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Lake water chemistry

Bernice Brewster BSc (Hons), FLS, MIFM, CEnv., CBiol., MRSB

Aquatic Consultancy Service

9 Charlton Lane, West Farleigh, Maidstone, Kent ME15 0NX

email: Bernice.aquatic.consultancy@gmail.com


t. 01622 815255

m. 07973 323494

This Report concerns North Troy Lake, Small Pynesfield Lake and Big Lake, which form part of the Rickmansworth Conservatives Angling Society, fishing waters.  In previous years, other lakes in the area have suffered fish deaths in the early autumn, following periods of oxygen supersaturation, followed by oxygen depletion.  This summer the dissolved oxygen concentration on Troy Lake has been persistently exceeding 100% saturation, often approaching 200% saturation and on one occasion in May, reaching 205% saturation.  The lake is completely land locked and last summer, the water levels were low due to the prolonged drought.  Gravel extraction works associated with the HS2 train connection are taking place within a few hundred metres of the lake, with one of the sites being backfilled recently.  Since the backfill of the gravel extraction site, the water level in Troy Lake has risen, despite the exceptionally low rainfall,  the submergent aquatic plant growth has died away and the lake has been subject to algae blooms and hypersaturation of the water with oxygen.

The purpose of visiting was to sample the water and plankton in all the lakes and compare the data obtained for the water quality in all the lakes.

Water Analysis

The water chemistry was tested using electronic meters for dissolved oxygen, conductivity, total dissolved solids, pH and temperature, Palintest photometer for chemical assay, plankton samples were taken using a net and 53µm filter. The results of the water chemistry are given in Table 1 and algae samples in Tables 2 & 3.

Table 1. Analysis of water samples


The measurement of dissolved oxygen concentration is only valid at the time of testing as so many factors impact on the solubility of this vital gas, some of the factors which affect oxygen are given below .  

  1. Temperature – the solubility of oxygen decreases as the water temperature increases.  This means in the summer months, when the fish are most active and their oxygen demand is greatest, the lake water physically holds less oxygen.
  2. Stock density –in heavily stocked lakes the dissolved oxygen concentration is at a premium, especially when the water temperature is highest in the summer.  A large population of fish in a lake can cause the dissolved oxygen concentration to fall to critically low levels, causing the fishing to slow down or even stop.
  3. Feeding fish – in the summer months when fish are feeding heavily, especially many carp waters where large amounts of bait are available, this may cause the oxygen concentration to drop significantly.  The digestion of food consumes large volumes of oxygen, when large numbers of fish are feeding their oxygen demand increases, affecting the available dissolved oxygen.
  4. Algae and aquatic vegetation or pond weeds - during the hours of daylight, all plants, including algae and aquatic weeds produce sugars using the energy from sunlight and carbon dioxide, which in the aquatic environment is dissolved in the water, a process known as ‘photosynthesis’. Oxygen is released as a waste by-product of photosynthesis and which in the aquatic environment results in the dissolved oxygen concentration increasing significantly through the daytime and may exceed 130% saturation.  Once it gets dark, photosynthesis ceases and respiration in algae and aquatic weeds dominates which means they consume oxygen and carbon dioxide is produced as a waste gas.   Most importantly, all aquatic plants are more efficient at extracting oxygen from the water overnight, the result of which is the oxygen concentration drops away and is at its lowest just before dawn.  The classic signs of overnight oxygen depletion are either the biggest carp are found dead in the morning, or there is a significant fish mortality overnight.  This rather indicates that aquatic weeds or algae are bad for a fishing lake, but this is not the case, the plants play an important role in adding vital oxygen to any lake the important factor is to ensure the weed growth is not prolific but contained through cutting if necessary.
  5. Die back of algae or submergent aquatic plants – if there is a sudden die back of algae or submergent aquatic plants, as they rot through the activity of bacteria and other micro-organisms there is an increase in the amount of oxygen consumed in the water.  The decaying plant life can cause an oxygen depletion in the water and critically low oxygen.
  6. Wind shadow – trees and densely planted shrubs planted around the margins create what is known as a ‘wind-shadow’.  The assumption is that wind hitting a group of trees will pass straight through the branches whereas when wind meets a bank of trees and shrubs, they form a complete barrier and the wind does not drop back onto the lake for a distance which is roughly 10 times their height.
  7. Successive overcast/dull days – when the weather is overcast over a period of days, the low light intensity means that aquatic plants cannot photosynthesize efficiently, respiration dominates and as a result over four or five days the oxygen concentration in the water declines, often to an unacceptably low level for the fish.
  8. Low air pressure – this climatic condition often accompanies dull, wet weather.  When an area of low pressure sweeps through, as it passes over water, it literally sucks out the dissolved oxygen.  The best description of the effect of low pressure is that it’s akin to opening a bottle or can of fizzy drink, with the accompanying rush of escaping gas.
  9. Spawning fish – when spawning the fish are hyperactive, which causes their oxygen consumption to increase but additionally, the release of hormones and gametes will also have a negative effect on the dissolved oxygen concentration in the water.
  10. Autumn leaf fall – when leaves fall into the water, they have an adverse effect on the oxygen concentration, possibly due to chemical reactions with the tannins and other residues which are found in leaves.
  11. Long term ice cover – ice obviously seals the water and prevents essential oxygen from dissolving into the water.  When ice is thin enough to melt during the daytime it represents little hazard, but we have had winters where a thick layer of ice forms over the surface for periods of days.  The problem with this ice cap is the dissolved oxygen can become depleted and the fish die.

It is usually presumed the initial sign of low oxygen is fish seen gasping or piping at the water surface, but fish will only show this behaviour when they are in severe distress and are within minutes of dying.  The first indication of low oxygen is when fish behave as though it’s winter, they stop feeding and become very lethargic – mechanisms designed to conserve oxygen and energy.

Ideally, the dissolved oxygen concentration should exceed 70% saturation (roughly 7mg per litre) for healthy fish, tissue repair and reproduction; 50% saturation (5mg per litre) is regarded as a minimum, values below this are becoming critical.

Interestingly at the time of visiting Troy Lake, the dissolved oxygen concentration was less than had been recorded in previous weeks.  When algae or submergent plant life is actively photosynthesizing, the pH of the water increases as the plants consume the carbon dioxide dissolved in the water, reducing the acidity and making the water increasingly alkaline.  It may be seen from the pH recorded on Troy Lake, this is consistent with reduced photosynthetic activity.  Similarly, the elevated dissolved oxygen concentration on Small Pynesfield Lake, has resulted in a higher pH, associated with the removal of dissolved carbon dioxide from the water, increasing the alkalinity.

Gas Supersaturation and Gas Bubble Disease

The recent exceptionally high dissolved oxygen concentration in Troy Lake has been a cause for concern amongst the membership.  Air is comprised of 78% nitrogen, 20.9% oxygen and the remaining 1.1% a mix of carbon dioxide, inert gases, water vapour and particulates, dissolved in water it is considered as the ‘total gas pressure’.  Oxygen is more readily soluble in water than nitrogen.  When using an oxygen meter, just the volume of this gas dissolved in water is measured.  Generally speaking, it is nitrogen which is the cause of gas bubble disease in fish and commonly encountered in aquaculture, either as a direct result of air entrainment or borehole water.

High oxygen dissolved oxygen concentration encountered in a lake is due to photosynthetic activity.  There are a limited number of records of dissolved oxygen causing gas bubble disease due to photosynthetic activity, but the measurements of oxygen were in excess of 300% saturation.

Gas bubble disease can affect fish mobility, although my experience in these instances have been in aquaculture and more likely due to the high dissolved nitrogen content, causing an equivalent of compression sickness, or ‘the bends’ in the affected fish.  Gas bubble disease is often visualized as blister like lesions, filled with gas in the fins and embolisms in the gills.

Phosphate and Nitrate

I have considered phosphate and nitrate together as their combination are the nutrients which promote the growth of plants.  Phosphate is regarded as the limiting factor as generally this is the nutrient which is found in low concentrations. The European Environment Agency regard a natural lake (not used for any recreational purposes) contains 0.03mg per litre phosphate.  Interestingly, the highest phosphate concentration was recorded from the bore hole!  As an aside, it might also be seen the ammonia concentration is also highest in the sample from the bore hole, the most likely source is bacteria and other micro-organisms at the surface of the pipe.

Phosphate tends to form stable compounds which become locked into the sediment, however, in those lakes which are primarily stocked with carp as these fish feed in the silty layers, the phosphate is recycled back into the water column and available once more as a nutrient.  This explains why lakes heavily stocked with carp become nutrient enriched (eutrophic, or hypereutrophic).

The lowest phosphate concentration encountered was on Small Pynesfield Lake, however, this lake is connected by water flow to adjoining lakes and phosphate is likely to be washed downstream as it is released from the sediment.

The nitrate concentration is notably highest on both sites sampled on Troy Lake, which has suffered both an algae bloom and high dissolved oxygen concentration.  Probably the phosphate concentration has been higher on Troy Lake, which has promoted the algae bloom.  Algae ‘luxury feed’, which means they absorb nutrients to excess more than they require, in other words, they greedily take in nutrients, a mechanism to deprive rivals of food and allow them to thrive.  This feeding pattern can strip the water of available nutrients, but of course when the algae die back, the nutrients are released into the water as they decay, allowing the cycle to be repeated.

Algae Samples

The algae identified from Troy Lake are given in Table 2, the algae identified from Small Pynesfield Lake and Big Lake are given in Table 3.

Table 2. Algae samples Troy Lake

Comments on algae populations in Troy Lake

It is quite unusual to encounter such diverse algae populations on the same lake.  The sample from the jetty on Troy algae are dominated by a species of green algae whereas in the car park area, Cyanobacteria (blue green algae) are dominant.  Usually the addition of aeration will disperse cyanobacterial blooms and the use of an aerator by the jetty is likely to have successfully controlled these nuisance algae.  

There appeared to be a certain amount of decaying algae in both samples, which I would suggest indicates the algae are going through a ‘bust’ cycle.  As stated above this does not imply the algae will not return as the nutrients are recycled as they decay.  It is notable the presence of Cyanobacteria does indicate an imbalance in the lake, due to nutrient enrichment.

Cyanobacteria although loosely grouped with algae and they certainly have chloroplasts for photosynthesis, they are more closely related to bacteria than plants.  The Cyanobacteria are also notably for producing toxins which are extremely harmful to all mammals, including humans.  Although four species of Cyanobacteria were dominant in the car park sample, the numbers present were below that which represent a health hazard.

Table 3. Algae on Small Pynesfield Lake and Big Lake

As may be seen from Table 3, the dominant algae present on Small Pynesfield Lake were diatoms and on Big Lake several species of green algae.  I would suggest this is typical of the succession of algal species which take place seasonally on many lakes, with nothing of significance to record here.  

It may be worth just making a comparison of the algal species identified on the above lakes, with those found on Troy.

Concluding Remarks

Quite clearly an incident has disturbed the chemical balance of the water on Troy Lake, giving rise to the dominance of a cyanobacterial bloom but these algae are associated with nutrient enrichment.  Unfortunately, once algae dominate the water column, it affects the light availability for the aquatic vegetation, which dies and as it decomposes, releases more nutrients into the water, further promoting the algae.  Whether the disturbance to the Troy Lake is due to the HS2 works is unknown but it may be worth considering mitigation to prevent further incidents affecting the water quality and fishing on this lake.

The dissolved oxygen concentration and algae samples would suggest there is an algae die back taking place currently.  Unfortunately, as the algae decomposes, it reduces the oxygen concentration in the water, but given the dissolved oxygen is monitored regularly on the site, it should prevent any catastrophic de-oxygenation event.  It is certainly beneficial to maintain aeration devices on the lakes.

Given the submergent aquatic plant growth has died on Troy Lake it would be advisable in the long term to have a strategy to control the nutrient content of the lake.  If the algae continues a die back and the water begins to clear, it may be feasible to transplant submergent aquatic vegetation from the Small Pynesfield Lake into Troy.  It may even be worth attempting to cultivate this aquatic vegetation in the margins of Troy where the water is clearer.  

Alternatively, there is always the possibility of installing pre-planted coir rolls and pinning these around the margins of Troy.  It may be worth considering the use of these coir rolls along the side abutting the HS2 works, reeds in particular, are very good at removing pollutants from the water.

Marginal or submergent aquatic planting is the long-term solution for nutrient enrichment of a lake but it is important to bear in mind there is no such thing as a quick fix in the aquatic environment.

Bernice Brewster

28th July 2019