Improving coastal water quality
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. Eelgrass health in turn depends on its access to clear, oxygenated water. In coastal areas globally, water quality may be reduced by high inputs of anthropogenically-generated nutrients (primarily nitrogen and phosphorous compounds) to coastal watersheds. Reducing such inputs may be critical for maintaining eelgrass populations.
In addition to supporting the water quality standards described in this article, sustaining the health of eelgrasses requires a holistic approach that includes restoring lost eelgrass beds as well as related coastal habitats such as salt marshes and shellfish beds, reducing physical impacts on existing eelgrass beds, implementing appropriate legal protections, and developing community ties to eelgrasses and other coastal ecological communities.
Effects of nutrient loading on eelgrass habitat and health
Nutrient loadings may be beneficial or problematic for eelgrass health and the health of its habitat more generally, depending in part on the concentration of nutrients in coastal waters. Eelgrass is able mitigate some level of anthropogenic nutrient load by utilizing the nutrients for its own growth and reproduction, thus reducing the amount that is available for its algal competitors. This keeps the water column clearer, allowing more light to penetrate to the sediment. As seagrasses in general have some of the highest light requirements of any flowering plants, by helping maintain water clarity the grasses; nutrient reductions support their own growth.
However, when the concentration of nutrients in the water and sediments in an eelgrass habitat exceeds the amount eelgrass and other species in the area can mitigate, algae and phytoplankton in the water column may instead use these nutrients for rapid growth. This may result in a reduction of the clarity of the water column as well as lower dissolved oxygen concentrations.
With continued inputs of nutrients into the waters, the eutrophication of the waters can lead to the deaths of many species of animals and plants, including eelgrass.
Sources of nutrients to coastal waters
There are many point and non-point sources of nutrients to coastal waters. These sources generally increase the concentrations of dissolved nitrogen (e.g. ammonia, nitrate, & nitrite) and phosphorous (e.g. phosphate) species. List of some sources of nutrients to estuarine ecosystems. (adapted from Inka Milewski)
Nutrient Discharge in Coastal Areas
Reduction of nutrient loads may focus on onshore (primarily fertilizer and wastewater) or offshore (waste water from boats) sources. The United States Clean Water Act of 1972 was an early piece of federal legislation that recognized the need to prevent various contaminants from entering our waterways. This Act (Section 312) requires the use of Marine Sanitation Devices (MSDs) on all commercial and recreational vessels that are equipped with installed toilets. MSDs are the on-board equipment for treating and discharging or storing boat sewage. The Federal Water Pollution Control Act of November 2002 more specifically restricts the ways that treated and untreated sewage may enter navigable waters.Individual states may also enforce even stricter standards for the discharge of waste from boats in their waters than what is outlined in these regulations. Massachusetts and other coastal states in New England have done so by designating “No Discharge Areas” (NDAs).
In NDAs, discharge of sewage from boats, both partially treated and untreated, is prohibited. Massachusetts State Law (323 CMR 2.07(6)) prohibits the discharge of untreated sewage from boats on or into waters of the Commonwealth—which typically extend three miles from shore.
Boats with functioning Type I and Type II MSDs may not discharge treated effluent in coastal in an NDA, and must appropriately secure their systems while within an NDA in order to prevent leakage. A Type III MSD or "holding tank" is the only type of MSD that can be used legally within an NDA. These measures can significantly reduce the chance of excessive nutrient loading to the shallow coastal areas where eelgrass could reside.
Nutrient loadings from either conventional sewage treatment or septic systems can be a problem in coastal areas, but solutions are possible. Boston Harbor, once infamous for its dirty waters, is an example of a success story for improvements to both water and sediment quality as a result of a set of projects that reduced the amount of treated and untreated sewage and storm drain water entering the coastal environment. The construction of a sewage effluent outfall pipe – stretched nine miles into Massachusetts Bay – as well as the closing of many combined sewer overflow (CSO) pipes have significantly reduce the amount of nutrients and various contaminants being poured into local water bodies.
Prior to the construction of the outfall pipe, the effluent from primary treatment of the sewage and storm drain water from much of the greater Boston area was released from very short outfalls at the Deer Island and Nut Island treatment facilities on the outgoing tide. If these facilities were at the maximum volume of sewage supported by the infrastructure, pressure would be released from the system by opening CSOs to drain untreated sewage throughout the Boston Harbor area.
With the modification of adding secondary treatment of the sewage and enhanced dilution of the effluent from Deer Island as well as the closing of some of the CSOs throughout Boston Harbor area, the loading of nutrients and other contaminants has dramatically decreased in recent years.
Other Sources of Nutrients
Besides direct discharges of nutrients from sewage treatment and dumping from boating activities, nutrients may travel into our waterways from land-derived sources (e.g. lawn fertilizers, runoff from agriculture). Increased development along coastal areas has reduced the amount of salt marshes and other wetland areas that are excellent natural buffers between land sources of nutrients and our bodies of water. The Codes of Massachusetts Regular “310 CMR 10” and Massachusetts General Law “M.G.L. c. 131, § 40” outline restrictions on pollution and development that help protect wetland areas throughout Massachusetts.
With enforcement of such laws, the limit on the loss of wetland areas in the state may help maintain that natural buffer of high concentrations of land-derived nutrients and coastal waters where eelgrass populations currently reside or may be able to reside in the future with restoration projects, particularly multispecies restoration projects, including shellfish for example. These may be more effective at mitigating nutrient loads than single species restoration projects alone.
- Eelgrass Fact Sheet.South Slough National Estuarine Research Reserve.
- Federal Water Pollution Control Act. 27 Nov 2002.
- Massachusetts Office of Coastal Zone Management. No Discharge Areas NDAs 30 Mar 2010.
- Massachusetts Trial Court Law Libraries.310 Code of Massachusetts Regulations.
- Massachusetts Water Resource Authority. The Massachusetts Bay Outfall
- Milewski, Inka. Nutrient Pollution:A survey of eelgrass beds in estuaries and coastal bays in northern and eastern New Brunswick .26 March 2010. Conservation Council of New England April 2004.
- No Discharge Areas in New England Waters. 2010. Region 1: EPA New England. 4 Jun 2010.
- Touchette, Brant W. et al. Variations in Eelgrass (Zostera marina L.) Morphology and InternalNutrient Composition as Influenced by Increased Temperature and Water Column Nitrate 26 March 2010.Vol. 26, No. 1