Further work recommended for the Fleet

Biological features


Nutrient levels

Nutrient sources

Point discharges (STWs)

Other aspects

Other water quality issues

The following outlines obvious areas of further work, prioritised where possible. It does not elaborate on work currently being undertaken, for example the water quality monitoring programme now in place and current research on environmental requirements of Lamprothamnium papulosum. The following should provide a steer to the management group for the statutory site and relevant authorities on priorities for further work but may also inform studies undertaken by members of the Fleet Study Group.

Overall, the emphasis should be on establishing an adequate baseline against which to changes can be assessed, including components of the community such as phytoplankton, and completing and refining our understanding of the nutrient budget in order to effectively target management measures.

Biological features

Condition monitoring: As a priority, monitoring of the condition of biological features likely to have been affected and or to respond to inputs of nutrients, eg Zostera, Ruppia and Lamprothamnium papulosum and green algae, particularly in the western part of the Fleet, should be undertaken.

Monitoring should build on previous work, e.g. see Holmes (1993) for methods and sampling sites, but should also take account of developments in monitoring, e.g. the use of remote sensing methods (such as aerial photography and CASI (Compact Airborne Spectral Imager)), which may prove to be cost effective, and provide a permanent record which can be re-analysed in future if required. However, care should be taken that new methods allow comparison with existing data, or are carried out in parallel with methods used in the past. Where quantitative and qualitative surveys of biota of conservation interest have been carried out in the past, these should be continued at suitable intervals, with the aim of detecting any changes should they occur.

Current work will contribute to establishing the current distribution of foxtail stonewort Lamprothamnium papulosum throughout the Fleet and to use of CASI. The condition monitoring programme planned for the site is likely to cover all of these attributes (see Annex C; English Nature 1999).


There is a need to undertake a baseline survey of plankton over at least a year to determine frequency of algal blooms and type of organism causing the blooms. Samples should be collected for plankton and preserved using iodine for later counting. These samples could then be analysed in batches as convenient. Such a time series of samples would help to characterise the plankton of the Fleet, including detecting blooms, and help to identify, in conjunction with nutrient monitoring, whether planktonic populations are nutrient limited, and by which nutrients.

It is suggested that as a minimum monitoring of phytoplankton as part of on-going monitoring (as opposed to survey to characterise the site) is achieved through measuring the surrogate attribute of light attenuation.

However, if funds allow, direct monitoring of phytoplankton would be preferable to identify which species are involved. Corresponding monitoring of zooplankton populations may indicate how they affect phytoplankton distribution. Consideration should be given to undertaken such monitoring in parallel with periodic surveys of fish populations mentioned, as these will in turn affect zooplankton populations.

Research: A number of aspects of the biota of the Fleet relevant to conservation interests merit study and include, in suggested priority order based on the degree of effort required and application of the results obtained:

Green algae: A review of Holmes= data from seagrass surveys carried out in 1983, =85 and =91 to look at spatial distribution and density of green algal growth on the seagrass beds in the Fleet may help in determining whether changes in green algal populations have occurred over recent years. When Holmes was carrying out his seagrass surveys, he recorded presence of benthic and epiphytic algae, but these data were not analysed fully for the reports in the FSG archive.

Historical data on L. papulosum: Investigation into the historic distribution of foxtail stonewort Lamprothamnium papulosum by analysis of sediment core samples for oospores will help to ascertain whether there has been a decline in distribution of this species, in particular in the Abbotsbury embayment, since the last century. This must be accompanied by attempts to establish the past nutrient status, particularly of phosphorus, of the Fleet. Current research is likely to contribute significantly to this topic (see Martin 1999).

Lamprothamnium physiology: If funds allow, distribution data for Lamprothamnium could be supplemented by quantitative survey of physiological performance along re-locatable transects so that trends can be more objectively assessed over time.

Historical data on phytoplankton: Palaeoecological investigations by analysis of sediment core samples from the Abbotsbury embayment for past history of plankton populations may assist in determining if plankton blooms have always been a feature of this part of the Fleet, or if they are a more recent phenomenon. The shallowness of the Fleet may make such analysis difficult to interpret, as water currents will affect the distribution of planktonic remains (Li, 1997).

In the case of studies using cores, it would be preferable to date levels within the cores to as fine a resolution as possible.

Nutrient levels

The data on nutrient levels within the Fleet could be investigated further with the following identified as a priority:

Current nutrient levels: It is suggested that it would be relatively easy and cost effective for Fleet Nature Reserve staff (and Swannery staff) to collect and freeze filtered and unfiltered water samples from one or two sites on a weekly basis over the summer months for later analysis of nutrients (nitrate, nitrite, ammonia, total nitrogen, soluble reactive phosphate and total phosphorus, with perhaps also silicate and carbon, and chlorophyll a). This could be undertaken in conjunction with the phytoplankton survey recommended above.

Nutrient sources

In order to address significant gaps in the nutrient budget and assist in focussing management measures, the following work should be undertaken in recommended priority order:

Sources not estimated

In situ (sediments): The role of sediments in nutrient flux with lagoon water is likely to be important and needs to be addressed, in particular for phosphates which tend to be bound onto sediments. Determining the sediment Equilibrium Phosphate Concentration around the Fleet, particularly in the vicinity of Abbotsbury, would be of value. There may also be considerable cycling of nitrogen between plant matter and the water column over any year as plant material dies back (and is eaten and excreted by wildfowl) over autumn and winter. It would also be of value to model the seasonality of the swannery inputs including the loadings to the sediment. Research work on these aspects is therefore recommended.

Groundwater: Groundwater movements and water quality could not be assessed during the current study, and were therefore excluded from the modelling process by EA. However, the study by WRc on diffuse sources of nutrient inputs to the Fleet identified this source as possibly significant. Study of groundwater movement and quality for the Fleet is therefore recommended, in order to either eliminate it as a significant source of nutrients, or include it in the modelling and consider whether management of groundwater quality should be considered.

With respect to sources that were estimated, more refined budget modelling, e.g. at the sub-catchment level, would be of value to verify where the nitrogen and phosphorus sources are and to understand the spatial and seasonal nutrient budget estimates of both diffuse and point source loads.

Sources estimated


Agriculture: Agricultural sources appear from the modelling done so far to be the most significant source of nitrates to the Fleet, particularly in winter, and also a significant source of phosphate, again particularly in winter. Further work involving analysis of remotely sensed images and field survey could be carried out to better refine the estimates of inputs from agricultural sources for the Fleet. This may need to be done in order to target particular areas for farming Better Management Practices (BMPs) as advocated by the EA. However, given that it has been demonstrated that agricultural sources of nutrients are significant for the Fleet, adoption of BMPs for the whole of the Fleet catchment, if not already in place, would be desirable. If this were the case, only limited further refinement of the estimates of inputs would be required.

Point discharges (STWs)

The modelling studies have indicated that Abbotsbury STW is the only point source discharge which is likely to be significant in terms of nutrient inputs to the Fleet. There is likely to be a summer increase in population served by Abbotsbury STW, as Dorset is a prime holiday area, but no information is available on this at present. Investigation of seasonal variation in nutrient concentrations with corresponding effluent flow data from Abbotsbury STW is therefore recommended. Information from this investigation can then be fed into the model developed by Cardiff University to further refine estimates of the significance of this source of nutrients and the likely ecological effect of decreasing discharges.

Wildfowl: Populations of wildfowl using the Fleet are already monitored on a regular basis. This monitoring should continue. In addition, further existing information on swan and other wildfowl populations should be obtained, as these appear to be a significant source of nutrients, in particular of phosphate, in the Abbotsbury embayment during summer. Have the summer populations of swans significantly increased in recent years? Has the spring/summer feeding regime changed? Could it be modified to reduce the quantities of feed and faeces entering the lagoon itself?

NB - It should be borne in mind, when devising a programme for, and considering results of, routine monitoring of water quality, that extreme events may be critical in influencing ecological issues for a system such as the Fleet, and may well be missed by routine monitoring. For example, exceptionally cold winters have had a profound influence on the distribution of Zostera spp. in the Fleet in the past. Storms may also reduce salinities and increase flushing on an irregular basis. Little can be done proactively to determine the effects of such events, but given a good baseline of survey data, ad hoc surveys following such events may help in determining their importance to the Fleet system.

Other aspects

Hydrographic modelling: To assist management, further testing and application of the hydrodynamic modelling is recommended (see section 4.6), particularly running simulations of nitrate and phosphate distributions and refinement of the model based on additional bathymetric data and influence of bed roughness including as a result of vegetation.

Hindcasting: One line of investigation not used in the Fleet which may be appropriate is that of hindcasting (see Johnes et al 1994 and Scott et al 1999). This could be undertaken using the technique of catchment nutrient export modelling. This requires determining the relationship between human activities and nutrient inputs from the catchment and then using historical information on changes in human activity, such as land use, to determine past inputs. Such an approach could build on the nutrient budget modelling undertaken thus far and further refinement of estimates of inputs as suggested above. However, such modelling does rely on good quality historical land-use data. It should be noted that there are concerns that the calibration method employed by Johnes et al is statistically not valid for hindcasting purposes (Carvalho pers. comm.).

Until current research is completed it is not possible to conclude whether the palaeoecological studies suggested above (section 4.11.1) could also contribute to hindcasting.

Use of a 'reference' site:. Another line of investigation not used in the Fleet study which may be appropriate would be to compare the Fleet with a similar lagoon site that is largely free of anthropogenic impact. Finding such Areference@ sites is notoriously difficult and in the case of the Fleet there are few lagoons of the same type and size (the most similar in physiographic terms is in northern Scotland). However, parts of the Fleet are likely to behave similarly to many lagoons and the Fleet supports communities and species found at other sites. It is recommended that monitoring data from other relevant lagoonal SACs is assessed in conjunction with data from the Fleet to assist in distinguishing between more global and more local changes to features of interest.

Other water quality issues

Pesticides: Investigation into use of terrestrial herbicides such as Atrazine on farmland adjacent to the Fleet should be carried out. Exposure to levels of Atrazine of 100 ng/l over 21 days has been found to result in growth inhibition and 50% mortality of Zostera marina in the US (Davison and Hughes 1998). Indications from the Environment Agency are that use of Atrazine may be significant for the Fleet, as it is used on maize crops, which are known to be cultivated in the Fleet catchment (M. Tucker pers.comm.). If this herbicide is used locally, literature study, followed if necessary by surveys, should be carried out with the aim of determining whether significant concentrations may enter the Fleet via run-off from agricultural land. Surveys should be carried out at a time when herbicides would be expected to leach into the Fleet or its freshwater streams, which will depend on timing of application, as well as on rainfall. Sampling of soils and Fleet sediments may also be advisable, if the herbicide is found to be likely to bind to particulate matter.

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