Oxygen dynamics in shelf seas sediments incorporating seasonal variability

N Hicks,G R Ubbara,D Sivyer,D J Mayor,E R Parker,H E K Smith,A Hatton,B Silburn,S Kröger,V Kitidis,H Stahl

doi:10.1007/s10533-017-0326-9

Abstract

Shelf sediments play a vital role in global biogeochemical cycling and are particularly important areas of oxygen consumption and carbon mineralisation. Total benthic oxygen uptake, the sum of diffusive and faunal mediated uptake, is a robust proxy to quantify carbon mineralisation. However, oxygen uptake rates are dynamic, due to the diagenetic processes within the sediment, and can be spatially and temporally variable. Four benthic sites in the Celtic Sea, encompassing gradients of cohesive to permeable sediments, were sampled over four cruises to capture seasonal and spatial changes in oxygen dynamics. Total oxygen uptake (TOU) rates were measured through a suite of incubation experiments and oxygen microelectrode profiles were taken across all four benthic sites to provide the oxygen penetration depth and diffusive oxygen uptake (DOU) rates. The difference between TOU and DOU allowed for quantification of the fauna mediated oxygen uptake and diffusive uptake. High resolution measurements showed clear seasonal and spatial trends, with higher oxygen uptake rates measured in cohesive sediments compared to the permeable sediment. The significant differences in oxygen dynamics between the sediment types were consistent between seasons, with increasing oxygen consumption during and after the phytoplankton bloom. Carbon mineralisation in shelf sediments is strongly influenced by sediment type and seasonality.

Highlights

Continental shelf sediments play a vital role in biogeochemical cycling, accumulating and burying organic matter (Jahnke et al 2005; Woulds et al 2007)
Despite the different experimental methodologies employed during this study, much of the oxygen dynamics are determined by the type of sediment (Silburn et al accepted, this issue), with the most permeable sediment (Site G) showing the deepest oxygen penetration depth (OPD) (Fig. 8), and the lowest Total oxygen uptake (TOU) rates
TOU was highest in May, coinciding with the flux of organic matter from the phytoplankton bloom recorded in late April-early May 2015, seen as a peak in chlorophyll-a at the sea surface (Kitidis et al 2017; Thompson et al submitted, this issue), and the associated increase in organic matter

Summary

Introduction

Continental shelf sediments play a vital role in biogeochemical cycling, accumulating and burying organic matter (Jahnke et al 2005; Woulds et al 2007). Shelf sediments receive up to 50% of primary productivity from the overlying surface waters (Stahl et al 2004a), and as this source of carbon reaches the benthos it is recycled, mineralised or buried (Canfield et al 1993; Glud 2008). In sediments below the photic zone, oxygen consumption exceeds production, due to the absence of photosynthesis, and in this case, the distribution and consumption of oxygen will vary with sediment type and the supply of organic matter from the surface waters (Stahl et al 2004a). The degradation and production of organic matter in the sediment can often be many orders of magnitude higher than the surface or overlying waters (Glud 2008), sediments still remain a net sink for surface primary production

Methods

Results

Conclusion