Algal glycans promote microbial turnover of organic matter through stabilization of subtidal permeable sandy sediments

Abstract Permeable sands on continental shelves host microbial communities that drive organic carbon turnover, oxygen fluxes and nitrogen loss. Advective porewater flow associated with sedimentary bedforms maintains these rates and fluxes. Modeling and laboratory studies suggest that advective porewater flow is tightly coupled to bedform stability. However, the impacts of in situ bedform stability on biogeochemical fluxes in subtidal sands remain unconstrained. We deployed a benthic lander at six stations in the North Sea to measure in situ bedform geometries, oxygen fluxes and primary productivity accompanied with ex situ incubations, glycan extraction and microscopy. We observed bedform migration velocities ranging between 0 and 3.2 cm h −1 , which were only 9–16% of expected values from mechanistic models. Bedform stability may be enhanced by high interstitial colloidal and particulate algal glycans concentrations (2–52 mmol C L −1 ) that are a representative component of extracellular polymeric substances. Benthic primary productivity as a source of carbon was negligible (< 0.003 mmol C m −2 d −1 ). Pore space glycan accumulation was attributed to algal biomass filtered out of the overlying water column by advective flow through the permeable sediment. Benthic glycan concentrations correlated significantly with oxygen consumption rates per volume porewater (17–207 μ mol L −1 PW h −1 ), whereas sediment oxygen fluxes (6–17 mmol m −2 d −1 ) significantly correlated with modeled porewater velocities. Overall, the impact of glycans as a proxy for pelagically derived algal biomass was twofold: (1) they drove higher oxygen consumption rates in surface sediments and (2) contributed to stabilization of bedforms, which almost doubled oxygen fluxes into the sediment.