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Abstract

Permeable sandy sediment are mostly found at the inner continental shelves, and their potential for advective pore water transport fuels a very active microbial community that intensely affects the carbon and nitrogen biogeochemical cycles. Previous studies have indicated that the grain size may play important roles in determining not only the physical sediment parameters but also the microbial abundance and activities in different types of sediment. Studying the influence of grain size of advection-dominated permeable sediment on the microbial abundance and activity will be of importance to our understanding of the global carbon and nitrogen cycling. In this study, North Sea permeable sediment was sieved into <355 µm, 355-500 µm, 500-710 µm, and 710-1000 µm fractions. The biological activities i.e. oxygen consumption and denitrification rates were determined in flow through reactors (FTR), using fiber-optic oxygen micro-sensors, and membrane-inlet mass spectrometry (MIMS). The total cell abundance of microorganisms was determined using ultra-sonication, cell staining and cell counting. The specific surface areas of the sediments were determined via nitrogen adsorption (BET method) and also calculated from the grain size distribution measured using a laser diffraction particle size analyzer. The NOx- production rate in the aerobic incubation and NOx-consumption rate in the anaerobic incubation were determined using a NOx analyzer. Other sediment parameters such as porosity, permeability etc. were also determined and compared to the microbial abundance and activities. The results show that cell abundance, oxygen consumption and denitrification rates in permeable sandy sediment were strongly correlated with the surface area to volume ratio of sediment. The surface area to volume ratio and porosity were not directly related to the grain size. The oxygen consumption rates linearly increased with water flux, while NOx-production and denitrification rates stayed stable. The close associations between surface area to volume ratio, water flux, and cell abundance and respiration rate may help us better understand the benthic oxygen uptake rate.