When will the lakes recover, how long will it take? This is what everyone wants to know. We have all heard of the word ‘Eutrophication’, but we have come to realise that the process of Eutrophication is often poorly understood, especially by those involved, both farmers and fishermen.

One reason for this apparent apathy is that it is rather like carbon emissions – I have always driven my car, and wasted heat from my house, so how come this has suddenly become a bad thing! And just as the Science of climate change is poorly understood, so the Science of Eutrophication is poorly understood. It is always, therefore, going to be an uphill struggle, suggesting that the circulation of substances within the environment such as Carbon Dioxide or Phosphorus should take precedence over long-established practices and patterns of behaviour. So, before anyone can properly understand how long the recovery will take, we first need to properly understand the issues.

Perhaps the first thing we should do is to properly define the word ‘Eutrophication’, which is:- the process in which there is an increase in the rate of addition of substances of Nitrogen and Phosphorus to a natural system, usually water.

This then is essentially what Eutrophication is all about:- two Chemical Elements.

The initial environmental response to that increase in the rate of addition of Nitrogen (N) and Phosphorus (P) is an increase in both Phytoplankton (algae) and macrophite (plant) growth. As the addition of N and P increases, so it becomes more likely that the algal state will come to dominate the aquatic environment, resulting in algal blooms, and a covering of filamentous algae, and a general lack of diversity. The quality of trout fishing is reduced, even to the point where it ceases entirely. Reduced insect life, reduced dissolved oxygen due to increased b.o.d., and increased zooplankton all inhibit the trout’s natural behaviour. Increased turbidity prevents the spotting of fish, while rafts of filamentous algae physically prevent fly fishing in extreme cases.

Both Nitrogen and Phosphorus are essential for the production of phytoplankton, but the practice has been to determine water quality, where Eutrophication is concerned, by the Total Phosphorus Concentration.

The Nutrient Status of water bodies is described as its Trophic Status, and there are a range of these classifications A Nutrient-poor lake is classified as Oligotrophic. A slight increase in Nutrient Level becomes Mesotrophic, a further increase becomes Eutrophic, and at high Nutrient Concentration Levels, a lake is classified as Hypertrophic.

The OECD Classification of lakes, 1982, sets out the Trophic Status of lakes, based upon average Concentrations of Total Phosphorus and the typical algal abundance associated with those Concentrations. These class bands have been broadly accepted.

These Classifications are important, but it does involve a fair bit of jargon – however, do bear with it.

The lower boundary for Eutrophic water is 35 micrograms Phosphorus per litre, expressed as 35ugPL-1.

The Mesotrophic range is 10 to 35ugPL-1.

These are known as the Fixed Boundary Classification.

The parallel probability-based Classification system indicates that, at a Concentration of 50ugPL-1, approximately 50% of water would be expected to be Eutrophic. 100ugPL-1 is the Hypertrophic boundary Concentration between Eutrophic and Hypertrophic where all waters would be expected to be Eutrophic.

Just to complicate matters a little, concentrations of Nutrients in lakes are necessarily lower than the inflowing streams, due to the capacity of lakes to act as ‘sinks’, and so for a lake to fall into the Eutrophic Category, the mean annual Total Phosphorus Concentration in inflowing streams could range from 210ugPL-1 for a well-flushed lake, to 320ugPL-1 for a lake with a retention time of one year.

To put Phosphorus into context:- in pristine Mesotrophic waters, Total Concentration will be of the order of few micrograms to a few tens of micrograms, expressed as Phosphorus Per Litre. For comparison, a single grain of table salt weighs a few milligrams, and a milligram contains a thousand micrograms, so we are dealing with minute Concentrations. Phosphorus is actually a rare substance in the natural environment.

So we know that vanishingly small concentrations of Phosphorus can cause Eutrophication, and that there is a Classification that defines the boundary limits, so the next obvious question is where does this substance come from? Phosphorus is liberated from decaying organic matter, animal and human waste, and from chemical fertiliser, as well as a contribution from dust and rainfall.

With so many potential contributions, if we really want to understand the problem, we obviously need a technique that can not only calculate the Phosphorus Input to a given Catchment, but we also need to be able to calculate how much of the Input is Exported from the Catchment into streams and lakes.

Such a technique is called Export Co-efficient Modelling, and is now the standard technique used to assess Catchment Nutrient Budgets across the country. Two leading authorities in this area are Professors Johnes and Moss.

The technique of Co-efficient Modelling has been carried out on many Catchments across the country, and there are numerous examples of Nutrient Budgets available for rural Catchments. In all examples, the largest contribution is usually from Livestock, often followed by Human Input where sewage works discharge into streams or rivers. Other significant contributions come from Cultivated Land and Permanent Pasture, both as a result of fertiliser Input. Inputs from such things as Rough Grassland, Woodland, and Rainfall, are tiny in comparison with the larger contribution.

In order to fully understand where the Phosphorus in the Brook Farm Catchment comes from, we conducted a Nutrient Budget using the same Co-efficient Model described. The results for our Catchment were exactly what you would expect, and mirrored closely the many published Nutrient Budgets available when comparing like with like. In our case, 98% of the Phosphorus Input to the Catchment came from Livestock.

That percentage initially looks incredibly high, but in fact it should come as no surprise. A cow produces between 7.65Kg and 17.6Kg of Phosphorus per head per year, and so a herd of 120 produces at least 950KgP per year. Sheep produce 1.47KgP per head per year, and so a notional number of 200 produce 294KgP per year. For comparison, the largest non-Livestock Input to our Catchment comes from the four people permanently residing; they produce a total of 4.64KgP per year. One of the more startling references from the Literature comes from ‘Ecology of Fresh Water’ by Professor Brian Moss, where he sums up the nature of Phosphorus Input as follows:- “A cow can produce as much Phosphorus (up to 18Kg per year-1) as up to 1,760 hectares of forest or 300 hectares of cropland”.

So, in our case, as in most rural Catchments, there can be no doubt about where virtually all of the Phosphorus came from – the only remaining discussion was:- how much of it entered the lakes, and by what means did it get there?

Here we enter the world of Export Rates, and Transport Mechanisms, and while the whole thing does become more complicated, it remains an exact Science, which can be calculated to a high degree of accuracy.

Due to the largely insoluble nature of Phosphorus, it is usually found bound to clay particles, and there were only two significant Export mechanisms from Manure into the stream. One was via surface run-off from the field into the stream, and the other was Direct Inclusion into or next to the stream from Manure.

The consequential difference between these two transport mechanisms was extremely significant. Cattle do not spend a huge amount of time in the stream - the Literature indicates between 4% and 8% of their day – and so something like 95% of the Manure produced was deposited in the Field, and not in the stream. However, when the Export Rates were applied, a totally different picture emerged.

The Field Export Rate used in our calculations for the Cotswolds area was 2.9%, although the Literature indicates perhaps half of this amount in areas of low Compaction.

The Export Rate of Phosphorus from Manure deposited directly into or next to the stream was, by definition, 100%

Cattle may spend a relatively short time in the stream compared to their time in the Field. However, the mathematical consequences are highly significant, because 100% of the smaller load, directly deposited into the stream, is far greater than 2.9% of the larger load Exported from the Field.

In our case, the Model indicated that 23.53 Kg of Phosphorus entered the stream directly from Manure when the cattle spent just 5% of their time there. This compared to 14.38 Kg Exported from the Field. In total, there was 41.2Kg of Phosphorus Exports from all sources. So we knew how much Phosphorus was produced, and we knew how much of it came our way, and by which means.

The next stage in the process was to calculate the Flow Rate Concentration of Phosphorus in the stream, which you do by dividing the amount of Phosphorus by the Mean Annual Flow Rate of the stream. This gave us a figure of 385ugPL-1 (micrograms of Phosphorus per litre) for the stream feeding the Brook Farm lakes.

This belied the fact that most of the Export in our case was from Direct Stream Inclusion during the six Summer months, when the Stream Flow Rate is lower, and the Exports are higher; it is not surprising, therefore, to see that the calculated 6-months Summer Flow Rate Concentration was 876 micrograms of Phosphorus per litre.

Having calculated the Flow Rate Concentrations, it was now possible to assess their significance, by reference to the OECD Trophic Status:- a pristine (Mesotrophic) lake might have an incoming Flow Rate Concentration of 60 to 90 micrograms of Phosphorus per litre. The Annual Flow Rate Concentration calculated in our case was 4 times higher than the Mesotrophic threshold, and the all-important Summer Concentration was 10 times higher!

Even allowing for a huge margin of error, these Concentrations were simply orders-of-magnitude above the Eutrophic Threshold. Using this technique, however, even the margin of error can be calculated, and the next stage was really neat. The trick is to compare your predicted Phosphorus Concentration, calculated by the Model, with actual Water Analyses taken over time from the stream. In our case, the results were very close, which indicated a high degree of accuracy.

So we had done our calculations, and we could now quantify the problem to a fair degree of accuracy. Armed with that knowledge, we could now return to the question of how long will the recovery take?

The obvious conclusion was that the Livestock were the cause of our Eutrophication, and that excluding them from the stream would reduce the yearly Export of Phosphorus by something like 23Kg. The remaining Export from the Field was more problematic because it will continue, and it amounted to 14Kg/year from Livestock, plus an additional 3.3Kg/year from other sources. However, the calculated Field Export Rate of 2.9% was, according to the literature, likely to be high, and that amount could probably be reduced, maybe by half. With the cattle excluded from the stream, the resulting Flow Rate Concentration of Phosphorus from Field Exports is likely to be between 160ugPL-1 and 80ugPL-1 and so the lakes will never be flushed with Phosphorus-free water. However, at the lower concentrations, a recovery can be expected over time, but it is border-line; the Phosphorus production of a herd of cattle is so enormous that the Field Exports alone remain significant. This is why it is so essential that Buffer Strips are provided, and that drinking arrangements provided for Livestock should be well away from streams, and certainly not on inclines draining into the stream. By comparison, if – hypothetically – there were no Livestock at all, the stream Flow Rate Concentration in our circumstances would be something like 30ugPL–1 would place the lakes into the Oligotrophic Classification, and would be too Nutrient-poor to support good plant life or insect life.

How much time the recovery takes also greatly depends upon the release of the Phosphorus accumulated in the stream, bank-side area, and lake sediment, which is a complicated process worthy of its own detailed description. Suffice it to say that, following the restoration of the lakes in 2002, the accumulation of Phosphorus in the lake sediment is confined to that period only, and we have already enjoyed one year without cattle at all, so the process is well under way.

The final consideration is that the Eutrophic response is not linear. For example, filamentous algae can maintain a strangle-hold on water plants simply by excluding sunlight, encouraged by the fact that it proliferates earlier in the Spring than macrophytes. This is why a plant-dominated condition can exist for some time in quite high Nutrient conditions, before suddenly switching to the algal-dominated condition. Conversely the algal-dominated condition can continue well after incoming Nutrient levels are reduced. Each state can be buffered and stable at varying levels of Nutrient Concentration. In our circumstances, once the switch is made from algal- to plant-dominance, we are very likely to remain in that condition. We will require some luck with the weather, however, before we see that switch.

Keen observers will be able to see the progress for themselves. The top lake will respond first, because it is the first to receive the cleaner water, just as it was the first to decline when it was the first to receive the Nutrient-rich water. There will be two things to look for:- firstly, look for a reduction in those water weeds that thrive in high nutrient levels, in our case this is especially the case for Fennel Pond Weed, and of course look for a reduction in algal bloom and filamentous algae. The second thing to look for is a recovery of the Stonewort (Chara). Stoneworts, collectively known as Charphytes, will only thrive when lake water Phosphorus Concentrations fall to about 50ugPL-1. This will be a really significant development when it happens, not only because it is a sure indicator of the low Phosphorus level, but also because Charaphytes are believed to release chemical substances that retard algal growth. It is our belief that we will start to see the Charaphytes make a recovery in the Top Lake this year, 2008, and when that condition is stable and buffered, we will then start to see the same progress in the Bottom Lake.

A lot will depend upon rainfall; floods bring with them suspended sediment, and wherever those particles come from within the Catchment, they will bring Phosphorus with them. We suffered a small flood two weeks before the current fishing Season opened, which was appalling luck, and that has set us back badly, so what we need going forward is plenty of rain to flush the system, but not sufficient to produce muddy floods. That’s not asking too much, is it ?!!

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