The world’s coastal ecosystems are under stress. The numbers tell the story: Over 50 percent of the global population resides within 100 kilometers of a coastline, a thin ribbon of land that includes three-fourths of the megacities with more than 10 million residents. Farms and factories needed to sustain all those people release fertilizers and toxic chemicals into our rivers, streams, and estuaries. Degraded water quality is the most obvious result, but the onslaught of nutrients has a more insidious effect. Fertilizer runoff causes coastal algae to proliferate; the algae take most of the oxygen in the water, causing fish to die and disrupting the food chain for humans and other organisms. As coastal populations keep increasing, the stresses are increasing with them.
The production of reactive nitrogen—from sources such as synthetic nitrogen fertilizers, legume and rice cultivation, and fossil fuel combustion—began in earnest during the Industrial Revolution and has increased sharply in recent decades; by 2005, global production reached 187 million metric tons of nitrogen. Since the 1970s, all that production has dramatically altered the flux of nutrients—not just nitrogen but also phosphorus from fertilizers, sewage, and detergents—from the landscape to receiving waters. The over-enrichment of such nutrients, commonly defined as eutrophication, has emerged as one of the leading causes of water quality impairments in coastal marine ecosystems.
Coastal ecosystems cover approximately 20 percent of the Earth’s surface and are extremely diverse. More than 415 of these areas worldwide are experiencing symptoms of eutrophication, according to a 2008 World Resources Institute study, and that number is growing. These areas represent half of the value of global economic services, such as tourism, recreation, coastal fisheries, and aquaculture.
Coastline waterways are important to everyone in some way: They harbor fisheries and other resources, prevent erosion, protect people from floods and storm surges, maintain ocean biodiversity, support recreation and tourism, and are culturally important to local heritages. Often the centers of activity for beachside recreation, these areas are also resources for minerals and geological products. Without these ecosystems, people would not be able to eat seafood, because 90 percent of global fishery activity occurs in coastal waters.
What is frustrating to us as marine scientists is that these problems are preventable: Human development and healthy ecosystems do not have to be at odds if communities think about sustainability. Marine and aquatic scientists are making progress in understanding the factors that control the distribution of nutrients and how they react or combine with other elements as they are transported from land to sea. Resource managers are working on strategies to manage this complex problem and identify changes that lead to coastal eutrophication in an effort to minimize the deleterious environmental and socioeconomic impacts.
The US Outlook
Some of the starkest examples of coastal eutrophication occur in the United States, where the phenomenon has multiple causes. Increased prevalence of sewage, fertilizers, and other chemicals for industry, household life, and agriculture are important contributors, but they are not the only way that people cause nutrient levels to rise. By cutting down trees, paving roads and parking lots, and developing in wetlands, nutrients are less likely to be taken up by plant roots and soil microbes or filtered through groundwater before reaching rivers and sea.
One of the most notorious examples of human-induced coastal eutrophication around the United States lies in the Mississippi River delta in the Gulf of Mexico on the continental shelf bordering Louisiana and Texas. As the Mississippi River runs through highly fertilized agricultural lands of southern Minnesota, Iowa, Illinois, Ohio, and Indiana, an estimated 1.6 million metric tons of nutrient fertilizer enters the Gulf annually from the Mississippi Basin, most of which is in the form of nitrate-nitrogen (NO3–N). Other major sources of nutrient inputs to the Gulf include residential fertilizers, nitrous oxide from fossil fuel combustion, and animal waste and sewage. Atmospheric N, fallen to the ground as dissolved compounds in rainfall and as adsorbed compounds on dust particles and debris, also contributes to increases in nutrient export. The over-enrichment of nutrients stimulates algae blooms and the development of seasonal low-oxygen zones or dead zones in the Gulf. Although these areas are called dead zones they are not actually “dead”—smaller invertebrates (like foraminiferans and nematodes) and microbes persist. Dead zones occur naturally in some coastal areas; however, the frequency and duration of this hypoxia (low oxygen) is increasing globally, particularly in coastal areas adjacent to densely populated areas. As the biological, physical, and chemical conditions of the coastal oceans change with increased nutrient pollution, the prevalence of low-oxygen zones is of increasing concern to fishermen and resource managers.
The loss of natural “sinks” for nutrients, such as wetlands and riparian zones, in coastal areas throughout the United States—largely due to agricultural expansion—have exacerbated the problem associated with coastal eutrophication, as well as the occurrence of flooding and erosion. A large fraction of the nutrients in runoff to streams and rivers ultimately makes its way to the sea. As a result, estuaries receive more nutrient inputs per unit surface area than any other type of ecosystem. As a consequence, the increases in nutrients have dramatic effects on algae biomass and species composition in coastal marine environments.
In 2002, the National Oceanic and Atmospheric Association (NOAA) reported the largest dead zone ever recorded in the Gulf of Mexico, stretching westward from the mouth of the Mississippi River in waters up to 60 meters deep and encompassing approximately 21,965 square kilometers—an area almost as large as the state of Massachusetts. In 2014 the Gulf’s dead zone, measured July 27 to August 2, was smaller than the five-year average of 14,373 square kilometers, but well above the 4,921 square kilometers that the Gulf of Mexico/Mississippi River Watershed Nutrient Task Force set as a goal for 2015—which means this year’s target will likely not be met. However, eutrophication is not just limited to the Gulf of Mexico. In the United States over half of the coastal estuaries—and even lakes—experience low-oxygen conditions, including the Chesapeake Bay, Lake Erie, Puget Sound, Narragansett Bay, and many other freshwater and marine centers.
The Global Scale
Coastal eutrophication is not just a US problem. Globally, nitrogen and phosphorus loading is expected to at least double by 2050 in coastal marine systems. In addition to more frequent occurrences of global coastal eutrophication, dead zones resulting from oxygen-depleted conditions are being identified in continental seas, including the Baltic, East China, and Black Seas. Some regions in which eutrophication is consistently present are in the Arabian Sea, the south Atlantic Ocean west of Africa, and the eastern Pacific Ocean off the coast of Africa. Many of the countries that rely heavily on marine resources in these regions will face future challenges associated with feeding their populations.
Declining dissolved oxygen levels were noted in the Baltic Sea as early as the 1930s, but it wasn’t until the 1950s that reports of this condition became widespread. During the 1940s and 1960s coastal hypoxia was reported for the northern Adriatic Sea, the northwest continental shelf of the Black Sea, and the Kattegat between Denmark and Sweden. In the 1990s, scientists reported coastal hypoxia in northern Europe, North America, and Japan. By the 2000s there were more such reports in South America, southern Europe, and Australia, as well as increasing dead zones in the Baltic Sea. As the world population increases and urbanization becomes more widespread, more coastal systems will likely become eutrophic, particularly in developing countries.
Because temperature and hydrology are inherently linked to eutrophication, climate change has important implications for the scope of this problem in the future. Changes to global and regional climates have the potential to make coastal and marine ecosystems even more susceptible and vulnerable to hypoxic conditions. Climate changes and extreme weather events may further exacerbate hypoxia in coastal waters. If runoff increases because of higher rainfall or flooding, more water, nutrients, and sediments will be delivered to coastal areas, where they are likely to amplify eutrophication through nutrient production, increased stratification, or both.
Even in the absence of global climate change, scientists anticipate more coastal systems to become eutrophic. As noted in a 2013 document by the Intergovernmental Panel on Climate Change (IPPC), “If current trends continue, human-related activities and the impacts of climate change are expected to further exacerbate human pressures on coastal ecosystems resulting from excess nutrient input, changes in run-off, and reduced sediment delivery.” The potential consequences could be devastating, since many coastal communities, both small and large, and biologically diverse hotspots depend on the vitality of these ecosystems.
Minimizing Coastal Eutrophication
Protecting coastal systems from the many adverse effects of eutrophication is one of the most pressing ecological issues facing the world in the next century. In response, scientific institutions and government agencies are working together to minimize the environmental and socioeconomic impacts of eutrophication to coastal ecosystems.
Ongoing efforts include large-scale restoration projects committed to reduce nutrient loading in the Gulf of Mexico, Chesapeake Bay, and Baltic Sea. Although substantial challenges arise when working across political jurisdictions and scientific disciplines, these groups have successfully developed watershed implementation plans that include best management practices and modeling tools for nutrient and sediment reduction. These comprehensive plans include actions that not only focus on reducing source pollution from agriculture but also include municipal and industrial wastewater and urban stormwater runoff mitigation strategies. The benefits of such projects not only focus on watershed health for the environment but improvements to commercial fisheries and recreation.
For example, the 2008 Gulf Hypoxia Action Plan described a national strategy to reduce and control nutrient runoff in the northern Gulf of Mexico and improve water quality in the Mississippi Basin. As noted in the 2013 Reassessment of the Action Plan, there has been great progress in nutrient reduction strategies: the United States Department of Agriculture (USDA) provides assistance for conservation measures; science and monitoring is ongoing; and all stakeholders are optimistic that the goal of reducing the dead zone remains reasonable.
Similar efforts are ongoing in the Chesapeake Bay. One of the most important goals of the Chesapeake Bay Program is to develop and implement watershed management plans for two-thirds of the total watershed acreage in Maryland, Pennsylvania, Virginia, and the District of Columbia. These plans help guide local communities in the protection and restoration of streams, forest buffers, wetlands, parks, and other natural resources. As of 2005, nearly 14 million acres of the Chesapeake Bay have developed watershed plans. Currently, partners are working on implementing plans to reduce pollution in the bay. Much progress has been made in restoring key wetlands on agricultural lands to reduce nutrient runoff. Between 2010 and 2013, approximately 6,098 acres of wetlands were established or reestablished on agricultural lands. In 2014 key stakeholders signed the Chesapeake Bay Watershed Agreement, which establishes goals, outcomes, and management strategies to guide the restoration of the bay, its tributaries, and surrounding lands. However, there is still work to be done. In 2015 the Chesapeake Bay Foundation reported minimal changes in the overall health of the bay, as improvements in water quality were offset by declines in fisheries.
Nonprofits play a vital role when working collaboratively with the public, private landowners, the scientific community, and decision makers on issues that affect natural resources. The Nature Conservancy’s Mississippi River Program and Gulf of Mexico Initiative are prime examples of a successful partnership with farmers and other stakeholders to slow or even reduce the growth of the Gulf’s dead zone—and its effects throughout the region—by promoting more effective use of fertilizers. For example, the Nature Conservancy helps farmers build two-stage drainage ditches that benefit both agriculture and the environment. The design incorporates a floodplain zone (decreasing water velocity), improves ditch stability by reducing water flow, and has the potential to maintain better habitat conditions. Since 2003, the Nature Conservancy has been working to design two-stage ditches in headwater sites throughout Indiana; these sites have the greatest potential to improve downstream water quality. In 2007, the Nature Conservancy received funds to continue their efforts in Indiana, Michigan, and Ohio to help improve water quality in the Maumee River—one of the largest sources of pollution to Lake Erie. In addition, the Nature Conservancy also helps restore wetlands and forests near waterways that transport nutrients to the Gulf.
What is frustrating to us as marine scientists is that these problems are preventable.
At the local level, state agencies are developing tools to help improve surface water quality. For example, the Wisconsin Department of Natural Resources Bureau of Water Quality developed the Erosion Vulnerability Assessment for Agricultural Lands (EVAAL) to help resource managers prioritize areas for conservation within a watershed that may be vulnerable to erosion and increased nutrient loss. EVAAL uses readily available GIS data sets, which include soil, topography, and land cover, to identify areas vulnerable to erosion. The output is a series of maps that show areas of soil vulnerability. This tool enables watershed managers to prioritize areas while increasing the probability of locating fields with high sediment and nutrient export.
Even more impressive is the successful collaboration between scientists, restoration ecologists, and engineers who are working to reconstruct strategically placed wetlands in the Mississippi-Ohio-Missouri Basin as a solution to the recurring hypoxic conditions in the Gulf of Mexico. William Mitsch of Ohio State University leads this effort; his group estimated that creation or restoration of 2.2 million hectares of wetlands is required to remove 40 percent of the total nitrogen discharging to the Gulf. Large-scale restoration projects, supported by the USDA, have begun in the Mississippi-Ohio-Missouri Basin. Currently, comprehensive monitoring to determine their effectiveness and applicability to other regions of the basin is under way. This large-scale wetland construction not only has the potential for ecological benefits to reduce Gulf hypoxia but also includes water quality improvements, reduction of public health threats, habitat creation, and flood mitigation.
Although these large-scale restorations have the potential to reduce a significant amount of nutrients before they enter coastal water bodies and millions of dollars are spent annually on stream and wetland restoration projects such as the ones described above, very few are monitored for their effectiveness and applicability to other regions. Many of our daily activities have the potential to cause nutrient pollution. Thus, to meet the challenges of protecting coastal ecosystems, government agencies and coastal communities need to continue to develop strategic watershed management plans that account for nutrient inputs, exports, and transformation in the system and include adaptive nutrient reduction strategies to minimize export. And they must work with local, state, and federal agencies to develop and implement cost-effective strategies that maximize nutrient loss on the landscape, building on the models established for the Gulf of Mexico Action Plan and Chesapeake Bay Program.
Above all, people need to recognize eutrophication as a problem. Local nutrient cycles and exports to coastal communities are intricately linked to changes in regional and global climate patterns. In the future the two must be viewed in a more synergistic manner to better anticipate threats and enact healing strategies for coastal communities.
- CENR. 2010. Scientific Assessment of Hypoxia in U.S. Coastal Waters. Interagency Working Group on Harmful Algal Blooms, Hypoxia, and Human Health of the Joint Subcommitee on Ocean Science and Technology, Washington, DC.
- Conley, D. J., et al. 2011. Hypoxia is increasing in the coastal zone of the Baltic Sea. Environmental Science & Technology 45:6777–6783.
- Diaz, R. J., and R. Rosenberg. 2008. Spreading dead zones and consequences for marine ecosystems. Science 321:926–929.
- IPCC. 2013. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge.
- NOAA. 1999. Trends and Future Challenges for U.S. National Ocean and Coastal Policy. Center for the Study of Marine Policy, University of Delaware, Washington, DC.
- Rabalais, N. N., et al. 2014. Eutrophication-driven deoxygenation in the coastal ocean. Oceanography 27:172–183.
- Smith, V. H. 2003. Eutrophication of freshwater and coastal marine ecosystems: A global problem. Environmental Science and Pollution Research 10:126–139.