Seawater carbonate chemistry and estuarine dissolved organic carbon export

Relative to their surface area, estuaries make a disproportionately large contribution of dissolved organic carbon (DOC) to the global carbon cycle, but it is unknown how this will change under a future climate. As such, the response of DOC fluxes from microbially dominated unvegetated sediments to individual and combined future climate stressors of temperature change (from delta −3 to delta +5 °C compared to ambient mean temperatures) and ocean acidification (OA, 2current CO2 partial pressure, pCO2) was investigated ex situ. Warming alone increased sediment heterotrophy, resulting in a proportional increase in sediment DOC uptake; sediments became net sinks of DOC (3.5 to 8.8 mmol C/m2/d) at warmer temperatures (delta +3 and delta +5 °C, respectively). This temperature response changed under OA conditions, with sediments becoming more autotrophic and a greater sink of DOC (up to 4 greater than under current pCO2 conditions). This response was attributed to the stimulation of heterotrophic bacteria with the autochthonous production of labile organic matter by microphytobenthos. Extrapolating these results to the global area of unvegetated subtidal estuarine sediments, we find that the future climate of warming (delta +3 °C) and OA may decrease estuarine export of DOC by ∼ 80 % (150 Tg C/yr) and have a disproportionately large impact on the global DOC budget.