Abstract Seasonal fluctuations of sedimentary Total Organic Carbon (TOC) and Total Kjeldahl Nitrogen (TKN) were investigated in two earthen ponds of a brackish-water extensive fish farm. One was subjected to periodic sediment “ploughing” (hydrocultivation), a practice performed by fishfarmers to oxidise sediments and fertilise the water column. Nutrients and chlorophyll-a were monitored for controlling the impact of hydrocultivation on water overlying treated sediments. The considered variables revealed a dystrophic event occurring in summer, when a dense biomass of macroalgae (7–10 kg m-2) died because of a rise in temperature (30°C and over). From July to August a large amount of labile organic matter settled to sediments increasing TOC and TKN from 600 gC m-2 and 38.5 gN m-2 to 1500–1800 gC m-2 and 50-68 gN m-2, respectively. A phytoplankton bloom occurred, increasing living organic matter in the water column, as pointed out by high values of total-N, total-P and chlorophyll-a. Low levels of dissolved ...