Nutrient profiles in the pore water of a deltaic lagoon: Methodological considerations and evaluation of benthic fluxes

Several aspects of the early diagenesis of major nutrients (silica, ammonia, phosphorus) and iron, based on sediment and porewater studies, were examined in Sacca di Scardovari (Po Delta, Italy). Radionuclide measurements (210Pb, 137Cs, 134Cs) were carried out in order to calculate modern sediment accumulation and mixing rates. The pore water obtained from two different sampling techniques is also compared. One method involves direct sampling through the use of dialysis samplers; the second one obtains samples through the centrifugation of sediment cores. The two methods compare well for all the elements considered, except for Fe values in the topmost centimetres of the sediment and for NH4 in the lowest 15 cm. These differences can be explained by alterations of porewater composition during the manipulation of core samples and by spatial heterogeneity. The distributions obtained for the different ionic species are typical of anoxic sedimentary environments, rich in organic matter. Evaluation of silica, ammonia and phosphorus fluxes at the water-sediment interface is conducted by applying a one-dimensional model: the sediment is considered as a two-layer system and a steady-state diagenesis is assumed. The model takes into account molecular diffusion and irrigation in a cumulative coefficient D1 or distinguishes irrigation effects in a separate term, I. The two schemes furnish different flux values, and explanations for these differences are proposed.