Effects of variable organic matter C:N:P stoichiometry on carbon cycling in the northwest European shelf seas

Abstract

Marine ecosystems couple the flows of carbon and nutrients from their assimilation by primary producers, through food webs and microbial ecosystems, to their eventual release through respiration back into the environment. Since Alfred C. Redfield first observed consistent ratios of carbon, nitrogen, and phosphorus across global phytoplankton communities and in the uptake of inorganic nutrients, the coupling of these elemental cycles in marine ecosystems has widely been described by the canonical Redfield ratios of C:N:P = 106:16:1 in biomass or C:N:P = 105:15:1 in the uptake of inorganic carbon and nutrients. This framework has informed the representation of living cells, including phytoplankton and heterotrophs, as well as non-living or detrital organic matter in regional and global biogeochemical models. However, observations over recent decades have revealed systematic deviations from these ratios, with differences among living cells and non-living organic matter, as well as across size and lability fractions. This variability challenges the assumption of constant stoichiometry in organic matter cycling in marine ecosystems, calling for a reassessment of existing model representations with fixed elemental ratios. This dissertation addresses this uncertainty in Redfield stoichiometry based model representations by examining the effects of observed variations in organic matter C:N:P stoichiometry on marine carbon cycling. It first synthesizes global observations across living cells, including phytoplankton and heterotrophs, as well as non-living dissolved and particulate organic matter. By evaluating the environmental drivers of this variability, exploring its implications for biogeochemical cycles, and reviewing past model implementations of variable stoichiometry, this review provides a foundation for subsequent model development and regional impact assessment, which form the main body of this work. Building on the global synthesis of observed stoichiometric variability, the dissertation explores its regional implications by quantifying the effects of variable organic matter C:N:P stoichiometry on carbon cycling in the northwest European shelf seas. This is achieved by incorporating variable stoichiometry into the representation of dissolved and particulate organic matter in a coupled physical–biogeochemical modeling system. Two process representations are implemented, including the preferential remineralization of organic nitrogen and phosphorus relative to carbon, and the extracellular release of carbon-rich dissolved organic matter by phytoplankton. The assessment focuses on how these processes, individually and in combination, affect the balance between carbon fixation and respiration, as well as seasonal and annual air–sea CO2 exchange, and cross-shelf exchanges with the northeastern Atlantic Ocean. The results show that variable C:N:P stoichiometry enhances seasonal biological drawdown of CO2 by strengthening pre-existing seasonal, vertical, and lateral gradients in net community production. These effects are most pronounced in the deep, seasonally stratified regions of the central and northern North Sea, the Norwegian Trench, and along the steep shelf edge, where biological CO2 drawdown is most efficient due to the vertical separation of production and respiration. The resulting increase in the seasonal vertical gradient of dissolved inorganic carbon drives a higher annual oceanic CO2 uptake by approximately 10–30 percent and enhances the cross-shelf export of dissolved inorganic carbon, accounting for roughly 60–90 percent of the additionally sequestered CO2. Impacts on organic carbon stocks and fluxes are smaller and more variable. In addition to the annual increase in oceanic CO2 uptake, the maximum uptake is shifted from winter to spring and summer due to increased biological control of the surface partial pressure of CO2. These insights demonstrate that regional physical and biogeochemical conditions strongly influence the effects of variable stoichiometry, emphasizing the need to represent C:N:P variability in both regional models and global representations of the coastal ocean. They also suggest that shifts in organic matter stoichiometry under environmental change can influence the efficiency of carbon export through the biological carbon pump. By quantifying the regional effects of variable C:N:P stoichiometry in the northwest European shelf seas, this dissertation offers a first glimpse into the regional variability of these effects and contributes to sustained improvements in the model representation of biogeochemical cycles at both regional and global scales.