Nutrient regeneration processes in bottom sediments in a Po delta lagoon (Italy) and the role of bioturbation in determining the fluxes at the sediment-water interface

Abstract

A study on nutrient regeneration processes and a measure of their fluxes at the sediment-water interface was carried out in two different stations of a shallow lagoon of the Po delta river (Italy). A few parameters on the solid fraction (grain-size, porosity, C, N) and pore water profiles of o-P, NH3, NO inf3 sup− , SiO2, Tot-CO2, SO inf4 sup2− , Fe, Mn, Ca, Mg, pH, Eh were determined. At both stations the results were typical for fine sediments rich in organic matter. The ratio of variations of sulphate (ΔSO inf4 sup2− ) to total carbonate demonstrates the main role sulphate reduction plays on the organic matter decay. The use of the ratios of variations of sulphate (ΔSO inf4 sup2− ) to ammonia (ΔNH3) and of sulphate (ΔSO inf4 sup2− ) to phosphate (Δo-P) in pore waters enabled us to calculate the C/N/P of the decomposing organic matter. Obtained C/N/P indicated an enrichment of N and P with regard to C/N/P ratios of the solid fraction, due to the selective stripping of N and P during organic matter mineralization. This phenomenon decreases with depth, where organic matter becomes more refractory. Calculations on saturation degrees have shown the possibility of authigenic calcite, apatite and rhodochrosite precipitation in sediments. Nutrient fluxes were estimated for SiO2, NH3 and o-P by means of benthic chambers and modelling the pore water profiles. The model used for the calculation of fluxes allowed us to account for the bioturbation-irrigation influence near the interface, by means of a cumulative diffusion coefficient. Directly measured fluxes proved to be always significantly greater than the theoretical ones. These differences seem to be due to surface processes which do not affect pore water concentrations (degradation of fresh materials at the interface; micro-bioturbation by small gasteropoda such as Hydrobia ventrosa) and/or to the different concept of the two methods in time and space. Number, size and biomass of macrobenthic species living in the sediment underneath the benthic chambers were determined. The comparison between data on macrobenthic populations and flux values showed a good relationship between the number of organisms and benthic fluxes within each station. However, flux variations between stations are to be attributed mainly to the different arrangement of the tubes of the polychaetes Polydora ciliata in the sediment.