From a nature conservation perspective, lagoons are considered to be important physiographic features and, together with lagoon-like habitats, support important biological communities. Lagoons are a rare habitat in the North East Atlantic biogeographic area. This is recognised by their inclusion under the Habitats Directive as a priority Annex I habitat and as a priority habitat under the UK's Biodiversity Action Plan.

Management of lagoons presents a number of issues including maintenance or restoration of the physiographic structure, e.g. physical barrier such as a shingle bar, and the hydrological regime, ie supply of both fresh and saline water inputs. Whilst these issues are not comprehensively understood sufficient is known for them to be addressed (see, for example, Bamber et al 1993).

Less well known is the role of the quality of the water entering and within a lagoon system and its effect on the ecology and biota (water quality is unlikely to affect the physiographic interest of the habitat). Although nutrient concentrations can vary naturally with the primary production cycle through the year, semi-enclosed and closed water bodies such as lagoons have low flushing rates and are therefore potentially sensitive to changes in water quality, including in relation to changed use around the periphery of the habitat. There is, therefore, a need to clarify the importance of water quality in the management of lagoon habitats and to provide guidance to site managers as to how the issue can be investigated and addressed.

It is apparent that there have been few site specific studies into water quality in saline lagoons and therefore little material that can be reviewed. The priority therefore was to initiate a demonstration study on a site with a perceived water quality problem, preferably a site with nature conservation interest and some data on which to build. Further to this, the Environment Agency has reviewed sites of recognised nature conservation interest in England to determine those with a perceived or known deterioration in water quality and/or where the EA have undertaken any surveillance or monitoring. The review highlighted two sites, the Fleet and Rye Harbour Lagoon, with the former subject to more detailed study. Whilst Rye Harbour is part of a Special Protection Area the Fleet was a more obvious candidate for the study not only because of the level of previous work but because: (a) it is a SAC (and one of the twelve demonstration sites within the UK Marine SACs Project), (b) there is a long standing, if not universally held perception of a water quality problem associated with nutrients which may be compromising the interests of the site, and (c) the biodiversity of the site is such that it supports a notably wide range of communities and species found in saline lagoons.

The study therefore focussed on investigating the Fleet system, building on previous work, to provide an understanding of water quality issues in the management of the site and to draw lessons from this for other sites.

The objectives of the study were therefore to:

Determine the current understanding of the nutrient status of the Fleet lagoon, attempt to confirm whether the site is affected (polluted) by excess nutrients, identify sources of nutrients, and recommend management and monitoring options and further work in the light of the study results. The study would also provide baseline information for future monitoring and environmental assessment of any developments around the site.

Provide generic guidance, derived from the Fleet study, on nutrient-related water quality issues in saline lagoons, covering the potential for impacts, their investigation and management, and subsequent monitoring.

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