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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 hyperact