Bioturbation in the Venice Lagoon: Rates and relationship to organisms

Experiments were carried out during autumn 1998 and spring 1999 at four selected sites in the Venice Lagoon in order to estimate the major bioturbation modes, and for quantitative analysis of the contribution of various taxa to these modes. Fluorescent sediment particles (63–350 micrometres) were supplied as a tracer pulse input at the sediment surface. Tracer depth profiles obtained after 15 and 20 days were simulated with a diffusion-advection-non-local transport model. This allowed the rates of biodiffusion, bioadvection, and RS, a non-local transport coefficient to account for the displacement of sediment by regeneration, to be estimated. A combination of fresh water and marine organisms were responsible for the recorded sediment reworking, which was dominated by both types of non-local transports (conveying and regeneration). Considering all the sampling sites and seasons, biodiffusion ranged from 0.87 +/-0.02 to 3.17 +/-0.92 sq cm/y-1, bioadvection from 0.12 +/-0.09 to 27.41 +/-2.47 cm/y-1 and RS from 0.00 +/-0.00 to 5.47 +/-1.09 g/cm-2/y-1 (mean +/-SE, n=3). A multiple regression analysis was applied to identify the contribution from individual species to sediment transport types. Biodiffusion resulted from the combined activity of polychaetes such as Spio decoratus and meiobenthic harpacticoïd copepods, while the polychaete Hediste diversicolor was mainly responsible for regeneration. Conveying processes were driven by another polychaete, Capitella capitata. Despite heterogeneity in the benthic community composition, biodiffusion and regeneration rates did not vary significantly between sites or season, with only bioadvection found to be higher in spring than in autumn.