Systematic Variation in Marine Dissolved Organic Matter Stoichiometry and Remineralization Ratios as a Function of Lability

Remineralization of organic matter by heterotrophic organisms regulates the biological sequestration of carbon, thereby mediating atmospheric CO2. While surface nutrient supply impacts the elemental ratios of primary production, stoichiometric control by remineralization remains unclear. Here we develop a mechanistic description of remineralization and its stoichiometry in a marine microbial ecosystem model. The model simulates the observed elemental plasticity of phytoplankton and the relatively constant, lower C:N of heterotrophic biomass. In addition, the model captures the observed decreases in the C:N of more labile dissolved organic matter (DOM) and the C:N of its remineralization with depth, which are driven by a switch in the dominant source of DOM from phytoplankton to heterotrophic biomass. Only a model version with targeted remineralization of N‐rich components is able to simulate the observed profiles of preferential remineralization of N relative to C and the elevated C:N of bulk DOM. The model suggests that more labile substrates are associated with C‐limited heterotrophic growth and not with preferential remineralization, while less labile substrates are associated with growth limited by processing rates and with preferential remineralization. The resulting patterns of variable remineralization stoichiometry mediate the extent to which a proportional increase in carbon production resulting from changes in phytoplankton stoichiometry can increase the efficiency of the biological pump. Results emphasize the importance of understanding the physiology of both phytoplankton and heterotrophs for anticipating changes in biologically driven ocean carbon storage. Key Points Explicit resolution of remineralization explains observed stoichiometric patterns and anticipates climate‐carbon‐cycle feedbacks The C:N of the more labile fraction of DOM decreases with depth as proportionally less phytoplankton biomass contributes to DOM Preferential remineralization is associated with “processing”‐limited growth, in contrast with consumption by C‐limited heterotrophs