The key reference used for the previous Coastal Challenges’ article on coral reefs was How to assess environmental impacts on tropical islands and coastal areas: South Pacific Regional Environment Programme (SPREP) training manual. The manual edited by Richard A. Carpenter and James E. Maragos was prepared by Environment and Policy Institute, East-West Center, in 1989. Asian Development Bank sponsored the venture.
The manual (not widely available in these days) presents useful conclusions for the management of environmental impacts in seagrass meadows everywhere in the tropics. (Together with mangroves and coral reefs, seagrass beds are the most important marine ecosystems of the tropical coastal zones. In addition, seagrass meadows are key components of shallow-water nature in temperate waters.)
The information presented by Carpenter and Maragos is summarised in a slightly edited form below:
Sustainable uses and values of seagrass meadows: The biological productivity is high; especially the fish and shellfish production is important. Seagrass beds are nursery areas for reef and mangrove species. These underwater meadows have useful functions in beach sand replenishment and act as a beach stabilisation area. Seagrass beds are also part of important synergistic interactions with onshore mangroves and offshore coral reefs. In addition, they are feeding areas of sea turtles and dugongs / manatees.
Sensitivity to environmental changes: Chemical pollution, changes in currents (leading to scouring or stagnation), changes in sedimentation patterns (leading to accumulation or burial), changes in longshore sand movements (disrupting the long-term balance of coastal sediment movements), dredging on offshore reefs (leading to lack of protection offered by the reef and resulting in changes in sediment movements), and cutting of onshore mangroves (resulting in loose sediments and lack of physical/runoff protection on the landward side of the seagrass beds).
Development hazards: Dredging, filling and construction of coastal structures commonly put the near-by seagrass beds at risk. Also oil and chemical spills can harm the seagrass meadows. In addition, damming and blocking the natural water flow and sediment movements may harm the seagrass communities.
Mitigation: The effects of projects on sediment and water movements need to be understood in advance. Damage to adjacent reefs and mangroves need to be avoided in projects leading to environmental impacts. In causeway construction, culverts and bridge openings help maintain the natural water circulation and sedimentation patterns thus protecting the seagrass meadows. Locating nonwater dependent facilities onshore is part of sound seagrass management.
The following Youtube video by Seagrasswatch.org is an excellent summary of the global importance of seagrass meadows: