Microbial-mediated recycling constrains the contribution of primary production to export flux in an oligotrophic system

This study presents the first multi-year assessment quantifying the contribution of primary production to vertical carbon flux in the ultra-oligotrophic southeastern Levantine basin of the Mediterranean Sea. Depth-integrated (0–180 m) daily primary productivity (PP) was 25% higher in the mixed winter period than in the stratified period (123 and 98 mg C m⁻² d⁻¹ respectively) and the predominant photoautotrophs contributed ~ 5%–11% of the bulk particulate organic carbon (POC). Time-resolved sediment trap data at 180 and 280 m from 2016 to 2020 showed POC fluxes ranging from 0.5 to 5.3 mg C m⁻² d⁻¹ (stratified-period) and 1.8 to 13.5 mg C m⁻² d⁻¹ (mixed-period), with primary producers potentially contributing 2.6%–7% of the POC flux at 180 m. Our calculated e-ratios are some of the lowest recorded in oligotrophic environments, reaching 0.061 during mixing and 0.026 under stratified conditions. High bacterial-to-primary production ratios and bacterial coupling to dissolved organic carbon (DOC) suggest that intense microbial recycling constrains the transformation of primary production to particulate export and reduces the biological pump efficiency. Our data show that applying generalized export models can overestimate export in the Levantine basin by overlooking microbial recycling and lateral carbon transport, underscoring the need for region-specific models that incorporate these processes under increasingly warm, stratified, and oligotrophic ocean conditions.