L'importance des estuaires

Les estuaires sont des endroits où les poissons, les oiseaux et les animaux de toutes sortes se réunissent pour se nourrir, trouver refuge, croître jusqu'à l'âge adulte, et organiser leurs migrations. 

Les estuaires sont des endroits uniques, fortement affectés par les marées, où la terre, la rivière et la mer se fusionnent en un complexe naturel dynamique. Même si au Canada Atlantique nous ne voyons pas de forêts tropicales et peu de récifs coralliens, nous vivons parmi et à côté de myriades d'estuaires qui soutiennent historiquement une abondante vie sauvage.

In coastal communities, the local "estuary" is called by any number of names -- bay, sound, lagoon, inlet, river mouth, salt marsh. 

Purchase:
Habitat Lost: Taking the Pulse of Estuaries in the Canadian Gulf of Maine

The Importance of Estuaries

Harvey, J., D. Coon and J. Abouchar
"Habitat Lost: Taking the pulse of estuaries 
in the Canadian Gulf of Maine." Fredericton: 
Conservation Council of New Brunswick Inc.
1998

n the Atlantic coast, estuaries are among the most important coastal features, both ecologically and with respect to human settlement and use (Environment Canada, 1987). Estuaries are semi enclosed bodies of water formed when fresh water from rivers and coastal streams flows into and mixes with salt water of the ocean. The fresh water is slowed from streaming into the open ocean by either surrounding mainland, peninsulas, barrier islands, or fringing salt marshes. This mixing of fresh and salt water creates a transition zone between land and sea known as an estuary. These are places where fish, birds and animals of all sorts congregate to feed, find refuge, grow to adulthood, and stage migrations. Estuaries are unique places, strongly affected by tidal action, where land and river and sea merge into a dynamic natural complex.

 
Western Head, Musquash
(photo: Musquash MPA Campaign website, CCNB)

Most definitions of estuaries do not reflect the uniqueness of these waters as habitats. They fail to convey the dynamic nature of the physical processes operating in estuaries, or to explain the roles that these processes play in shaping the character of aquatic and terrestrial life in and around estuaries (Lippson et al, 1979). Many commercially and recreationally important species depend on estuaries. However, since commercial and recreational fishing occurs predominantly offshore or in rivers, the vital life history and energetic connections with estuaries have largely been disregarded.

Estuary Productivity and Function

Estuaries rank along with tropical rainforests and coral reefs as the world's most productive ecosystems, more productive than both the rivers and the ocean that influence them from either side. While we in Atlantic Canada see no rainforest and few coral reefs, we live amidst and beside myriad and diverse estuaries that support our historically abundant wildlife.

In an estuary, nutrient-rich river
waters combine with warmer, 
light infused shallow coastal waters and the upwelling of nutrient-rich deep ocean waters to generate primary productivity. The mixing of lighter fresh water and heavier salt water trap and circulate nutrients such that they are often retained and recycled by benthic (bottom dwelling) organisms to create a self-enriching system.

Estuaries can generate year-round primary production from macrophytes (seaweeds, sea grasses and marsh grasses), benthic microphytes (mud algae) and phytoplankton. Estuaries are also the beneficiaries of energy subsidies through the tidal transport of food and nutrients and removal of wastes.

This vast primary productivity is the cornerstone of the estuarine food chain, providing food for large populations of shellfish such as clams, mussels, oysters or quahogs (Environment Canada, 1987). In some estuaries, where productivity exceeds what can be used within the estuary, the action of regular tidal flushing moves nutrients and organic materials to adjacent coastal waters thereby increasing their productivity.

In areas of extremely high tides such as the Bay of Fundy, this export of nutrients to adjacent waters does not seem to occur. Instead, the estuaries themselves use up everything they produce, making them incredibly efficient in their production of marine life. The primary source of nutrients in the Bay of Fundy proper are the upwellings that occur at the mouth of the Bay. These are such prolific nutrient generators that they export nutrients out of the Bay and into the larger Gulf of Maine.

Estuary Habitat

While every species has unique needs that must be served by its habitat, there are a number of general habitat types in Gulf of Maine estuaries which define the ecology of each estuary. These classes of habitat are: fresh/brackish marsh and water; dunes and vegetated beach ridges; sand flats; salt marsh and salt ponds; mudflats; oyster and mussel bars; rockweed (found on bedrock); beaches (sand, gravel, cobble and boulder); sea bottom (mud, sand, gravel, cobble, boulder or bedrock); shallows (ledges, bars, reefs, shallow bays); seaweed beds (i.e. kelp) and eelgrass beds. All these different types of estuary habitat support vigorous marine food chains and attract a vast array of fish, marine mammals and birds. Some of the more familiar types of intertidal (between the low and high water marks) habitats in Bay of Fundy estuaries include rocky shores, mudflats and salt marshes.

Habitat is the combination of physical features and living organisms that provide food, nesting and resting areas, and shelter for fish and wildlife.

Estuaries and Fish

Estuaries provide a nursery for the larval forms of some marine fish species, and provide shelter and food for many young and adult fish and shellfish. These in turn provide food for other levels of the food chain including shore birds, waterfowl, larger fish and marine mammals. Many seafood species such as lobster, herring, menhaden, gaspereau, crab, oyster and clam rely on the rich food supply of estuaries during some part of their life cycle (Environment Canada, 1987).

Many species of fish live their whole lives in northern estuaries; others migrate short distances into or out of estuaries; still others migrate from offshore and from estuaries into fresh water to breed. Non-migratory estuarine fish are mostly small fish that rarely venture into the open coast but usually range far up the estuary into brackish and even fresh water. These include: tomcod; white perch; smooth flounder; mummichog; silverside; pipefish; sticklebacks (three-spined, four-spined and nine-spined); and winter flounder. Migrant fish that pass through estuaries on spawning runs include: smelt; Atlantic salmon; sea sturgeon; sea lamprey; gaspereaux (alewife and blueback herring); shad; striped bass; brook trout; and American eel.

In marine areas of the lower estuary, common bottom dwellers include lobsters, oysters, crabs, mussels, clams, scallops, flounder, hake and cod. Cod and flounder spawn in marine waters just outside estuary boundaries (Maine Coastal Program, 1991). Striped bass spawn in fresh or brackish waters; males feed predominantly near the waters where they were hatched and females return to the waters where they were born to spawn (U.S. Department of the Interior, 1993).

In the US, the National Oceanic and Atmospheric Administration (NOAA 1994) compiled information on the distribution and abundance of fishes and invertebrates in North Atlantic estuaries, with Passamaquoddy Bay in New Brunswick (Quoddy Region) the most northerly study area. Of 13 shellfish and 45 finfish studied, seven shellfish and 18 finfish are categorized as abundant or highly abundant during at least one life history stage in Passamaquoddy Bay.1

Five of the fishes are abundant in Passamaquoddy Bay during all five life stages - adult, spawning adult, juvenile, larvae, eggs. Of these five, Atlantic tomcod, mummichog and winter flounder are abundant in the fresh tidal or mixing zone of Passamaquoddy Bay (encompassing the St. Croix river and estuary) during at least part of the year.

Mummichog (Fundulus heteroclitus)

(photo: MRRI | NOAA CSC)

According to research done in the US, estuaries are essential to the fishing industry, providing spawning, nursery and feeding grounds for 60-90 percent of the fish species of commercial interest along the northern coast. Each acre of Atlantic coast estuaries is estimated to produce up to 125 pounds of commercial fish. Approximately 87 percent of the dollar value of US finfish harvests are species whose life cycles depend on coastal habitats. The combined commercial and recreational catch of these species contributes over $30 billion US annually to the economy of the United States (US Environmental Protection Agency, 1992).

While relatively little research has examined the importance of Atlantic Canadian estuaries specifically, we can assume that they make comparable contributions to commercial and recreational fisheries in the Maritimes. Berrill and Berrill (1981) report that well-mixed estuaries are particularly valuable as feeding grounds for young fish because phytoplankton and zooplankton populations remain dense throughout the summer months. Because of the tidal action in Fundy and adjacent estuaries, they are indeed very well mixed.

References

Berrill, M. and D. Berrill (1981). A Sierra Club Naturalist's Guide: The North Atlantic Coast. San Francisco: Sierra Club Books.

Environment Canada (1987). A profile of important estuaries in Atlantic Canada. Environmental Quality Division, Conservation and Protection, Atlantic Region. Dartmouth, NS.

Lippson, A. J. et al. (1979). Environmental atlas of the Potomac Estuary.

NOAA (National Oceanic and Atmospheric Administration) (1994). Distribution and abundance of fishes and invertebrates in North Atlantic estuaries. US Dept. of Commerce, Maryland.

US EPA (Environmental Protection Agency) (1992). Protecting coastal and wetland resources: A guide for local governments. US EPA Office of Water; Office of Wetlands, Oceans and Watersheds; Office of Policy, Planning and Evaluation.