Carp were cultivated in polyculture with grass carp, silver carp and European catfish. The chemical parameters (pH, oxygen, ammonia nitrogen, nitrate nitrogen, nitrite nitrogen, total nitrogen, phosphates, total suspensions, chemical oxygen demand CODMn) of discharged water and four pond layers were measured during the autumn harvest. The study indicated that when the water was discharged the amounts of nitrogen compounds, phosphates and especially total suspension increased from 40 mg dm⁻³ in the first layer to 152 mg dm⁻³ in the last layer and that chemical oxygen demand CODMn increased from 10 mg dm⁻³ in the first layer to 176 mg dm⁻³ in the last layer. The results of both physical and chemical tests on the water discharged from ponds during two periods of the autumn harvest were used to calculate the magnitudes of the loads of organic matter, total suspension and nutrients. The loads of organic matter (expressed as CODMn in an acidic medium), nitrogen, phosphorus and suspensions in the discharged water were determined per ha of area. The highest values were observed in T-type ponds (1,500 specimens ha⁻¹) at an average of 1,214 kg ha⁻¹ and in G3-type ponds (4,000 specimens ha⁻¹ ) with an average of 1,198 kg ha⁻¹ . This data allowed the chemical parameters (kg ha⁻¹) of waters discharged from the ponds in autumn to be compared with the level of CODMn and the amount of nutrients which are introduced to the ponds in spring, as well as to determine the degree of nutrient and organic matter retention in the pond. In the majority of cases nitrogen is fully retained, phosphates are either fully or partially retained and there is an increase in water chemical oxygen demand CODMn especially during the harvest in September.
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