Shallow Water Habitats

Physical Characteristics: Light

Light is a key limiting factor in all aquatic habitats.  Light regulates photosynthesis, which is the process that allows plants to produce organic matter from carbon dioxide.  We call the organisms that produce organic matter by photosynthesis “autotrophs”, which means self-feeding organisms.  Heterotrophs (other-feeding) use the organic matter produced by autotrophs for food and energy in a process called respiration. In addition, photosynthesis also releases oxygen, which animals and some bacteria require for respiration.

Light is diminished (called attenutation) as it passes through water.  Some attenuation, especially of red light, occurs due to the nature of water itself.  In coastal and estuarine areas, both dissolved and particulate materials further absorb light, giving the water its characteristic green to brown, and sometimes even black appearance.  Many estuaries and coastal regions have suffered from increasing turbidity as human activities increase in the watershed.  These activities may directly reduce light availability in shallow water areas by causing increased runoff of organic compounds and sediments from the land. 

Light penetration affects the depths at which photosynthesis can occur. When water is very clear, light penetrates to greater depths than when water is cloudy (turbid).  While light typically does not penetrate more than a few meters in turbid estuarine habitats, many estuaries have vast shallow subtidal areas where light can reach the bottom. 

As a result, a variety of photosynthetic organisms, such as benthic microalgae, macroalgae, phytoplankton, and submerged aquatic vegetation, can live in these habitats. We call these plants and algae primary producers.  They support consumers (also called secondary or tertiary producers) that make up higher trophic levels in estuarine and coastal food webs.

Indirect effects of human activities on light availability can occur when undesirable algae are stimulated by excessive nutrient availability or there are other changes in the ecosystem.  Increased phytoplankton production in the overlying water due to nutrient enrichment may cause shading of benthic primary producers. Stimulation of epiphytic growth on submerged aquatic vegetation (SAV) by excessive nutrient inputs often has detrimental effects, because SAV require a high light environment.

For further information about light and its effects in estuarine and coastal marine habitats refer to the following:

Lalli, C. M. and T. M. Parsons. 1997. Biological Oceanography, An Introduction (2nd Edition).  The Open University. 314 pp. ; Chapter 2.1 - Solar radiation.

Valiela, I. 1995. Marine Ecological Processes (2nd Edition). Springer; Chapter 2 - Factors affecting primary production.

OzCoast and OzEstuaries website