Abstract
AbstractTemperate shelf seas are productive areas with the potential to export high quantities of particulate organic carbon (POC), as sinking particles, to the sediments or off‐shelf to the open ocean. The amount of carbon which can be exported depends partly on the amount of POC produced and on the remineralization processes occurring on the sinking material. Here, we assessed the relative seasonal importance of microbial respiration and bacterial production associated with suspended, slow‐ and fast‐sinking particle fractions. The three fractions were collected in the Celtic Sea above and below the seasonal thermocline in November 2014, April and July 2015 using Marine Snow Catchers. The slow‐sinking fraction had higher microbial respiration and bacterial production rates than the fast‐sinking fractions, and these two fractions sustained rates of microbial respiration and bacterial production between 1 and 3 orders of magnitude lower than the suspended fraction. This low contribution of the slow‐ and fast‐sinking fractions was consistent with their low contribution to the POC concentration at the two depths sampled. The POC‐specific respiration rates associated with the slow‐ and fast‐sinking fractions were low (median 0.17 and 0.08 d−1, respectively), indicating low‐sinking particle degradation. Our results indicate that ∼5% of the POC in surface waters can be exported below the thermocline.
Highlights
Temperate shelf seas are important areas due to their role in the uptake of atmospheric carbon dioxide (Frankignoulle & Borges, 2001; Kitidis et al, 2019), their high productivity (Behrenfeld et al, 2005; Wollast, 1998), and the existence of abundant particulate organic carbon (POC) (Dunne et al, 2007)
The slow- and fast-sinking fractions sustained between 1 and 2 orders of magnitude lower microbial respiration rates than the R associated with the suspended fraction at the two depths (Figures 1a and 1b)
Assuming that bacteria were responsible for 50% of the R in our study, our average estimates of bacterial growth efficiencies (the balance between the biomass produced per unit of substrate assimilated, BGE = bacterial production (BP)/ (BP + bacterial respiration)) would be 0.21 ± 0.03, 0.15 ± 0.02, and 0.06 ± 0.01 for the suspended, slow- and fast-sinking fractions, respectively
Summary
Temperate shelf seas are important areas due to their role in the uptake of atmospheric carbon dioxide (Frankignoulle & Borges, 2001; Kitidis et al, 2019), their high productivity (Behrenfeld et al, 2005; Wollast, 1998), and the existence of abundant particulate organic carbon (POC) (Dunne et al, 2007). In situ studies have reported higher carbon-specific respiration rates in the Southern Ocean (Cavan & Boyd, 2018) than in warmer areas of the North East Atlantic (Bach et al, 2019) indicating that factors other than temperature, such as the microbial community and particle composition, play an important role (Bach et al, 2019; Fontanez et al, 2015; Johnson et al, 2020; Kiørboe, 2001) This is consistent with the wide range in estimates of the percentage of sinking carbon that is respired in the surface layer.
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