5.2. How an Estuary Works

Under natural conditions, the mixing of tides and river flows acts like a pump bringing nutrient-rich bottom water to the surface and the sun.In the estuary, dead plants, plankton, fish, and other organic material sink to the bottom and decay. This produces nutrients such as nitrogen and phosphorus for new plant growth. However, these nutrients must be brought to the surface to be recycled. There, the sun, penetrating the water, encourages the growth of small microscopic plants (called phytoplankton). These plants are extremely important as food sources for many animals. In fact, phytoplankton is very important for human survival. Without them we would not have fish, and without fish, many people in the world would be without a food source.

These important phytoplankton that form the basis of food chains in estuaries are extremely dependent on the constant mixing of the water in the estuary.

Three main forces create this mixing: water movement created by river run off, the daily tidal movement, and wind currents.

Water Movement Created by River Run-Off
Fresh water is lighter than salt water and flows in by way of the river. Because it is lighter, it floats over the surface of the sea water. As it moves out to sea, it entrains salt water making the outward flow many times greater than the river flow. This draws saltwater in along the bottom of the estuary to replace the water moving out across the surface. This creates a upward mixing area near the head of the estuary which mixes the water like a pump, bringing the bottom water to the top. This mixing moves the bottom water with its nutrients to the surface feeding the phytoplankton. This mixing is strongest in spring and fall when the river flows are the strongest. Many fish species have adapted to take advantage of this abundant source of food, laying their eggs in the spring so that the newly hatched young will have a rich food source. The young ride in on the bottom flow, come to the top and feed as they ride out, dropping down again to repeat the cycle. There is lots of food with very little effort.

Tidal Changes
Daily tide changes, flowing in and out twice a day, also bring nutrients to the surface. This daily mixing is important in large estuaries where small amounts of fresh water are flowing in, or in places of high tides (the Bay of Fundy). Tides moving over banks on the sea bottom also cause upward mixing of nutrients. In this way, ocean banks (such as the edge of the Scotian Shelf or George Bank) produce more fish.

Wind Currents
When a southwest wind blows along the coast of Nova Scotia for a few days, the surface waters along the coast turn to the right and are blown out to sea. This water is replaced by the nutrient-rich bottom water, which up-wells along the coast bringing cold nutrient rich water to the surface.

Parts of the estuary and coastal zone are very important places for food production. These are estuaries comprised of a fresh water source (river), tidal wetlands (tidal fresh, brackish, salt marshes), rocky intertidal, mudflats, kelp beds, and eelgrass flats. These areas are often buried or reduced in size by coastal developments. This reduces the juvenile habitat for many species of anadromous and marine fish