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Jak se mění kvalita vody na odtoku z rybníků při jejich výlovu
VÁGNER, Martin
The goal of this diploma thesis was to characterise the change of pond effluent quality during fish harvesting period. The change of water quality variables were measured and the pond draining stage at which fish harvesting activities affect most the quality of pond effluent was identified. Results and observations should help with the development of technology for retaining the sediment and nutrients discharged from ponds during fish harvest. The monitoring of pond effluent quality was carried out in twelve ponds managed by Krajské školní hospodářství České Budějovice in South Bohemian region. At different fish harvest phases, water temperature, concentration of dissolved oxygen, conductivity, turbidity and pH were measured in situ with a multiparametric probe YSI EXO 2. At the same time, water samples were also taken to quantify the concentration of suspended solids (SS), total phosphorus (TP), particulate and dissolved phosphorus (PP, DP) and total nitrogen (TN). Water samples were processed in the laboratories of the state enterprise Povodí Vltavy in České Budějovice. Our results confirmed that draining and fish harvest have significant influence on the quality of both the pond effluents and probably on the water bodies located downstream of the ponds. The largest changes of monitored parameters were caused by the time of active movement of fish and people in fish harvesting area. The active movements include setting the tanks for fish sorting, fish scaring, and seining. The highest concentrations of suspended solids, total phosphorus and total nitrogen were detected at the stage of catching fish with hand nets and surface outflow after the end of fish harvest. The changes of values during the fish harvest were considerable (% higher than pond water) and was fast. Therefore, best management practices aiming at reducing the concentration of SS, TP and TN may be implemented starting from the stage of fish netting until the closure of the pond outlet. Dissolved phosphorus has been identified as the key parameter causing eutrophication. The results of our study revealed that fish harvest is not so important problem from this point of view. During fish harvests obviously dominated particulated phosphorus which part was in average more than 99 %. The quality of effluent discharged during fish harvest was mainly influenced by the biomass and type of fish. Weather, water level regulation and type of outlet device influenced the effluent quality as well but at a lesser extent.
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