Review of historical data and interpretation

The Fleet Study Group (FSG) was established in 1975 in recognition of the view that management and protection of the Fleet lagoon and Chesil Bank could best be achieved from a well informed position. The FSG has a diverse membership of academic and scientific practitioners, land managers and others. Its aim is to encourage research into all aspects of the Fleet (and more recently Portland Harbour), although until recently the emphasis has been towards biological, natural history, palaeoenvironmental and physiographical studies.

An accessible archive of work carried out by members of the Fleet Study Group and others is held at Weymouth College Library, Dorset. It includes over 250 reports, journal articles, letters, and other items of relevance to all types of studies on the Fleet from 1933 to present. The archive has been reviewed as part of the current project in order to extract any relevant information on current or past water quality, particularly in relation to nutrients. Other current and recent studies have also been reviewed as part of the project. A summary of the archive information in relation to understanding past and present water quality of the Fleet is presented here.

Information in the FSG archive concerns primarily the biological and historical interest of the Fleet. Few references contain any information on water quality, and those which do have very limited data relating to spot samples of a few sites, and none have reliable data on nutrients. There have been no comprehensive surveys of water quality in the Fleet prior to the Environment Agency investigations from 1996 onwards (see following report sections). Limited data on nutrient content of sewage effluent inputs to the Fleet exists from 1990 to present. It has, therefore, not been possible to identify the past nutrient status of the Fleet, nor to identify any reliable indicators of an increase or decrease in nutrient levels with time, due to lack of data. Data exist for temperature and salinity in the Fleet where measurements were made at the same time as biological studies. Review of other recent reports supplied by English Nature (several still in early draft form) which are not yet in the FSG archive has also been carried out, including the thesis (John, 1995) from which the following information on phytoplankton and nutrients was extracted. A list of references from the FSG archive which might have had information of relevance to water quality have been reviewed, and are listed and summarised in Annex D.

John (1995) measured salinity and temperature, and took samples for analysis for nutrients and plant pigments, together with enumeration of plankton species. Samples were taken during July-August 1995 from eight sites along the length of the Fleet. Two algal blooms occurred at Abbotsbury during the sampling period, the first at the end of July composed of the dinoflagellate Oxyrrhis sp., the second towards the end of August of the dinoflagellate Glenodinium foliaceum, which caused red colouration of the water. Elevated numbers of Oxyrrhis sp. were observed in samples from further eastwards to Moonfleet and Chickerell on days following the first bloom. The second bloom occurred on the last day of sampling at Abbotsbury, with high numbers of Glenodinium foliaceum also detected in the Clouds Hill sample. Lower numbers of Glenodinium foliaceum were recorded from samples taken eastwards in the Fleet as far as Chickerell Hive Point. Neither of these two dinoflagellates were detected in samples from the Narrows or further eastwards on any sampling occasion, and neither are known to be toxic or produce toxins which might harm fish or animals.

Measurements of plant pigments (including chlorophyll a) in the samples reflected the concentrations of phytoplankton found in the samples, with the exception of the bloom of Oxyrrhis sp., as this organism is not pigmented. On the day of the second bloom, a tributary entering the Fleet at Abbotsbury Swannery (referred to as Mill Stream (Abbey Barn) for EA sampling) was also sampled, and found to contain high numbers of single celled blue green algae.

Measurements of nutrient content of the water column indicated generally average concentrations for brackish waters of nitrate, phosphate and ammonium along the Fleet, ranging from 0-10 µM. However, peaks of nitrate were observed on one day at Abbotsbury and Clouds Hill (25 and 6 µM respectively, equivalent to 350 and 84 µg/l-N), on another day at Langton Herring (9 µM, or 126 µg/l-N) when dinoflagellates were found in the plankton samples, and again at Abbotsbury (10 µM, or 140 µg/l-N) on the last day of sampling when the bloom of Glenodinium foliaceum occurred. Extremely high levels of nitrate were also detected in the stream water on the day of the Glenodinium bloom, at a concentration of 266 µM (3724 µg/l-N), the concentration decreasing by an order of magnitude in the short distance from the stream to the Abbotsbury Embayment itself. Regression analysis indicated that the elevated total inorganic nitrogen levels observed at Abbotsbury were significantly correlated with the increased population density of Glenodinium foliaceum observed during the bloom in August.

An attempt was made to determine if phytoplankton populations were nutrient limited or not, by comparison of the ratios of different plant pigments (carotenoids and chlorophyll a). However, the ratio varied considerably, and did not correspond to variations in nutrient content of the water. The variation was attributed to the fact that the populations were of mixed species of phytoplankton, so no clear conclusions could be drawn from the comparison.

Whilst there are few references which contain information on water quality, several raise concerns about possible impacts on the Fleet including eutrophication, e.g. Elton (1991), Holmes (1983), John (1995).

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