Nitrogen and Phosphorus Budgets in the Northwestern Mediterranean Deep Convection Region

Abstract

The aim of this study is to understand the biogeochemical cycles of the northwestern Mediterranean Sea (NW Med), where a recurrent spring bloom related to dense water formation occurs. We used a coupled physical‐biogeochemical model at high resolution to simulate realistic 1 year period and analyze the nitrogen (N) and phosphorus (P) cycles. First, the model was evaluated using cruises carried out in winter, spring, and summer and a Bio‐Argo float deployed in spring. Then, the annual cycle of meteorological and hydrodynamical forcing and nutrients stocks in the upper layer were analyzed. Third, the effect of biogeochemical and physical processes on N and P was quantified. Fourth, we quantified the effects of the physical and biological processes on the seasonal changes of the molar NO3:PO4 ratio, particularly high compared to the global ocean. The deep convection reduced the NO3:PO4 ratio of upper waters, but consumption by phytoplankton increased it. Finally, N and P budgets were estimated. At the annual scale, this area constituted a sink of inorganic and a source of organic N and P for the peripheral area. NO3 and PO4 were horizontally advected from the peripheral regions into the intermediate waters (130–800 m) of the deep convection area, while organic matter was exported throughout the whole water column toward the surrounding areas. The annual budget suggests that the NW Med deep convection constitutes a major source of nutrients for the photic zone of the Mediterranean Sea.

Vertical Mixing Effects on Phytoplankton Dynamics and Organic Carbon Export in the Western Mediterranean Sea

“…Regarding the vertical distribution of chlorophyll in convection regions, Kessouri et al () found a good agreement between model results and DeWEx cruise observations. Moreover, the time evolution of the vertical distribution of phytoplankton in the Stratified (southern) region is consistent with climatological observations reported by Lavigne et al (): the model correctly reproduces the formation of a deep chlorophyll maximum (DCM) after the winter mixing period and its deepening between 50 and 80 m in summer (presented in section 4.2).…”

“…A detailed validation of the coupled hydrodynamic and biogeochemical models over the study period is described in Estournel et al () and Kessouri et al (), respectively. Estournel et al () conducted several comparisons with observations, such as the spatial distribution of water column stratification in winter at the scale of the northwestern subbasin (their Figure ) or time series of the potential temperature profile in the center of the Gulf of Lions (their Figure 11).…”

“…The period between the two estimates (based on surface chlorophyll and vertically integrated biomass) corresponds to the increase in the MLD from the DCM to the nutricline. In our case, this period lasted 1 month, which appears long if we consider the proximity of the DCM to the nutricline (about 20 m, see Kessouri et al, ). However, in the present case, it corresponded to the period between two strong wind and heat loss events at the end of October and the end of November (Figure ), each of which coincided with a sudden deepening of the mixed layer (see Estournel et al, , their Figure 11 for the temperature evolution in the 150 m upper layer).…”

“…The model structure used in this study has been previously described in Ulses et al (), Auger et al (), and Herrmann et al (). Most parameter values are based on the study of Ulses et al (), but a new calibration was carried out for some parameters using several observational data sets (Kessouri et al, ).…”

“…Estournel et al () conducted several comparisons with observations, such as the spatial distribution of water column stratification in winter at the scale of the northwestern subbasin (their Figure ) or time series of the potential temperature profile in the center of the Gulf of Lions (their Figure 11). In Kessouri et al (), a comparison of the biogeochemical model results with nutrients and chlorophyll concentrations measured during the DeWEx winter and spring cruises was presented. This comparison showed that the model correctly reproduced the horizontal and vertical chlorophyll and nutrient distribution in these two seasons.…”