There are seven freshwater inputs flowing into the Fleet draining a very small catchment of 28km² (see linked figure for locations):

East Fleet Stream;

West Fleet Stream;

Herbury Stream;

Rodden Stream;

Portesham Mill Stream;

Mill Stream B Abbey Barn;

Coward's Lake.

Of these, limited nutrient data from 1990 to the present exists for all inputs except Herbury Stream. Although sparse, data from 1990 to '97 has been subject to long term trend analysis (EA 1998a), the results of which are summarised below. However, because of the lack of long term data and infrequency of sampling, analysis for statistical significance was not performed, and the conclusions reached are therefore tentative.

Cowards Lake had a mean concentration for ammonia of 0.03-0.09 mg/l from 1990-96. This then increased to 0.4 mg/l for 1997, due to a suspected farm pollution incident skewing the data. Nitrate and TON also displayed a general increase in concentration over the period. TON lowest mean was 3.33 mg/l in 1992, highest mean was 7.80 mg/l in 1995. Orthophosphate ranged from low to high during the period with means of from 0.04 mg/l in 1992 to 0.18 mg/l in 1995.

Mill Stream - Abbey Barn showed a general increase in concentration of nitrate, TON and ortho-phosphate from 1990-97, with mean ammonia from 1990-96 remaining less than 0.16 mg/l. In 1997 mean ammonia increased to 0.68 mg/l, again, attributable to one sample skewing the data. This one sample with high ammonia was due to a farm pollution incident (enforcement action was taken by EA). TON lowest mean was 4.75 mg/l in 1992, highest mean was 7.78 mg/l in 1997. Orthophosphate ranged from moderately low to high during the period with means of from 0.08 mg/l in 1996 to 0.18 mg/l in 1991 and >95.

Portesham Mill Stream receives effluent from Abbotsbury STW, and has shown a general decrease in ammonia from 1990-97, except during 1995 when levels were elevated. Nitrate and TON showed a general increase in levels, ortho-phosphate was in equilibrium. TON was high, with lowest mean at 7.48 mg/l in 1994, highest mean of 14.55 mg/l in 1996. Orthophosphate was higher than all other stream inputs throughout the period with means of from 0.55 mg/l in 1995 to 0.94 mg/l in 1992.

Rodden Stream (which receives effluent from Langton Herring STW) showed a decline in ammonia and ortho-phosphate from 1990-97, with an increase in nitrate and TON (mirroring Langton Herring STW effluent trends). TON was high, with lowest mean of 9.65 mg/l in 1990, highest mean of 13.67 mg/l in 1991. Orthophosphate was also somewhat elevated for some of the period with means of from 0.04 mg/l in 1996 to 0.63 mg/l in 1990.

No nutrient data are available for Herbury Stream; its water quality is assumed to be similar to that of East Fleet stream.

West Fleet Stream is dry for 3-4 summer months of the year (a winterbourne). Ammonia concentrations were similar to those in the above streams, and exhibit a similar pattern with relatively stable low concentrations, with an occasional elevated peak. However, the nitrate and TON concentrations were much higher than those of other streams. The highest concentrations were recorded in 1997 (36.2 mg/l nitrate and 36.28 mg/l TON), at two to three times greater than East Fleet, Portesham Mill stream and Rodden stream (the other streams with high TON). Lowest mean TON was 12.00 mg/l in 1995, highest mean TON was 36.28 mg/l in 1997. Orthophosphate concentrations were moderate to high with lowest mean of 0.07 mg/l in 1996 and highest mean of 0.37 mg/l in 1991.

East Fleet Stream is also dry for 3-4 months a year, and exhibits a similar pattern to that of the West Fleet Stream, with somewhat lower concentrations of nutrients, but still with TON higher than either Portesham Mill stream or Rodden stream. TON lowest mean was 11.59 mg/l in 1994, highest mean was 17.33 mg/l in 1995. Orthophosphate remained relatively moderate to high throughout the period with means of from 0.08 mg/l in 1996 to 0.41 mg/l in 1990.

In terms of nutrient concentrations, without account being taken of flows of each stream, West Fleet stream had by far the highest nitrogen concentrations. East Fleet stream, Portesham Mill stream and Rodden stream also had high nitrogen concentrations, with Cowards Lake and Mill stream (Abbey Barn) being much lower. Both West Fleet and East Fleet streams are winterbournes (i.e. dry for several months of the year). In terms of phosphorus concentrations, Portesham Mill stream (which receives effluent from Abbotsbury STW) and Rodden stream (which receives effluent from Langton Herring STW) had highest concentrations of orthophosphate, followed by East and West Fleet streams, and, again, Cowards Lake and Mill stream (Abbey Barn) with the lowest.

The relative contribution of each of these streams to nutrient loads to the Fleet lagoon itself has been modelled, taking into account estimated flows for each stream as well as nutrient concentrations. The results of the nutrient load modelling are considered below.

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