This paper presents a case study of a massive fish mortality during a Cylindrospermopsis raciborskii bloom in Aleksandrovac Lake, Serbia in mid-December 2012. According to a preliminary investigation of the samples taken on November 6 before the fish mortalities and to extended analyses of samples taken on November 15, no values of significant physicochemical parameters emerged to explain the cause(s) of the fish mortality. No industrial pollutants were apparent at this location, and results excluded the likelihood of bacterial infections. Even after freezing, the dissolved oxygen concentration in the water was sufficient for fish survival. High concentrations of chlorophyll a and phaeophytin occurred in the lake, and phytoplankton bloom samples were lethal in Artemia salina bioassays. A bloom of the cyanobacterium C. raciborskii was recorded during November. Although the A. salina bioassays indicated the presence of toxic compounds in the cyanobacterial cells, the cyanotoxins, microcystins, cylindrospermopsin and saxitoxin were not detected.

This paper deals with the methodology used for conducting a water resources assessment as the core component of water resources master plans. It is basically a quantitative and qualitative water resources balance (WRB) and an essential element of any short or long-term planning of sustainable and environmentally sound river basin development and management. The use of a water resources management balance instead of the water resources balance (water budget) that includes the water withdrawals and discharges in the balance equation should be favoured in water resources assessment. By using a water resource management balance, the multiple use of a given volume of water is accounted for so that it is possible to satisfy larger water demands than by using the natural water budget approach. This approach forces planners to look at a much wider scope of alternatives (reservoirs versus water recirculation and conservation for example) to meet the demands and also reinforces the role of water quality in water resources assessment.

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