Abstract
Abstract. 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 Δ−3 to Δ+5 ∘C compared to ambient mean temperatures) and ocean acidification (OA, ∼ 2× current 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 m−2 d−1) at warmer temperatures (Δ+3 and Δ+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 (Δ+3 ∘C) and OA may decrease estuarine export of DOC by ∼ 80 % (∼ 150 Tg C yr−1) and have a disproportionately large impact on the global DOC budget.
Highlights
The aquatic dissolved organic carbon (DOC) pool is one of the largest pools of organic carbon on earth (Hedges, 1987), roughly equivalent in size to the atmospheric CO2 reservoir (Siegenthaler and Sarmiento, 1993)
Mean rates calculated from overlapping control cores present in each week were compared to establish whether the two sets of incubations were directly comparable, and whether changes attributed to high pressure of CO2 (pCO2) levels were truly due to that treatment, not just a temporal shift in how the sediments were behaving
There were no significant differences between controls for light or dark production of dissolved inorganic carbon (DIC) or DOC
Summary
The aquatic dissolved organic carbon (DOC) pool is one of the largest pools of organic carbon on earth (Hedges, 1987), roughly equivalent in size to the atmospheric CO2 reservoir (Siegenthaler and Sarmiento, 1993). Phytoplankton in the surface ocean are the main source of DOC globally, with an estimated production of around 50 Pg C yr−1, only 0.3 % of the DOC they produce reaches the ocean interior (Hansell et al, 2009). More recently has the coastal zone been considered a major source of DOC export to the open ocean and deep sea (Duarte et al, 2005; Maher and Eyre, 2010; Krause-Jensen and Duarte, 2016). The shallow coastal zone accounts for 1 to 10 % of global net primary production (NPP) (Duarte and Cebrián, 1996), with up to 33 % of the associated DOC exported offshore and stored in the ocean interior (KrauseJensen and Duarte, 2016)
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