A progress report on research undertaken on the chemical budget of a lake, outlining the importance of nitrogen and phosphorus in governing the production of life in freshwater. The report uses the Rivers Brathay and Leven, which flow into Windermere, as examples. The report also refers to the Rivers Rothay, Troutbeck and Cunsey. A table is including which shows the monthly average nitrate content (mg per litre) of the River Brathey and River Leven for 1937 into 1938. The report also includes a figure showing Windermere lake levels, discharge and rainfall during 1937. It also briefly considers possible anthropogenic influences on water quality

Mortimer, C.H.

Aquatic Commons

THE CHEMICAL BUDGET OF A LAKE. 
by C. H. Mortimer. 
The dependence of life on chemical changes which take place in 
the surface rocks, water and air, strikes one forcibly when considering 
the conditions governing the production of life in lakes. Here, as on 
dry land, the plants, either the rooted vegetation along the shore, or 
the microscopic algae in the water, utilising the energy in sunlight, 
build up from simple inorganic chemical substances the complex 
organic compounds which are the ultimate source of food for animals. 
These organic compounds consist in the main of the chemical elements 
carbon, hydrogen and oxygen, and to a lesser extent nitrogen and 
phosphorus. Other essential elements, a supply of which the plant 
must have, are calcium, magnesium, sodium, potassium, sulphur, iron, 
chlorine, etc., all of which are almost universally present in rocks. The 
dissolving power of rain-water, enhanced by the presence of carbonic 
acid dissolved in it, ensures that there is a ready supply of these elements 
in normal water. There is usually no lack of hydrogen, oxygen and 
carbon for plants, but nitrogen and phosphorus are less available and 
may limit growth. The phosphorus-bearing minerals, especially 
apatites, are distributed fairly evenly but in extremely small quantities 
throughout the rocks of the world, so that the supply, although regular, 
is scarce. Recent work on plant nutrition in soil, in the sea, and 
in fresh water, all tends to show that the lack of phosphorus is definitely 
holding up plant production, and that no excess of the other essential 
plant foods can make up for this lack ; hence the importance of basic 
slag and other phosphate manures in agriculture. 
Nitrogen, which can also under certain circumstances become a 
limiting factor in plant production, is in a class by itself. Although as 
39 
a gas it makes up four-fifths of the atmosphere, plants can use it only 
in its combined forms such as nitrate, nitrite, or ammonia. The 
ultimate source of the combined nitrogen used by plants is, neglecting 
negligible amounts of nitrate produced by electrical discharge in the 
atmosphere, the fixation of atmospheric nitrogen by bacteria, which live 
freely in soil and water, or in the root-nodules of leguminous plants. 
The nitrogen fixed in the body substance of the bacteria only becomes 
available to plants after the dead bacteria have been decomposed by 
other bacteria, releasing the nitrogen as ammonia in the process. 
Ammonia is the first product of decomposition; it is normally oxidised 
by one group of bacteria to nitrite and by another to nitrate. The 
resulting nitrate is very soluble in water, and, unless absorbed by plants 
on the spot, may be washed out of the soil to form one of the main 
sources 6f nitrogen for life in fresh water. 
This cycle of events is demonstrated by the seasonal variation of 
nitrate content in the water of River Brathay, one of the main streams 
flowing into Windermere, as shown in the table below. The decomposing 
plant remains in the autumn are worked upon by the various groups of 
bacteria to produce nitrate. During the winter much of the nitrate is 
not used by land plants, but is washed out of the soil by rain, and 
appears in high concentration in the river waters during the early 
months of the year. The lower values in summer may be the result of 
the increased nitrate demand of land plants in the drainage basin. 
Nitrate content of a typical inflow (River Brathay) and 
the outflow (River Leven) of Windermere (monthly 
averages in milligrams per litre). 
1937 Brathay Leven 1937-1938 Brathay Leven 
Feb. 1.22 1.54 Aug. 0.46 0.64 
March 1.25 1.65 Sept. 0.56 0.62 
April 1.10 1.57 Oct. 0.55 0.78 
May 0.57 1.13 Nov. 0.83 1.09 
June 0.50 1.18 Dec. 1.05 1.30 
July 0.80 0.68 Jan. 0.80 1.22 
40 
The activity of these groups of bacteria, nitrogen-fixing and 
nitrifying, although clearly important to-day, must have had a con-
trolling influence on life in these lakes just after their formation (see 
previous article). The bare rocks or glacial clay beds left after the 
recession of the ice would presumably first be colonized by nitrogen-
fixing forms, which would flourish more readily as the weathering of 
the rocks proceeded and soil was laid down in the valleys. The 
controlling influence of the amount of cultivated soil on the nitrate 
production of the drainage system can be clearly seen to-day. At the 
outset, even though the other plant foods were present in ample 
quantities, production could not proceed because of nitrogen deficiency, 
and so the first deposits in the newly formed lake contained little or no 
life. As nitrate was produced in increasing quantities, both in the lake 
and its drainage area, more organic matter would fall to the bottom to 
decompose there and yield up its nitrogen for further consumption. 
It seems probable that production increased from this early stage, 
like compound interest, until phosphorus deficiency made itself felt 
as a limiting factor and the production rate fell off. Analyses of the 
different layers of the mud cores may throw light on this hypothesis. 
At the present time nitrate is rarely used up completely, even during 
the spate of algal growth in the spring, while the phosphorus in the 
form available for plants (phosphates) often appears to be completely 
absent from surface water. 
The course of events in the lake itself and the importance of the 
mud as a stabiliser and regulator of production was outlined in the last 
annual report (article on the changes taking place in the mud of the 
lakes) so that only the main facts need recapitulation here. The bulk 
of the dead organic matter falls to the mud and decomposes there; in 
all but the surface film of mud the oxygen is almost completely used 
up by decomposition so that nitrification of the ammonia produced 
cannot take place; the thickness of the surface film of mud, which can 
usually be distinguished by its reddish brown colour, due to the 
precipitation of ferric hydroxide (iron rust), is determined by the depth 
to which oxygen penetrates. In this layer nitrification and other 
important bacterio-chemical changes take place, and the resulting 
products such as nitrates diffuse into the water. Other factors may 
impose a seasonal cycle on the rate at which these substances become 
available to plants; they may for instance remain in the deeper water 
during the summer period of temperature stratification, or they may be 
evenly distributed throughout the lake by the complete mixing of the 
41 
water in the winter. During this period they appear in the greatest 
amount in the surface waters, forming a store ready for plant growth in 
spring when the conditions of light and temperature are more favourable. 
This sequence of events is shown by the figures for the nitrate content 
of Windermere's outflow water, the River Leven, which contains 
surface water from the south basin of the lake (see table above). 
Windermere, in common with all other lakes and reservoirs in 
the British Isles, cannot however be regarded as a closed system, 
because the cycle outlined above is dependent on the rate at which 
the lake is losing its nitrogen and phosphorus to the sea and gaining 
them from its affluent streams. In order to ascertain these factors, it 
is necessary to know the amounts of inflowing and outflowing water, 
and their concentrations of nitrogen and phosphorus. Data for the 
calculation of daily flows in the main inflow rivers (Brathay, Rothay, 
Troutbeck and Cunsey) and the outflow (Leven) have been obtained 
and periodic analyses made of these waters for 1937. The apparatus 
used in the flow measurements was kindly lent by Captain McClean, 
whose assistance has been most valuable also in checking the results. 
From river-flow measurements at the outflow the total discharge 
of Windermere at various lake levels has been computed. The 
variation of lake level during 1937 is shown in figure 5 on which is 
FIGURE 5. Windermere lake levels, discharge and rainfall during 1937. 
42 
inserted the appropriate scales in million gallons and million cubic 
metres per day. The monthly totals of rainfall at Ambleside are 
superimposed on these. Combining the figures for the Leven dis-
charge with those for nitrate concentration one arrives at a total nitrate 
loss of 570 metric tons during the year. The average nitrate con-
centration of the outflow water is 1.09 milligrams per litre, while those 
of the two main inflows are, Brathay 0.72, and Rothay (which receives 
the treated sewage of Ambleside) 0.90 milligrams per litre. These 
two rivers may be taken to be representative of the inflow water 
generally. Since the average discharge is approximately 1.4 million 
cubic metres (308 million gallons) per day these figures represent an 
excess of 143 metric tons of nitrate in the outflow water above the 
amount brought in by the main inflow streams. This result does not 
necessarily mean that the surplus is due to biological agencies such as 
nitrogen fixation in the lake itself, because there are human and other 
agencies yet to be taken into account. One of the important factors 
yet to be carefully assessed is the influence of sewage on the lake water. 
Applying figures arrived at in Berlin (17.5 grams of nitrogen in raw 
sewage per head of population per day : the equivalent of 77 grams 
nitrate), the approximately 6,000 inhabitants of Windermere, doubled 
to include summer tourists, would represent, assuming the extreme 
that all the sewage enters the lake, an addition of 340 metric tons of 
nitrate per annum. The effect of evaporation from the lake's surface 
has yet to be measured. A discussion of the significance of these 
results must clearly await a critical review of all possible factors, based 
on the results of a further year's study. 
The volume of the lake is approximately 347 million cubic metres. 
At the average discharge rate mentioned above, water entering the lake 
remains in it on the average for nine months. This explains why a 
heavy rainfall in the first three months of any year is followed by a 
large diatom crop in the spring, as noted by Pearsall; in years with 
little rainfall during this period, only small crops are produced. A 
large algal crop may be interpreted as the result of the washing into the 
lake of relatively large amounts of phosphorus and nitrogen by floods, 
the effect being enhanced by the fact that nitrogen and possibly phos-
phorus are available to a much greater extent in the soil during the first 
three months than at any other time of the year. This shows clearly 
that a lake cannot, from a chemical view-point, be considered as a unit 
apart from its drainage system. 


The chemical budget of a lake

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The chemical budget of a lake

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