Nutrient (N, P and Si) and carbon partitioning in the stratified NW Mediterranean

The distribution of nutrients and carbon in the different pools present in the three functional layers (the upper, biogenic layer, the thermocline layer, and the deeper, biolythic layer) of the stratified NW Mediterranean Sea was examined. The stoichiometry between dissolved inorganic nutrients, which had low concentrations in the surface waters, indicated a deficiency in nitrogen, relative to phosphorus, and an excess nitrogen relative to phosphorus within the thermocline, as well as a general silicate deficiency relative to both N and P, even extending to the biolythic layer. The dissolved organic matter was highly depleted in N and, particularly, in P relative to C, with average DOC/DON ratios >60 and DOC/DOP ratios >1500 in all three layers. The particulate pool was also depleted in N and P relative to C, particularly in the biolythic layer. The concentration of biogenic silica was low relative to C, N and P, indicating that diatoms were unlikely to contribute a significant fraction of the seston biomass. Most (>80%) of the organic carbon was present as dissolved organic carbon. Total organic N and P comprised 50-80% of the N and P pool in the biogenic layer, and decreased with depth to represent 10-25% of these nutrient pools in the biolythic layer. The high total N:P ratios in all three depth layers (N/P ratio >20) indicated an overall phosphorus deficiency in the system. The high P depletion of the dissolved organic matter must derive from a very rapid recycling of the P-rich molecules within DOM, and the increasing C/N ratio of DOM with depth indicates that N is also recycled faster than C in the DOM. Because of the uniform depth distribution of the total dissolved nitrogen concentration, the increase in the percent inorganic N and the decline in the percent dissolved organic N with depth indicates that there must be biological transformations between these pools, with a dominance of DON production in surface waters and remineralisation in the underlying layers, from which dissolved inorganic nitrogen is supplied back to the biogenic layer. Downward fluxes of DON and DOC were estimated at 200-250 μmol N m^-2 d^-1 and 1.4-2.1 mmol C m^-2 d^-1, respectively, while there should be little or no export of P as dissolved organic matter. The downward DON flux exceeded the diffusive DIN supply of about 145 μmol N m^-2 d^-1 to the biogenic layer, suggesting that allochthonous N inputs must be important in the region.