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Sources of salinity in coastal environments
Sources of salinity in coastal environments
Salinity is a measure of the concentration of dissolved salts in the water. In coastal environments the main salt present is sodium chloride (NaCl), which is common table salt.
Fluvial
Fluvial
Fluvial river systems contain fresh water, defined as naturally occurring water with low concentrations of dissolved salts.
Fresh water contains less than 0.05% dissolved salts.
Marine
Marine
Marine systems contain seawater which has high concentrations of dissolved salts.
Seawater contains about 3.5% dissolved salts.
Brackish
Brackish
Brackish water is water having more salinity than freshwater, but not as much as seawater. It occurs as the result of seawater mixing with fresh water together, as in estuaries.
Brackish water contains between 0.05% and 0.3% dissolved salts.
How excess salinity affects plants
How excess salinity affects plants
Water diffuses into the roots by the process of osmosis, which involves the movement of water from regions of low salt concentration (in soil) to regions of high salt concentration (inside the root cells). When salt concentrations in the soil are high, the movement of water from the soil to the root is slowed down. When the salt concentrations in the soil are higher than inside the root cells, the direction of water movement reverses and the soil draws water from the root causing the plant to wilt and die.
The damaging effects of salt on plants are caused not only by osmotic forces, but also by toxic levels of sodium and chloride. Sodium ions are particularly toxic to plants in high concentrations.
Plants vary in their sensitivity to salt. Those with the lowest salt tolerance include tomatoes, onions and lettuce. At the other extreme are 'halophytes' (meaning 'salt plant'). Halophytes comprise around 2% of plants and occur most frequently in coastal river and marine ecosystems including seagrasses, mangroves and saltmarshes.
Salinity influence on Minnamurra River's plant communities
Salinity influence on Minnamurra River's plant communities
The seagrasses, mangroves and saltmarsh plants growing along the Minnamurra River are facultative (as opposed to obligate) halophytes. This means they do not rely on the salty environment to survive but they exist there because they have adaptations allowing them to survive such harsh conditions. These plants would also thrive in freshwater environments but are not typically found there due to competition from freshwater specialist plants.
The seagrass, mangrove and saltmarsh plant communities growing along the Minnamurra River are exposed to twice daily tidal fluctuations of brackish water. Not only do these plants need to cope with fluctuating salinity due to tides, they also need to cope with intermittent periods of low salinity after flood events and high salinity due to evaporation in high temperatures and drought conditions.
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