Benthic processes in fresh water fluffy sediments undergoing resuspension

In the Po river plain relict freshwater wetlands are characterised by a low free water to emergent macrophyte surface ratio, rapid infilling and fluffy sediments, undergoing frequent resuspension. Particle mixing should alter the steep gradients of dissolved gas, nutrient and organic matter quality that generally characterise sediments, with implications for benthic processes. Sediment features and solute fluxes were studied from December 2003 to February 2005 within the Busatello swamp complex (Northern Italy) by means of a combination of core incubation, porewater extraction and microprofiling. At the study site, along a 10 cm vertical profile, sediment organic matter content (32.5%), porosity (0.94) and density (1.02 g cm–3) were nearly constant. Oxygen demand measured by dark core incubation (12.7-56.9 mmol m–2 d–1) was strongly correlated with water temperature. Rates agreed reasonably well with diffusive oxygen fluxes calculated from microprofiles while ammonium and phosphorus regeneration rates predicted from porewater gradients (-309.3-43.4 and -0.1-0.7 μmol m–2 d–1, respectively) were significantly lower than rates measured via core incubations (-129.8-5420.5 and -120.4-35.4 μmol m–2 d–1, respectively). This is a probable consequence of insufficient vertical resolution of nutrient profiles and lack of steady state conditions. A sediment resuspension experiment, carried out under controlled laboratory conditions, indicated large instantaneous oxygen and nitrate consumption, a transient increase of denitrification rates and the rapid release of ammonium and soluble reactive phosphorus. In unconsolidated sediments, resuspension and mixing of sediment particles are key processes regulating mineralisation rates and benthic-pelagic coupling.