Pathways of organic matter degradation in marine coastal sediments: implications to ecosystem functioning

Abstract

The main mechanisms that regulate organic matter (OM) mineralization in marine sediments include distribution and concentration of electron acceptors, quantity and quality of OM and bioturbation of benthic macrofauna. Degradation processes that occur in the sediment play a key role on coastal ecosystem functioning, for example to return nutrients to the water, to establish benthic-pelagic coupling, to stimulate pelagic and benthic primary production and to yield energy flux. Very little is known about the biogeochemical processes involved in the OM degradation and total rates of carbon and nitrogen mineralization in sediments of Brazilian coast. It is thus important to understand the mechanisms regulating the OM degradation, since these processes are vital to ecosystem functioning. The main goal of this project is to investigate the OM degradation pathways in several sediments types of Ubatuba coastal area, Sao Paulo and the major factors altering these processes, such as the participation of different electron acceptors and benthic fauna assimilation and stimulation. Laboratory experiments will apply the latest methods and analysis to study the project goal. Fluxes of O2, CO2, dissolved organic carbon and nutrients in the sediment-water interface will be measured in cores maintained in laboratory conditions similar to in situ. The same solutes will be quantified in the porewater to calculate the amount of compounds produced, accumulated in sediments and exported to the water column. Oxic and reduced conditions in the sediment will be accessed by microelectrodes of O2 and redox. In another experimental set-up, enriched microalgae (13C), will be added to the sediment surface of cores to access the amount of organic matter assimilated by fauna and bacteria. The isotope analysis of 13C in the animal tissues at the end of the experiment will reveal the amount of OM consumed by fauna, while the 13C isotope analysis in bacterial lipid biomarkers will reveal bacterial incorporation. The chemical and isotope analysis will be done in collaboration with Institute of Biology, University of Southern Denmark and Netherlands Institute of Marine Ecology. We expect to find high reactivity and mineralization rates in sandy sediments, since the oxic layer should be broader and degradation via O2 is more efficient. Fauna participation should vary according to sediment type and the structure of benthic community.