General Introduction to physical and chemical parameter in estuarine systems

Estuarine Environments:

An estuary is defined as a semi-enclosed body of water where river water mixes with oceanic water. Estuaries are sometimes thought of as filters for riverine waters entering the coastal zone. The mixing between fresh and saline waters results in rapid physical and chemical gradients which can affect the constituents of the water body. There are four main physical and chemical parameters that vary with estuaries.

1. Salinity

Within estuaries salinity typically varies from marine salinity at the tidal inlet to fresh water at the mouth of the river. Less saline water will always attempt to float on top of more saline (denser) water resulting in three different types of estuaries:

  1. Salt wedge
  2. Partially mixed
  3. Well-mixed

Due to the intrusion of freshwater into seawater in the estuary, there is the presence of freshwater-saltwater interface (FSI). This FSI is a region of sharp salinity and density gradients (Uncles & Stephens, 1993). At the location of the FSI, the water column is highly stratified with fresh water flowing seaward over more dense sea water (Schumann et al., 1999).

2. Temperature

Temperature plays a key environmental, ecological and morphological role within an estuary by determining the range of fauna and flora on an annual time period (Beukema, 1990). Observed temperatures within estuarine systems are predominantly controlled by conservative mixing of coastal and river waters (Miller, 1999). Variation in water temperature is therefore observed in estuaries. For example during spring, summer and autumn, maximum temperatures are often observed in the upper reaches of estuarine systems but exhibit minimum temperatures during winter (Uncles & Stephens, 2001).

3. Dissolved Oxygen

Dissolved O2 (presented either as % saturation or as mg l -1) enters the estuarine water body via diffusion across the water/atmosphere. The solubility of O2 is impacted by both temperature and salinity, with lower solubilities occurring at higher temperatures. However biotic processes of photosynthesis and respiration will lead to both seasonal and diurnal variations in dissolved O2. For example, during intense phytoplankton growth (“blooms” arising as a result from nutrient inputs) super-saturation during the day of O2 may be apparent, with a depletion during the night as a result of respiration. Post bloom, subsequent decomposition of organic detritus by bacterial may also lead to a depletion in O2 concentrations. Such depletion may lead to lethal / sub-lethal effects on the wider biotic estuarine communities.

4. Suspended solids

Estuaries are fed by river discharges which results in the availability of high load of suspended particles as compared to other natural water systems. This occurrence of high loads of suspended particles in estuaries is exhibited by the presence of turbidity maximum (Uncles & Stephens, 1989; Uncles & Stephens, 1993). Processes that are known to facilitate the generation and maintenance of turbidity maximum include complex interactions between the tidal dynamics, gravitational circulation and erosion and deposition of fine sediment (Dyer, 1988). Flocculation and precipitation of dissolved riverine constituents occurs as the salinity increases, and in some estuaries this, coupled with tidal resuspension of sediments, leads to the existence of a turbidity maximum zone. At the turbidity maximum zone of an estuary, there occurs a gradual change in freshwater environment to a marine environment (Flameling & Kromkamp, 1994).

References

Beukema, J. J. (1990) Expected effects of changes in winter temperatures on benthic animals living I soft sediments in coastal North Sea areas. In: Beukema, J. J. (Ed.). Expected Effects of climatic Change on Marine Coastal Ecosystems. Kluwer, Dordrecht, pp.83-92.

Dyer, K. R. (1988) Fine sediment particle transport in estuaries. In: Dronkers, J. & van Leussen, W. (Eds.) Physical Process in Estuaries. Springer-Verlag, Berlin, pp.427-445.

Flameling, I. A. & Kromkamp, J. (1994) Responses of respiration and photosynthesis of Scenedesmus protuberans Fritsch to gradual and steep salinity increases. J. Plankton Res. Vol. 16, pp1781-1791.

Miller, A. E. J. (1999) Seasonal investigations of dissolved organic carbon dynamics in the Tamar Estuary, U. K. Estuarine, Coastal & Shelf Sci. Vol. 49, pp.891-908.

Schumann, E. H., Largier, J. L. & Slinger, J. H. (1999) Estuarine hydrodynamics. In: Allanson, B. R., & Baird, D. (Eds.). Estuaries of South Africa. Cambridge Academic Press, Cambridge, pp.99-52.

Simpson, J. H. & Nunes, R. A. (1981) The tidal intrusion front: An estuarine convergence zone. Estuarine, Coastal & Shelf Sci. Vol. 13, pp.257-266.

Uncles, R. J. & Stephens, J. A. (1989) Distributions of suspended sediment at high water in a macrotidal estuary. J. Geophys. Res. Vol. 94, pp.14395-14405.

Uncles, R. J. & Stephens, J. A. (1993) The freshwater-saltwater interface and its relationship to the turbidity maximum in the Tamar Estuary, United Kingdom. Estuaries. Vol. 16, pp.126-141.

Uncles, R. J. & Stephens, J. A. (2001) The annual cycle of temperature in a temperate estuary and associated heat flux to the coastal zone. J. Sea Res. Vol. 46, pp.143-159.