This post was originally published under the title "Grass Roots Biology" in the December 2015 issue of The Biologist, the Royal Society of Biology's magazine. If you're a member of the RSB you can also view the article on their website here. The images are not those used by the RSB nor are they mine, the copyright belongs to their credited owners and most are from ARKive.org.
|Neptune grass (Posidonia oceanica) meadow in the Mediterranean © M. San Felix, from livescience.com|
Grass Roots Biology
Home to myriad species and acting as massive carbon sinks, seagrass meadows are key marine habitats – but they are disappearing fast, reports Xaali O'Reilly Berkeley
The Biologist 62(6) p16-19
Seagrasses are a group of flowering plants adapted to live in salt water. They grow, flower and pollinate completely submerged in estuaries and along shallow coastal waters around the globe, both in temperate and tropical environments. Although they physically resemble grasses and grow in large expanses called meadows, the term seagrass actually refers to the ecological niche the plants occupy rather than a taxonomical group. There are four different families of seagrass, all thought to have adapted to their marine and estuarine lifestyles independently from one another. Together, they form the foundation of one of the most productive ecosystems on Earth and cover somewhere between 300,000 and 600,000 square kilometres of seabed. Yet many people have never heard of them.
|A West-Indian manatee (Trichecus manatus) grazing in a turtlegrass (Thalassia testudinum) meadow. © Doug Perrine|
Seagrass meadows are by no means as structurally diverse or as superficially spectacular as coral reefs or mangrove forests – they really do resemble a submerged pasture. However, between their elongated leaves dwells an incredible diversity of marine flora and fauna. The blades of most seagrasses become encrusted with many different microscopic algal species. These algae are epiphytic, which means they grow and live on a plant without deriving their nutrition from their host. In addition to the nearly 500 species of epiphytic algae documented in seagrass meadows, several hundred species of animal are associated with seagrass beds. Some of the smaller species spend their entire lives sheltered in seagrass meadows and, as such, are utterly dependent on the plants and their algae – either directly, through herbivory, or indirectly from predating herbivores. Many more species depend on seagrasses for part of their life cycle, using the meadows as nursery habitats or as feeding grounds. Sea turtles and manatees are the most notable megafauna that graze on seagrass. Cod, plaice, pollock, herring, brown tiger prawn, blue crab, spiny lobster, queen conch and sea bass are just a few of the commercially important species that also spend time in seagrass meadows. While terrestrial ecosystems are studied in great depth to ensure our fruit and vegetables grow, seagrass meadows silently provide a suite of ecosystem services largely unknown to most of us.
In addition to providing habitats to species we value, seagrass meadows modify the seascapes they inhabit. Their leaves attenuate the water flow around them, and their roots and rhizomes hold fast the sand or mud beneath them, fixing sediments that would otherwise drift away suspended in the water column. By slowing down water flow, they may contribute, along with reefs and mangroves, to reducing wave velocity and, thus, storm potential [1,2]. Many of the nutrients from natural coastal run-off are absorbed by seagrasses, their epiphytes, and associated substrate bacteria, effectively filtering continental waters of phosphorous and nitrogenous compounds as they reach the sea.
In the context of our changing climate, perhaps the most interesting function seagrasses perform is carbon storage. The sheer biomass that seagrasses, especially their roots and rhizomes, compose worldwide represents an important carbon sink. However, most of the carbon captured by the meadows is buried beneath them. Carbon can be packed beneath the soil in this way for millennia, in the same way that forests store carbon on land.
|A common octopus (Octopus vulgaris) in a seagrass (don't know which species) meadow. © Jean-Michel Mille|
Despite the various ways in which we benefit from seagrasses, they are one of the most rapidly disappearing ecosystems in the world. At the beginning of the 20th century, the total area of seagrasses was falling by 1.5% per year. At last year's International Seagrass Biology Workshop, it was announced that an estimated 7% of meadow area is now being lost annually. The main cause of their disappearance appears to be coastal development. Dredging, construction and traffic along the coast physically disrupts and destroys meadows. However, it is the consequent pollution of seas and estuaries that causes most damage to seagrass. Although it can absorb nutrients from the water and soil, too much nitrogen or phosphorus has a number of negative effects on seagrass.
First, there is some evidence to indicate a serious nitrogen overload can cause growth problems in the plants themselves. More importantly, however, the excess of nutrients in the water column (eutrophication) leads to spikes in the abundance of both planktonic and epiphytic algae. These algal blooms reduce the amount of light available to the seagrass for photosynthesis, reducing seagrass productivity and causing meadow thinning, which leads to habitat impoverishment.
The impoverishment or loss of a foundation species such as seagrass can have huge impacts on the communities that depend on them. When a seagrass meadow is destroyed, the organisms that inhabit it either perish or move elsewhere in search of more suitable conditions, which can have severe consequences for local fisheries. Local species associated with seagrass have been estimated to make up 50% of the fish catch consumed in the coral triangle (the area surrounding Indonesia, Malaysia, Papua New Guinea and the Philippines) and contribute 30–40% of the value of fisheries in the Mediterranean. Although these are very specific cases and can't be extrapolated to the rest of the world, it exemplifies how important seagrass meadows can be to local sources of protein.
Coastal communities aside, the global market would suffer were there to be declines in seagrass-associated species. Although most seagrass animals do not exclusively occur in the meadows, losing feeding grounds or nursery sites would probably affect the numbers of many of these species.Another consequence of the loss of seagrass beds is increased erosion in shallow waters and resuspension of sediments. Finally, the disappearance of seagrass meadows will not only eliminate a carbon sink, but will release the carbon stored up over millennia into the ocean and atmosphere as carbon dioxide.
There is hope for seagrasses. There are numerous restoration projects aiming to replant seagrass, although many sites that previously boasted meadows may still be unsuitable for the plants, and the road to a healthy, productive system is long. Nonetheless, some restoration projects are showing promise, especially in the US.
In Chesapeake Bay, the Virginia Institute of Marine Science has been working on how best to restore temperate seagrass meadows since 1978, while the National Oceanic and Atmospheric Administration and the Florida Fish and Wildlife Conservation Commission have been working on restoring tropical meadows in the Florida Keys and Tampa Bay.
A new challenge, however, in protecting and restoring seagrass beds worldwide is our changing climate and ever more acidic oceans. Research has shown genetic diversity may have an important role to play in conveying resistance to meadows in the face of change. This could be an important consideration for future restoration projects, yet it reminds us that there is still much to learn about seagrass ecology.
The future challenges for seagrass biologists include better understanding the role of genetic diversity in the ecology of seagrasses, while continuing to quantify the ecosystem services these meadows provide. For now, as a global community, the best thing we can do for seagrass is the same favour that would benefit most other ecosystems: reduce pollution and carbon emissions.
1 Chen, S.-N. et al.
4 Unsworth, R. K. F. et al. Food supply depends on seagrass meadows in the coral triangle.