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RCAS Water and Invertebrate Survey

Prepared by Bernice Brewster

Aquatic Consultancy

Aquatic Consultancy

9 Charlton Lane

West Farleigh



ME15 0NX


This Report follows a site visit to RCAS Lakes on 29th June to conduct a water analysis and invertebrate survey on Troy, Little and Big Lakes.  Some concern had been expressed following a number of mortalities of carp in Big Lake.

Site visit 29th June

At the point of visiting it was understood that some eight carp had died in total on Big Lake, these were thought to be amongst the older members of the stock and had been dead for some time and floated to the surface in the warmer weather.  There had been no fresh mortalities and Big Lake was fishing well.

a) Water chemistry

The water chemistry was tested using dissolved oxygen, temperature, conductivity and dissolved solids electronic meters, other parameters were measured using standard titrimetric kits.  T


Overall the water chemistry is perfectly acceptable for the welfare of coarse fish.  It is important to recognize the solubility of oxygen is strongly dependent on the water temperature as well as other factors such as air pressure, nuisance algae growth such as blanket weed, phytoplankton blooms (green water) and decaying organic material such as a when a phytoplankton bloom crashes.  Whilst the dissolved oxygen concentration at the time of visiting was good, as the summer progresses, the oxygen concentration is likely to drop on all three lakes.

Oxygen is readily soluble in cold water but as the water temperature increases, this vital gas becomes only sparingly soluble. Algae and submergent plants, even oxygenating species may also affect the dissolved oxygen concentration.  During the hours of daylight all plants including algae produce sugars using the energy from sunlight and carbon dioxide and oxygen is given off as a by-product, a process known as  ‘photosynthesis’.  At night plants respire, consuming oxygen and releasing carbon dioxide. Tthe presence of excessive submergent plants or heavy algae blooms the water can be stripped of dissolved oxygen over night during periods of hot weather, when oxygen is less soluble in water.  

If an algae bloom suffers a sudden die back, the decaying phytoplankton may also lead to low dissolved oxygen concentration.

Light intensity may also affect the ability of plants to add oxygen to the water.  On successive dull summer days, photosynthesis is inhibited and oxygen consumption increases through respiration, which may also lead to low dissolved oxygen.  

Coarse fish such as carp, tench and roach have a minimum dissolved oxygen concentration requirement of 6mg per litre, although both will survive if the oxygen dips below this value and will tolerate values as low as 3mg per litre dissolved oxygen for short periods of time. Perch and pike are less tolerant of low dissolved oxygen concentration and often succumb first as a lake suffers an oxygen crash.

The site tested on Troy was in a peg where there was quite extensive Canadian pond weed (Elodea canadensis) growing in the margin, which would cause the dissolved oxygen concentration to have increased and the carbon dioxide concentration to drop due to photosynthesis (see above).

The alkalinity is not a fixed value but will be affected by the rate of photosynthesis and plant respiration, which is why this appears to be lower on Troy than the other lakes.  Alkalinity will also vary seasonally.

Similarly, the pH is not a fixed value and varies both daily and seasonally being affected primarily by the rate of photosynthesis.  None the less the pH is within the range of 6.5 – 8.5 for coarse fish.  In the south east the average pH of most natural waters is between 7.5 – 8.5 and that of the lakes tested here falls within this range.

There is a slight but detectable ammonia concentration on Little Lake, however, at the peg where the water was sampled the pond weed had been cut back.  The decaying vegetation would cause the slight rise in the ammonia concentration in the water.  Also present in the water on this site was Lemna trisulca, larger than the other species of duckweed and certainly one which is unlikely to cause a nuisance problem.  I am a great advocate of aquatic plants, which play such an important role in lake ecology, helping to maintain good water quality and as a refuge for aquatic invertebrate life.  

Interestingly Little Lake also recorded a low level of nitrate in the water, possibly through bacterial degradation of the decaying weed, via the nitrogen cycle.  Nitrifying bacteria break down ammonia firstly to nitrite and then nitrate.  It is worth pointing out that nitrate is generally regarded as harmless to aquatic life.

Other parameters recorded for these three lakes were within the ranges which I would associate with fisheries.

b) Invertebrate survey

Because the lakes surveyed are large, the invertebrate survey carried out should be considered to be a snapshot of the aquatic life found in the fisheries, the survey should not be considered as complete as other insects and bugs would be found in other habitats within the lakes.

The peg from which the invertebrates were surveyed on Troy was relatively low in plant life and the results clearly demonstrate the benefit of aquatic plants, where the animal life on both Little and Big Lakes was more extensive as the samples were taken around the vegetation.

The survey method employs the ‘Biological Monitoring Working Party’ (BMWP) system, where the invertebrates are scored according to how intolerant they are to poor water quality.  The highest scores are accorded to those invertebrate species which require the best water quality.  So, for example species of the hog louse, Asellus sp. is tolerant of very poor water quality and even on the most heavily stocked fishery are likely to be encountered and therefore scores 3, whereas many caddis species will only be found in good quality water.  Irrespective of the number of different species of invertebrate found in each family, it scores only once.  This will be seen on the sample from Troy, where there are two caddis species in the family Leptoceridae but the score remains as 10 (the highest BMWP score).

It is also worth pointing out the BMWP system is selective in the bugs and beasts which are awarded a score and not all the invertebrates will count, so for example water fleas, Daphnia spp. although abundant are non-scoring.  Similarly species such as fish lice, Argulus foliaceus do not score as they will parasitize the fish populations irrespective of the water quality.


The samples from both Little and Big Lakes were obtained from the marginal and submergent vegetation and I hope these results demonstrate how much invertebrate life is found amongst the plants.  The invertebrate populations form the staple diet of the fish in these lakes and therefore are essential to the ecology of the lake.  In Big Lake, the self seeded trees on the gravel bars will also form habitat for the invertebrates.

Although there has been concern with regard the to the water quality particularly in Big Lake, following the fish mortalities it may be seen from the results of these surveys that Big Lake produced the highest BMWP score and average score per taxon.  Both Little and Big Lakes had high scoring taxa, those species which are sensitive to poor water quality.  In the event of a pollution or deterioration in water quality these high scoring taxa are usually the first to die.

Big Lake is turbid, however, examination of a sample of the water had a number of single celled organisms such as Paramecium sp. but no phytoplankton (microscopic algae, which tend to make the water discoloured) although there was a large amount of suspended organic debris.  It is possible a previous algal bloom has died back, causing the suspended organic material.

Bernice Brewster

2nd July 2010