Receiving waters

The lack of data on nutrient concentrations in streams, combined with concerns regarding the eutrophication status of the Fleet, led the Environment Agency to initiate a project to monitor water quality within the Fleet lagoon itself as part of its routine monitoring programme from 1996 to the present. Information obtained was intended to facilitate more informed decisions on the future management of the Fleet. Monitoring was also required in order to provide routine nutrient data to allow possible designation of the Fleet as a >polluted water (eutrophic)= under the EC Nitrates Directive (91/676/EEC).

Two day surveys were undertaken, covering both spring and neap tides, four times a year, corresponding to spring, summer, autumn and winter. Five sites from within the lagoon itself were sampled from mid channel (see linked figure):

Abbotsbury Swannery;

Langton Hive Point;

Chickerell Hive Point;

the Narrows;

Smallmouth.

Seven freshwater input sites were sampled (see linked figure):

Cowards Lake

Mill stream (Abbey Barn) (upstream of Abbotsbury Swannery restaurant discharge);

Mill stream (Horsepool) (downstream of Abbotsbury STW discharge);

Rodden stream (upstream of Langton Herring STW discharge);

Rodden stream (downstream of Langton Herring STW discharge);

West Fleet stream;

East Fleet stream.

Herbury stream was the only stream input not sampled.

A number of water quality determinands was measured for each sample, including temperature, dissolved oxygen, pH, salinity, ammonia, nitrate, nitrite, orthophosphate, silicate, total organic nitrogen (TON) and chlorophyll a.

Summary data for nutrients for the Fleet sites are presented below, although it should be noted that the data set is limited and figures should be treated with caution.

The data indicate that the Fleet off Abbotsbury Swannery has by far the largest nutrient concentrations, with concentrations reducing eastwards. The sites at the Narrows and Smallmouth had by far the lowest nutrient concentrations. Variations in results were high, in particular at the three sites with the highest concentrations (Abbotsbury Swannery, Langton Hive and Chickerell Hive).

Levels of orthophosphate from marine waters exceeded 10 g/l-P at Smallmouth on all but one sampling occasion from 1995 to 1997, with much higher levels (well over 15 g/l-P) further up the Fleet measured on most sampling occasions. At Chickerell Hive and Langton Hive orthophosphate concentrations reached 45 and 68 g/l-P respectively in 1997. Concentrations at Abbotsbury were much higher throughout this period (EA 1998a). The relevance of these observations to Lamprothamnium papulosum is explored elsewhere.

Concentrations of chlorophyll-a (as an indicator of planktonic algae) followed a similar pattern in distribution to nutrients, with high levels at the Abbotsbury end of the Fleet, and lower levels at the eastern end.

Elevated chlorophyll-a levels at Abbotsbury in August 1997 corresponded with high levels of planktonic algae. Water samples taken at this time showed bloom levels of the dinoflagellate Prorocentrum micans, which caused red discoloured water in the Abbotsbury area. Prorocentrum lima was also detected in samples in low numbers from Abbotsbury as well as Langton Hive and, to a lesser degree, at Chickerell Hive. The bloom duration lasted some time, at least four weeks (the interval between sampling occasions). Both the occurrence and duration of the bloom is of possible concern given the associated potential increase in light attenuation and the fact that P. lima can cause toxicity though the food chain above a certain level (>100 cells/l) including Diarrhetic Shellfish Poisoining (Environment Agency 1997b).

Summary nutrient data for the Fleet, April 1996-August 1997

 

Site

Ammonia

g/l

Nitrate

g/l

Orthophosphate

g/l

TON

g/l

Salinity

g/kg

Mean

st.dev.

Mean

st.dev.

Mean

st.dev.

Mean

st.dev.

Mean

st.dev.

Abbotsbury swannery

14.83

13.08

1920

1550

135.67

134.93

1960

1600

15.51

4.14

Langton Hive

27.33

45.03

440

680

30.67

26.15

460

720

25.84

2.91

Chickerell Hive

9.57

5.83

310

520

21.00

13.49

320

550

26.87

4.94

The Narrows

8.14

4.10

180

220

16.43

6.55

190

230

29.00

5.28

Smallmouth

6.83

3.35

91

72

9.75

5.72

94

76

33.90

1.37

Source: Environment Agency 1998a

Summary data for chlorophyll-a for the Fleet, April 1996-August 1997

 

Site

Chlorophyll-a (g/l)

Mean

st.dev.

Abbotsbury swannery

58.83

44.93

Langton Hive

9.50

5.68

Chickerell Hive

8.49

9.81

The Narrows

5.40

7.61

Smallmouth

2.94

2.70

Source: Environment Agency 1998a

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