The Philippines' Forgotten
Resource:
Seagrass and its Management

Seagrass ecosystems have very high primary productivity and play an important role in the food chain in the Philippine marine environment. While coral reefs and, more recently, mangroves have become the focus of many management interventions, however, seagrass ecosystems have generally been forgotten in the preparation of management plans, largely because their values and functions are not as well recognized.

(Extracted from Philippine Coastal Management Guidebook Series No. 5: Managing Coastal Habitats and Marine Protected Areas by AT White, R.S. Nakashima, and Mary Gleason; published by the Department of Environment and Natural Resources, Bureau of Fisheries and Aquatic Resources of the Department of Agriculture, Department of Interior and Local Government, and the Coastal Resource Management Project. To download complete publication, click here)

he 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.

Seagrasses that usually inhabit sandy and soft-bottom areas are the only submerged flowering plants in the marine environment. Thriving in the shallow waters lining the shore, they have adapted to life in saline waters with a root system that can withstand wave action and a reproductive system that distributes pollen by water. They possess erect leafy shoots and creeping stems or rhizomes that aid in propagation. Unlike other marine plants like seaweeds or algae, they flower, develop fruit, and produce seeds. They are normally found in areas where light can easily penetrate (shallow, clear, calm waters) enabling photosynthesis to occur. Vast seagrass meadows are often found between coral reefs and the coastal mangroves, colonizing the soft, shallow, sandy-muddy bottom. They can be several hundred meters wide and cover large expanses of the reef flat. They can also include algal beds of Sargassum or other seaweeds interspersed in the shallow reef flats.

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).

Fig. 1. Food chain in Philippine seagrass ecosystems (Fortes 1989). Click image to enlarge

Seagrass beds slow down water velocity so that sediments settle out of suspension before they can be washed onto the reef. Without the seagrass roots anchoring the bottom sediments, they become loose, erode, and smother benthic populations. Scallops, clams, crabs, and many fish species suffer from the loss of protective seagrass habitat and from the sedimentation and erosion of the bottom (Fortes 1989).

Major distributions of seagrass beds in the Philippines occur in Bolinao Bay in Luzon, Palawan, Cuyo Islands, the Cebu-Bohol-Siquijor area, Zamboanga, and Davao. Other beds are scattered throughout the coastal expanse of the other islands (Fortes 1989).

Resource uses

Seagrasses have a long history of use in the Philippines. Traditional uses for seagrasses as well as a list of potential contemporary uses are given below. A primary ecological function of seagrass beds is to provide nursery areas for a variety of fish and crustaceans that migrate to other habitats as adults. This contributes significantly to fisheries. Soft-bottom areas, being primary habitat for many mollusks, worms, and some fish have corresponding resource values.

Table 1. Traditional and contemporary uses for seagrasses.

Management issues

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.

1. 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.
2. 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).
3. 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
4. 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

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.

Legal and institutional framework

Except for inclusion in a sanctuary or other management scheme there are few specific regulations for seagrasses. Nevertheless, many of the potential development impacts on seagrass beds and soft-bottom areas are controlled under the environmental impact requirements of the DENR.

References

Fortes, M.D. 1995. Seagrasses of East Asia: Environmental and management perspectives. Regional Coordination Unit, East Asian Seas Action Plan Technical Report Series No. 6. United Nations Environment Programme. 79p.

Fortes, M.D. 1995. Seagrasses: A resource unknown in the ASEAN Region. ICLARM Education Series 5, International Center for Living Aquatic Resources Management, Manila, Philippines, 46p.

Alan T. White, Ph.D., Chief of Party, Coastal Resource Management Project

R. Steven Nakashima, MPA, MSPH, Environment Specialist, New Rochelle, New York

Mary Gleason, Ph.D., Tetra Tech EM Inc.

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