*This article is excerpted from oneocean.org booklet (Why protect seagrass beds)
Seagrasses are seed-producing marine plants that occur in shallow, nearshore waters, the only group of submerged flowering plants in tropical and marine environments. Thriving in the shallow waters lining the shore, they have adapted life in saline waters with a root system that can withstand wave action and a reproductive system that distributes pollen by water. They are normally found in areas where light can easily penetrate (shallow, clear and calm waters) enabling photosynthesis to occur. Seagrass beds are often found between coral reefs and mangrove areas, colonizing the soft, shallow and sandy-muddy bottom.
Seagrasses have very high primary productivity that helps support and provides nutrients and physical habitat to a variety of organisms. Their main role as a nutrient source occurs when the dead seagrass decomposes and releases it nutrients to the water. Important fish species such as rabbitfishes (siganids), rely completely upon the seagrasses. Shrimps, sea cucumbers, sea urchins, seahorses, crabs, scallops, mussels and snails are economically important and abundant. Many resident and transient species also use the seagrass for refuge, spawning and nursery activities.
The Philippines has extensive seagrass and algal beds and soft-bottom communities that often occur in close proximity to mangroves and coral reefs. Soft-bottom communities made up of sand or muddy substrates occur in many shallow sub-tidal areas. Some are dominated by seagrass and algal beds while others are not vegetated. Organisms that inhabit soft-bottom areas are influenced by particle size and stability of the sediment, light, and temperature. Although not obvious, the unvegetated soft-bottom areas have a variety of organisms that are in-fauna, benthic animals that burrow or dig in the sediment. Some animals live on the surface such as clumps of seaweeds, mollusks, flat fishes, and rays. Typical in-fauna include various worms, bivalves, heart urchins, sand dollars, some sea cucumber, and shrimps. All depend to some degree on detritus for food and are easily disturbed if the sediment is moved or churned up
Seagrass ecosystems have very high primary productivity. It is this capacity which helps to support and provide nutrients and physical habitat to a variety of organisms. Seagrasses can grow quickly without fertilizers or modern cultivation techniques. Some species can grow as much as 8 cm/day (Fortes 1995). They also produce multiple crops (2-4 times annually). Their high productivity includes not only their own high growth rates but also the many small plants and other organisms that attach to their surfaces and live among them.
Relatively few animals actually eat seagrasses. The main role of seagrasses as a nutrient source occurs when the dead seagrass decomposes and releases its nutrients to the water. The seagrass food web is illustrated in Fig. 1 Important fish species, such as some rabbitfishes (siganids), rely completely upon the seagrasses. Shrimps, sea cucumbers, sea urchins, seahorses, crabs, scallops, mussels, and snails are economically important and abundant in seagrasses. Many resident and transient species use the seagrasses for refuge, spawning, and nursery activities. Large animals like dugong and green sea turtles also graze extensively in seagrass beds. Seahorses, a tourist attraction and of medicinal value, reside in seagrass beds (see Fig. 1)
The Value of Seagrasses.
“Within seagrass communities, a single acre (half a hectare) of seagrass can produce over 10 tons of leaves per year. This vast biomass provides food, habitat, and nursery areas for a myriad of adult and juvenile vertebrates and invertebrates. Because seagrasses support such high biodiversity, and because of their sensitivity to changes in water quality, they have become recognized as important indicator species that reflect the overall health of coastal ecosystems” (Smithsonian Marine Station).
“Fish species such as rabbit fish spend t heir whole life cycle in seagrass beds; where they spawn, protect their eggs from predators and grow to maturity”
Seagrass Resources in the Philippines.
The Philippines with its 18,000 km coastline, has sizeable seagrass areas spread discontinuously along the shallow portions of its coastline. Sixteen species that have been identified (Fortes 1986) are variably distributed in all parts of the country… From surveys in 96 sites, a total of 978 sq km of seagrass beds have been identified in the country, mostly in northwestern, western and southern portions, with outlying islands having sizeable beds.
A significant portion of the coastal habitats is at high risk of being lost in the next decade. About half have either been lost or severely degraded during the past 56 years (Fortes and Santos 2004).
Table 1. Traditional and contemporary uses for seagrasses.
|Traditional Uses:||Potential Contemporary Uses:|
| Woven into baskets|
Burned for salt-making and heat
Stuffing for mattresses
Upholstery and packing material
Compost for fertilizer
Insulation for sound and temperature
Fiber substitute for making nitrocellulose
Piles to build dikes
Bio- filters for sewage|
Source of useful chemical fertilizer and fodder
Food and medicine for people
Source: (Fortes 1989)
Seagrass beds and soft-bottom communities are often forgotten during preparation of various management plans because their values and functions are not as well recognized. The many coastal activities having major impacts on littoral basins also cause loss of seagrass habitat or damage to soft-bottom communities.
- Encroachment and seagrass modification. Encroachment by “land reclamation” for development of shore facilities has reduced the habitat for seagrasses. Both dredging and filling greatly disturb the bottom and largely remove that area as a potential seagrass area and area for soft-bottom organisms. Shrimp and fish farming have displaced many hectares of seagrass beds and mangroves.
- Sedimentation. Excessive sedimentation can physically smother the seagrasses or it can cause such turbidity in the water that photosynthesis is impaired and filter feeders die. Major sediment sources include improper mining, agriculture, or forestry practices. Large areas of seagrasses have been smothered by mining runoff in Marinduque Island and other areas (Fortes 1989).
- Introduction of waterborne pollutants. Pollution of nearshore waters occurs from domestic waste, oil and gas from boats and ships, and the accumulation of solid waste in shallow areas. In highly polluted waters, concentrations of herbicides, heavy metals, and detergents may be elevated enough to cause tissue damage to seagrasses (Fortes 1989). However, the major long-term threat to seagrasses in the world is from coastal eutrophication. Surface water runoff and groundwater containing excessive nutrients from fertilizers or sewage create conditions that promote algal blooms. Excessive algal growth shades the seagrasses on the bottom, interfering with light passage and the photosynthesis process. In addition, excessive amounts of dying algae will strip the oxygen from the water causing anoxic (no or low oxygen content) conditions in waters and sediments. The two actions combined may severely limit seagrass survival, either killing everything or leaving only those species that are hardy enough to survive
- Destruction of submerged and fringing vegetation. Destruction of fringing vegetation, such as mangroves, allows additional sediment and pollutants to enter the water. It also removes significant sources of nutrients that help to sustain the seagrasses and coral reefs. Blast fishing gouges large holes in the bottom, not only killing the plants but also creating erosion sites that may remove more plants. Dragging boats, nets, anchors, and other gear can dislodge seagrass as can the churning of shallow waters from small boats and jet skis. In addition, planting of mangroves in seagrass beds is destructive to seagrasses and not appropriate.
Management interventions for addressing the loss of seagrass and soft-bottom habitats include:
- Mapping and identification of beds to catalogue the extent and location of the resource;
- Zoning to prioritize use of space between pristine seagrass meadows versus those that are disturbed, altered, or newly emergent;
- Controlling of fishing methods to ban bottom trawling, blast fishing, and other methods of harvesting which tear up the bottom and cause turbidity;
- Reducing pollution by enforcing prohibitions against discharge of urban and industrial effluent and sea dumping of solid waste or dredge spoils and by reducing the amount of impervious surface area in the upland areas abutting the shoreline. Maintaining vegetated buffers along the shoreline and around disturbed sites to filter the runoff and promote infiltration of water into the ground; improve logging, mining, and agriculture practices to prevent erosion;
- Control coastal construction and beach nourishment;
- Transplanting shows signs of success from experimental transplanting; however, careful selection of the transplant site in regard to light, nutrients, and sediment type and stability is important while considering relative cost and benefits; and
- Recreation and tourism opportunities can provide opportunities for alternative sources of income to replace income generated by activities that degrade seagrass beds.
An example of a successful intervention to protect seagrass beds is at the village of Handumon on Handayan Island in Getafe, Bohol, where a municipal seahorse sanctuary was established with assistance from the Visayan Seahorse Project of Haribon Foundation. Ordinarily, a fisherman would receive Php10 per seahorse from a broker feeding a lucrative market supplying Chinese folk medicines. Through the Visayan Seahorse Project, fishermen learned to let the animals mature to reproductive size and to allow pregnant males to deliver their young. Tourists can arrange to go with a fisherman on a seahorse catch and release night expedition. The Php300 fee compensates the fisherman (Php250) for his catch and contributes Php50 to the project. Visitors stay in basic accommodations built by the project that are priced high enough (Php750 including meals) to increase benefits to the community. Additional bungalows are added as profits from the venture permit. Local residents also are learning to create alternative products for sale, including woven bags and pillows stuffed with old fish nets, rattan products, wooden boat models, and assorted fish and squid “snack foods” for local consumption.