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Temporal variations of sedimentation in shallow freshwater systems

Nineteen new traps which made it possible to sample almost the whole water column in shallow systems were exposed on a monthly basis during 18 months in two ponds. Quantitative and qualitative temporal variation of settled particles was shown even though seasonal variation was generally masked by bottom sediment resuspension in these shallow aquatic systems. Sedimentation rates fluctuated on a seasonal basis. They were minimum in spring and maximum in summer. These fluctuations were linked to the high primary production observed. These new traps showed that decaying submersed macrophytes can be an important source of organic matter, as well as decay of blue-green algal blooms which have always been considered responsible for anoxic problems in fish ponds. Interference by the fish farmer at various times of the year also caused sudden high sedimentation rates (>200 g m -2 d -1 ). Sedimentation in the fish ponds was controlled by natural mechanisms partly similar to those that occur in lakes, and by mechanisms specific to shallow systems and to aquatic systems managed by man.