N/P ratio of nutrient uptake in the Baltic Sea

Abstract. The N/P ratio of nutrient uptake, the change of dissolved inorganic nitrogen (DIN) relative to the change of dissolved inorganic phosphorus (DIP), is a key parameter for many ecological models. In the Baltic Sea ecosystem, the N/P ratio of nutrient uptake varies among different basins and different seasons. The N/P ratio of nutrient alteration, i.e., the ratio of DIN to DIP altered before and after spring blooms, is not the same as the N/P ratio of nutrient uptake, but the former can be regarded as an indicator of the latter in the Baltic Sea. Based on the observed N/P ratio of nutrient alteration, we hypothesize a non-Redfield N/P ratio of nutrient uptake. The 3-D-ecosystem model ERGOM coupled with the circulation model DMI-BSHcmod was used to test this hypothesis. When the Redfield ratio was used in the model, the DIP surplus after spring blooms was too high and resulted in excessive growth of cyanobacteria and too much nitrogen fixation. When the non-Redfield ratio was used in the model, these problems tended to disappear. In summary, we show that: (1) the Redfield N/P ratio of nutrient uptake in the Baltic Sea tends to be too high; (2) a N/P ratio of 10:1 appears to work better than the Redfield value; and (3) the N/P ratio of nutrient uptake in the Baltic Proper during spring blooms is around 6:1. As the model limitation using one identical value for two N/P ratios for nutrient uptake and remineralization, the quantitative conclusions are only convincing as a model parameter even though it obviously improves model predictions. Whether this model parameter is consistent with the biological nutrient uptake is worth being further verified with some laboratory investigations or simulations using a more sophisticated model with independent N/P ratios for nutrient uptake and remineralization.