The Effect of Pig Manure and Mineral Fertilization on a Eutrophic Lake Ecosystem

Summary

A study was carried out in the shallow eutrophic Lake Warniak (area: 40 ha, max. depth: 3.8 m) in the Masurian lakeland, northern Poland. Enclosures made of clear plastic on a wood and iron framework extending 25 cm into the sediments and 25 cm above the water surface were used. The study lasted for a month (June 6–July 5). Additions of a pig manure slurry and/or mineral fertilizers were made to some of the enclosures at six day intervals. Some enclosures were stirred for five minutes each day to simulate wave action.

Daily measurements were taken of transparency, temperature, conductivity, pH. Weekly measurements were made of oxygen concentration, seston, phytoplankton, zooplankton, primary production and various forms of nitrogen and phosphorus. Samples of bottom sediments, bethons and periphyton were taken at the end of the study period.

The applied loading of P was several g/m^-2 and that of N about 20 g/m^-2, exceeding Vollenweider’s dangerous limit many times.

Although the experiment was repeated in 1979, this paper presents only selected general results from the 1978 study. The objective of our study is to elucidate some features of the structure and function of hypertrophic ecosystems in situations where the hypertrophic state was achieved in various ways by the addition of organic or mineral substances, in varying quantities, and with varying frequency.

Periphyton development was intensive in all enclosures. Distinct differences were noted between the different enclosures used in the experiment.

Even the heaviest load of nutrients which we added did not destroy the functioning of the ecosystem until an “ecological catastrophe” resulted. Such a catastrophe—full oxygen depletion and its consequences—happened in the enclosures with the largest addition of pig manure slurry. These enclosures were quite different from the others during the month of study in that the seston increased greatly in the virtual absence of zooplankton. However, even these enclosures became biocenotically balanced after about two months from the moment of addition of the manure.

The control enclosures and that to which the largest addition of manure was made were the only two groups of enclosures which reacted clearly to the large drop in temperature experienced in the middle of the study period. The lack of clear response in the other enclosures can be attributed to their degree of ecological balance.

Some enclosures reacted very differently, sometimes even in the opposite way, to the stirring of the water. It seems that these reactions are to a great extent compensating: increasing the trophic state in the poor environment of the control and it in the richly fertilized enclosure. The simulated wave action resulted in very distinct and important differences in the structure and functioning of the experimental ecosystems.