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Natural and Human-Induced Disturbance of Seagrasses

(With Y. Luc, M. Ostrovsky, and A. Persaud)

Seagrasses are marine flowering plant species found on the bottom of estuaries and along coastal margins throughout the world (Short and Wyllie-Echeverria, 1996). They are an integral part of coastal ecosystems, supporting a complex trophic food web and a detritus based food chain (Short and Wyllie-Echeverria, 1996). In addition, they provide sediment and nutrient filtration, sediment stabilization, and breeding and nursery areas for aquatic animals (Short and Wyllie-Echeverria, 1996) such as the blue crab, Callinectes sapidus (Orth and Moore, 1983). Disturbances, either natural or human-induced, greatly affect the productivity of seagrasses, and can result in degradation or loss.

Natural disturbances, including disease, hurricanes, earthquakes, and grazing by herbivores, have constantly exerted pressure on seagrasses without causing a drastic decline. Naturally disturbed areas can recover because of the phenotypic and genotypic plasticity of the species present, which depends on species diversity. Therefore, in an area with high species diversity of seagrasses, there should be a greater chance of recovery after a disturbance.

Nevertheless, a severe decline in seagrasses has occurred and continues to occur. Scientists, such as Short and Wyllie-Echeverria (1996), have attributed this decline to increased anthropogenic inputs to coastal areas of oceans. This increased anthropogenic input arises primarily from human population expansion. Over the past ten years, ninety thousand hectares or more of seagrasses have been lost (Short and Wyllie-Echeverria, 1996). Of documented seagrass losses, natural disturbances caused three in 1970-82 and human disturbances caused six. In 1983-94, the losses were five and twenty-eight, respectively (Short and Wyllie-Echeverria, 1996).
Humans disturb seagrasses through nutrient and sediment loading, dredging and filling, pollution, upland development and some fishing practices (Short and Wyllie-Echeverria, 1996). Sediment loading occurred in South-East Asia after the second world war. There was increased deforestation, and sediment eroded into the coastal ecosystems. This reduced the amount of light reaching the seagrasses and increased the nutrient load, which negatively affected seagrass growth (Terrados et al., 1998.) Lack of light penetration and nutrient enrichment also caused seagrass losses in Chesapeake Bay (Short and Wyllie-Echeverria, 1996). Gallegos and Kenworthy (1996) discussed how color and turbidity of the water negatively affected seagrass growth in Indian River Lagoon in Florida. Dredging and filling, or fishing practices such as using a boat propeller to dig up clams, can also reduce light and cause mechanical damage to the seagrasses. They also bioaccumulate toxic compounds from the water or sediment, and this may cause seagrass loss (Short and Wyllie-Echeverria, 1996).

Seagrasses are ecologically important to shallow water ecosystems. Thus, humans must protect seagrasses from human-induced disturbance at the habitat level. Low cost efforts are already in effect in Chesapeake Bay, such as using physical structures and vegetation to prevent excess discharge of nitrogen and phosphorus into seagrass habitat. In addition, over the last ten years, the reconstruction of the seagrass community through retransplantation and restoration has proven successful in maintaining seagrass populations. Since eutrophication and suspended sediments are the greatest long- term threats to the health and survival of seagrasses, humans must further develop and enforce regulations to reduce nutrient loading from point sources. This may include designing better septic systems or regulating fertilizer use (Short et al., 1991). This will reduce contamination of estuaries and thus reduce loss of seagrasses. Humans must also protect seagrasses from anthropogenic coastal activities to maintain populations. The fate of many coastal ecosystems depends on the protection of seagrasses.

References

Gallegos, C. L. and W. J. Kenworthy. 1996. Seagrass Depth Limits in the Indian River Lagoon (Florida, U. S. A.): Application of a Water Quality Model. Estuarine, Coastal and Shelf Science, 42:267-288.

Orth, Robert J. and Kenneth A. Moore. 1983. Chesapeake Bay: An Unprecedented Decline in Submerged Aquatic Vegetation. Science, 22:51-52.

Short, Frederick T. and Sandy Wyllie-Echeverria. 1996. Natural and human-induced disturbance of seagrasses. Environmental Conservation, 23(1):17-27.

Short, F. T., G. E. Jones and D. M. Burdick. 1991. Seagrass decline: problems and solutions. Pp. 439-53. In H. S. Bolton (editor). Coastal Wetlands. American Society of Engineers, New York.

Terrados, J., C. M. Duarte, M. D. Fortes, J. Borum, N. S. R. Agawin, S. Bach, U. Thampanya, L. Kamp-Nielsen, W. J. Kenworthy, O. Geertz-Hansen and J. Vermaat. 1998. Changes in Community Structure and Biomass of Seagrass Communities along Gradients of Siltation in SE Asia. Estuarine, Coastal and Shelf Science, 46:757-768.

 

(October 1998)

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