Eelgrass (Zostera marina) provides a variety of essential ecosystem services in coastal areas, including habitat for shellfish and finfish, food chain production, and sediment stabilization. However, eelgrass populations are declining worldwide, suffering from a variety of natural and anthropogenic stressors. How might we best restore and sustain these systems? Since the causes of the decline are varied, including loss of historically connected ecological communities such as shellfish and salt marshes, reductions in water quality due to nutrient loading from agriculture and waste water treatment, and direct physical destruction of eelgrass from dredging, mooring, and other maritime activities, supporting the health of these populations requires a holistic approach that addresses the full range of concerns they face.
The articles in this Sustaining Eelgrass Reader describes some elements of such an approach, including restoring lost eelgrass beds as well as related coastal habitats such as salt marshes and shellfish beds, reducing physical impacts on existing eelgrass beds, improving water quality, implementing appropriate legal protections, and developing community ties to eelgrasses and other coastal ecological communities.
Eelgrass: ecosystem engineer
Unlike salt marsh grasses that are directly visible to beach goers, eelgrass normally grows subtidally. Its thin green leaves are most often seen in mid-summer washed up on the beach as wrack, an unprepossessing sight. But eelgrass is among the world’s most productive ecosystems. It provides habitat for many fish species of commercial and recreational value, including offering juveniles protection from predators. It also serves as a feeding ground for fish, shellfish and migratory birds, contributing more than half the primary production in some estuaries. It is an important factor in secondary and tertiary production as well, as eelgrass detritus may be transported a great distance to serve as a source of food offshore. According to various estimates, eelgrass exports between 7.30 to 73.00 g/cm2/year of NPP to neighboring ecosystems.
Eelgrasses are considered “ecosystem engineers” for the changes they make in their physical environments. By reducing the flow rate of water in their vicinity, the plants trap suspended particles around their roots and rhizomes, thus building and stabilizing sediment, particularly uring the growing season. In areas where eelgrasses have been lost, subsequent erosion of sediments has been documented. The stabilizing process in turn supports plant growth by improving water clarity and thus access to light.
Global loss of eelgrass
Eelgrass and other valuable seagrass ecosystems are declining, putting the ecological communities they support at risk. In the last 100 or so years, globally 29% of all seagrass area has been lost. It is now disappearing at a rate of 110 square kilometers per annum. Eelgrass populations specifically are declining due to both naturally occurring wasting disease and human impacts. These impacts are primarily the direct and indirect effects of development, including the introduction of excess nutrients from agriculture, and from stormwater and wastewater discharge. Increased nutrients in coastal waters increase the stocks of phytoplankton and algae, reducing water clarity and thus the amount of sunlight reaching the eelgrass. This is particularly worrisome as there are few known examples of seagrass recovery following the loss of beds to this cultural eutrophication.
Holistic approach to sustaining eelgrass
As there is no single cause to eelgrass decline, there is likely no single solution to its restoration and the ongoing support of its health. A holistic approach, one that address physical, ecological, and social factors, may be most successful. The approach as described in this reader focuses on Massachusetts, particularly within Boston Harbor.
A holistic approach starts from asking the question: How will managers or communities know that eelgrass beds are likely to be sustainable in their areas? There are a variety of environmental and social indicators that may be useful here. Physical indicators, such as water temperature, concentration of dissolved nutrients (nitrogen, phosphorous), sediment quality, and the attenuation of solar radiation (or the total suspended solids), can indicate whether eelgrass can physically grow in the particular coastal waters. A diverse benthic community also indicates a healthy eelgrass ecosystem. Socio-economic indicators, such as the density of human populations, the type of land cover, and the amount of fertilizer used in the watershed, can point to potential areas of concern for eelgrasses such as likely nutrient loadings. Finally, cultural indicators, such as the presence of eelgrass in community watershed-related publications or festivals, show whether there is likely support for policies protecting the grasses.
This Reader grows largely out of work done in Massachusetts, particularly in Boston Harbor, including through the Green Boston Harbor project founded by author Frankic. Each coastal community will adapt a holistic approach to the conditions and needs of that ecosystem, however in order to address the above physical, socio-economic, and cultural concerns such an approach will likely need to include:
- Reducing direct and indirect physical impacts to eelgrass beds, resulting from development, dredging, or other maritime activities
- Restoring lost eelgrass beds as well as related coastal habitats such as salt marshes and shellfish beds
- Improving water quality, primarily through the reduction of both onshore and offshore anthropogenic nutrient sources
- Ensuring appropriate legal measures, on necessary jurisdictional levels, to protect and restore eelgrass habitat
- Encouraging human communities interest in eelgrass and related habitats through education and cultural activities
Eelgrasses are usually hidden from direct sight, with perhaps only leaf tips showing at low tide to indicate their presence. But they are felt in human communities as fish catches, as the presence of feeding shorebirds, and as the stabilization of otherwise more muddy shores. A holistic approach to sustaining these grasses can ensure that these gifts are passed down to future generations of human and ecological coastal communities.
- Bos, A., et al. (2007). "Ecosystem engineering by annual intertidal seagrass beds: Sediment accretion and modification." Estuarine, Coastal and Shelf Science 74(1-2): 344-348.
- Burkholder, J. A. M., et al. (2007). "Seagrasses and eutrophication." Journal of Experimental Marine Biology and Ecology 350(1-2): 46-72.
Heck, K., et al. (2008). "Trophic transfers from seagrass meadows subsidize diverse marine and terrestrial consumers." Ecosystems 11(7): 1198-1210
Hemminga, M. A and C. M. Duarte. (2000) Seagrass Ecology. Cambridge University Press; Cambridge, UK.
- Masahiro, H. (2007). "Review of the effects of within-patch scale structural complexity on seagrass fishes." Journal of Experimental Marine Biology and Ecology 350(1-2): 111-129.
- Orth, R., et al. (2006). "A global crisis for seagrass ecosystems." Bioscience 56(12): 987-996.
- Short, F. T., et al. (1987). "Eelgrass wasting disease: cause and recurrence of a marine epidemic." Biological Bulletin 173(3): 557.
- Waycott, M., et al. (2009). "Accelerating loss of seagrasses across the globe threatens coastal ecosystems." PNAS 106(30): 12377-1238
Note: This article is part of a series of articles by the author known as A Sustaining Eelgrass (Zostera marina) Reader