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Abstract

Dual purpose reservoirs for fish culture and field crop irrigation allow a better utilization of the scarce water resources during the dry Israeli summer. The system is characterized by an increase in organic loading due to fish growth and the corresponding increase in feed input, together with a decrease in the water level due to irrigation. Temporal (monthly and daily) and spatial (horizontal and vertical) water quality variability in a shallow (4 m deep) reservoir in the coastal area is herein analyzed in relation to biological transformations, hydrological processes, and management procedures. The main two factors of water quality variability identified were organic-inorganic matter and oxygen balances. The first balance results from photosynthesis and decomposition, and the second from photosynthesis and respiration, biological processes which only partially overlap in space and time. Management decisions related to fish stocking rate and water level regime determine organic-inorganic and oxygen levels through the year in the reservoir. On this strong long term trend, biological and hydrological processes and management procedures related to feeding and manuring introduce variability at small time-space scales. These processes and their time-space variability should be considered when making management decisions such as location and timing of feeding, manuring and aeration, whether to include fish cages and where, etc.