Seasonal fluctuations of the organic load of sediments in two ponds of a brackish-water fish farm ("valle") located in the Po River Delta (Italian North Adriatic)

Seasonal fluctuations of sedimentary Total Organic Carbon (TOC) and Total K jeldahl Nitrogen (TKN) were investigated in two earthen ponds of a brackish water extensive fish farm. One was subjected to periodic sediment "ploughin g" (hydrocultivation), a practice performed by fishfarmers to oxidise sedim ents and fertilise the water column. Nutrients and chlorophyll-a were monit ored for controlling the impact of hydrocultivation on water overlying trea ted sediments. The considered variables revealed a dystrophic event occurring in summer, w hen a dense biomass of macroalgae (7-10 kg m(-2)) died because of a rise in temperature (30 degreesC 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), respec tively. A phytoplankton bloom occurred, increasing living organic matter in the water column, as pointed out by high values of total-N, total-P and ch lorophyll-a. Low levels of dissolved oxygen recorded in August attest to th e danger represented by high macroalgae biomasses for the considered ecosys tem, mainly because of the high oxygen demand induced by rapid decompositio n of macroalgae tissues. In mid-October TOC dropped to 870-890 gC m(-2), and TKN to 42-43 gN m-2. In the two ponds no important difference in variable trends has been pointed out, despite the periodic "ploughing" interventions, probably because hydro cultivation cannot affect dystrophic environments dth such a heavy organic matter recycle.