Sensitivity of different lagoons

Potentially, percolation and isolated lagoons are likely to be most sensitive to changes in water quality because of their very limited water exchange (flushing) with the sea or fresh waters. Silled and sluiced lagoons will be less sensitive, but generally still have very restricted water exchange. Overall, lagoonal inlets may be expected to be the lagoon type least sensitive to changes in freshwater quality or anthropogenic inputs, as seawater enters on each tide. However, inlets are still potentially sensitive, as tidal flushing to some parts of a site is usually poor, and the site as a whole is still sensitive to changes in water quality of marine inputs.

Given that larger sites may be able to withstand or respond more readily to water quality changes, it is worth noting the comparative size of the different lagoon types (see table below. Average size reinforces the general order of potential sensitivity from isolated and percolation types to lagoonal inlets. Note, however, that there is a large size range for each type. In addition, it is likely to be the case that once an impact does occur on a site it may be more difficult to reverse that change or restore conditions on larger rather than smaller sites.

Size of different lagoon types in the UK (data from Bamber et al 1999, Covey 1999, Covey et al 1998, Smith and Laffoley 1992, Thorpe 1998 and Thorpe et al 1998)

Lagoon type

Number of sites

Average size/ha

Size range/ha




<0.1 - 44.0




<0.1 - 21.0




<0.1 - 850.0




0.3 - 300




0.1 - 860

Stratification, and factors affecting this, will also influence the response of a lagoon system to various inputs, particularly nutrients. The nature of the isolating barrier can lead to stratification, eg sills can favour faster exchange and circulation of surface water at the expense of deeper water. The potential for stratification is greater in relatively deep lagoons, ie more than 2 metres, although transient stratification has been reported at shallower depths (Hodgkin & Birch 1986); deeper sites tend also to be larger sites. In considering factors affecting nutrient retention in estuarine systems, Scott et al (1999) noted that sills can play a part in trapping nutrients particularly where the sill is shallower than the photic depth. In such conditions the bottom water nutrients cannot leave the system without first entering the photic zone where nutrients can be utilised by phytoplankton.

Owing to their hydrodynamic properties many lagoons are characterised by finer sediments. Certain introduced contaminants to lagoons, such as phosphorus and organic material, will bind to, or be stored within, fine sediments such as mud more than other substrata such as sand or rock.

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