Phosphate and iron stress control global surface ocean dissolved organic phosphorus concentrations

17 18 Dissolved organic phosphorus (DOP) has a dual role in the surface ocean as both a product of 19 primary production and as an organic nutrient fueling primary production and nitrogen fixation, 20 especially in oligotrophic gyres. Though poorly constrained, understanding the geographic 21 distribution and environmental controls of surface ocean DOP concentration is critical to 22 estimating distributions and rates of primary production and nitrogen fixation in the global ocean. 23 Here we pair DOP concentration measurements with a metric of phosphate (PO 43- ) stress (P*), and 24 satellite-based chlorophyll a concentrations and iron stress estimates to explore their relationship 25 with upper 50 m DOP stocks. Our results show that PO 43- and iron stress work together to control 26 surface DOP concentrations at basin scales. Specifically, upper 50 m DOP stocks decrease with 27 increasing phosphate stress , while alleviated iron stress leads to either surface DOP accumulation 28 or loss depending on PO 43- availability. Our work suggests an interdependence between DOP 29 concentration, inorganic nutrient ratios, and iron availability, and establishes a predictive 30 framework for DOP distributions in the global surface ocean.

estimating distributions and rates of primary production and nitrogen fixation in the global ocean. 23 Here we pair DOP concentration measurements with a metric of phosphate (PO4 3-) stress (P*), and 24 satellite-based chlorophyll a concentrations and iron stress estimates to explore their relationship 25 with upper 50 m DOP stocks. Our results show that PO4 3and iron stress work together to control 26 surface DOP concentrations at basin scales. Specifically, upper 50 m DOP stocks decrease with 27 increasing phosphate stress, while alleviated iron stress leads to either surface DOP accumulation 28 or loss depending on PO4 3availability. Our work suggests an interdependence between DOP 29 concentration, inorganic nutrient ratios, and iron availability, and establishes a predictive 30 framework for DOP distributions in the global surface ocean. 31

Main 33
In oligotrophic gyres, dissolved organic phosphorus (DOP) is the dominant form of phosphorus 34 (P) in surface waters 1-3 and supports phytoplankton growth when the preferred substrate, 35 phosphate (PO4 3-), is scarce 4-10 . Estimates from the Biogeochemical Elemental Cycling (BEC) 36 model indicate that global marine net primary production (NPP) and di-nitrogen (N2) fixation rates 37 are ~8% and ~33% higher, respectively, and better match observed rates, when they include DOP 38 as an assimilative P source 11 . Like dissolved organic carbon and nitrogen (DOC and DON) in the 39 ocean, DOP is a product of primary production 12 . However, a clear understanding of both the 40 distribution of DOP in the global surface ocean as well as the controls on those distributions is 41 limited by the relatively small number of DOP concentration measurements in the global ocean 11 . 42 A mechanistic framework to understand the controls on surface ocean DOP concentrations would 43 thus improve model-based estimates of the rates and distributions of marine NPP and N2 fixation. 44

45
Given that DOP is both produced and consumed by photosynthetic organisms, we explored 46 relationships between DOP distributions and metrics of primary production (i.e., DOP production), 47 S1), respectively, all of which are statistically significantly lower than the mean values in the 79 Eastern Pacific and Gulf of Mexico (Fig. S1). Below we explore the interdependence of surface 80 ocean DOP concentrations, primary productivity, PO4 3stress (P*), and surface ocean iron 81 availability. 82 83 PO4 3as the primary control on surface ocean DOP concentrations 84 The primary control on surface ocean DOP concentrations is PO4 3stress, gauged by P*, with upper 85 50 m DOP stocks in all data sets significantly positively correlated with P* (Fig. 1b) (Table. 1). 86 Global regions of net DOP production in the Eastern Pacific are associated with elevated P* values, 87 typically >0.2 µM (Fig. 1b), generated by subsurface denitrification and anammox in the oxygen 88 deficient zones (ODZs) in and upstream of these sampling locations 23 . Surface waters with high 89 P* values thus correspond to regions with "excess PO4 3-", or low PO4 3stress, relative to supplies 90 of NO3and "Redfieldian" phytoplankton N and P demands 15 (Table 1), which we interpret to reflect enhanced DOP production when iron 109 stress is alleviated in high-chlorophyll a upwelling regions (Fig. 2). A significant negative 110 correlation between upper 50 m DOP stocks and iron stress is also found on the West Florida Shelf 111 in the Gulf of Mexico (Table 1) (Fig. 2). On the global scale, the Eastern Pacific appears unique 112 as a region of net DOP production in upwelling-associated, relatively low-iron stress surface 113 waters, with that DOP subsequently advected west away from regions of net DOP production and 114 accumulating due to low PO4 3stress in the eastern portion of the Pacific. 2). We interpret the higher DOP stocks in these waters with higher iron stress to reflect iron 122 limitation of primary productivity, and thus decreasing demand for DOP as an assimilative P 123 source, and/or iron limitation of alkaline phosphatase activity 14,13 , thus limiting the ability of phytoplankton to use DOP. The low DOP concentrations (<0.15 µM) in the North Atlantic and 125 Western Pacific are found in regions with relatively high chlorophyll a concentrations (i.e., 126 typically >0.1 mg chl a m -3 ), low P* values (<0.1 µM), and low iron stress (Fig. 2), consistent with 127 DOP use by phytoplankton associated with oligotrophic environments as an assimilative P source 128 when iron is available and PO4 3is scarce. The North Atlantic Ocean in particular receives high 129 rates of dust deposition 26 , and the South Western Pacific Ocean may receive significant 130 hydrothermal iron fluxes 27 , lowering iron stress in these regions. These regions are also associated 131 with high rates of N2 fixation 28-31 , which may be due to certain diazotrophs (e.g., Trichodesmium 132 spp.) being particularly well-adapted to utilizing DOP when PO4 3is scare 5,24,32,33 . 133 134 Finally, we note that DOP appears to accumulate in other specific regions of the ocean due to 135 convergence of surface currents where iron stress is also high and thus limits productivity. 136 Specifically, in the convergence zone of the South Pacific ( Fig. 2d and S2), we observed elevated 137 DOP concentrations (~0.3 µM) in waters with high iron stress, low P*, and low chlorophyll a 138 concentrations (Fig 2). The same scenario was also found in the surface convergence zone of the 139 South Atlantic, where the DOP concentration is higher than that of the gyre boundary ( Fig. 2j and 140 S2). We suggest here that DOP will accumulate in the surface convergence zone of the North 141 Pacific, as well, which is another region with high iron stress, low P*, and low chlorophyll a (Fig.  142   S2). Further sampling would test this hypothesis. 143 144

Conceptual model of DOP accumulation and loss in the surface ocean 145
According to the observed relationships between upper 50 m DOP stocks, surface chlorophyll a 146 concentrations, and PO4 3and iron stress, we propose a simple conceptual model of the factors 7 influencing DOP distributions in the surface ocean (Fig. 3). On the global scale, DOP loss is 148 enhanced under elevated PO4 3stress, which increases to the right along the x-axis in Fig. 3, with  149 surface DOP concentrations decreasing to the right and increasing to the left. Additionally, iron 150 stress promotes both DOP production and consumption, depending on PO4 3stress. In Figure 3, 151 iron stress increases up the y-axis. The four quadrants in Fig. 3 correspond to different PO4 3and 152 iron stress regimes reflecting the corresponding role of DOP as either a product of or substrate for 153 primary productivity in specific ocean regions. 154

155
As discussed above, the Eastern Pacific is a highly productive region with low PO4 3stress, 156 resulting in net DOP production and accumulation ( Figs. 1 and 2), and is represented by the two 157 pink quadrants in the left of Fig. 3. Iron stress further influences the magnitude of DOP 158 accumulation under low PO4 3stress. When both iron and PO4 3stress are alleviated, such as in 159 surface waters overlying ODZs, enhanced primary production will lead to significant net DOP 160 accumulation, so that DOP has a "production" signature. In contrast, the upper left light pink 161 quadrant reflects regions in the Eastern Pacific Ocean exhibiting more muted net DOP 162 accumulation due to surface ocean convergence of DOP produced in "upstream" regions and to 163 the lower, iron-limited rates of primary productivity locally. Global surface ocean regions with the 164 lowest DOP concentrations are associated with high PO4 3and low iron stress, such as the North 165 Atlantic and Western North and South Pacific Oceans (Fig. 1). These regions are represented by 166 the dark blue, lower right quadrant in Figure 3, and correspond to regions of net DOP loss that we 167 interpret to reflect use of DOP as an assimilative P source sustaining productivity, perhaps 168 especially by diazotrophs. Finally, the upper right, light blue quadrant corresponds to regions 169 experiencing elevated PO4 3and iron stress, potentially including the South Atlantic. The South Atlantic Ocean receives relatively low atmospheric dust inputs 26 , and the lack of significant rates 171 of water column denitrification and/or anammox in the eastern portion of the basin leaves low P* 172 values in the surface waters (Figs. 1, 2). Here, net changes in DOP distributions are small, with the 173 potential co-limitation of primary productivity by PO4 3and iron. We performed a correlation analysis between upper 50 DOP stocks and P*, chlorophyll a 256 concentration, or iron stress by using a Type II linear regression model.    (100˚W -130˚W), samples from the surface convergence zone are within the red box in the map (j); (m, n and o) the AMT17, AMT 14 and 36N cruises in the Atlantic, where the surface convergence zone is circled with a red box in the map (m); (p, q and r) the KH12-3 cruise in the Western North Pacific. All black lines are lines of best fit determined with a Type II linear regression model. Note the different scales for the x-and y-axes in the panels. Details of correlations and sample sizes from each cruise are listed in Table 1.