Non‐Redfieldian Dynamics Explain Seasonal pCO₂Drawdown in the Gulf of Bothnia

Abstract

High inputs of nutrients and organic matter make coastal seas places of intense air‐sea CO2 exchange. Due to their complexity, the role of coastal seas in the global air‐sea CO2 exchange is, however, still uncertain. Here, we investigate the role of phytoplankton stoichiometric flexibility and extracellular DOC production for the seasonal nutrient and CO2 partial pressure (pCO2) dynamics in the Gulf of Bothnia, Northern Baltic Sea. A 3‐D ocean biogeochemical‐physical model with variable phytoplankton stoichiometry is for the first time implemented in the area and validated against observations. By simulating non‐Redfieldian internal phytoplankton stoichiometry, and a relatively large production of extracellular dissolved organic carbon (DOC), the model adequately reproduces observed seasonal cycles in macronutrients and pCO2. The uptake of atmospheric CO2 is underestimated by 50% if instead using the Redfield ratio to determine the carbon assimilation, as in other Baltic Sea models currently in use. The model further suggests, based on the observed drawdown of pCO2, that observational estimates of organic carbon production in the Gulf of Bothnia, derived with the 14C method, may be heavily underestimated. We conclude that stoichiometric variability and uncoupling of carbon and nutrient assimilation have to be considered in order to better understand the carbon cycle in coastal seas.

Remineralization rate of terrestrial DOC as inferred from CO<sub>2</sub> supersaturated coastal waters

“…The pCO 2 was measured during 25 cruises, spanning January to October 2012, with the TransPaper cargo (Fransson et al, 2019). The TransPaper cargo sails from Gothenburg on the Swedish west coast, through the Baltic Proper and northwards through the Bothnian Sea and the Bothnian Bay to the ports of Oulu and Kemi in Finland.…”

Remineralization rate of terrestrial DOC as inferred from CO<sub>2</sub> supersaturated coastal waters

“…The third set contains an experiment (1YS), which is similar to the 1Y experiment, but a tDOC dependent light parameterization is used instead of a salinity dependent one, as described in Fransner et al (2018). The aim of 1YS is to investigate the potential indirect effect tDOC could have on the pCO 2 by dampening phytoplankton growth and carbon fixation.…”

Combined climate change and nutrient load impacts on future habitats and eutrophication indicators in a eutrophic coastal sea

“…Additionally, parameterization of nutrient and Chl : C ratios according to, for example, Geider and La Roche () and Macintyre et al () will be considered in future work. It has been shown that models with constant molecular plankton ratios show poorer performance in the northern Baltic Sea (Eilola et al ), while Fransner et al () showed that including non‐Redfieldian dynamics in biogeochemical models may improve the representation of the carbon dynamics in these boreal waters. The contrasting results indicate that more research is needed to fully understand the causes for deviations and to implement new process knowledge in future scenarios.…”