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Light availability and productivity

Light is important in wetlands because it is necessary for plants (algae and macrophytes) to undertake photosynthesis.

During photosynthesis, inorganic carbon (CO2) is transformed into carbohydrates.

Ambient sunlight available to aquatic plants is limited by 2 principal factors:

  • shading by riparian vegetation
  • turbidity of the water itself.

Littoral benthic algae Photo by Water Planning Ecology Group, DSITIA

Quick facts

Turbidity
is strongly influenced by river flows and run-off. Turbidity recorded in the Darling River during a drought increased twenty-fold during a flood event.[5]

Phormidium cf puteale Photo by Water Planning Ecology Group, DSITIA

Surrounding vegetation shades wetlands, decreasing the amount of sunlight reaching the water surface and reducing daily and seasonal extremes of water temperature. Water temperature influences pH and dissolved oxygen concentration, which affects the species composition and abundance of invertebrates and fish[4].

Shading controls primary productivity within the wetland by reducing light availability. The degree of shade created by vegetation is influenced by several factors, including canopy height, foliage density, wetland width and orientation, topography, latitude and season. The effect of shading on the structure and function of wetland ecosystems is greatest in small wetlands[2].

Suspended particles are the dominant influence on light penetration in most natural waters[3], with the exception of highly coloured waters where absorption can be more significant. Turbidity is a measurement of the degree to which light travelling through the water column is scattered by suspended organic and inorganic particles.

Turbidity increases with a greater suspended load and therefore reduces the depth to which sunlight penetrates the water. The higher the turbidity the greater this effect. The depth to which light penetrates is the photic depth of the water body. Below this depth there is insufficient light for plants to photosynthesise. The photic depth is shallow in highly turbid wetlands with primary production limited to floating or emergent plants and benthic, planktonic or submerged plants only within the shallow photic zone.

Rates of primary production in deeper areas can be 2 orders of magnitude lower than in shallow littoral areas[1]. Primary production is then limited to the uppermost layers of the water. Where turbidity is high and light penetration subsequently low, phytoplankton assemblages are usually dominated by either flagellated eukaryotic algae or vacuolate cyanobacteria which are selectively favoured due to their ability to actively position themselves within a suitable light environment.

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References

  1. ^ Bunn, SE & Davies, PM 1999, 'Local run-off, Paroo floods and water extraction impacts on wetlands in Currawynya National Park', A free flowing river - the ecology of the Paroo River, p. 55-66, NSW Parks and Wildlife, ed. R T Kingsford.
  2. ^ Bunn, SE, Mosisch, T & Davies, PM 2002, 'Chapter 3: Temperature and light', Riparian Land Management Guidelines, Volume One. Part A: Principles of Sound Management, Land and Water Resources Research and Development Corporation (LWRRDC), Canberra, eds. S Lovett & P Price.
  3. ^ Davies-Colley, RJ & Smith, DG 2001, 'Turbidity, suspended sediment, and water clarity: A Review', Journal of American Water Resources Association, vol. 37, pp. 1085-1101, Wiley.
  4. ^ Rutherford, JC, Marsh, NA, Davies, PM & Bunn, SE 2004, 'Effects of patchy shade on stream water temperature: how quickly do small streams heat and cool?', Marine and Freshwater Research, vol. 55, pp. 737-748, CSIRO.
  5. ^ Shafron, M, Croome, R & Rolls, J 1990, 'Water Quality', in N Mackay & D Eastburn (eds), The Murray, Murray Darling Basin Commission, Canberra, pp. 147-145.

Last updated: 22 March 2013

This page should be cited as:

Light availability and productivity, WetlandInfo, Department of Environment and Science, Queensland, viewed 11 February 2019, <https://wetlandinfo.des.qld.gov.au/wetlands/ecology/processes-systems/productivity/>.

Queensland Government
WetlandInfo   —   Department of Environment and Science