Streams entering the Fleet
There are seven freshwater inputs flowing into
the Fleet draining a very small catchment of 28km2
figure for locations):
East Fleet Stream;
West Fleet Stream;
Portesham Mill Stream;
Mill Stream B
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
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