Coastal and subcoastal non-floodplain sand lake—Perched

Coastal and subcoastal non-floodplain sand lake—Perched – Hydrology

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• Hydrology
• Geomorphology
• Fauna
• Flora
• Nutrients
• Associated habitat types
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• Flooding
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Although there is a great deal of variability between lake types, the water of dune lakes are generally characterised by being acidic (pH of less than 6) and have extremely low levels of dissolved solids, suspended solids and nutrients. They can be crystal clear, clear and tea-coloured, or opaque because of the presence of dissolved organic matter. Water that is tea-coloured or brown is also known as ‘black water’ and this is more typically found in perched lakes. ‘White water’ that is clear and colourless arises from regional groundwater aquifers and this is more commonly associated with window lakes. Factors that contribute to the physical variation between lakes include lake age and size, how it was formed, proximity to the sea, surrounding vegetation, and the extent to which leaf litter accumulates and decays within it.

Rainfall quickly infiltrates sand, leaching nutrients from the soil profile, and leaving very little surface water available for run-off. The lack of streams or geomorphological features associated with overland flows is evidence of the rapid infiltration rates and low overland flows in these environments.

Rapid infiltration indicates that the groundwater system is likely to be a critical component in the environmental processes of perched wetlands in sandmass environments.

Perched lakes are hydrologically complex systems that can fill rapidly in response to local rainfall and associated infiltration through the sand mass. They are bounded by an organic layer that is semi-permeable to water, thus creating their own local groundwater systems where free water can accumulate in the soil profile. The topography, stratigraphy (layering), permeability and other properties of the local semi-permeable layer can be very complex and heterogeneous, making these systems highly diverse, complex and difficult to predict. They are generally more acidic (they have a lower pH) than window lakes as they tend to have a higher organic matter content.

Stratification

Stratification is driven by density differences within the one waterbody.

Temperature has a significant effect on water density - warmer water is less dense and therefore will float on top of colder water (of the same salinity).

1. Sunlight warms the upper surface of the lake (the epilimnion) making it less dense than the water below, allowing it to float and discouraging mixing (hence inducing stratification).

Only a small temperature difference is required to prevent mixing between layers, depending on the lake’s surface area, shape and wind fetch.

2. The stratification can break down through a decrease in surface temperature from cooler air temperatures that occur either at night time or seasonally, depending on the lake’s climate and other characteristics.

The decreasing density of the cooling surface water, along with wind action cause this surface water to sink and mix with the deeper water. During this time period, most of the lake water is at the same temperature, and surface and bottom waters can mix freely.

Mixing

Stratified lakes allow very little mixing between the lake surface and the lake depths, virtually cutting off the oxygen supply to the unmixed bottom layer (the hypolimnion).

When there is little oxygen input from submerged plants (due to low light levels, for example) or when a large amount of organic matter has stimulated the growth of oxygen-consuming bacteria, the water in the unmixed bottom layer (hypolimnion) can become extremely low in oxygen (anoxic) over time.

Anoxia has a range of impacts. Not only does it make conditions harder for organisms that require oxygen for respiration to survive, it impacts on a range of chemical processes.

Anoxia and nutrients

In anoxic conditions, the nutrients phosphorus and nitrogen (in the form of ammonia) become more soluble (dissolvable) and are released from the bottom sediments into the water column. Ammonia cannot be converted to nitrate without oxygen and will therefore accumulate in anoxic water, especially under high pH (basic/alkaline) conditions. During the summer, stratified lakes can sometimes partially mix (such as with the passing of a cold front accompanied by strong winds and cold rains), allowing some of these nutrients to ‘escape’ up into the well-mixed surface waters and stimulate an algal bloom if light and temperature conditions permit. Ammonia can also directly affect animals, such as fish, that are sensitive to ammonia and are repelled by high levels in the water.

Hyporheic flow

Water flowing through a river can move through the sediments beneath and adjacent to the channel. This is termed the hyporheic zone, where surface water and groundwater mix. The water that flows through this zone is termed hyporheic flow, subsurface flow or base flow, and can be an important source of water for floodplain lakes and swamps.

Due to filtering by alluvial sediments (e.g. sand) hyporheic flow tends to be less turbid (clearer) than surface water flow.

Stratification zones

Under certain conditions related to temperature, wind mixing, salinity, waterbody depth and vegetation cover, Queensland lakes can stratify into different layers.

The upper layer of a waterbody is a warmer (and therefore lighter), well-mixed zone called the epilimnion.

Below this is a transitional zone called the metalimnion, where temperatures rapidly change with depth. The thermocline is a horizontal plane within the metalimnion at the zone of greatest water temperature change. The metalimnion is very resistant to wind mixing.

Beneath the metalimnion, and extending to the lake bottom, is the colder, heavier, darker and relatively undisturbed hypolimnion.

More hydrology information is available in the chronology of a flooding lake.

Last updated: 22 March 2013