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Estuaries in NSW

Estuarine Water Quality

Nutrients

There are basically three types of vegetation growing in water: free floating microscopic algae (phytoplankton and diatoms); attached large algae (macroalgae); and rooted plants (aquatic macrophytes). Free floating and attached algae obtain their nutrients directly from the water mass. Rooted plants obtain their nutrients from the root medium, as well as from the water mass through their leaves.

Plant growth requires many different types of nutrients. However, the nutrients nitrogen and phosphorus are generally the "limiting" nutrients determining the rate and amount of growth of aquatic plants. (Silicon may also be a limiting nutrient for diatoms).

If nitrogen and phosphorus are present in estuarine waters in excessive levels ("eutrophic" conditions), they can promote the uncontrolled growth of phytoplankton (an "algal bloom") and the excessive growth of attached macroalgae and aquatic macrophytes. It should be noted that "algal blooms" appear to be natural events in many parts of the world.

Increased nutrient levels in estuarine waters originate from the discharge of raw and treated sewage effluent, from urban and rural runoff and from some industrial discharges. Urban stormwater contains animal faeces and garden fertilisers. The widespread and inefficient use of agricultural fertilisers can result in high nutrient levels in rural runoff. Excessive nutrient levels are mainly a problem in those estuaries that are poorly flushed by the tides or freshwater flows.

Excessive algal growth can have a number of adverse effects on estuarine ecosystems and aesthetic quality, which include:

Increased diurnal fluctuations in dissolved oxygen concentration in the water mass. Algae, like all plants, are net producers of oxygen during daylight hours and net consumers during darkness. Excessive algal growth causes saturated or super-saturated levels of dissolved oxygen during the daytime, but minimal levels during the night, which can lead to physiological stress in fish and other organisms. If the algal biomass is sufficiently large, all the dissolved oxygen will be consumed during the night time, leading to the death of fish and other oxygen breathing organisms (a "fish kill"). Dead fish and dead vegetation then tend to wash up on the shores, where they decompose and produce unpleasant odours.
Changes to the species of aquatic flora and fauna and numbers of individuals inhabiting the affected waters. For example, the fish that do best under eutrophic conditions are not highly valued by anglers and commercial fisherman; other algal types, such as blue-green algae, are encouraged.
Diminished aesthetic appeal of estuarine waters because of the discolouration and turbidity caused by the high levels of phytoplankton. (The excessive growth of certain species of diatoms can impart a "muddy" colour to the waters). This in turn results in a diminished recreational amenity, because poor clarity renders the water less attractive and less safe for pursuits such as swimming and diving (AEC, 1987).

Quite apart from nitrogen and phosphorus levels, a number of other factors influence algal growth. Meteorological conditions are significant (bright sunlight and still conditions appear to promote rapid growth); certain "micronutrients" or other substances may be necessary to trigger growth.

Phosphorus

Phosphorus exists in a range of chemical forms in the aquatic environment. The two most common measures are "filterable reactive phosphorus" (FRP) and "total phosphorus" (TP). FRP is a measure of the level of phosphorus that is readily recycled through the organic biomass; TP includes FRP together with phosphorus bound to particulate matter, etc. Total phosphorus is the most meaningful determination of the element in terms of nutrient levels.

Catchment runoff is the main source of phosphorus in estuaries. Phosphorus is exported from estuaries by being flushed into coastal waters or in the bodies of migratory animals. All phosphorus which remains in an estuary eventually becomes permanently trapped in undisturbed sediments in the form of insoluble calcium compounds (Emsley, 1980).

Figure 1 shows the main links in the phosphorus cycle within an estuarine system. Phosphorus is found in the water body as inorganic phosphates (PO₄^3-, HPO₄^2- and H₂PO₄^-) and as a variety of organic phosphates. Compared with a terrestrial system, the turnover of phosphorus in a water body is very rapid, especially through bacteria, phytoplankton and zooplankton (Emsley, 1980). There is usually very little soluble phosphorus in the water because of rapid uptake by organisms, adsorption onto colloidal particles and precipitation to the sediments as calcium, aluminium or iron compounds.

Estuarine consumers (filter feeders, deposit feeders, grazers and carnivores) obtain phosphorus from their food and release it as solid or soluble excreta, and from their decaying bodies. Most of the detrital phosphorus is rapidly recycled through the food web.

Low oxygen and low pH levels are required to release phosphate that is bound to sediments. These conditions are found beneath the sediment surface, but as soluble phosphate diffuses upwards into the oxygenated layer it tends to react and bind with metal ions. It may require de-oxygenation of the bottom waters, often caused by salinity stratification, to allow sufficient phosphate to enter the water column to stimulate phytoplankton growth (Bulleid, 1983).

Macroalgae, although generally attached to the substrate, obtain nutrients such as phosphorus directly from the water. Macrophytes, such as seagrasses, can absorb phosphates through their leaves, but obtain most nutrients through their roots from the sediments, as do mangroves and other emergent plants. Some phosphates are available from decaying detritus. Phosphate is also mobilised from sediments by root respiration. This process lowers oxygen levels and increases the acidity of the surrounding sediment by releasing carbon dioxide.

Nitrogen

The major forms of nitrogen occurring in estuarine waters and sediments are (Day et al, 1989):

nitrogen (N₂) and nitrous oxide (N₂O) gases
soluble inorganic salts of ammonium (NH₄⁺), nitrite (NO₂^-) and nitrate (NO₃^-)
soluble organic salts including urea and uric acid, and
other organic compounds.

Although soluble inorganic salts are the primary nutrient source for the growth of phytoplankton and other plants, the rapid turnover of nitrogen within the estuarine system makes it more practical to use total nitrogen as the criterion for water quality.

Nitrogen enters an estuary via catchment runoff, tidal transport and diffusion from the atmosphere. It is lost from the system mainly by tidal and freshwater flushing and diffusion of gases to the atmosphere. The nitrogen cycle within an estuary is complex, but the most common pathways are shown in Figure 2.

All of the different forms of nitrogen are present in land based runoff and can also be carried into an estuary by tidal transport. Nitrogen fixation, i.e. the conversion of gaseous nitrogen into ammonium ions and thence to organic forms, is carried out by bacteria and some forms of blue-green algae.

A variety of bacteria use inorganic ammonium, nitrite and nitrate ions in a number of energy producing pathways. The dominant pathways vary between different parts of the water column and sediments, depending on the concentration of oxygen and other substances. Some nitrate is converted into a gaseous form by denitrifying bacteria. This is the main way by which nitrogen is lost from the estuarine ecosystem.

Phytoplankton and other plants take up inorganic nitrogen and use it in the synthesis of larger organic molecules. Animals obtain nitrogen from their food and excrete it in the form of ammonium, urea and uric acid.

The decomposition of dead animals and plants releases nitrogen as both organic and inorganic molecules. Dissolved organic nitrogen is further broken down by bacteria to release ammonium ions.

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