Community experiments using benthic chambers: Microbial significance in highly organic enriched sediments

The structure of the benthic microbial loop was studied in order to understand heterotrophic pathways in the suboxic sediments of the Rapallo Harbour in autumn, 1996. Sediments were characterized by the large accumulation of organic detritus (17.2-21.4 μg chloroplastic pigment equivalents (CPE)g-1; carbohydrates and proteins: 7.8-16.7 and 6.7-7.5mg g-1). Due to the high organic load, benthic bacteria and protozoa displayed extremely high densities (1.4 × 109 cells g-1 and 26.9 × 105 cells g-1). Meiofauna, protozoa and bacteria showed an approximate biomass ratio of 1:2:20. The presence of large amounts of organic matter appeared to determine a shift of the benthic size structure toward the increasing dominance of the smaller components of the benthic food webs. These data indicate that the sediments of the Rapallo Harbour were dominated by microbial biomasses to a larger extent than in non-food limited environments, characterized by a lower organic contamination. On the results presented in this study, the microbial dominance in highly organic enriched sediments can be explained with: (1) a reduction of the top-down control and grazing pressure of meio- and macrofauna on the microbial components; (2) the opportunistic composition and high metabolic activity of the microbial components.