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)

Abstract

Seasonal fluctuations of sedimentary Total OrganicCarbon (TOC) and Total ~eldahl Nitrogen (TKN) were investigated in two earthen ponds of a brackishwater 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 sedimen ts.The considered variables revealed a dystrophic event occurring in summer, when a dense biomass of macro algae (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 ffi"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 oxygen recorded in August attest to the danger represented by high macroalgae biomasses for the considered ecosystem, mainly because of the high oxygen demand induced by rapid decomposition 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 hydrocultivation cannot affect dystrophic environments with such a heavy organic matter recycle.