Phosphorus cycling in the upper waters of the Mediterranean Sea (PEACETIME cruise): relative contribution of external and internal sources
Authors: Pulido-Villena Elvira , Desboeufs Karine , Djaoudi Kahina , Van Wambeke France , Barrillon Stéphanie , Doglioli Andrea , Petrenko Anne , Taillandier Vincent , Fu Franck , Gaillard Tiphanie , Guasco Sophie , Nunige Sandra , Triquet Sylvain , Guieu Cécile 
Published: November 2021

Abstract. The study of phosphorus cycling in phosphate-depleted oceanic regions, such as the
Mediterranean Sea, has long suffered from methodological limitations, leading
to a simplistic view of a homogeneous surface phosphate pool with
concentrations below the detection limit of measurement above the
phosphacline. During the PEACETIME (Process studies at the air-sea interface
after dust deposition in the Mediterranean Sea) cruise, carried out from 10
May to 11 June 2017, we conducted co-located measurements of phosphate pools
at the nanomolar level, alkaline phosphatase activities and atmospheric
deposition of phosphorus, across a longitudinal gradient from the west to
the central Mediterranean Sea. In the phosphate-depleted layer (PDL),
between the surface and the phosphacline, nanomolar phosphate was low and
showed little variability across the transect spanning from 6 ± 1 nmol L−1 in the Ionian basin to 15 ± 4 nmol L−1 in the
westernmost station. The low variability in phosphate concentration
contrasted with that of alkaline phosphatase activity, which varied over 1 order of magnitude across the transect. Nanomolar phosphate data revealed
gradients of phosphate concentration over density inside the PDL ranging
between 10.6 ± 2.2 µmol kg−1 in the westernmost station to
values close to zero towards the east. Using the density gradients, we
estimated diapycnal fluxes of phosphate to the PDL and compared them to
atmospheric deposition, another external source of phosphate to the PDL.
Phosphate supply to the PDL from dry deposition and diapycnal fluxes was
comparable in the western part of the transect. This result contrasts with
the longtime idea that, under stratification conditions, the upper waters of
the Mediterranean Sea receive new P almost exclusively from the atmosphere.
The contribution of atmospheric deposition to external P supply increased
under the occurrence of rain and Saharan dust. Although this finding must be
taken cautiously given the uncertainties in the estimation of diapycnal
fluxes, it opens exciting questions on the biogeochemical response of the
Mediterranean Sea, and more generally of marine oligotrophic regions, to
expected changes in atmospheric inputs and stratification regimes. Taken
together, external sources of phosphate to the PDL contributed little to
total phosphate requirements which were mainly sustained by in situ
hydrolysis of dissolved organic phosphorus. The results obtained in this study show a highly dynamic
phosphorus pool in the upper layer of the euphotic zone, above the
phosphacline, and highlight the convenience of combining highly sensitive
measurements and high-resolution sampling to precisely depict the shape of
phosphate profiles in the euphotic zone with still unexplored consequences
on P fluxes supplying this crucial layer for biogeochemical cycles.