Basis for management response

Management options

From the study of the Fleet there is little direct evidence that nutrient inputs are having a detrimental effect on the conservation interest of the lagoon, except possibly Lamprothamnium papulosum, or that nutrient inputs have significantly increased in recent years and from which sources. Both of these conclusions are due to a lack of historical information rather than being based on available evidence. There is, however, circumstantial evidence for both scenarios (see, for example, Dean 1996, Elton 1991, Holmes 1983, and John 1995), and there have been changes in land use around the Fleet, particularly intensification of agriculture, that would be expected to increase nutrient inputs. Furthermore, knowledge of lagoons and lagoonal biota indicate that features of conservation interest in the Fleet, as defined in the site=s conservation objectives, are sensitive to nutrient enrichment.

From a site management point of view, the following combination of points should be noted:

• there is some evidence for an impact on the foxtail stonewort and circumstantial evidence for increases in green and planktonic algae (see references in previous paragraph and EA 1997c, EA 1999) which would have an impact on several conservation interests such as seagrass;
• circumstantial evidence suggests that nutrient inputs from anthropogenic sources have increased over time, i.e. the last few decades;
• nutrient budget and distribution studies indicate probable nutrient enrichment, the timing and location of which could cause eutrophication;
• several features of nature conservation importance in the Fleet are sensitive to impacts from nutrient enrichment;
• several features of nature conservation importance in the Fleet are therefore vulnerable to impacts from nutrient enrichment;
• lagoon features which may have been impacted or which could be impacted are of high (international and national) nature conservation interest as reflected in the conservation objectives for the European marine site;
• based on studies elsewhere, a system such as the Fleet may enter a self-perpetuating condition of nutrient enrichment and eutrophication through internal recycling.

Taking these points together, a precautionary approach would suggest that, even though some further work is required, steps should be taken to reduce nutrient inputs to the Fleet that are proportionate to the likely costs of taking no action. Other further work recommended in Section 4.11 is directed towards helping to identify appropriate management measures. The question then arises as to the priorities for management.

The studies thus far clearly indicate that the priority is for reducing inputs to the western part of the Fleet. Whilst it is likely that phosphorus is of more concern than nitrogen, consideration should be given to reducing inputs of both nutrients. Taking account of the sources of nutrients identified thus far, whether manifest as peaks in summer or winter, management measures should address, in priority order, agricultural sources, Abbotsbury STW and the swannery.

Point sources, i.e. STWs and to a lesser extent the swannery, are more amenable to control than diffuse sources such as from agriculture and wildfowl not based at the swannery. In all cases, it is recommended that use is made of the model developed by Cardiff University to determine the effects of changing inputs and to assist in verifying prioritisation of potential changes. This will also assist in assessment of the cost effectiveness of any such changes.

Agricultural sources: Since agricultural inputs of both nitrogen and, particularly phosphorus, have been found to be significant, at least in winter, implementation of agricultural Best Management Practices, as already advocated by the EA, is recommended as a first step. Effects of such changes in agricultural practices on nutrient inputs to the lagoon should be modelled by running the Cardiff University model developed for the Fleet under different scenarios, to determine the priorities for action. A mechanism for this may be to target agri-environment funds at the catchment, which would ease implementation of less intensive agricultural practices.

Abbotsbury STW: Options for nutrient reduction in the effluent from Abbotsbury STW should be considered as, although it is not the major source of phosphorus to the Fleet, it discharges to the most sensitive area, and provides bio-available nutrients during summer when algal and plant growth are at a maximum. The mechanism for achieving this is full appropriate assessment under the Habitats Regulations which is, therefore, recommended. The assessment should confirm whether nutrient reduction is required and the best option for achieving this. If required, there are a number of options for reducing phosphorous inputs which would need to take account of the fact that this is a small works (see Mainstone et al 2000).

In addition to addressing current sources of nutrient inputs, it is recommended that there should be a presumption against any new discharges to the Abbotsbury embayment. Any application for a new discharge should be subject to an appropriate assessment, as would normally be the case, during which due consideration should be given to the potential effects and precautionary approach set out herein.

Wildfowl: Consideration of any practical measures which might reduce nutrient inputs from the swannery should be made, as it was identified by the modelling as a significant source of phosphorus during summer. Such measures might include changing the location of feeding stations, collection of runoff from feeding and nesting areas, and simple treatment of runoff (eg by settlement) before discharge to the Fleet. It is not clear, however, how much of the phosphorus input to the lagoon from wildfowl is derived from recycled phosphorus from herbivorous wildfowl feeding on the seagrass and algae, before excretion into the lagoon; this may merit further investigation. It is also apparent that swannery managers are already putting into place measures to reduce inputs as much as possible.

In addition to the above management measures, current monitoring of water quality parameters should continue and be adapted as appropriate.

A range of work is also recommended to further investigate relevant issues within the Fleet, to help determine other priorities for action, and to assist with management of the site. These are elaborated on below.

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