Abstract
An accurate understanding of the biogeochemistry of dissolved phosphate pool in the upper waters of P-depleted oceanic regions is constrained by the low sensitivity of routine phosphate measurements. In this study, by using the sensitive Liquid Waveguide Capillary Cell method, we report the first extensive cross-basin survey of nanomolar dissolved inorganic phosphate (DIP) and dissolved organic phosphate (DOP) concentration in P-depleted surface waters of the Mediterranean Sea during the stratification period. In the north western Mediterranean Sea (NWMS), DIP above the mixed layer depth (MLD) ranged between 4.9 and 26.5 nM. Along an E-W transect crossing Ionian and Tyrrhenian Seas (E-W transect), DIP above the MLD was lower, ranging between 0.9 and 11.4 nM. Contrarily to the traditional view of a depleted and invariant surface dissolved phosphate pool, a significant vertical variability of DIP and DOP was revealed in upper waters. A positive gradient of DIP was observed above the phosphacline, between the MLD and the deep chlorophyll maximum (DCM) depth, suggesting a potential diffusion of new phosphate to near-surface waters, even under stratified conditions. Interestingly, despite this apparent DIP availability, a significant negative gradient of DOP concentration was observed in the same layer. Finally, the positive gradient in DIP coincided with a significant increase in N:P ratio, suggesting a higher rate of increase of N than of P. The results obtained in this study indicate that acquiring nanomolar DIP data is a sine qua non condition for the comprehension and prediction of the biogeochemical functioning of P-depleted oceanic regions, such as the Mediterranean Sea.
Highlights
Nitrogen (N) and phosphorus (P) are essential macronutrients for sustaining marine organisms (Redfield, 1958)
By using the LWCC, all dissolved inorganic phosphate (DIP) concentrations in the upper waters of the Mediterranean Sea obtained in this study were above the LOD of 0.8 nM
This study reports the first cross-basin distribution of nM DIP and dissolved organic phosphate (DOP) in the P-depleted Mediterranean surface waters
Summary
Nitrogen (N) and phosphorus (P) are essential macronutrients for sustaining marine organisms (Redfield, 1958). N has traditionally been viewed as the limiting nutrient of primary production in marine waters (Ryther and Dunstan, 1971). The Redfield N:P ratio of 16:1 has been considered as a fundamental tenet in marine biogeochemistry. Deviations from this canonical ratio have been used to provide insights, for instance, on the efficiency of carbon sequestration in the ocean (Sigman and Boyle, 2000), or on nutrient limitation of primary production (i.e., Falkowski and Raven, 1997; Moore et al, 2013). In some oligotrophic regions, such as the North Atlantic subtropical gyre, N:P ratios in near-surface waters are higher than 16:1, suggesting a depletion of P before that of N (Wu et al, 2000; Ammerman et al, 2003)
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