When the sea is over-fertilized, not all dead plankton is broken down within the surface layers, but instead rains down over the sea bottom. There, on the bottom there are many creatures that filter their food out of the water, i.e. blue mussels. These creatures consume some of these decaying particles that rain down on them, while the majority of this material is taken care of by bacteria. During this decaying process, nutrients are again released into the environment, but for this process to be effective, oxygen is needed - the bacteria consume oxygen during the breaking down of organic material. If a situation should arise where the down pour of organic material is excessive, larger quantities of oxygen are required and there is a risk that the oxygen content of the water can decrease to a critical level for the inhabiting organisms. This can occur when surface water is prevented from mixing with underlying water because of a thermocline. In the end, a situation arises where the bacteria can no longer break down the decaying material and that the oxygen contents is so low that bottom-living organisms are injured. In such a situation, there is much sediment on the dead bottom. This has for example occurred in the Bay of Laholm (on the southwest coast of Sweden) and in the deeper areas of the Baltic.     Increased sources of nutrients also cause increased numbers of plant plankton in the water, which results increased turbidity with poor visibility where sunlight has difficulty penetrating to deeper levels. Perennial macroscopic attached algae are also effected. It is believed for example, that bladder wrack no longer lives as deep as it did at the beginning of the centuary. The discharge of fibres from cellulose plants and different soil particles also increase water turbidity.     The increased production of plant and animal plankton also results in an increase of food resources for fish and other creatures, thus increasing fish populations in the open water. Since the 1920´s, catches of herring and cod have steadily increased from about 100 000 tons annually to about 800 000 tons during the late 70´s. Thus we can deduce that this increase is a result of increased food resources, but also improved fishing methods.     When a lack of oxygen arises on the bottom, poisonous hydrogen sulphide is produced. Hydrogen sulphide is a slag product from bacteria, so called sulphur bacteria, that break down organic material in an oxygen free environment. Oxygen scarcity occurs mainly under the thermocline because oxygen delivery from above is impeded. This eventually leads to the elimination of bottom-living organisms below the thermocline and the reduction of food resources for fish.     When organic material is broken down on oxygen rich bottoms, phosphate ions that are disengaged are bound to iron and create insoluble compounds that stay in the sediment. If a situation should arise where the lack of oxygen increases, these compounds change and became soluble so the phosphate ions are released and come out into the water. In this way, a vicious circle exists, where the phosphates that were once (wholely or in part) responsible for over-fertilization, once again cause the same condition. Page 16 of 28
A splendid outing ?
Problems and expectations
”It was better before"
Seawater and soluble salts
The sea moves
Coastal waters are close to us
What is eutrofication?
Sources of over-fertilization
How the open masses of water are effected
How shallow bays are effected
What can we do?
Alga harvest

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Bo Johannesson | Martin Larsvik | Lars-Ove Loo | Helena Samuelsson