Sediment heterogeneity shapes spatial variability of resuspension-induced CO2 production

Abstract

Demersal fishing is a major anthropogenic disturbance to marine sediments, with global implications for benthic carbon cycling and greenhouse gas emissions. Resuspension of sediment organic matter during bottom trawling enhances oxic mineralisation, converting stored organic carbon into aqueous CO2 and reducing the long-term carbon storage potential of the seafloor. Sediment heterogeneity likely plays a role in the vulnerability of sedimentary organic carbon to resuspension, but spatial estimates CO2 release from resuspended sediment rarely accounts for this heterogeneity. We conducted a large-scale survey in the Hauraki Gulf, New Zealand, to assess how sediment characteristics affect resuspension-induced CO2 production (RCO2 P). Using a resuspension assay at 57 sites, we quantified RCO2 P accompanied by measurements of sediment grain size, organic matter content and quality, and phytopigments. Boosted regression tree analysis revealed that organic matter content has the strongest influence on RCO2P variability, followed by coarse grained sand content and water depth. Non-linear relationships with RCO2 P further indicate context-dependent mechanisms controlling RCO2 P and allowed for the identification of three clusters with differing levels of vulnerability to resuspension impacts and different environmental influences. Overall, risk of resuspension-induced CO2 release was moderate to very high in sediments with > 3 % organic matter, < 8 % coarse grained sand, and at depths > 56 m, comprising 73 % of our sampling sites. Multiple "hotspot"locations were found in the Hauraki Gulf, likely driven by an interplay of organic matter bioavailability and hydrodynamic conditions. Our results demonstrate that accounting for sediment heterogeneity in resuspension impact assessments will create more realistic and ecologically relevant estimates of C vulnerability over regional scales to inform spatial fisheries management.