On the origin of eutrophication in the Baltic Sea
Eutrophication refers to clouding of water and dense growth of aquatic plants due to excessive nutrient loading. This may cause water systems to become overgrown, growth of filamentous algae along shorelines, major algal blooms, oxygen depletion in winter and changes in stocks of fish and other organisms. The cause of eutrophication is an increase in the nutrients required by plants, principally phosphorus and nitrogen, which reach the water system for a variety of reasons. Nutrients that enter the water system from the surroundings are described as external loading. The state of oxygen depletion that arises in deep waters may also convert nutrient sediments back into soluble form. This phenomenon is known as internal loading, and its effect has begun to be regarded as an increasingly important reason for the current state of the Baltic Sea. (Lehtoranta 2003)
Officials and environmental activists have repeatedly argued that marine eutrophication is due to nutrient discharges. This opinion is firmly held and even regarded as a fact (see, for example, the 19 May 2007 edition of Turun Sanomat). Does this fact asserted by experts rest on anything other than the observation that nutrient discharges have increased and that marine eutrophication has occurred since the end of the 20th century? Where has any study been conducted into the quantitative and temporal relationship between eutrophication and nutrient discharges? These questions remain unanswered, even after reviewing hundreds of pages of relevant literature.
The extent of eutrophication can be measured by studying the concentration of chlorophyll in water. The conclusions of this presentation are based on a long series of measurements taken by the Finnish Institute of Marine Research (FIMR), the Yearbook of Forest Statistics and my own calculations.
Nutrient discharges insufficient as the source of
eutrophication
Some large figures are required to describe both the Baltic Sea and nutrient loading caused by human beings. The Baltic Sea has a total capacity of 21,000 cubic kilometres. The run-off area of the Baltic Sea is loaded by 85 million people, and it may be assumed that several million tonnes of fertiliser are used within this region. Even in waters affected by eutrophication, on the other hand, nutrient concentrations are very small – of the order of microgrammes per litre. It is not at all easy to interpret these figures.
The loading caused by human beings may be better understood by considering a 1:1,000 scale model of the Baltic Sea. For this miniature model to function in global conditions in the same way as the real Baltic Sea, it is essential for the density of materials and nutrient concentrations to remain constant. This means that the scale for surface area is 1:1,0002 and the scale for cubic capacity and mass is 1:10003. A square kilometre becomes a square metre, a cubic kilometre becomes a cubic metre, and 1,000 tonnes becomes a gram.
The Baltic Sea, with an area of 415,000 square kilometres, yields a rounded model pond of diameter 720 metres and average depth 55 mm, containing 21,000 cubic metres of water with a surface of 41.5 hectares and depth not exceeding 45 cm. It is immediately apparent from these figures that the Baltic Sea is very shallow. This must be of the greatest importance to its ecology. In terms of the volume of water, the pond would correspond most naturally to a small lake of diameter 340 metres with a maximum depth of 2 metres.
85 million people live in the run-off area of the Baltic Sea. The annual loading of these people would shrink to one billionth in the scale model. This is the loading caused by one person in 31 days. The Baltic Sea model would therefore require just one resident spending a holiday month on the lakeside.
Some 20 kilogrammes of phosphorus is used per hectare of land in Finland, i.e. 2 grams per square metre, and if the same kind of intensive agriculture is practised elsewhere on fields in the Baltic Sea run-off area, then a total of 680,000 tonnes of phosphorus will be used every year. The corresponding figure for the model is 680 grams, sufficing to fertilise 340 square metres, or a circular field of diameter 21 metres. Would undomesticated animals, fish and vegetation load the pond to a substantially greater degree than one holidaymaker with a smallholding?
In my Turun Sanomat article (16 May 2007) on the effects of phosphorus discharges on eutrophication in the Gulf of Finland I argued that such discharges are currently insufficient to cause this effect. Some 6,000 tonnes of phosphorus reach the Gulf of Finland annually. Human beings cause half of this, with the remainder coming from natural sources. In the absence of any natural cleansing process such a discharge level would cause a phosphorus concentration of 30 mg per cubic metre. A healthy marine environment includes effective cleansing mechanisms that are quite capable of eliminating at least 100 kg of dissolved phosphorus per square kilometre per annum (20 per cent), and so the concentration would remain less than 15 mg per cubic metre and no eutrophication could even begin to occur. The concentration of phosphorus in very pure natural water is about 10 mg per cubic metre. Measurements taken by the Finnish Institute of Marine Research (FIMR) suggest that a high phosphorus concentration of about 50 mg per cubic metre is required for blue-green algal blooms to appear.
Eutrophication of the Gulf of Finland began in 1970 and
the situation deteriorated until 1985
As my calculations indicated that nutrient discharges were not enough to cause marine eutrophication, I began to consider possible sources for the massive discharges that brought about the substantial change in the state of the Gulf of Finland in 1970. Eutrophication has to be a more complex and extensive process than mere nutrient discharges. Several metres of nutrient sediment have accumulated on the seabed over thousands of years. The sediments of the Gulf of Finland contain about 200 grams of phosphorus per tonne, which amounts to millions of tonnes in total. Any phenomenon that could release even a few thousandths of this phosphorus would provide an adequate explanation.
Phosphorus is assumed to be the most important nutrient antecedent for eutrophication. The general principle is that nitrogen-fixating plants will rapidly flourish in any area where there is phosphorus but no nitrogen. While there is always a small amount of phosphorus in water, aquatic plants consume nitrogen and are nitrogen-dependent. This is why common alder grows at the waterside, as this plant has its own means of procuring the nitrogen that it requires. This is also the situation in the sea, with nitrogen-fixating blue-green algae flourishing when phosphorus reaches a sufficiently high concentration of about 50 mg per cubic metre. Only when there is sufficient nitrogen will other plants take over the area.
Some Swedish biologists have even proposed the release of nitrogen into the sea, as the phosphorus would then be consumed and the sea would cleanse itself. This might not work, however, as current specialist literature suggests that the dead algae would sink to the bottom causing oxygen depletion and renewed release of phosphorus. The experts even suggest that the Baltic Sea has now reached a stage at which eutrophication is self-stoking. This cannot be entirely correct, as the Baltic Sea has presumably suffered from naturally occurring eutrophication before and has subsequently recovered. My calculations also suggest that the quantities of algae involved would not suffice to consume seabed oxygen reserves entirely.
I have also heard a theory that propeller streams from ships stir up nutrients from the seabed. This would be negligible. Simple calculations show that even minor daily variations in atmospheric pressure cause currents in water exceeding those that would arise from the engine output of tens of thousands of ships.
Storms stir up the waters of the Gulf of Finland thoroughly, and these have become increasingly frequent. However, the phosphorus is also chemically fixated and not directly available to plants. It must first be reduced and rendered soluble, which can only occur in anaerobic conditions. This is precisely the view that the environmental guardians have now begun to report (see Helsingin Sanomat, 11 June 2007). It suggests that dead algae cause oxygen depletion on the seabed, and that phosphorus is released under these conditions. This has only a minor impact, however.
Marine oxygen depletion and its effects
Reduction of phosphorus occurs on the anaerobic seabed, and measurements indicate that this causes the release of 5 tonnes of soluble phosphorus per square kilometre annually. The opposite reaction occurs when the environment is oxygenated and phosphorus fixates into the seabed sediment. A substantial proportion of the 30,000 square kilometre seabed of the Gulf of Finland suffers from oxygen depletion. The logical conclusion is that sufficient phosphorus is released due to oxygen depletion – with about 50 mg per cubic metre corresponding to 50,000 tonnes – to give rise to the present poor condition. This is the evident starting point for eutrophication.
Some really massive discharges of oxygen-consuming substances of organic origin are required for oxygen depletion and eutrophication to get under way. Industry is the first suspect to investigate in this respect. At its peak the Finnish forest industry discharged 500,000 tonnes of oxygen-consuming substances annually. How much of this reached the sea has not been investigated. Discharges from the forest industry only ever polluted rivers and lakes in the near vicinity and coastal waters. This matter is well known, and the industrial discharges were never sufficiently large to cause oxygen depletion. Nowadays these discharges have fallen to only a few thousand tonnes, and the state of the inland water system has improved, but the sea is still doing badly.
The role of humus in oxygen consumption
Natural processes cause the discharge of about one million tonnes of humus to the sea in Finland’s river waters each year. Humus is a little known, highly stable material of organic origin that forms a brown colloidal suspension in water. On reaching the saline seawater humus coagulates and sinks to the seabed. Microbes oxidise this precipitated humus by using it as an energy source, resulting in oxygen depletion and an anaerobic state when enough humus is present. The chemical composition of humus suggests that it can consume a roughly equal mass of oxygen. The Baltic Sea has absorbed the one million tonne annual humus loading from Finland’s mires for many hundreds of years without suffering from eutrophication. Indeed the sea can withstand major discharges. We must therefore look for an event that led to the marine discharge of several million tonnes of oxygen-consuming material before 1985 when the blue-green algae situation in the Gulf of Finland reached its peak.
Humus discharges began increasing massively in the 1950s
Large-scale mire drainage in Finland began in 1950 and reached a peak in the 1970s. Some 65,000 square kilometres of mires representing one fifth of the whole country were drained or re-drained. On our scale model this drained area would correspond to a circle 280 metres in diameter, and criss-crossed with a very fine network of drainage ditches. And Bingo! – we have our answer. This is a huge area in which the ground has been violently churned up. All we need to do is estimate the quantities involved. Are these enough to consume benthic oxygen more rapidly than it can be replenished from the atmosphere?
We know from extensive tests that mire drainage involves a 250-fold increase in discharges of solid materials . After peaking, these discharge rates gradually fall to normal over the following decade, and we may calculate that a total of a couple of hundred million tonnes of humus may well have been liberated towards the sea during the era of mire drainage by mechanical ditch digging that began in the 1950s.
About one hundred million tonnes of humus were discharged towards the Gulf of Finland. Following the mire drainage fifteen years earlier, eutrophication of the Gulf of Finland began in 1970 and reached a peak in 1985 as discharges from mire drainage consumed oxygen reserves more rapidly than they could be replenished from the atmosphere. There is a significant statistical dependency between mire drainage and eutrophication, admittedly over a fifteen-year delay. It takes a few years for the humus to reach the sea. It is even likely that the process of humus oxidation, oxygen depletion, reduction of phosphorus and large-scale growth of algae takes about ten years. The highest phosphate concentrations were measured in the Gotland trench in about 1989. Both temporally and quantitatively, the problems of the Baltic Sea are very closely associated with mire drainage. The correlation between drainage discharges in 1957-77 and the number of spring algal blooms in 1972-1992 is 0.68. The 95 per cent confidence interval obtained for the correlation by a Fisher’s z' transformation is 0.34-0.86. The diagram shows how the mire drainage trend that began in 1950 is followed by eutrophication after an interval of 15 years.
It is difficult to find any statistical dependency between eutrophication and the history of nutrient discharges other than the fact that both of these occurred at the end of the same century. I think that nutrient discharges were selected as the source of eutrophication on quite inadequate grounds and without seriously considering the natural self-cleaning capacity of a healthy sea. If the wastewater from a large city can be cleaned by chemical oxidation and biological treatment in tanks of only a few thousand cubic metres, then why should the same processes be unable to do likewise in a marine basin of a few cubic kilometres? The concentrations involved in the marine process would be only one millionth of those that are found in wastewater treatment tanks.
Statistical analyses suggest that the Baltic Sea would support an annual humus loading from Finland of nearly 2 million tonnes, but that anything in excess of this would cause oxygen depletion and a surprisingly sharp increase in the level of phosphorus.
The Baltic Sea is ill and intensive forestry may exacerbate
the condition
The same kind of loading of marine oxygen reserves is caused by the humus discharges that occur when forest harvesting machinery breaks up the soil in forests and woodlands covering hundreds of thousands of square kilometres in the run-off area of the Baltic Sea. The effects of using this machinery are clearly visible when the water in Lapland rivers turns brown. These discharges at least retard the recovery of the Baltic Sea.
Forest machinery was first introduced on a large scale in the 1960s, followed by combined tree harvesters in 1980, and spring algal blooms began to increase again at around the turn of the century. New oxygen-consuming organic material may have reached the Gulf of Finland and caused this change. We must develop less violent methods of forestry in order to reduce oxygen-depleting discharges. A clean Baltic Sea would be a valuable and substantial source of healthy fish oils and proteins, and so concern for this objective should override the benefits of intensive forestry in all decisions.
Commentary
It was initially surmised that eutrophication was a direct consequence of nutrient discharges from agriculture etc. The debate then turned to the issue of internal loading, because reductions in nutrient discharges did not seem to lead to a fall in eutrophication. By reviewing studies of eutrophication I have concluded that it is possible that from the very outset the eutrophication process is due to internal loading, and that local problems have arisen only in the vicinity of major point loading. For example, river estuaries and large cities have caused pollution of coastal waters.
To those who agree that this subject should be studied, I would say that the matter is already one of urgency.
To those who ask whether it has been studied, I would respond by asking why this has not been done.
