Shallow Water Habitats

Nutrients, especially nitrogen and phosphorus, are key indicators of water quality in estuaries and other shallow coastal regions. Plants require many nutrients (e.g., carbon, nitrogen, phosphorus, oxygen, silica, magnesium, potassium, calcium, iron, zinc, copper) to grow and reproduce. Of these, nitrogen and phosphorus are the most essential for aquatic plants and algae. 

Nitrogen and phosphorus naturally enter estuarine and coastal waters when freshwater runoff passes over geologic formations rich in phosphate or nitrate, or when decomposing organic matter and wildlife waste get flushed into rivers and streams. Manmade sources of nutrients entering marine and estuarine ecosystems include sewage treatment plants, leaky septic tanks, industrial wastewater, acid rain, and fertilizer runoff from agricultural, residential and urban areas.

Too much nitrogen and phosphorus acts as a pollutant in the water. This leads to explosive blooms of both micro- and macroalgae that cloud the water, shade sea grasses and the benthos and deplete both the water column and sediments of the oxygen that is critical for aquatic animals. The process of overfertilization has many effects on an estuary and is called eutrophication.  Excessive nutrient concentrations have been linked to hypoxic (very low oxygen) conditions in more than 50 percent of U.S. estuaries. Under the worst conditions, the waters of an estuary can become anoxic (having no oxygen)

Phytoplankton require nitrogen and phosphorus in an approximate ratio of 16:1 (called the Redfield ratio).  At the freshwater head of estuaries, phosphorus tends to stick to particles and is less available to support phytoplankton production, while dissolved nitrogen is often in excess.  During transport down the estuary, nitrogen uptake by phytoplankton reduces dissolved nitrogen while salinity down-estuary allows for the release of phosphorus from particles. 

Thus, phosphorus tends to limit phytoplankton production toward the head of estuaries, while nitrogen usually limits phytoplankton production toward the mouth and in coastal waters.  When both nitrogen and phosphorus are in excess, silicate may limit production of some phytoplankton (especially diatoms), which dominate the water column in most estuaries.  This can cause a shift in the composition of the phytoplankton community. High nitrogen concentrations coupled with low silicate concentrations have been linked to algal blooms such as red and brown tides, some of which produce harmful toxins.

For further information about nutrients and their effects in estuarine and coastal marine habitats refer to the following:

Mann, K. H. 2000. Ecology of coastal waters, with implications for management.  Blackwell Publishing; Chapters 2 -7 have relevant sections on nutrients and their effects.

OzCoast and OzEstuaries website

OzCoasts and OzEstuaries Conceptual Models