Ecomare

In order to end the pollution in the fresh water, sewage purification plants were built. In addition, overfertilization on land was dealt with. The water that was discharged after all these regulations clearly contained fewer nutrients. But strange enough, the lakes were not any cleaner; they continued to be cloudy. What was the problem? The lake bottoms were the villain. One of the nutrients, phosphate, readily combined with mud particles on the bottom. When the water became cleaner, the water supplemented its phosphate supply by extracting it out of the bottom. The buffer supply of phosphate in the bottom was so great that it fed the lakes for years, allowing the algae growth to continue.

When the phosphate finally disappeared out of the bottom, the water was still cloudy. The lakes continued to appear polluted and biologists racked their brains how that was possible. The solution came from Czechoslovakian fish farmers, who described how their nurseries became clearer every time they removed the fish. As an experiment in the Netherlands, a large majority of the fish were removed from a few small lakes. Fish such as carp and bream continually burrow in the bottom in search of freshwater aquatic worms, mosquito larvae and other food. Because the fish also emitted the bottom mud via their gills, they would leave a trail of 'dust' behind them, which created the muddiness. In addition, the burrowing actions released other nutrients which stimulated the growth of phytoplankton. After removing a major amount of the fish in the experimental lakes, the water became crystal-clear, a spectacular result. By removing the fish, not only was the water cleaner because there were fewer mud particles in the water, but the water fleas were able to keep the algae under control since there were fewer young fish to eat them up.

One would expect that when the fish population has grown after a few years, that the water fleas would be threatened by the young fish and the water would lose its clarity again. Just the opposite happened. It appears that the clean water will remain clean for years to come as long as the water does not exceed the critical amount of nutrients. The secret is in the plants. When the water is clean, the aquatic plants on the bottom get all the opportunity to grow. These aquatic plants, such as water lilies, water soldier and coontail hold down the bottom and act as a wave breaker. Less mud is whirled around. In addition, they gobble up the nutrients via their stems and leaves before the phytoplankton can use them. The water fleas can hide better among the plants from the fish. They only appear at night to filter the water. In this way, the aquatic plants provide a positive feedback: the clear water becomes even clearer with the presence of the aquatic plants.