the St. Inigoes Creek at the mouth where it enters the St. Mary’s River at the Potomic. Did most of my undergraduate work at Saint Mary’s College of Maryland. It is a beautiful area, but the politics are terrible. Couldn’t live there now on a bet.

(I have a post awaiting moderation, but I think I have removed the word that triggered the dirty word filter.)

Brian Pratt
January 1, 2021 6:50 am

Great essay. Minor correction: sea grasses are thought to have evolved in the Late Cretaceous, so more like 70–100 million years ago.

Reply to  Brian Pratt
January 1, 2021 9:00 am

Good catch. Indeed I meant to say 100 million years ago.

H. D. Hoese
January 1, 2021 7:38 am

Good article, always great to see homework, important way to ‘cure’ the “crises.” I spent several decades on and off studying the turtle grass beds behind the Chandeleur Islands, Louisiana, the only ones of consequence in the exceptionally productive state, save different species in low salinity, often impounded, areas. System is ephemeral, main loss is from storms, while Camille in 1970 took out a lot, Katrina both scoured and filled much more of the beds. The islands are remnants of an old delta, grass bed fossils occur deep under New Orleans.

Seagrasses also get transported when the whole plant is eroded, that is if they find the proper place to root. Most of Louisiana is too fresh and turbid but species with more salinity tolerance than turtle grass do rarely turn up in a couple of places for awhile.

Don’t know if the Science paper knows the history, but Hand once lived down the block from me. [Fisher, M. R. and S. C. Hand. 1984. Chemoautotrophic symbionts in the bivalve Lucina floridana from seagrass beds. Biological Bulletin.167:445-459.] There are always certain bivalves as also snails, crustaceans and others found in such. Grassbeds are pretty, but only widespread in clear estuarine shallow waters, lots more of open turbid, productive water doesn’t always get its due. “…Large scale seagrass loss (> 1000 km2) in other seagrass dominated systems such as Chesapeake Bay..” from “Extreme climate events lower resilience of foundation seagrass at edge of biogeographical range” While the range edge is susceptible, sorry, lots of turbid, open water there, while important wouldn’t call them dominant. Overall fisheries production there didn’t get the message.– [Kemp, W. M. and 17 other authors. 2005. Eutrophication of Chesapeake Bay: historical trends and ecological interactions. Marine Ecology Progress Series. 303:1-29. ] “The absence of clear evidence of declines in fish productivity does not necessarily in fish productivity does not necessarily indicate thatthere have been no effects of eutrophication on Bay fisheries.”

Used to take students to Florida to see clear water, they have lots of seagrass bays, even in the open low energy Big Bend area around Cedar Key. As to heat south Texas beds often get very high temperatures, low sea levels sometimes expose them to the delight of birds. Darn birds!

Mickey Reno
January 1, 2021 9:07 am

In the Everglades and across S. Florida, another human cause of sea grass loss is road building. The roadbed for what is called ‘Alligator Alley’ that goes from Miami on the East coast to Naples on the West coast, was raised mainly by dredging. This raised the roadbed and created an artificial canal next to the road. The so-called “river of grass” stopped flowing, because there was too little egress for the fresh water north of Interstate 595 and the smaller state highways, to get to the south side of the roads. This meant too little fresh water flowing into Florida Bay, which messes with the salinity of the water, making it less enjoyable for the sea grass.

But people are aware of this problem, and now there are a couple of new viaducts where the water can flow under the freeway, and I believe more are planned. People don’t want to damage nature, they did it because they were ignorant back when homes and boat canals were dredged by the mile back when Florida became a desirable place to live. And people are willing to pay to mitigate the effects of such ignorance, up to a point. Not having a road is not one of the prices most are willing to pay.

This was not the ONLY problem for Florida Bay’s salinity levels. Major hurricanes like Irma can cause very high tides and pile up water even inside the reefs, water is also too salty for sea grass. And there’s little humans can do to stop hurricanes. Anyone who really thinks not driving or switching to electric cars or somehow crippling the human economy will stop hurricanes from forming needs to have their friggin’ head examined, and yet, the CAGW alarmist “cause” pimps this implication at every turn, from Florida to, well, all around the globe and back to Florida, from the NPR all the way to BBC.

Reply to  Mickey Reno
January 1, 2021 11:08 am

The Courtney-Campbell Causeway in North Tampa Bay is another man-made impediment to nature water flow, with a huge effect on the Bay eco-system.
But don’t blame it ALL on man…I have snorkeled many times in Kings Bay at Crystal River in Florida. Forty years ago the manatees were in clear water, and sought us out to “play” with us. Now the water is usually so murky one cannot find the openings for the many springs–except Three Sisters, protected by a one-way flow outfall.

The local people tell me the Bay is over-populated with manatees, who have eaten all the sea lettuce, and destroyed their own habitat. This is being studied.
Years ago I worked with an active leader of the Chesapeake Bay Foundation. We discussed the need for fundraising, and I suggested selling a “Keep Lake Erie out of the Chesapeake” bumper sticker. My suggestion was adopted, but with the motto “Save the Bay.” (ugh!)

January 1, 2021 11:33 am

Sea grass leaf structure should be understood as having some different characteristics than terrestrial grasses. For example they do not have spongy mesophyll & thus their leaves do not naturally scatter light (despite light scattering sounding “bad” it actually promotes better interior light usage). This sea grass difference means that the cell’s own pigments partially shade out some of the light that had been absorbed by the leaf. Another difference is while land plant leaves have tubular palisade cell features the sea grass do not & this impacts the dynamic of penetration by light that has gotten inside the leaf.

The Original Post highlights sea grass requirement for synthesized oxygen. There is generally a difference in the amount of light (photons) required by temperate sea grass & tropical sea grass to evolve a molecule of O2 oxygen. For example about 14-20 photons are needed to be by temperate sea grass to synthesize 1 oxygen molecule & for tropical sea grass it is about 9-10 photons needed to synthesize 1 oxygen molecule; whereas land plant in general only need to get 8 photons to evolve an O2.

Reply to  gringojay
January 1, 2021 12:28 pm


Thanks for adding to the reader’s understanding of seagrass physiology. My article is intended for the general public and a quick newspaper read so it doesn’t go too deep into the weeds. However WUWT has a far more knowledgeable readership, and I count on people like you to take a deeper dive into the science.

January 1, 2021 7:23 pm

It’s been my observation that ecosystems are opportunistic. Nature fills a vacuum. For example, we had some lake side property. The lake was deep along one side and shallows extended quite far out along another. I began setting rocks out in straight lines from shore in the shallows to encourage the trapping of sand, using wave action to create a nice, sandy wading bottom for the children. It worked. For years the children had a nice area of shallow water to play in. Slowly the sand in high areas began growing moss and various weeds. Years later the area was taken over by rushes, which trapped more sand and debris. It wasn’t long before different plants and brush colonized the newly created land inshore of the rushes. Today that nice sandy wading beach is but a memory. Nature has changed it utterly. I can’t find it in myself to suggest that that changing “ecosystem” is bad. It evolved, and still is. I may have given it a bit of a shove but nature took over and nature continues to express itself. Nature is change too, and I don’t think it’s bad ecology when it does. Ecologist need to stop believing that the status quo is the norm.

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