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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The seasonal variability of plankton metabolism indicates how much carbon is cycling within a system, as well as its capacity to store carbon or export organic matter and CO2 to the deep ocean. Seasonal variability between November 2014, April 2015 and July 2015 in plankton respiration and bacterial (Bacteria+Archaea) metabolism is reported for the upper and bottom mixing layers at two stations in the Celtic Sea, UK. Upper mixing layer (UML, >75 m in November, 41 - 70 m in April and ~50 m in July) depth-integrated plankton metabolism showed strong seasonal changes with a maximum in April for plankton respiration (1.2- to 2-fold greater compared to November and July, respectively) and in July for bacterial production (2-fold greater compared to November and April). However UML depth-integrated bacterial respiration was similar in November and April and 2-fold lower in July. The greater variability in bacterial production compared to bacterial respiration drove seasonal changes in bacterial growth efficiencies, which had maximum values of 89 % in July and minimum values of 5 % in November. Rates of respiration and gross primary production (14C-PP) also showed different seasonal patterns, resulting in seasonal changes in 14C-PP:CRO2 ratios. In April, the system was net autotrophic (14C-PP:CRO2 > 1), with a surplus of organic matter available for higher trophic levels and export, while in July balanced metabolism occurred (14C-PP:CRO2 = 1) due to an increase in plankton respiration and a decrease in gross primary production. Comparison of the UML and bottom mixing layer indicated that plankton respiration and bacterial production were higher (between 4 and 8-fold and 4 and 7-fold, respectively) in the UML than below. However, the rates of bacterial respiration were not statistically different (p > 0.05) between the two mixing layers in any of the three sampled seasons. These results highlight that, contrary to previous data from shelf seas, the production of CO2 by the plankton community in the UML, which is then available to degas to the atmosphere, is greater than the respiratory production of dissolved inorganic carbon in deeper waters, which may contribute to offshore export

a It is widely accepted that mankind derives benefits from ecosystem services provided by the marine environment. It is less clear how these benefits can be quantified in order to make objective and responsible environmental management decisions. Providing an economic quantification of these benefits is one approach that can help. While it is relatively simple to derive monetary values for benefits accruing from activities with an economic basis, such as food provision and tourism, different approaches must be taken to provide economic values for services with less obvious links to economic activity. Here we demonstrate how a range of approaches may be used to derive economic values for three very different ecosystem services provided by marine environments, namely the bioremediation of waste by marine biota, the fixation of carbon dioxide by photosynthetic marine organisms, and the sea defence role provided by wetlands in coastal areas of the UK.

Organic matter (OM) plays an important role in productive shelf seas and their contribution to global carbon (C) and nutrient cycles. We investigated the impact of storm mixing on OM dynamics in the seasonally stratified Celtic Sea. After the storm, OM production was decoupled from consumption in the euphotic layer. Over the 15 day study, dissolved OM (DOM) became phosphorus (P) rich relative to C, whereas particulate OM (POM) became P-deplete relative to C. Upward diapycnal phosphate fluxes were accompanied by reciprocal downward mixing of dissolved organic P (DOP) and particulate P (PPhos). Transfer of DOP and PPhos below the thermocline accounts for 22% and 26%, respectively, of the upward phosphate flux. Given the changes in stoichiometry of POM and DOM after the storm, the form in which OM is transferred below the thermocline has important implications for the efficiency of elemental transfer, impacting C cycling and storage in the ocean.