Modelling nitrogen fluxes in oligotrophic environments: NW Mediterranean and NE Atlantic

Stocks and fluxes of nitrogen in two oligotrophic ecosystems are evaluated using a vertically resolved turbulence-driven ecological model. The model simulates the intra-annual variability of the ecological processes in the upper 300 m of the water column in the Catalan Sea (CS), northwestern Mediterranean, and in the subtropical northeast Atlantic (NEA). Higher irradiance and daylight length making the euphotic layer thicker, together with a higher mixing layer depth explained that the phytoplankton maximum was deeper in NEA than in CS during summertime. In general, the amount of irradiance appears to control the chlorophyll maximum depth while the amount of nitrate transported from the bottom boundary is mainly constraining the phytoplankton stocks. The summer chlorophyll maximum was found at depths below the thermocline and close to the nitracline. In both locations, zooplankton grazing controlled the late winter phytoplankton bloom but reduced more than expected the subsurface phytoplankton concentration in summer. The nitrite maximum was successfully simulated to be close to the chlorophyll maximum, explained by the phytoplankton reduction of nitrate in the dark. The yearly estimates of upward fluxes of nitrate to the euphotic zone were variable depending on the horizon at which fluxes were computed. Just below the euphotic zone, the upward fluxes of nitrate were 0.64 and 0.22 mol N m−2 in the CS and NEA, respectively, more closely connected to the variation of the nitrogen gradient than to the density field.