Tracing terrestrial DOC in the Baltic Sea—A 3‐D model study

Abstract

The fate of terrestrial organic matter brought to the coastal seas by rivers and its role in the global carbon cycle are still not very well known. Here the degradation rate of terrestrial dissolved organic carbon (DOCter) is studied in the Baltic Sea, a subarctic semienclosed sea, by releasing it as a tracer in a 3‐D circulation model and applying linear decay constants. A good agreement with available observational data is obtained by parameterizing the degradation in two rather different ways: one by applying a decay time on the order of 10 years to the whole pool of DOCter and one by dividing the DOCter into one refractory pool and one pool subject to a decay time on the order of 1 year. The choice of parameterization has a significant effect on where in the Baltic Sea the removal takes place, which can be of importance when modeling the full carbon cycle and the CO2 exchange with the atmosphere. In both cases the biogeochemical decay operates on time scales less than the water residence time. Therefore, only a minor fraction of the DOCter reaches the North Sea, whereas approximately 80% is removed by internal sinks within the Baltic Sea. This further implies that DOCter mineralization is an important link in land‐sea‐atmosphere cycling of carbon in coastal and shelf seas that are heavily influenced by riverine DOC.

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

“…Subjecting tDOC to a decay, as in the 1Y and 10Y experiments, results in higher remineralization per volume unit where the highest concentrations of tDOC occur. Consequently, in the North-Eastern Bothnian Bay, where the highest tDOC concentrations are found (not shown here, but in Fransner et al (2016)), the remineralization rates are also the highest ( Figure 6). It is in the areas with the highest remineralization that the largest impacts on the pCO 2 are seen (Figure 2 and 6).…”

“…Like autochthonous POC it is degraded by bacteria with a time scale of 10 days. The 10Y and 1Y experiments are based on Fransner et al (2016). These experiments are the same as the TP experiment, but with the addition of tDOC.…”

Key processes in the coupled carbon, nitrogen, and phosphorus cycling of the Baltic Sea

“…Marine and terrestrial contributions to DOC in different areas of the Baltic Sea have been estimated based on carbon isotope signatures (Alling et al 2008;Deutsch et al 2012). Two model studies (Gustafsson et al 2014a;Fransner et al 2016) used these estimates to calibrate sink terms for terrestrial DOC in the system and concluded that the majority (*60-80%) of terrestrial DOC must be removed by internal sinks within the Baltic Sea in order to reproduce observed concentrations. The relative importance of the two sink terms (i.e., mineralization in the water column and flocculation followed by sedimentation and burial/ mineralization in the sediments) is however not clear.…”

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

“…In the Bothnian Bay, on the other hand, the DIP assimilation is 11% lower than in REF. The sensitivity experiments did however show that this has minor effects on the pCO 2 dynamics and the air‐sea CO 2 exchange in this area, which can be related to the larger remineralization of terrestrial DOC in the North (Fransner et al, ). The high C:P and N:P ratios needed to simulate the pCO 2 and nutrient dynamics in the Bothnian Bay, and the C:nutrient ratios to simulate the pCO 2 in the Bothnian Sea, strongly suggest the need for variable stoichiometry models in order to adequately simulate the biogeochemical cycles in this region.…”

“…The terrestrial organic matter is introduced as a new state variable R ð7Þ c;n;p . It is subject to a linear decay rate k with a time scale of 1 year (Fransner et al, 2016). The decayed material ends up in the inorganic carbon, and the ammonium and phosphate pools:…”