Benthic fluxes of dissolved organic carbon in three temperate Australian estuaries: Implications for global estimates of benthic DOC fluxes

Abstract

Light and dark benthic dissolved organic carbon (DOC) fluxes and sediment characteristics (total organic carbon (TOC), total nitrogen (TN), TOC isotope ratio (δ13C), phospholipid fatty acid (PLFA) concentration, and compound‐specific δ13C) were measured seasonally in a range of habitats in three warm‐temperate Australian estuaries to determine what factors control benthic DOC fluxes. Benthic DOC fluxes were seasonal with the highest rates generally observed during summer. Most habitats displayed uptake of DOC during the dark and efflux during the light, and were a net source of DOC to the overlying waters over the diel cycle. Macrophyte habitats (seagrass and macroalgae) had the highest benthic fluxes of DOC (up to ∼50 mmol C m−2 d−1). Organic matter supply to the sediment was dominated by in situ production, and both quantity and source exerted control over benthic DOC fluxes. DOC flux was tightly coupled to bacteria and algal PLFA concentrations during summer, and algal PLFA concentrations during winter, indicating a strong influence of microbial interactions on benthic DOC flux and a temperature‐related decoupling of algae and bacteria under cooler conditions. Estimates of the global estuarine benthic DOC flux indicate that these areas are a significant source of DOC to the ocean (1–41 Tg C yr −1). Inclusion of macrophyte, salt marsh, mangrove, and intertidal benthic DOC flux estimates resulted in a higher global continental margin benthic DOC flux (106–416 Tg C yr −1) than the previous estimate (90 Tg C yr−1).