Assessing the fate of organic matter in subtidal sandy sediments using carbon and nitrogen stable isotopes as deliberate tracers

In this thesis, several subtidal sandy sediments of the coastal ocean are investigated to unravel the structural, ecological and biogeochemical interactions at the microbial and metazoan scales. The first study illustrates the coupling between the bacterial compartment and primary producers. The availability of nitrogen in sediment phytodetritus for seagrass plants was investigated in 5 tropical species from Indonesia. In an in situ experiment, 15N-labelled phytodetritus was injected into the sediment and the appearance of 15N in the roots, the rhizomes and the leaves of the plants was measured over time. The combined efficiency of seagrass canopy-induced trapping and retention of sestonic particles and root-uptake, results in the acquisition of nutrients released upon mineralization of particulate organic matter, giving them a competitive advantage over other primary producers in oligotrophic environments. The second study provides a detailed snapshot of the microbial compartment of a coastal permeable sandy sediment in the photic zone, where the pathways and the fate of C and N were studied in a stable isotopes pulse-chase experiment. This experiment demonstrates the efficient transfer of inorganic carbon and nitrogen to the benthic community through the activities of photo- and chemoautotrophic microorganisms, and the role of extracellular polymeric substances (EPS) as a carrier for energy to the benthic microbial food web. The third study focuses on the benthic food web and provides a complementary picture to the second study. Carbon and nitrogen flows within the food web of a subtidal sandy sediment were studied through a dual stable isotope approach. First, the natural abundance of 13C and 15N stable isotopes of consumers and potential benthic and pelagic resources suggested trophic network relying on MPB and suspended particulate matter. Second, the MPB of sediment cores was labelled through a pulse-chase experiment with 13C-bicarbonate and 15N-nitrate. The fate of the MPB was followed in the different heterotrophic compartments. Heterotrophic bacteria contributed most to the total heterotrophic incorporation of 13C. Significant labelling of metazoans allowed us distinguishing between utilization of MPB and phytoplankton derived carbon. The combined natural abundance and deliberate tracer addition approaches thus provided complementary information on the key role of MPB in structuring benthic communities in sandy sediments. In the last study, the relative significance of phytodetritus deposition and autochthonous MPB production for benthic consumers in an organic carbon-poor sandy sediment was assessed in two consecutive tracer experiments. In a first experiment, sediment cores received a pulse of 13C-labelled phytodetritus and the fate of that organic matter was followed in the benthic food web over a period of 72 h. In a second experiment, the MPB was labelled with 13C-bicarbonate and its fate was followed over a period of 96h. Coupled to a detailed survey of the natural abundance of 13C in the different benthic organisms, this study revealed complex trophic interactions among benthic consumers. Relative dependence of consumers on phytodetritus and MPB was estimated and revealed that benthic heterotrophs in these sandy, permeable sediments rely more than 95% on MPB