Comparing model scenarios of variable plankton N/P ratio versus the constant one for the application in the Baltic Sea

Observation data on surface dissolved inorganic nutrients in 2000–2009 at 15 stations in the Baltic Sea were used to analyze the ratio of nitrogen change to phosphorus change (N/P) before and after spring blooms. The ratios of nutrient N/P before and after spring blooms vary from 6.6:1 to 41.5:1. To estimate the spatially varied plankton N/P ratios, the observed nutrient N/P ratios as proxies for plankton N/P ratios are used to extrapolate a spatial pattern, and then the spatial pattern is adjusted by comparing observations and model results and the best fit spatial pattern is selected to discern the horizontal variability of plankton N/P, i.e., low in the center of the Baltic, relatively high away from the center. To examine the potential impact of spatially varied N/P ratios on phytoplankton and nutrients, a three dimensional physical–biogeochemical coupled model is used to compare two scenarios: spatially varied plankton N/P ratios versus a constant N/P ratio. When comparing model results to observation data, model results with spatially varied N/P ratios showed consistent improvements over model results with a constant N/P ratio, specifically in dissolved inorganic nitrogen, dissolved inorganic phosphorus, chlorophyll. Therefore, we concluded that the spatially varied N/P ratios can feature the horizontal distribution of plankton N/P in the Baltic Sea. Furthermore, the impacts of the variable plankton N/P ratio on primary production and nitrogen fixation are also investigated using the 3D ecosystem model. The estimated primary production and nitrogen fixation using the constant N/P ratio of 16:1 are 38% and 317% higher, respectively, than those estimates using the variable N/P ratio.