Benthic pH gradients across a range of shelf sea sediment types linked to sediment characteristics and seasonal variability

Abstract

This study used microelectrodes to record pH profiles in fresh shelf sea sediment cores collected across a range of different sediment types within the Celtic Sea. Spatial and temporal variability was captured during repeated measurements in 2014 and 2015. Concurrently recorded oxygen microelectrode profiles and other sedimentary parameters provide a detailed context for interpretation of the pH data. Clear differences in profiles were observed between sediment type, location and season. Notably, very steep pH gradients exist within the surface sediments (10-20 mm), where decreases greater than 0.5 pH units were observed. Steep gradients were particularly apparent in fine cohesive sediments, less so in permeable sandier matrices. We hypothesise that the gradients are likely caused by aerobic organic matter respiration close to the sediment-water interface or oxidation of reduced species at the base of the oxic zone (NH 4 ? , Mn 2? , Fe 2? , S -). Statistical analysis suggests the variability in the depth of the pH minima is controlled spatially by the oxygen penetration depth, and seasonally by the input and remineralisation of deposited organic phytodetritus. Below the pH minima the observed pH remained consistently low to maximum electrode penetration (ca. 60 mm), indicating an absence of sub-oxic processes generating H ? or balanced removal processes within this layer. Thus, a climatology of sediment surface porewater pH is provided against which to examine biogeochemical processes. This enhances our understanding of benthic pH processes, particularly in the context of human impacts, seabed integrity, and future climate changes, providing vital information for modelling benthic response under future climate scenarios.

Oxygen dynamics in shelf seas sediments incorporating seasonal variability

“…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). One of the determinants of burial efficiency of organic matter is the presence and the depth of oxygen penetration (Burdige 2007;Glud 2008;Woulds et al 2007).…”

“…In coastal sediments and shallow waters, oxygen production through photosynthesis may exceed consumption. However, 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), although sediments still remain a net sink for surface primary production.…”

Patterns of Carbon Metabolism, Storage, and Remineralization in Seagrass Ecosystems

“…Porewater exchange of solutes through coarse sediments leads to faster turnover rates of organics introduced to sediments and lower C org retention (Huettel et al, 2014). Fine-grained, muddy sediments have lower porosity and pore water exchange, leading to decreased solute penetration and a reliance on sulfate reduction rather than aerobic decomposition (Cook et al, 2007;Silburn et al, 2017). Limited solute renewal and delivery in fine sediments decreases OM remineralization and thus increases OM preservation .…”

“…Oxygen uptake rate by microbial decomposition on the sediment surface is negatively correlated with grain size, thus sediments containing higher C org content and greater surface area for microbial substrate have faster breakdown rates Huettel et al, 2014). Faster decomposition and consumption of solutes at the sediment surface in fine sediments can lead to fewer solutes being transported through the sediments for bacteria to use when remineralizing buried C org ; this compounded with limited permeability of muds, creates a large gradient in oxygen consumption and decomposition down the sediment profile (Silburn et al, 2017).…”

“…The modified hydrology due to seagrass canopy can reduce average sediment grain size by allowing for the accumulation of fine grained particles that would not otherwise occur . Finer sediments promote O 2 consumption at the sediment surface through enhanced microbial activity while limiting the diffusive ability of solutes through to porewaters (Cook et al, 2007;Silburn et al, 2017). As a result, anoxia has been shown to develop millimeters beneath the sediment surface (Borum et al, 2005) and the density of seagrasses reduces the oxygen input via pore-water advective transport .…”