Phosphorus is an essential nutrient for all life on earth. In the ocean, the most bioavailable form of phosphorus is inorganic phosphate, but in the extensive subtropical gyres, phosphate concentrations can be chronically low and limit primary productivity and nitrogen fixation. In these regions, organisms produce hydrolytic enzymes, such as alkaline phosphatase (AP), that enable them to utilize the more replete dissolved organic phosphorus (DOP) pool to meet their cellular phosphorus demands. In this study, we synthesized data from 14 published studies and present our own findings from two research cruises (D326 and D361) in the eastern subtropical Atlantic to explore the relationship between AP activity (APA) and nutrients, Saharan dust and trace metals. We found that below a threshold phosphate concentration of ∼30 nM, APA increased with an inverse hyperbolic relationship with phosphate concentration. Meanwhile, DOP concentrations decreased with enhanced APA, indicating utilization of the DOP pool. We found APA rates were significantly higher in the subtropical Atlantic compared to the subtropical Pacific Ocean, even over the same low phosphate concentration range (0-50 nM). While the phosphate concentration may have a first order control on the APA rates, we speculate that other factors influence this basin scale contrast. Using bioassay experiments, we show that the addition of Saharan dust and zinc significantly increased the rate of APA. To our knowledge, our results are the first direct field-based evidence that APA is limited by zinc in the subtropical ocean. Further work is required to explore the relationship between trace metals such as iron and zinc, which are co-factors of phosphohydrolytic enzymes, specifically PhoX and PhoA, respectively, and APA in the ocean.

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Highlights: Seasonal phosphorus uptake and dissolved organic release examined in the Central Celtic Sea  Uptake highest in spring bloom, with biomass-normalised uptake equal in spring and summer  Release high in November and late spring, with efficient P-retention in summer  Strong phytoplankton influence on spring P-uptake, whilst bacteria influential in summer  Relatively C-rich uptake in November and late April, strongly P-rich in summer AbstractThe seasonal cycle of resource availability in shelf seas has a strong selective pressure on phytoplankton diversity and the biogeochemical cycling of key elements, such as carbon (C) and phosphorus (P). Shifts in carbon consumption relative to P availability, via changes in cellular stoichiometry for example, can lead to an apparent 'excess' of carbon production. We made measurements of inorganic P (P i ) uptake, in parallel to C-fixation, by plankton communities in the Central Celtic Sea (NW European Shelf) in spring (April 2015), summer (July 2015) and fall (November 2014). Short-term (<6 h) P i -uptake coupled with dissolved organic phosphorus (DOP) release, in parallel to net (24 h) primary production (NPP), were all measured across an irradiance gradient designed to typify vertically and seasonally varying light conditions. Rates of P i -uptake were highest during spring and lowest in light-limited fall conditions, although biomass-normalised P i -uptake was similar in spring and summer. The release of DOP was highest in November and declined to low levels in July, indicative of efficient utilization and recycling of the low levels of P i available. Examination of turnover times of the different particulate pools, including phytoplankton and bacteria, indicated a differing seasonal influence of autotrophs and heterotrophs in P-dynamics, with summer conditions associated with a strong bacterial influence and early spring with fast growing phytoplankton. These seasonal changes in plankton composition, coupled with changes in resource availability (P i , light) resulted in seasonal changes in the stoichiometry of NPP to P i -uptake (C:P ratio); from relatively C-rich uptake in November and late April, to P-rich uptake in early April and July. Overall these results highlight how the entire plankton community, both autotrophs and heterotrophs, influence the relative uptake of C and P and that any excess C-consumption relative to the P-rich uptake must be balanced by C-rich process such as the heterotrophic remineralisation and/or consumption of organic material.

Alkaline phosphatase activity (APA) is traditionally a proxy for phosphate (DIP)-limitation because it is induced by DIP-limited microbes to access the labile ester fraction of the organic phosphorus (OP) pool. Here, we present multi-year summertime depth distributions of APA and enzyme kinetics in the DIP-replete Celtic Sea. Our findings support the cumulating body of evidence that APA has a potentially widespread role in OP remineralization through the water column. APA and V max were positively correlated with depth and DIP, with total APA being threefold higher below (0.93 6 0.32 nM P h 21 ) compared to above the thermocline (0.30 6 0.24 nM P h 21

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