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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.
