Spatial variability of phosphate and nitrate in the Mediterranean Sea: A modeling approach

•Phosphate gradient corresponds to the east-west trophic gradient.•Nitrate presents south-north signature, higher values in the northern reaches.•Nutrients limit plankton growth for large part of the year.•Phosphorus is limiting, the exception is Alboran Sea (nitrogen limited).•Atmospheric deposition strongly affects the nitrogen in the upper surface layers.

We present an analysis of the spatial distributions and seasonal dynamics of phosphate and nitrate in the Mediterranean Sea. The analysis involves observations under present conditions and relies on statistical analysis of the output of a state-of-the-art corroborated numerical model. Model results are corroborated with available data from cruises and fixed monitoring stations. The results capture and quantitatively detail the main spatial patterns observed in the Mediterranean Sea, with an east-west gradient in the surface layer for phosphate and a north-south gradient for nitrate. The model results highlight that plankton growth in Mediterranean waters is usually nutrient limited and that such limitation is less frequent in the western sub-basin and is usually due to a lack of phosphate, rather than nitrate, with the exception of the areas close to the Atlantic Ocean, where nitrogen limitation is important. The model simulation illustrates the benefit of a variable stoichiometric formulation of the phytoplankton cell, showing how the ratio of the internal N:P quota adapts to different limiting conditions.