What Are Estuaries

What are estuaries?

Estuaries are semi-enclosed bodies of water where fresh water from rivers or streams mingles with the salt water of the ocean. Estuaries trap nutrients and sediment that is carried from the land by rivers and from the ocean by tides. Estuaries are characterized by a constant mixing of these nutrients with the rise and fall of the tide. These conditions create an enormous amount of plant and animal life. Estuaries are therefore one of the most productive types of ecosystems on earth.

Estuaries can be found in flat river deltas or steep-sided coastal fiords. They may have different components, such as tidal mud flats, lagoons, salt marshes and sand dunes. Although estuaries make up a small percentage of coastline, they are used by almost ALL of all wildlife species on the coast. Estuaries are also popular places for people, and unfortunately many of them have been lost to urban and agricultural development.

How are estuaries formed / shaped?

Estuaries can be formed in a number of different ways. After the most recent glaciation (~29,000 to 10,000 years ago), melting ice caused the sea level to rise. Valleys that were previously carved out by glaciers and/or streams were flooded, and shallow, protected inlets and estuaries were created.

Sediment (i.e. fine-grained mineral and organic material) also plays a role in forming estuaries. Streams and rivers often carry sediment, due to natural or human-caused erosion of streams and upland areas. When the water reaches the ocean and is no longer constricted in the stream channel, it slows down, and the sediment drops out. In areas that are protected from the scouring action of large waves, this sediment can build up and spread out, eventually forming mud flats. This process is at work in Portage Inlet, which is gradually becoming more shallow as the sediment builds up. Although the sedimentation occurring in this area is natural, it has likely been accelerated by human-induced erosion.
Vegetation such as eelgrass and salt marsh grasses often colonize shallow muddy areas, and in turn trap more sediment. Salt marsh grasses themselves add organic material, and may eventually form a dense mat of vegetation that is laced with tidal channels. Thus an estuary can have many forms, and is constantly changing.

What form an estuary takes depends on the pre-existing form of the surrounding landscape, and the forces that act upon it. Controlling forces include tides, wind, waves, and currents. Longshore drift, caused by waves that contact a shoreline on an angle, transports sediment along a shoreline and can form a spit that helps to shelter an estuarine area; such is Esquimalt Lagoon.

What kinds of wildlife & plants are found in estuaries?

Estuaries support a great abundance of life. The following organisms are only some of the many that live, feed or develop in estuaries:

· Aquatic plants such as eelgrass and sedges; terrestrial plants such as dunegrass and salt grass

· Microorganisms including fungi, bacteria and protozoa

· Shellfish such as mussels, clams, snails, limpets and Native Oysters

· Segmented worms, ribbon worms, flatworms, and bristleworms

· Crustaceans such as shrimp, crabs, copepods, amphipods

· Fish such as Pacific Salmon, herring, Cutthroat Trout, stickleback and sculpins

· Mammals such as bears, cougars, river and sea otters, harbour seals, sea lions, mink and raccoons

· Birds such as Great Blue Herons, plovers, Black Oystercatchers, killdeers, sandpipers, mergansers, kingfishers, widgeons, pintails, Canada Geese, Black Brant Geese, Mallards, Surf Scoters, cormorants, eagles, ospreys and owls (an estimated five million birds use estuaries along the coast of BC)

Estuaries have been called the “nurseries of the sea” for their important role in providing sheltered habitat and food for juvenile fish such as salmon, sea-run Cutthroat Trout and herring. In these protected environments, young fish can quickly grow and gradually become accustomed to salt water.

Why are estuaries important?

Estuaries provide many “ecosystem services.” These are functions performed by natural systems that are necessary for human welfare. In the past, these functions were often overlooked and estuaries were considered useful only after they were “reclaimed” for human use. The following are some examples of how we benefit from estuaries.
Erosion control and storm surge protection: Vegetation in estuaries helps to anchor sediment and soil along river banks and shorelines. This prevents stream flows, rainwater and waves from scouring away the land. Estuaries also build up deposits of mud, silt and sand. This natural barrier helps to dissipate the energy of large waves that can otherwise inflict serious damage on human life and property.

Water Quality

Estuaries function as natural water purification systems. Vegetation and fine sediments in estuaries filter water as it flows from the land to the ocean. Bacteria living in the sediments of estuaries can also help to break down certain pollutants.

Atmospheric Gas Regulation

Estuaries tend to be “carbon sinks,” since carbon dioxide is absorbed in the photosynthesis carried out by the prolific plant growth. Carbon dioxide is released when wood or fossil fuel is burned, or when estuaries are filled in. It is one of the greenhouse gases that are thought to be at least partly responsible for climate change.

Nutrient Cycling

Estuaries help to regulate concentrations of nutrients such as nitrogen and phosphorous in the marine environment. These nutrients are needed for plant growth but in excess can cause harmful algae blooms and rob the water of oxygen.

Why are estuaries important?

Habitat for Plants, Shorebirds & Other Animals

As described above, many species of animals spend all or a portion of their lives in estuaries. Some of these species, such as Pacific Salmon, have direct commercial value. Others are important as part of the overall function of many ecosystems. Estuaries contribute greatly to the biodiversity of the marine environment.

Education, Recreation & Tourism Opportunities

Estuaries are peaceful, beautiful landscapes. Artists, canoeists, bird/wildlife watchers, hunters, fishers, photographers, scientists, children and teachers are all attracted to estuaries. The special qualities of estuaries make them ideal study and teaching environments.

Estuaries are not only one of the most biologically productive types of ecosystems, they are also one of the most valuable. It is difficult to attach monetary worth to these ecosystem services, especially since most are needed for life on earth and are not replaceable with human-made systems, at any cost. Nevertheless, some people believe that estimating the value of ecosystems, even imperfectly, will help people to make better land use decisions. Thus in a 1997 study, a conservative estimate of the value of ecosystem services performed by estuaries was US$22,832 per hectare per year. (This would give Esquimalt Lagoon a value of over $2 million per year.)

What threatens estuaries?

The main threat to estuaries is urban and agricultural development. People have practiced agriculture in the rich soils of floodplains and estuaries for thousands of years. However, modern methods that involve diking, damming and filling-in estuaries to create farmland severely affect the ecological function of the land. Today, some restoration efforts focus on removing dikes and re-introducing tidal influences. For example, one the world’s largest tidal marsh restoration projects is currently underway in Delaware Bay, USA.

Some estuarine areas, particularly where large cities now exist, have been so heavily modified that restoration would not be feasible. This makes it all the more important to protect estuaries that have not yet been lost.

Some existing estuaries are affected by poor water quality, caused by excess sediment, nutrients or chemical pollution. Activities such as logging and construction in the watershed can cause erosion and deposition of excess sediment. Too much sediment can smother eelgrass and bottom-dwelling animals. Excess nutrient levels may be caused by fertilizers or sewage in the water. Large areas of impervious surfaces prevent the natural percolation of water into the soil, and transport pollutants such as oils from road surfaces into streams and estuaries.

Invasive species also degrade estuaries by altering the natural flows of water and nutrients, and providing less habitat and food for wildlife. These effects in turn decrease the biodiversity in estuaries.