Case Study - The Fleet

Background and objectives to case study

Concern has been expressed that the Fleet may be subject to eutrophication (see relevant references in Annex D for example). It is not known to what extent the extensive growth of benthic green algae that characterise the western basin of the lagoon in early summer may be due to direct or diffuse anthropogenic inputs, nor whether the planktonic algal blooms which have occurred have been due to natural events or to anthropogenic inputs. The difficulty is that some lagoons are naturally rich in nutrients, and a high green algal biomass and plankton blooms can typify unpolluted lagoons. However, as discussed in Section 3, research undertaken in other systems with seagrass beds has shown that nutrient enrichment can tilt the balance in favour of benthic, epiphytic and planktonic algal growth at the expense of the seagrass beds. This does not appear explicitly to have happened in the Fleet to date. However, due to the poor flushing and high retention times within the lagoonal basin, there is concern that even small additional inputs of nutrients could have deleterious effects, and that these effects may become self sustaining due to nutrient cycling within the lagoon system. This is reflected in statutory advice for the Chesil and the Fleet European marine site (English Nature 1999) which identifies nutrient enrichment as an operation which may cause deterioration of natural habitats or the habitats of species, or disturbance of species, for which the site has been designated.

In order to determine the nutrient status of the Fleet and potential related impacts on the conservation interests of the lagoon, and to inform management of the site, a number of specific studies have been undertaken. These studies were undertaken through both LIFE-Nature funding and contributions from the Environment Agency, Cardiff University and English Nature. The studies, which are summarised in the following sections of this report, are:

review of historical information on the Fleet from the Fleet Study Group archive to collate any historical data on water quality, trends in water quality parameters and related changes in biota;

investigations into the current and recent water quality status of the Fleet and its inputs, including data to support nutrient budget modelling (Environment Agency 1998a, 1998b; Mainstone and Parr 1999);

modelling of tidal currents, salinity and solute distribution to determine the flushing characteristics of the Fleet and assist prediction of the fate of nutrient inputs (Westwater, Falconer and Lin 1999);

modelling of nutrient budget of the Fleet and its inputs to assist in identifying causal factors and in determining priorities for future management of the site (Murdoch 1999).

Introduction to the Fleet

Review of historical data and interpretation

Water quality monitoring and investigations

Nutrient load analysis and modelling

Modelling tidal currents and solute distributions

Relevant studies of parameters other than nutrients

Summary and interpretation of nutrient budget, distribution and fate

Assessment of change in conservation interests in response to nutrient levels

Implications for management of the Fleet

Further work recommended for the Fleet