The Elizabeth River is a short tidal estuary at the southern end of Chesapeake Bay in southeast Virginia in the United States. It is located between the cities of Portsmouth and Norfolk. The estuary provides significant military and commercial port facilities for the two cities. Source: U.S. Navy photo 040213-N-7412M-001 Portsmouth, Va. (Feb. 13, 2004) by Chief Photographer’s Mate Greg McCreash.
If you visit or live by the coast, then you have undoubtedly taken advantage of your nation’s estuaries. Estuaries are the bodies of water “where the river meets the sea.” They are:
- located next to many of our largest and oldest cities
- used for recreational fishing, boating, swimming, and bird watching
- and provide many ecosystem services that bolster our quality of life and economy.
Americans use these coastal ecosystems intensely, which affects how the ecosystems function, and their ability to provide services. The feedback loop between ecosystems supporting human activities through ecological functioning, and the impacts of humans on the ability of ecosystems to function naturally (see Figure 1), is prevalent throughout the world and across many ecosystem types.
The feedback is particularly strong in estuaries because of:
- the dense populations of humans in their area
- human activities in coastal areas
- and the rich diversity of natural services provided by estuaries. Thus, the wise management of estuaries depends on recognizing the feedback and mitigating the anthropogenic (i.e., human-caused) influences.
What is an estuary?
The simplest definition of an estuary is where the river meets the sea. But that definition does not capture the entire range of ecosystems called estuaries. In fact, scientists have no single definition of an “estuary” that is accepted universally. Consider the following diverse examples of American coastal ecosystems that could be called estuaries:
- the massive Chesapeake Bay that links several major rivers to the Atlantic Ocean through a wide mouth nearly 20 kilometers wide;
- Albemarle-Pamlico Sound, behind the outer banks of North Carolina and at the mouths of several major rivers, which is largely isolated from the Atlantic Ocean, except for a few small inlets;
- the Mississippi River delta that has resulted from geological shifts in the River and has a plume of turbid river water extending into the Gulf of Mexico;
- the Laguna Madre, Texas, which sits behind Padre Island and has no major river flow, so salinities often exceed those of the Gulf of Mexico; and
- Old Woman Creek, Ohio, which drains into Lake Erie and that has no saline waters.
In 1967, a somewhat restrictive definition of “estuary” was offered: An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which seawater is measurably diluted with freshwater derived from land drainage.1 This definition is still used today, sometimes slightly modified, and it is close to the “river meets the sea” definition because it highlights the dilution of seawater.
Many, but not all, estuaries have salinities (i.e., concentration of salt in water) intermediate between seawater and freshwater. This dilution process may be fostered by tides. Tidal “waves” move water in and out of the estuary from the ocean and may help mix the fresh and salt waters. When the tidal energy is not strong enough, the less dense, fresh, water from rivers may ride above the denser salt water from the sea, forming what is called a salt wedge or stratification. Each estuary has its own pattern of dilution and stratification based on the intensities of river flow and tides and connection to the sea. Some estuaries, such as the Laguna Madre, even concentrate salt through excess evaporation compared to those estuaries where dilution is the pattern.
One of the broadest definitions of an estuary, which removes the interaction of fresh and seawater is: An estuarine system is a coastal indentation that has a restricted connection to the ocean and remains open at least intermittently.2 This definition still does not include coastal systems of the Great Lakes, such as Old Woman Creek, that have no neighboring ocean. These systems are sometimes called “lacustrine” or “lake” estuaries.3
To further complicate the definition, estuaries often have considerable wetlands associated with them:
- Some wetlands exist within the body of the estuary at depths below the low-tide mark. These are dominated by a variety of plant species, collectively called submerged aquatic vegetation; and many are considered sea grasses.
- Other wetlands are flooded with estuarine water—at least periodically.
- In temperate areas, most of these wetlands are marshes—although in fresh and low salinity regions, wetland forests may be found.
- In subtropical and tropical regions, mangrove trees may dominate the wetlands.
While the definitions focus on the “water” of estuaries, the wetlands are generally considered part of the estuary for both scientific and environmental management reasons. Many of the estuarine animal species use the wetlands for food, habitat for breeding, or refuge from predation.
Although the definition of estuary may not be established universally, most researchers agree that estuaries are an important interface where the activities of land and large water bodies interact. Further, they are the sites where human activities interface with high levels of biological productivity and ecological activity.
Ecosystems process materials and energy to grow, develop, and respond to outside forces, as well as to maintain themselves. Major, recognized processing occurs in three ways:
- Cycling of the various natural elements through different chemical forms and among organisms and their physical environment. Cycling of carbon, nitrogen, phosphorus, oxygen, and sulfur, is particularly important, as these are found in all living organisms in high concentrations. Carbon serves as the “backbone” of organic matter. The availability of the nutrients nitrogen and phosphorus often controls the amount of organic matter production through photosynthesis. Oxygen availability may determine the livability of a habitat because it is required by all animals and because of the prevalence of its chemical reactions with other elements. Forms of sulfur can determine other reactions, as well; and hydrogen sulfide may stress and damage organisms.
Interactions within food webs, via trophic dynamics. Energy from the sun promotes the primary production of organic matter within plants and algae, and these organisms store some of that energy. These plants and algae provide food for others: directly as they are eaten; indirectly after death and processing of the resultant “detritus”; or indirectly, through the feeding of predators on prey who have derived food directly or indirectly from the plants and algae. The seafood we harvest from our estuaries comes from these food webs.
Water motion, storage, and associated energy for moving or depositing materials. Each estuary has its own morphology, regime of fresh and saltwater inputs, and tidal signal, which controls how materials move around the estuary; how long this takes; and ultimately, how the estuary is shaped. The biological activities of elemental cycling and food webs, as well as how ecosystems are maintained as sea-level changes, are dependent on these features.
All of the above activities are linked to the behavior of organisms. Collectively, these activities have been called “ecosystem functions.” We now understand that humans benefit from these ecosystem functions. Thriving, healthy ecosystems generally provide more benefits than stressed and damaged ecosystems. Healthy estuaries are known to be particularly beneficial through the ways that we use estuarine resources. A widely cited and controversial article estimated that the functions of estuaries have the highest financial value per area of any ecosystem on Earth.4 These functions and resources provide the “services” of the ecosystems. The protection and management of estuaries depends upon the recognition we place on the value of these services.
Estuaries and their wetlands provide the following services to humans:
- Food. Many of the seafood species we eat are harvested from an estuary, or depend on estuaries for some part of their life—these species include both shellfish and finfish. Estuaries support both commercial and recreational fisheries.
Waste and runoff treatment. Nutrients and organic matter enter estuaries from a variety of sources. Wastewater treatment plants and septic systems decompose much of human wastes, but some nutrients continue to escape into our water ways. Stormwater runoff from urban and agricultural land use is often untreated, allowing additional nutrients into the estuary. The primary production, decomposition, and elemental cycling within estuaries remove and sequester these nutrients and remove organic matter.
Recreation. Recreational fishing, boating, swimming, and bird watching depend on both the physical and biological integrity of estuaries.
Protection from natural hazards. Coastal systems are highly susceptible to a variety of storms capable of rearranging the landscape and damaging human lives and structures. Estuaries, and their wetlands, can mitigate the impacts of these storms by dissipating river discharge, dampening wave action, and storing water temporarily.
Transportation and marine operations. One reason that many of our oldest cities were located on estuaries was for access to ready ship transport that were linked to safe ports. This is still true, and this commerce depends on a continued connection to the sea. Refuge from storms is also important for marine commerce.
Pressures on estuaries
Estuaries are subject to many pressures from human activities and stresses from natural phenomena, which may result from activities on or adjacent to estuaries, or even activities distant from the areas. For example, by sitting at the mouth of rivers, estuaries receive pollutants entering waterways from entire watersheds. By providing ports for ocean-going ships, estuaries receive non-native species from ballast water, and these organisms can establish themselves in ways to disrupt normal food webs. The Coastal Ocean Observations Panel listed the “drivers of change” on coastal aquatic ecosystems,5 and with some modification, these serve as a good summary of pressures on estuaries:
- Physical restructuring of the environment
- Alteration of the hydrological cycle
- Harvesting living and non-living resources
- Alteration of elemental cycles (including acidification of sea water)
- Sediment inputs
- Chemical contamination (including nutrients causing eutrophication)
- Inputs of pathogens
- Introductions of non-native species
- Global warming and sea-level rise
- Storms and other extreme weather events
- Seismic events
- Regional scale ocean currents
- Waves, tides, and storm surges
- River and groundwater discharges
Given the diversity of estuaries and environmental conditions surrounding them, each estuary is impacted to different degrees by these drivers and with different relative importance. For example, an estuary with a heavy concentration of oil refineries would be subject to different dominant pressures than one with a large infrastructure for tourism, or one associated with artisan fishing by tropical islanders. This means that both environmental assessments of condition and management options cannot take a “one size fits all” approach.
In science, however, one looks for commonalities. Ivan Valiela, of the Boston University Marine Program has studied the various drivers of change on coastal ecosystems.6 He found that loss of habitat and eutrophication (when excess nutrients deplete oxygen) were the most import drivers, and these are caused within the system or watershed. Global stressors of temperature increase and sea-level rise were among the next most important group. While this prioritization may not be appropriate for all estuaries, it serves as a useful reference by which a specific estuary can be gauged.
Several Federal agencies have jointly assessed estuarine natural resources, and hence, the results of these drivers in the United States during recent years.7,8 They identified five indicators of ecological health through the following indices:
- Water quality (largely measured by chemical concentrations)
- Sediment quality (chemical condition of sediments)
- Benthic quality (condition of the community of organisms living on and in sediments)
- Coastal habitat (based on wetland and tidal flats)
- Fish tissue contaminants (concentrations of chemical contaminants in fish and shellfish)
Water quality had the highest score in the two studies. Much of our management efforts have gone to improving water quality, and it seems that these are paying off. More efforts are required to address the other areas.
Management: observe, assess, preserve, and restore
It is clear there is strong feedback between the estuaries’ abilities to provide ecological services to humans through ecosystem functions, and the impacts of humans on the ability of these ecosystems to function naturally (Figure 1). Improving and maintaining the environmental quality of estuaries is not altruistic, as this helps to ensure that we will obtain the services we rely on. How do we make such assurances?
It is also clear that estuaries associated with human activities are subject to numerous pressures of global climate change—locally and within watersheds. The long history of humans living on the shores of estuaries has led to few pristine estuaries. Conditions in these ecosystems are likely to worsen, at least in the near future, as populations and standards of living increase throughout the world. Environmental management differs among countries; we know much about what needs to happen, but it is unclear if the political will is strong enough to foster management now and into the future.
The approach to managing estuaries is similar to most, if not all, ecosystems: evaluate the condition of the system and the pressures on it, and then act to reduce those pressures and mitigate their impacts. Environmental monitoring programs are mandated at many governmental levels. In recent years, “observing systems” have expanded the nature of these monitoring systems by linking these to other systems and to mechanisms that can communicate findings and predictions to the government, as well as non-governmental organizations, industry, and the general public.5 Local assessments on individual estuaries are conducted based on similar data as used in the national reports. In addition, international assessments have been conducted;9 thus, there is a growing recognition for the need for both long-term and comprehensive observations and assessments at numerous scales.
Once the problems are known and assessed, environmental management strategies need to be put in place and carried out effectively. A major challenge to environmental management of estuaries is to weaken the feedback loop that fosters the damage of estuarine resources as pressures continue to build. Generally, management strives to preserve environmentally healthy conditions through permitting and control on activities. Various levels of government have laws and regulations to help in this way. For example, the overharvesting of fish may be curbed by managing the fishing season, limiting licenses, or setting restrictions on catch; and mineral extraction may require an extensive permitting process. Once damage occurs, regulations may be in place to restore conditions. As an example, at least in the United States, wetland loss often requires that other, previously impacted wetlands be restored. Strategies are evolving from addressing single issues at a time to addressing overall ecosystem health through “ecosystem-based management.” Actual regulations, however, often involve the permitting or mitigation for a single practice. Because of the intense pressures of human populations, and their desires to make use of the ecological services of the coast, it is difficult to strike a balance between stewardship and exploitation.
Acknowledgments: Support for this effort was partly funded by NSF grant DEB-0621014 to the Virginia Coast Reserve Long-term Ecological Research site and by NOAA grant NA05NOS4781184.
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