Estuarine Ecosystems

The special habitat of an estuary is governed by the tide which pushes 3,000,000,000 gallons or 13,600,000,000 litres of the North Sea in and out of Montrose Basin twice each day. (See Basin Tide Tables)

Estuaries are very rich environments that produce comparatively large amounts of plant and animal material compared to the adjacent environments. The meeting of fresh water from rivers and run-off from fields with sea-water produces a region of intermediate salinity that changes radically with the tides. (Fresh water contains less than 0.5% salt and sea water averages 35% salt). These exacting conditions of changing salinity reduce the number of different species that can survive but supports very large numbers of each type.

This change in salinity with the tide produces the greatest stress for the estuarine creatures that have permeable outer bodies as the differences in salinity between body fluids and surrounding water creates osmotic pressure across the body wall. This results in fluid being forced from the low concentrations to high concentrations. i.e. an organism with a high salt concentration in low salinity water will swell as water is forced into the body through the body wall and ultimately rupture it and kill the organism. Also the opposite situation can arise where fluid is forced out from an organism into the surroundings with similar result. Osmoregulation is the process of changing the bodily salt concentration to prevent this osmotic stress.
Learn more about mud organisms.

A large amount of detritus (see definition below) is produced in estuaries from adjoining salt marsh (plant material fragments) and from the FSI (Fresh water-Sea water Interphase) where river phytoplankton is ruptured due to the osmotic pressure of increasing salinity and sea borne phytoplankton lyses due to decreasing salinity. All this raw material is acted upon by bacteria which releases the constituents in a form suitable for the detrivores. The contribution to detritus from the sea is greater than that from fresh water sources.

The detritus supports the main estuarine food web where the detritus feeders are themselves food for predators such as birds and fish. All stages in this web will eventually produce more detritus when individuals die and are decomposed by bacteria.

Research on the Ythan estuary has shown that fish consume 3 times the amount of food taken by birds.


Detritus - All types of biogenic material in various stages of microbial decomposition. i.e. decomposing animals and plants