Wetlands 1: Water - the Key Factor

WETLANDS 1: Water - the Key Factor

Mamukala Wetlands, Kakadu National Park (Ian Morris)

Water is the key factor in any wetlands system. Wetland plants, and indirectly animals, are affected by the soil types and by the water regime of their wetlands. The water regime of a wetland means the amount, the quality and the timing of water entering the system. The timing will depend on the weather patterns of the region and will usually be either runoff from local rains or from rain falling some distance away. Some water will enter wetlands from underground sources, such as springs, or in some cases from irrigation storage dams. In more complex wetlands, water may enter from a number of sources.

Figure 1. Dynamics of the coastal wetlands environment.

The quality of water depends on what is carried along with it as it enters the wetlands. Water can transport soil, silt and sand and floodplains are built up from these materials, which have often been carried long distances from their source. The clarity of the water is affected by suspended soil particles: turbid water is very cloudy, and water plants find it difficult to carry out their food-making processes, photosynthesis, which depend on sunlight. One cause of turbid, muddy water is a high level of soil erosion in the catchment area, and this underlines the fact that the effects of careless land use, like overgrazing, clearfelling, may be often felt some distance away from the cause.

Salinity & Acidity
Salinity of the water is an important factor. Some plants and animals are tolerant of a range of salinities, whilst other species are sensitive to the slightest changes in the salt content of water. Salinity may vary from season to season, decreasing with the wet season rains (November-April) in the Top End. Conversely, salinity levels increase as water evaporates during the dry season (May-October) in the Top End.

The first rains of the Top End's wet season usually dissolve salts that have gathered on the soil surface during the dry season. This makes the first water flow considerably more acidic than normal, sometimes resulting in kills of the freshwater fish in the waterways. The water regains its normal acidity soon after the beginning of the wet and the fish populations are quickly restored.

Dissolved Chemicals & Pollutants
Dissolved chemicals in waterbodies come from the use of pesticides, herbicides and fertilisers in a catchment area. On the East Coast of Australia, phosphates from fertilisers may be beneficial in small amounts, but too much will enrich the water, causing algal blooms that result in the use of all available oxygen in the water. This is a process called eutrophication, and whilst it is also natural, is greatly speeded up by human activity . When waters become nutrient rich, plant growth and algal blooms increase dramatically in response. When the nutrients become depleted, or return again to normal levels, the plants and algae die off, and are decomposed by bacteria that deplete the water of oxygen - this killing other aquatic life. The result is a stagnant and putrid waterbody, that is unable to sustain life. If these phosphate enriched waters reach the ocean, the lack of oxygen severely affects the coral reefs ability to grow and survive. 

The geology of an area may affect the amounts of heavy metals (e.g. lead, copper, zinc) and radioactive materials (radium and uranium) which are carried in the water. Mining can accelerate the release of these into the environment, as well as other chemicals used in mineral processing. Uranium mines handle radioactive waste materials and the substances used in normal gold mining operations include sodium cyanide, sodium hydroxide, liquefied petroleum gas, hydrochloric acid, ammonium nitrate and dieseline. Uranium and gold mines operate in the Top End and although regulations governing the handling of harmful materials are very strict, the potential for accidental pollution of natural water sources does exist.

Temperature of the water is another factor affecting quality. In the Top End, shallow billabongs become warmer in the dry season and the water temperature may reach 40 degrees celsius, with a consequent loss of dissolved oxygen. This brings about high stress levels for aquatic insects and fish and also impacts on aquatic plants. In still, deep billabongs a cold layer tends to form in the lower levels of the billabong, affecting plant growth.

  2. Types of Wetlands in the Top End 3. An Environment Subject to Change 4. Activities

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