Additional investigations of water quality within the Fleet

Chemical sampling of Fleet lagoon and stream inputs

Stream flow measurements

Diurnal variation of discharges

Continuous monitoring

Algal sampling

Sediment sampling

Additional investigations of the Fleet itself and inputs were carried out in 1998 (EA 1998b), to provide additional data, in particular for use in the nutrient budget modelling.

Chemical sampling of Fleet lagoon and stream inputs

Monthly chemical sampling (April to October 1998) in the Fleet lagoon (5 previous sites plus Cloud=s Hill) and six freshwater input sites (excluding Herbury stream) (results not included in EA 1998b but used in nutrient budget modelling).

Stream flow measurements

Monthly flow measurements (March to September 1998) for six streams (excluding Herbury stream).

Diurnal variation of discharges

Investigation into diurnal variation of Abbotsbury and Langton Herring STW discharges over 24 hours 2-3rd August 1998 (BOD and nutrients).

Diurnal variations in nutrient concentrations from the two main sewage works discharging into streams into the Fleet (Abbotsbury and Langton Herring STWs) were investigated over two days. At Abbotsbury, little diurnal variation in concentrations of any of the determinands was observed. Approximate concentrations were:

BOD 4-5 mg/l;

ammonia 1-3 mg/l;

TON and nitrate 20-25 mg/l;

nitrite <1 mg/l; and

ortho-phosphate around 7 mg/l.

At Langton Herring, diurnal variation was greater, in particular for BOD. Approximate concentrations were:

BOD generally 3-6 mg/l;

ammonia 7-12 mg/l;

TON and nitrate 1 mg/l;

nitrite <1 mg/l; and

ortho-phosphate around 6 mg/l.

Whilst BOD was generally around 3-6 mg/l, there was an isolated peak of 19 mg/l around 18.30hrs on 2nd August, and from 7.00-9.00am on 3rd August BOD increased markedly to 168 mg/l. At this time ammonia concentration dropped, and TON and nitrate increased to around 6 mg/l. This latter noticeable change in determinand concentrations was attributed to the settlement tank being cleaned at the works at that time, and concentrations of all determinands were back down to their previous levels by between 19.00 and 23.00hrs on 3rd August.

The principal difference between the effluents from the two works was the higher nitrate concentration of Abbotsbury STW effluent, and the slightly greater diurnal variation in effluent quality at Langton Herring STW, in particular in terms of BOD and ammonia concentrations. Higher nitrate concentrations in the STW effluent indicate that the works is nitrifying, i.e. converting ammonia and nitrites to nitrate. The concentrations recorded are not particularly high for a sewage works effluent.

Continuous monitoring

Continuous monitoring of dissolved oxygen, salinity and turbidity in Fleet waters at Abbotsbury and Chickerell over 15 days in October 1998.

Continuous monitors for dissolved oxygen (DO) concentrations were sited in the Abbotsbury embayment and in the channel off Chickerell Hive Point to gather data on summer diurnal variations in DO. These results have not been interpreted in EA 1998b. However, preliminary examination of the continuous monitor trace indicates that at Abbotsbury, tidal variation is not seen in the trace for salinity, with levels for approximately 12 days at around 24 ppt, dropping to around 12 ppt on 25.10.98, and increasing to 15-18 ppt for the last 2 days of the record. This drop in salinity corresponds approximately to a series of peaks in turbidity on 25.10.98, and is assumed to be due to a rainfall event. For dissolved oxygen, there does appear to be some diurnal variation, but the trace is not regular, varying between <80% and well over 100%. Around 25.10.98 and for the following two days, diurnal variation was barely discernible, with DO consistently around 80% saturation.

At Chickerell, tidal influence can clearly be seen, with variations in salinity from around 34 to 35.5 ppt presumably corresponding to tidal influx of higher salinity water. Dissolved oxygen at this site does not vary with the tidal cycle, but there does appear to be diurnal variation, with higher levels (>100% saturation) during the middle of the day, and lower levels of 70-80% during the night. This variation is indicative of algal or plant photosynthesis increasing water oxygen concentrations during the day, with lower levels at night. Turbidity was fairly constant at a low level, with the exception of a few groups of peaks of up to 600 NTU (Nephelometric Turbidity Units). These groups of peaks corresponded to disruptions to the tidal and diurnal variations in salinity and DO. At the same time as some of the peaks in turbidity, salinity is reduced, and the diurnal variation in DO levels is less obvious. They may therefore be associated with rainfall events, however, there is no data on rainfall in the report (1998b).

Algal sampling

Algal sampling with corresponding chemical sampling (chemical results not included in EA 1998b).

Low numbers of diatoms were observed in samples taken during April and May 1998. No potential toxin or nuisance organisms were observed or reported during this time. In July, the sample from Abbotsbury Swannery showed the presence of bloom numbers of Alexandrium sp. (a toxin-producing dinoflagellate which can cause paralytic shellfish poisoning or PSP). The duration of this bloom was unknown, but a sample taken a month later was clear. Very low numbers of Prorocentrum sp. (a potential diarrhetic shellfish poison producer) were observed in a sample taken at Ferrybridge in July >98. CEFAS results showed the presence of ASP (amnesic shellfish poison) cells in April and August >98, and a sample taken by CEFAS in July showed the presence of PSP cells, with numbers lower at Ferrybridge than at Abbotsbury. No toxins were detected in shellfish flesh.

Sediment sampling

Sampling from mid channel at Abbotsbury Swannery, Langton Hive and Chickerell Hive to assess phosphate concentrations in surface sediments during October 1998. See the table below for results.

Sediment nutrient data for Fleet samples, 15th October 1998


(mg/kg dry weight)

Abbotsbury Swannery

Langton Hive Point

Chickerell Hive Point





Total phosphorus




Total organic carbon




Total organic nitrogen












Dry matter (%)




Source: Environment Agency 1998b

It should be noted that with respect to phosphorus, concentrations within surface sediments are meaningless because phosphorus is mobile within sediments and significantly affected temporally by redox reduction. To derive an indication of the importance of phosphorus and likelihood of uptake and release by sediments it would be necessary to measure the Equilibrium Phosphate Concentration within the Fleet, i.e. a measure of the propensity of the sediment to leach phosphorus (see Mainstone et al 1996).

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