When the morning sun rises over the sea, a myriad of tiny floating plants – too small for the eyes to see – start to capture the solar energy that fuels marine life. These plants, known as phytoplankton, are nothing like the plants we see on the land (known as vascular plants), though they are relatives. They have evolved in ways that enable them to stay in the shallow part of the sea and/or to find the essential nutrients they require for growth. Some species have little appendages known as flagella that enable them to swim slowly; others can even produce toxic substances to ward off the animals that predate on them (zooplankton).

The essential nutrients that phytoplankton need to help them to grow include compounds of nitrogen, phosphorus, iron and in some cases, silicon. Phytoplankton also need several organic compounds such as vitamins. You can find more information about all of these substances in the Fact Sheet on chemistry in the Study Pack. There are three major sources of these substances:

1. their release by phytoplankton at night, dead phytoplankton, or from other marine organisms (as waste products);
2. arrival from deeper waters of the sea by mixing; and,
3. transport from land by rivers and streams and through the atmosphere.

Though (1) and (2) are the main source of nutrients in the Black Sea, a significant amount of nutrients come from the land, either directly or through the major rivers draining parts of 17 countries across Europe.

In the past, before humans caused massive alterations in the landscape of our continents, the amount of phytoplankton in the sea depended mainly on the degree of mixing of deeper, nutrient rich, water with the shallow layer where sufficient light penetrated to allow these plants to grow (this layer is known as the euphotic zone). There were seasonal variations according to the climate as this governs the amount of wind energy for mixing, variation in the temperature, rainfall and the strength of the sun. The overall system adjusted to these changes. Fish for example, spawn just prior to seasonal increases in phytoplankton so the small larval hatchlings would have plenty of food to eat and a better chance of survival.

On the land, plants also need nitrogen and phosphorus compounds in order to grow. These are usually supplied by the soil but continuous cultivation depletes these substances in the soil and reduces the capacity of the land to produce crops. About half way through the 20 th century, relatively inexpensive ways of making nitrogen and phosphorus fertilisers were discovered. When these are added to the soil, plants grow more quickly and crop yields increase. Adding fertilisers however, also results in changes to soil quality, making it more susceptible to erosion as the plants gradually use up the natural organic matter present.

Question for group discussion : How did farmers maintain agricultural production before chemical fertilisers were introduced?

Another problem however, is that some of the nitrogen and phosphorus added to the sea gets washed into streams and rivers and eventually to the sea. In the sea, it enhances the natural growth of phytoplankton causing a ‘bloom' – a dense patch of phytoplankton that may change the colour of the sea to green, red of brown. Decaying cells from patches consume oxygen (see the Chemistry fact sheet for details) and can lead to the death of all animals that require oxygen and that cannot escape from the area. The phenomenon of over-enrichment is called eutrophication .

Classroom quiz : Divide the group into two teams, A and B and stand in two lines. There are two lists below. The first is a list of human activities or substances that may introduce nutrients to the environment and the second is a list of possible ways of avoiding or reducing them. The teacher will write the second list on a board in a place that both teams can see and explain the options. He/she will then read out the items in the first list one at a time. The objective is to decide whether or not the activity or substance could contribute to eutrophication in the Black Sea . Group A will answer the first question with ‘yes' or ‘no'. If they are wrong they will lose a member of the group to the other team. If the substance or activity can contribute to eutrophication, Team B will then be invited to suggest a good option for removing the risk from the list on the board. If they are wrong they will lose a team member to group A. The second question will be directed to Team B and team A will be allowed to suggest a solution if appropriate. The questions are asked to alternative teams until the list is finished. The team with the most players at the end of the game is the winner. List one: List two 1. Human sewage 1. Wastewater treatment plant 2. Traffic exhaust 2. Use of alternative substance 3. Bus factory 3. Restoring a wetland 4. Washing detergent 4. Applying waste to fields 5. Animal farms 5. Improving agricultural practices 6. Nuclear power station   7. Oil refinery   8. Potato farm

Another alternative for producing food

technique known as organic farming (or ecological farming) is becoming an increasingly popular way to produce food that has a high nutritious value and a low environmental impact. This is based on traditional methods of farming combined with low-cost modern technology and requires no added chemical substances. Soil quality is improved by adding animal manure and compost and by changing the crops planted in each field every year. In some years, plants such as beans or clover are planted that can use nitrogen from the atmosphere and convert it to forms that remain in the soil and can supply other plants the following year. Organic farming requires great skill and much more effort than ‘chemical' farming but crops can be sold for higher prices.

Eutrophication in the Black Sea

During the late 1960s, there was a major change in agricultural production often called the “Green Revolution”. This involved the use of large amounts of fertilisers and pesticides to sustain high crop yields. Intensive animal farms were also established to provide a cheaper source of meat (one farm in Romania , for example, had more than a million pigs). Discharges of waste nutrients from these agricultural activities, and from domestic and industrial sources across the Black Sea basin, entered rivers and streams and eventually found their way to the Black Sea itself. The seagrass and algal beds of the north-western shelf (see the box on Zernov's field) were unable to absorb such large amounts of nutrients and large quantities of phytoplankton began to grow, shading the light from the larger plants below. Deprived of light, the meadows began to die. The huge amount of additional decaying organic matter at the sea floor and the associated bacteria, used up the dissolved oxygen. This resulted in a dead-zone where the entire bottom dwelling fauna was asphyxiated.

The effects of eutrophication were felt across the entire Black Sea . Though it is estimated that 70% of the dissolved nutrients produced by human activity came from the Danube River alone, there are no innocent countries and coastal eutrophication is observed in areas well away from the influence of the Danube . Indeed, the whole sea suffered changes in the structure of its ecosystem during the last three decades and eutrophication is one of the factors which have contributed to this change. Organisms which are specialised in feeding on surplus organic matter have appeared in large numbers all around the Black Sea coast but these are often regarded as “dead end” species as they do not serve as fodder for zooplankton and the rest of the food chain. In many respects, the “fertilisation” of the sea with nutrients has made it poorer and not richer.

An underwater meadow called Zernov's field …

In shallow areas of the sea, where the seabed is bathed in light, larger plants and algae may grow in underwater meadows. These too can form the base of a food-chain but also provide shelter for a large number of animals which live attached to the sea floor or close to it, or arrive as visitors, sometimes remaining during an important stage in their reproductive cycle. The North-western part of the Black Sea is largely below one hundred metres depth and always received a good supply of nutrients from the rivers Danube and Dnieper , Europe 's second and third largest rivers. It was virtually covered with underwater meadows. One species alone, a red algae called Phyllophora , dominated a vast area. The meadow, named Zernov's field after its Russian discoverer, was the home to a unique and highly productive ecosystem of plants and animals. Incidentally, the red algae were also harvested by humans for their agar, used as an ingredient for ice cream!
Sadly, the above text uses the word “was”. During the 1970s and 1980s, as a consequence of eutrophication, the NW Shelf ecosystem rather suddenly and catastrophically collapsed. Vast amounts of dead plants and animals covered the beaches of Romania and western Ukraine ; between 1973 and 1990, losses were estimated as 60 million tons of bottom animals including 5 million tons of fish. At market prices, the fish alone might have been worth US$ 2 billions, but a monetary value cannot be placed on the real loss of such a unique ecosystem.

Studies by the Danube Basin Environmental Programme suggest that about half the nutrients discharged to the river are from agriculture, one quarter from industry and a similar proportion from domestic sources. Because of economic difficulties in the downstream countries and improved environmental controls in the upper basin countries, in recent years the current loads of nutrients entering the Black Sea from the Danube has fallen. The Black Sea ecosystem is gradually recovering but the reprieve may be a temporary one. It is widely considered that nutrient discharges are likely to rise again, with consequent damage to the Black Sea , unless action is taken to implement nutrient discharge control measures as part of the economic development strategies.

Eutrophication from space

n the photograph shown on the opposite page, there is an image taken from a satellite orbiting the earth. The colours are enhanced to show the chlorophyll concentration in the sea. Chlorophyll is the light collecting substance that exists in all plants, terrestrial and marine. In the sea, more chlorophyll indicates more phytoplankton. In the photograph, higher concentrations of chlorophyll are shown by darker green colours. This beautiful image clearly shows the eutrophication associated with river inputs such as the Danube and Dnipro ( Dnieper ) and in the Sea of Azov . It shows how the whirlpool-like eddies carry the nutrient-rich water deep into the Black Sea . The deeper blue water near the coast of Crimea indicates the least eutrophic water of the Black Sea.

The moral maze – working to reduce eutrophication - what do you think?

There is a close relationship between the way we live and the problem of eutrophication in the sea. Farming techniques that use fertilisers may keep the cost of food down but this is often at a high cost to the environment through eutrophication. Losses of nutrients to the environment can be avoided with better farming techniques but these have to be paid for by increased food prices. Nutrients from human sewage can be reduced by full sewage treatment but this is also quite expensive to operate. Removal of phosphate from detergents has been achieved in many parts of the world but this also results in higher prices. Much of our current lifestyle is achieved by using rivers and the sea as a cheap alternative for disposal. For some politicians, it is regarded as acceptable to pollute the environment in order to achieve badly needed economic recovery and reduce poverty. They argue that the mess can be cleaned up when the economy has recovered. Others argue that this is unacceptable and that it is better to slow down the rate of recovery but to make it sustainable from the beginning. This may result in many people being poorer for a longer time. What do you think? Are there any other alternatives?

Source : SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE