Atmospheric deposition and river runoff stimulate the utilization of dissolved organic phosphorus in coastal seas

Domain: earth_science
Published: 2024-01-30
Editorial outcome: minor_revision

Abstract
In coastal seas, the role of atmospheric deposition and river runoff in dissolved organic phosphorus (DOP) utilization is not well understood. Here, we address this knowledge gap by combining microcosm experiments with a global approach considering the relationship between the activity of alkaline phosphatases and changes in phytoplankton biomass in relation to the concentration of dissolved inorganic phosphorus (DIP). Our results suggest that the addition of aerosols and riverine water stimulate the biological utilization of DOP in coastal seas primarily by depleting DIP due to increasing nitrogen concentrations, which enhances phytoplankton growth. This “Anthropogenic Nitrogen Pump” was therefore identified to make DOP an important source of phosphorus for phytoplankton in coastal seas but only when the ratio of chlorophyll a to DIP [Log10 (Chla/DIP)] is larger than 1.20. Our study therefore suggests that anthropogenic nitrogen input might contribute to the phosphorus cycle in coastal seas.

Introduction
Phosphorus (P) is an essential element for microorganisms such as phytoplankton due to its relevance for the synthesis of nucleic acids, phospholipids, and the main energy currency in cells, adenosine 5′-triphosphate1,2. Dissolved inorganic phosphorus (DIP) is regarded as the main bioavailable phosphorus and its concentration is often low in the surface open oceans with the exception of coastal seas3,4. Low DIP concentrations have been widely observed in open oceans such as the Atlantic and the Pacific Oceans, and sometimes also in coastal seas4–6. In contrast, dissolved organic phosphorus (DOP) in the oceans is more abundant relative to DIP7,8. Thus, microbes including phytoplankton have evolved to efficiently utilize DOP through extracellular hydrolysis, which converts DOP into DIP for cellular uptake5,9 and therefore mitigates phosphorus limitation under conditions of DIP scarcity.

The most important DOP hydrolase is the alkaline phosphatase (AP), which can be active in different subcellular compartments such as the extracellular space (e.g., phycosphere), the cell wall, the plasma membrane, or as part of the cytoplasm of diverse microbes including phytoplankton1,10,11. The activity of AP (APA) is mainly composed of inducible APA and constitutive APA. Although there is evidence of background APA at relatively low levels (constitutive APA) independent of DIP concentrations12–14, AP usually is induced by low concentrations of DIP. Thus, the APA usually is inversely correlated with DIP concentrations, and inducible AP therefore becomes activated for the utilization of DOP when DIP concentrations fall below a certain threshold15–17. In addition to the DIP concentration, other factors such as the abundance and diversity of microbiomes, N:P ratios, seawater temperature, and the availability of essential enzyme co-factors such as Fe and Zn also affect APA11,18–20. Hence, these variables might therefore have an impact on the DIP threshold for the activation of AP as previously shown11,13,21. This complex system of variables impacting the activation of AP likely is the reason why there is no consensus yet in terms of environmental conditions that induce DOP utilization in the ocean and especially in coastal seas.

Atmospheric deposition and river runoff are considered important sources of nutrients22–25 for the surface ocean as they usually contain significant amounts of nitrogen (N) and trace metals (e.g., Fe) but relatively low concentrations of P26–30. Studies have shown that atmospheric deposition can contribute between 40% and 70% of the total terrigenous N input in the North Pacific, North Atlantic, and Mediterranean31–33. Similar, riverine input can contribute 19–37 Tg N year−1 to the global coastal seas and is considered an important reason leading to coastal eutrophication24,34. Furthermore, atmospheric and riverine inputs are known to be indispensable sources of Fe in the global ocean26,30. Hence, atmospheric deposition has been estimated to stimulate phytoplankton growth and therefore to support between 3% and 5% of global marine net primary productivity35,36, while river runoff, of which the impact is mainly concentrated in the coastal waters, has been estimated to enhance the net primary productivity by ~14% in the global coastal seas over the 1905–2010 period34. Atmospheric deposition can also supply nutrients to the deeper ocean where it supports phytoplankton growth in the deep chlorophyll a maximum layer (DCM)37,38. In the recent past, the N input through atmospheric deposition and river runoff has increased due to increasing economic activities27,34. This might lead to an excess of N especially in seawater closest to the source of anthropogenic pollution39–41. Hence, coastal seas are likely more affected than the open ocean in terms of an increased input of N, which can lead to elevated N:P ratios24,42 and therefore potentially a gradual shift from N deficiency to P deficiency43. Under such biogeochemical conditions, DOP might become an important source of P for phytoplankton to alleviate limitation by DIP in excess of N1,5,44. However, despite decades of research on the impacts of atmospheric deposition and riverine input of essential nutrients on marine biogeochemistry28,34,36,45, their role in DOP utilization in coastal seas remains elusive.

Thus, only a few studies so far have focused on meso-/eutrophic coastal waters when it comes to the role of atmospheric deposition and riverine input in regulating DOP utilization18,46,47. Most studies have focused on oligotrophic open oceans where only atmospheric deposition can be considered because the geographical reach of rivers is more limited due to significant mixing and advection in coastal seas. Atmospheric deposition in oligotrophic open oceans can enhance DOP utilization and therefore alleviate P stress, but primarily by supplying essential enzyme co-factors of AP such as Fe and Zn11,17. In contrast, our recent work has provided preliminary evidence that atmospheric deposition can stimulate phytoplankton growth in the China Coastal Seas even though the concentrations of DIP in this geographical area were unable to meet the cellular requirements of phytoplankton for growth46,47.

Westerly winds in East Asia can carry aerosols into the China Coastal Seas, including the Bohai Sea, the Yellow Sea, and the East China Sea48–50 and therefore significantly contribute to atmospheric deposition in these coastal seas. Furthermore, multiple rivers such as the Yellow River, the Xiaoqing River and the Yangtze River significantly contribute to riverine input of terrestrial nutrients in these coastal seas. In the relatively enclosed Bohai Sea, both atmospheric deposition and river runoff cause excess macro-nutrient concentrations (e.g., dissolved inorganic nitrogen, DIN) and elevated concentrations of trace metals51,52. Consequently, P has now become the main limiting nutrient53–55. The Yellow Sea and the East China Sea are marginal seas connected to open waters of the Northwest Pacific Ocean, with typical biogeochemistry spanning from eutrophy to oligotrophy with increasing distance from the East Asian continent56. In contrast to the increased anthropogenic input of N, DIP concentrations have been continuously decreasing since the mid-1990s, hence causing P deficiency43,57. Thus, these coastal seas provide an ideal testbed for studying the effect of atmospheric deposition and river runoff on the utilization of DOP in relatively nutrient-rich coastal waters, which contributed disproportionally to global biogeochemical cycles58,59.

To address this knowledge gap, we carried out a series of on-board microcosm experiments in the China Coastal Seas covering a gradient from eutrophic to oligotrophic conditions with the aim to reveal the impact of atmospheric deposition and riverine input on DOP utilization and therefore the role of N-rich aerosols and riverine waters in biogeochemical cycles of coastal seas (Fig. 1).

Results
Characteristics of atmospheric aerosols, riverine water, and seawater of the China Coastal Seas
The concentration of DIN (0.99–2.14 μmol·m-3) in aerosols was much higher than that of DIP (0.66×10-3–8.36×10-3 μmol·m-3), resulting in high DIN:DIP ratios (>100), which significantly exceeded the Redfield ratio (N:P=16:1) (Table S1). Similarly, the concentration of DIN in the riverine water was high (330.54 μM) and that of DIP was low (0.65 μM), resulting in a high N:P ratio of 509 (Table S1). In addition, the salinity of the 20 L incubation system decreased about 0.05 PSU for the River-low group and 0.2 PSU for the River-high group, which had negligible impacts on phytoplankton growth60. The DIN:DIP ratios were also high (>100) in the seawater at B1Spr (127) and B1Sum (107) in the Bohai Sea, E1 (151) in the East China Sea, and Y1 (200) in the Yellow Sea, while closer to the Redfield ratio at Y3Spr (12), Y1-DCM (18), and Y2 (15) in the Yellow Sea.

Generally, seawater in this study was characterized by low DIP concentrations (0.01–0.11 μM) with the highest value measured at the DCM layer. In contrast, the DIN (0.23–9.83 μM), Si(OH)4 (0.20–8.47 μM), and Chl a (0.14–4.47 μg·L−1) concentrations varied widely (Table S2). However, we were able to divide the nutrient status of the seawater used in this study for the microcosm experiments into three categories: I. Eutrophic-like: At B1Sum and DCM layer of Y1, Chl a (1.21–2.94 μg·L−1), DIN (1.99–3.60 μM), and Si(OH)4 (3.10–8.47 μM) concentrations were relatively high. At Y3Spr and B1Spr, the Chl a concentrations were also high (3.07–4.47 μg·L−1), even though DIN (<1 μM) and Si(OH)4 (0.20–0.79 μM) concentrations were relatively low. This is likely because the seawater was collected during a spring bloom. In contrast, DIN (9.83 μM) and Si(OH)4 (3.18 μM) concentrations were high, while the Chl a concentration was relatively low at E1 (0.43 μg·L−1), which corresponded to the end of the spring bloom. II. Mesotrophic-like: The seawater of Y1 and Y3Sum had a medium level of Chl a (0.47–1.16 μg·L−1), DIN (0.23–1.53 μM), and Si(OH)4 (1.04–1.69 μM) concentrations. III. Oligotrophic-like: The seawater of Y2 had low nutrient concentrations (DIN, 0.27 μM; DIP, 0.02 μM; Si(OH)4, 1.02 μM), and lowest Chl a concentration (0.14 μg·L−1) (Table S2).

The response of phytoplankton to additions of aerosols and riverine water
Even though the in-situ Chl a and nutrient concentrations in the sampled seawater varied over an order of magnitude at different stations (Table S2), a significant response of Chl a concentrations to the addition of nutrients, aerosols, and riverine waters was found at most stations (Figs. S1 and S2). However, considering that seawater samples were collected from different nutrient regimes (eutrophic to oligotrophic) and phytoplankton growth stages (Figs. S1, S2, and Table S2), it was challenging to delineate the response of Chl a under different experimental treatments conducted with the microcosms. ΔR, which has been used to quantify the relative change of Chl a after diverse treatments in previous studies61, was therefore introduced to determine the significance of the response of Chl a to additions of nutrients, aerosols, and riverine waters. According to our ΔR calculations for nutrient treatments, phytoplankton was generally limited by N in spring, and by either P or N+P in summer (Fig. 2a). Note that phytoplankton at E1 and Y1-DCM were not growth limited by either N or P (Fig. 2a). However, additions of aerosols or riverine waters appear to have increased Chl a and ΔR in almost all experiments (p < 0.05) irrespective of the limiting nutrient, and the addition of aerosols and riverine waters with same DIN contents shows a similar increase of Chl a (Figs. 2b and S2). Even under the limitations of P and N+P, the additions of aerosols or riverine waters with minimal DIP supply relative to the concentrations in natural seawater, increased Chl a concentrations over time (Figs. 2, S2, and S3).

The activity of alkaline phosphatases and DOP utilization in microcosm experiments
After the addition of aerosols or riverine waters, the DIN and DIP concentrations decreased in most incubations (Fig. S3). The APA in the surface seawater varied greatly during spring- and summertime. Low APAs were observed at E1 (0.4 nM P·h−1) and B1Spr (0.9 nM P·h−1) in spring with lower seawater temperature (10–17 °C), whereas much higher (9.0–179.1 nM P·h−1) APA was measured in the summer with higher seawater temperature (28–32 °C), due to the increased severity of P deficiency except for Y1-DCM (2.1 nM P·h−1) that had relatively high DIP concentrations (Table S2). This seasonal difference was most pronounced at B1 in the Bohai Sea, of which the APA in summer was more than two orders of magnitude higher than in spring (Table S2).
The APA generally showed a positive response to the addition of DIN (Figs. S4a, c and S5a-b), aerosols (Fig. 3) and riverine waters (Fig. S6a, b) whereas it decreased after the addition of P or N+P (Fig. S5). A significant increase of Chl a normalized APA (APA/Chl a) was also found at B1Spr, B1Sum, and Y3Sum (Fig. S7).
Compared with concentrations in surface seawater, The concentration of total dissolved phosphorus (TDP) changed by -42.5%–5.9% during the incubations (Fig. S8), indicative of substantial concentrations of P in seawater. The net utilization of DIP and DOP by phytoplankton were estimated through the change of DIP (ΔDIP) and DOP (ΔDOP), respectively, during the incubations.
There were three patterns observed with respect to the utilization of P:
I. DIP was the main source of phosphorus. In experiments using eutrophic seawater with relatively high DIP concentrations (0.07–0.11 μM) including E1 and Y1-DCM, DIP was the main bioavailable P nutrient, and 0.04–0.05 μM DIP was consumed by phytoplankton in the control groups (Fig. 4a and S9a, f). The DIP utilization changed by -5.5%–91.0% after aerosol addition, which was positively correlated with the added amount at Y1-DCM (Fig. S9f).
II. DIP was the main source of phosphorus in the control and DOP was the main source of phosphorus in the aerosol treatments. DIP was also the main P nutrient in the control at Y3 and Y2 (Fig. S9b, g). Except for Y1-DCM which had higher DIP, DOP utilization was induced after the aerosol addition, leading to small uptake (≤0.02 μM) of DOP during the incubations in the aerosol treatments at E1, Y3Spr, and Y2. Aerosol addition contributed to DOP becoming the main P nutrient for phytoplankton growth, which accounted for 51.9%–59.7% of P uptake in the high-content aerosol treatment at Y3Spr and in the low- and medium-content aerosol treatments at Y2 (Fig. 4b and S9a, b, g).
III. DOP was the main source of phosphorus. At B1Spr, B1Sum, Y1, and Y3Sum, where DIP was highly deficient (0.01–0.03 μM), DOP contributed 82.4%–92.5% of P and became the primary P nutrient for phytoplankton growth in the control groups (Fig. 4c). However, although DOP was still the main P nutrient in the treatment groups, no significant enhancement of ΔDOP was found after aerosol additions (Fig. S9c, d, e, h).

Note that ΔDOP as net DOP utilization did not show a significant response either to aerosol or riverine water additions in most experiments (Fig. 4, S6c-d, and S9), which was likely ascribed to the high turnover rate of easily degradable DOP in coastal seas62,63. We therefore defined ΔDOP* (ΔDIN/16–ΔDIP, Eq. 3) on the basis of DIN (ΔDIN) and DIP (ΔDIP) utilization (Eq. 2), and the Redfield ratio for N:P requirement (16) of phytoplankton. This enabled us to estimate the potential maximum uptake of DOP (versus net uptake given as ΔDOP)11,44, which was positively correlated with ΔDOP and APA (Fig. 5a). At E1 and Y1-DCM, where DIP was the main P nutrient, ΔDOP* was low (-0.06–0.02 μM) (Fig. 4a). At Y3Spr and B1Spr with ΔDOP between -0.02 and 0.02 μM, positive ΔDOP* could be found. Moreover, at B1Sum and Y3Sum with high APA (15.5–513.7 nM P·h−1) and ΔDOP between 0.04–0.06 μM, phytoplankton had a larger potential to take up DOP (ΔDOP*, up to 0.29 μM), indicating that the total uptake of DOP during the incubations may be up to 37 times higher than the calculated net DOP uptake (ΔDOP, up to 0.06 μM) (Fig. 4, S6, and S9). In addition, ΔDOP* significantly responded to aerosol and riverine water additions at Y3Spr, B1Spr, Y2, Y3Sum, and B1Sum where DOP was the main P nutrient or might have become the main P nutrient after aerosol additions, of which the response was positively correlated with the amount added (Fig. 4b-c, S6, and S9). ΔDOP* at B1Sum in the Bohai Sea was even higher than the DOP concentration at the end of the incubations, which is in accordance with an increased turnover of DOP in the treatments with aerosol and riverine water (Figs. 4, S6, and S9). As an exception, although APA was high at Y1 (26.2–121.0 nM P·h−1), ΔDOP* did not significantly respond to aerosol additions. This is possibly due to the contribution of bacteria to DOP hydrolysis, as the Chl a normalized APA (APA/Chl a) at Y1 didn't significantly respond to aerosol addition, and was much higher than that of other stations with significant DOP utilization (B1Spr, Y3Sum, B1Sum) (Figs. S7 and S9).

It should be noted that although phytoplankton can alleviate P limitation by increasing the N:P uptake ratio under P-limited conditions64, the higher ΔDOP* relative to ΔDIP was also identified using three (ΔDOP*48=ΔDIN/48–ΔDIP, generally indicative of obvious P deficiency) and even if five (ΔDOP*80=ΔDIN / 80–ΔDIP) times the Redfield ratio (Fig. S10). Thus, our results do not appear to be significantly impacted by differences in the Redfield ratio, which therefore corroborates the robustness of the parameter ΔDOP*.

Methods
Collection and chemical analysis of atmospheric aerosols and riverine water
Atmospheric aerosols were collected at the Atmospheric Environment Monitoring Station in the Laoshan campus of the Ocean University of China, Qingdao, China (36.16°N, 120.50°E), about seven kilometers off the coast of the Yellow Sea (Fig. 1). The sampling was conducted under haze weather (visibility <10 km, relative humidity <90%) in January 2018 (Aerosol 1), January 2019 (Aerosol 2), and June 2019 (Aerosol 3) (Table S1). The aerosols were collected onto an acid washed filter (Whatman, 41#) through a high-volume sampler (KC-1000, Laoshan electronic instrument company of Qingdao) operated at a flow rate of 1.0 m3·min−1 for about 20 hours uninterruptedly, and a blank was simultaneously collected at a flow rate of 0 m3·min−1 under the same condition74.

The aerosol sample was divided up into several aliquots for chemical analyses and the on-board microcosm experiments. For chemical analyses, two parts of the filter were extracted ultrasonically with deionized water (18.2 MΩ cm) in an ice-water bath for 40 minutes and the extracts was filtered instantly through a PES membrane (aperture 0.45 μm). The first extract was used for nutrient (NO3-+NO2-, NH4+, DIP, and Si(OH)4) analysis using an QuAAtro continuous-flow analyser (SEAL Analytical)47. The second extract was digested using 15.5 mol·L−1 HNO3 and then evaporated. The solution was used for dissolved trace metal analysis using an Agilent 7500c ICP-MS75. The detection limit was 0.4 μg·L−1 for Fe and was 0.09 μg·L−1 for other metals. The third part of the filter was used for aerosol amendments in the microcosm experiments. It was extracted before the microcosm experiments took place following the same procedure as described above for the nutrient analysis76,77.

The riverine water was collected using a surface-water collector in the Xiaoqing River in 2019. The riverine water was filtered through acid-washed acetate fiber membranes (0.45 μm pore size) and stored in HDPE bottles at -20 °C. It was defrosted in the dark at room temperature (18–20 °C) before measuring the chemical composition. Nutrient concentrations (NO3-+NO2-, NH4+, DIP, and Si(OH)4) of riverine water were then measured using the QuAAtro continuous-flow analyser (SEAL Analytical)47.

Setup of microcosm experiments
Eight on-board microcosm experiments were carried out at five stations in the East China Sea, the Yellow Sea, and the Bohai Sea using surface seawater collected in spring (March—April) and summer (July—August) during the cruises 2018 by R/V Dongfanghong 2 and 2019 by R/V Beidou, respectively (Fig. 1). Surface seawater (3–5 m depth) as well as water from the DCM layer of Y1 were collected by Teflon-coated Niskin bottles assembly on a Sea Bird CTD and pre-filtered through 200 μm acid-washed nylon mesh to eliminate large zooplankton. They were transferred into transparent polycarbonate bottles, and instantly spiked with the extract of aerosol sample (20 L bottle experiments, Nalgene 2251-0050) and combinations of nutrients / trace metals (KNO3, 2 μM; KH2PO4, 0.2 μM; FeCl3, 2 nM) (2 L bottle experiments, Nalgene 2015-2000), respectively (Table S4). Aerosol 1 was used for the microcosm experiments at Y2, Y3Spr, and Y3Sum, Aerosol 2 for those at E1 and B1Spr, and Aerosol 3 for those at Y1 and B1Sum (Table S4).
Riverine water amendments were also set up in the 20 L bottle experiments at B1Spr and B1Sum, to make a comparison with aerosol treatments under the condition of same DIN supply. The added amount of aerosol and riverine water was calculated based on their DIN (NO3- + NO2- + NH4+) concentrations (0.5, 1, 2 μM, Table S4). Three parallel experiments (n=3 samples) were setup for each treatment.
The bottles were placed in three microcosm tanks and incubated for 4–6 days for 20 L bottle experiments and 3–5 days for 2 L bottle experiments with flow-through surface seawater to keep the temperature stable and like in-situ conditions. The experiments were covered by photomask to attenuate photosynthetically active radiation by around 40%78.

Sampling and measurements
To mix the substances in bottles adequately, samples were taken after gentle shaking. Subsamples for the Chl a and nutrient analysis were taken every 24 h during most of the incubations. Samples for APA analysis were mostly taken at the beginning, middle and the end of the incubations, and samples for the DOP measurements were taken at the beginning and end of the incubations.

Chlorophyll a measurements
Triplicate samples of 200–300 mL were filtered through 20 (nylon, Millipore), 2 (polycarbonate, Whatman) and 0.2 (polycarbonate, Whatman) μm aperture sized membranes in time series, and total Chl a concentrations were calculated by combining size-separated Chl a concentrations for the 20 L incubation systems, and 100–150 mL were filtered through a GF/F membrane (Whatman) to measure total Chl a for the 2 L incubation systems. The pigments on the membranes were extracted with 90% acetone at -20 °C for 20 h in the dark. The extract was used for measuring Chl a using a 7200-000 Trilogy fluorometer (Turner Designs)47.

Inorganic and organic nutrients
100 mL samples of 20 L bottle experiments were filtered through 0.45 μm acid-washed acetate membrane, and the filtrate was stored in acid-washed HDPE bottles at -20 °C. The samples were melted at room temperature 48 hours before measuring them. 50 mL of filtrate was used for determining inorganic nutrients (NO3- + NO2-, NH4+, DIP, and Si(OH)4) using a QuAAtro continuous-flow analyser (SEAL Analytical)47. The detection limits of NO3-+NO2-, NH4+, and Si(OH)4 were ≤0.02 μM, ≤0.02 μM, and ≤0.03 μM, respectively47. The long optical fiber technology was used to measure DIP concentration and its detection limit was estimated to be ≤0.005 μM79. The error margin for the DIP analysis was 1.39% of the average value of 0.06 μM, which represents the relative standard deviation of five replicates. To measure total dissolved phosphorus (TDP) and dissolved organic phosphorus (DOP), 40 mL filtrate and 4 mL oxidizing agent were put into a Teflon digestion tank and were disintegrated at 120 °C for 20 min in an autoclave to enable the detection of TDP. The oxidizing agent was made by dissolving 10 g K2S2O8 and 6 g H3BO3 in 200 mL prepared NaOH solution (0.375 mol·L−1)80. The detection limit of TDP was 0.02 μM, and the error margins for TDP analysis was 0.77% of the average value of 0.20 μM through the same calculating method as that of DIP. DOP was estimated by subtracting DIP from TDP. The detection limit of DOP was 0.025 μM (DL DIP + DL TDP). The error margin for DOP analysis (1.27% of the average value of 0.14 μM) was calculated using the standard deviations (SD) and average values (avg) of five replicates for TDP and DIP analysis (Eq. 4).

Activity of Alkaline Phosphatase (APA)
4-methylumpholone phosphate (MUF-P) was used as the substrate to analyze APA. MUF-P was converted to PO43- and 4-methylumpholone (MUF) by the alkaline phosphatase, in which MUF showed fluorescence when pH is above 1081. For 2 L bottle experiments in 2019 and 20 L bottle experiments in 2018 in summer and in 2019, 45 mL sample with 200–1000 nM MUF-P was incubated in the microcosm tank in the dark after gently mixing11. The fluorescence and therefore the APA (nM P·h−1) were calculated estimated by time-course measurements (0, 1.5, and 3 h) using the 7200-000 Trilogy fluorometer (Turner Designs) with the long wavelength UV module (P/N 7000-967) after adding borate buffer (pH = 10.8–11.2) to the samples82. The concentration of MUF corresponding to the fluorescence was calculated using the standard curve (0–1 μM) configured with MUF-2Na5,82.

Data analyses
As the seawater was collected at different nutrient status and phytoplankton growth stage, the Chl a concentration and its response to different treatment were variable (Figs. S1 and S2). The response of phytoplankton to nutrient, aerosol, and riverine water additions was there quantified by ΔR61, which was expressed as:
ΔR=Log10(Chla t-avg / Chla c-avg)/t (1)
where Chl a c-avg and Chl a t-avg stand for the average Chl a concentration in the control and N, P, N+P, aerosol, or riverine water treatment groups, respectively, over the time course during the microcosm experiments (n=3 samples); t represents duration of microcosm experiment in days.
Nutrient utilizations were estimated by the decrease of nutrient concentrations (DIN, DIP, or DOP) over time during the incubations:
ΔX=X0 - Xt (2)
where the X0 and Xt represent the nutrient concentrations at the beginning and the end of the incubations, respectively (n=3 samples) (Figs. S3 and S8).
P*=DIP–DIN/16 is often used to determine excess of available dissolved inorganic phosphorus (DIP). A positive correlation between surface DOP concentration and P* was found on a global level11,44. The potential maximum DOP utilization during the incubations was therefore calculated according to:
ΔDOP*=ΔDIN/16–ΔDIP (3)
where the ΔDIN and ΔDIP represent the change of DIN and DIP concentrations, respectively, during the incubations. The number 16 represents the standard Redfield Ratio for N:P requirement of phytoplankton. Although the nutrient uptake ratio can vary83, the POC:PON ratios in this study (4.07–10.50, mean=6.96) were close to Redfield ratio (C:N=6.63) and positive correlations were measured between ΔDOP*, ΔDOP, and APA, indicating the robustness of the parameter ΔDOP* in this study (Fig. 5a and S14).
The error margin for DOP analysis was calculated using the standard deviations (SD) and average values (avg) of five replicates for TDP and DIP analysis according to:
√((SD TDP^2 + SD DIP^2) / (TDP avg - DIP avg)) * 100% (4)

Statistical analyses
The response of Chl a to nutrient additions (Fig. 2a), the response of APA to nutrient, aerosol, and riverine water additions (Figs. 3 and S4–7), and the differences in P utilization (Fig. 4, S6, and S9) were evaluated using one-way ANOVA. A Tukey HSD post hoc test was used to identify statistically significant differences. We used the “aov” function for one-way ANOVA, and the “TukeyHSD” function for Tukey HSD post hoc test in the R package “stats”61. Unpaired t-test (two-sided) was used to identify statistically significant differences between the response of Chl a to treatments (aerosol and riverine water) in spring and summer (Fig. 2b), and between the P utilization in control and treatments (aerosol and riverine water) (Fig. 4). We used the “t.test” function for unpaired t-test in the R package “stats”84. Nonparametric Spearman correlation test (r) (two-sided) was used to evaluate the correlation between parameters (Chl a, ΔDIN, ΔDIP, ΔDOP, and ΔDOP*) that could impact DOP utilization (Fig. 5a). Linear regression was used in Figs. 5b, 6a, and S12. To explore the statistically significant threshold of APA and DIP concentrations, a segmented regression was conducted through R package “segmented”85. To investigate Log10 (Chl a / DIP) in the global surface ocean (upper 20 m, 1°×1°), we obtained Chl a concentration data at 9-km spatial resolution from SeaWiFS86 and MODIS-Aqua87 ocean color observations and the climatological mean for DIP data from World Ocean Atlas 201888.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "REVIEW of a manuscript enfitled \"Atmospheric aerosols sfimulate the ufilizafion of dissolved organic phosphorus in coastal seas\" by Haoyu Jin et al. Jin and his colleagues conducted a study that demonstrated how aerosol addifions to the coastal waters near China enhance the ufilizafion of DOP by phytoplankton. This finding is relevant due to the substanfial increase in anthropogenic N input to this region of the Pacific. While one would expect an increase in phytoplankton biomass in response to higher anthropogenic N input, the exact mechanism for this biomass increase has not been fully addressed. Specifically, the hypothesis that the addifion of N input maximizes the ufilizafion of the DOP pool to increase biomass has not been experimentally proven unfil now. The paper aimed to invesfigate this outstanding quesfion and, to some extent, the authors achieved their goal. I am hopeful that the authors’ efforts will yield further progress. In preparing the revised manuscript, I have several reservafions that the authors should consider. (1) One crucial aspect to address in this study is the shift in the nutrient regime shift in the marginal seas of the northwestern Pacific, parficularly the transifion from N limitafion to P limitafion, which can be aftributed to the addifion of anthropogenic N input. A recent L&O paper by Moon et al. (66, 2021, 914-924) provides valuable insights in this regard. The authors observed a change in nutrient dynamics during the 1990s, where the prevalence of N deficiency decreased over fime. Concurrently, they found the emergency of P deficiency in the vicinity of the Changjiang estuary, which progressively expanded beyond the estuary. Given the significance of the P-limitafion condifion in this work, including a paragraph outlining this nutrient regime shift would not only jusfify your work but also emphasize the importance of your finding. (2) I challenge use of log (Chl a/DIP): The use of log (Chl a/DIP) = 1.2 appears to be subjecfively chosen. To me this rafio simply indicates the condifions of low DIP concentrafion and high chl a concentrafion. I suggest using N* = [N] – 16 [P] as an alternafive approach. N* is direct measure of either N excess (P deficit) or N deficit (P excess). Can you recalculate all data used in Figure 5a? N and P concentrafion data in most locafions are likely available. I like to see how the results based on N* differ from those based on log (Chl a/DIP) = 1.2. (3) Addifion of river water (rich in N relafive to P) to seawater samples can be problemafic in microcosm experiments: The introducfion of river water into the seawater samples resulted in a decrease in salinity. This alterafion in salinity subsequently impacted the growth of phytoplankton How did you take this fact into considerafion when you interpret the results? (4) Delta DOP* is not enfirely clear to me relafive to delta DOP: Please provide more detailed explanafion in comparison with delta DOP. (5) Figures 2 and 3 are very hard to follow: I am extremely unhappy with Figures 2 and 3, as they lack intuifive appeal. Considering that these figures encompass the key findings of this work, I strongly encourage the authors tom invest considerable effort in revising them. Upon reading the figure legends, I encountered considerable difficulty in interprefing the figures. Ideally, the figures should be self-explanatory by visual examinafion along.", "reviewer_2": "This is an interesfing paper which addresses growing interest in the cycling of nutrients in marine systems. The data are robust and will add to knowledge of the biogeochemical processes involving P and interrogate the relafionship between DIN and DOP and the role of N. My reservafions on recommending the paper for publicafion are mainly how much this comprehensive dataset advances knowledge as the idea that low DIP or an increased N:P rafio will induce APA and ufilisafion of DOP is well established and accepted, though as I have noted there are also reports of consfitufional uptake of DOP (see separate table of comments). Rees at al (2009) doi:10.1016/j.ecss.2008.12.005 observed an increase in APA in the Western English Channel and this corresponded with a salinity decrease related to increased riverine inputs, increasing the N:P rafio in the sampling area. If these inputs are riverine or atmospheric it stands to reason that DOP ufilisafion could be sfimulated. The last sentence on curbing nitrogen inputs is vague and could be constrained using esfimates of anthropgenic inputs, such as Jickells et al (2017) doi:10.1002/2016GB005586. This could strengthen Figure 5, where N deposifion is key.", "reviewer_3": "Jin et al present the results from shipboard nutrient, aerosol, and river water amendment microcosm experiments to invesfigate the controls on alkaline phosphatase acfivity (APA) and DOP ufilizafion by phytoplankton in the coastal waters off the East Asian confinent. The fitle and abstract focus on the role of aerosols and the macro and micro nutrients they contain in sfimulafing APA and DOP ufilizafion in the microcosms. The authors conclude that “atmospheric aerosols sfimulate the ufilizafion of dissolved organic phosphorus in coastal seas” and that this arises primarily by deplefing ambient DIP when stoichiometric larger addifions of DIN are added by aerosols, with phytoplankton turning to DOP to safisfy their P demand. In my opinion, the authors fail to convincingly demonstrate 1) that DOP ufilizafion is sfimulated by aerosol addifion, 2) that APA is sfimulated by aerosol addifion which would be required for DOP ufilizafion, and 3) that the driving mechanism leading to enhanced DOP ufilizafion by phytoplankton is the alterafion of the ambient DIN:DIP by aerosol addifion. The study concludes that DOP ufilizafion is sfimulated by aerosol addifion. The data in support of this is presented in Figure 3. Of the 8 stafions presented for aerosol addifion microcosms, only two conclusively demonstrate a DOP ufilizafion effect that is greater than nil within the presented error bars (stafions Y3 and B1). River water amendment microcosms were conducted at B1 with the magnitude of the DOP ufilizafion effect comparable to that from aerosol addifion, weakening the authors conclusion that aerosols alone are important for sfimulafing DOP ufilizafion in coastal seas. As acknowledged by the authors, due to the weak support from interprefing enhanced DOP ufilizafion in the microcosms by differences in measured DOP concentrafions, the authors present a derived metric, DOP*, to support the nofion that DOP ufilizafion is enhanced by aerosol addifion. DOP* represents the theorefical potenfial maximum in DOP ufilizafion that might have occurred in the microcosm after compufing the residual between the expected phytoplankton P demand (computed from the observed DIN ufilizafion and Redfield N:P stoichiometry) and the observed DIP ufilizafion. Using DOP* as the metric to gauge sfimulafion of DOP ufilizafion by aerosol addifion, raises the number of stafions exhibifing a posifive effect from 2 out of 8 to 4 out of 8, sfill not a very conclusive result in my opinion. The APA data presented in Figure 3 generally support the interpretafions from using delta DOP or delta DOP*; i.e. in microcosms where a sfimulafion effect was observed for aerosol addifion on DOP ufilizafion, APA is higher and vice versa. As for the conclusion that elevated DIN:DIP is responsible for elevated DOP ufilizafion, the data in Figure 4b largely supports this by idenfifying a significant posifive correlafion between the relafive APA sfimulafion (expressed as a percentage relafive to the controls) versus DIN:DIP as the predictor variable. However, dissolved iron has also been idenfified as an important co-control on DOP ufilizafion in marine surface waters (see Browning et al. 2017 Nat. Comm., Liang et al., 2022 Nat. Geo.). The authors need to demonstrate data and/or other arguments to include or rule out the potenfial control of enhanced DOP ufilizafion by Fe addifion from aerosols and possibly river water. The authors should report the limit of detecfion and analyfical uncertainty for DOP determinafions after considering the addifive uncertainfies from the separate determinafions of TDP and DIP. The precision on TDP analyses is unreported so it is impossible as a reviewer to determine if the delta DOP values reported in Figure 3 on the order of -30 to +90 nM are even larger than the analyfical uncertainty on DOP determinafions. Lastly, I am excited by the idenfified log10(chl/DIP) metric for predicfing APA and potenfially DOP ufilizafion (Fig. 5). I believe many marine biogeochemists will find its predicfions at the global scale to be useful in helping predict the role of DOP in the local to regional biogeochemistry across the global ocean."}, "author_response": "Response to reviewers’ comments. Text in italics (copied from revised manuscript) represents how we have addressed the reviewers’ suggestions in our revised manuscript. Our response in blue. Reviewer #1: 1. One crucial aspect to address in this study is the shift in the nutrient regime shift in the marginal seas of the northwestern Pacific, particularly the transition from N limitation to P limitation, which can be attributed to the addition of anthropogenic N input. A recent L&O paper by Moon et al. (66, 2021, 914-924) provides valuable insights in this regard. The authors observed a change in nutrient dynamics during the 1990s, where the prevalence of N deficiency decreased over time. Concurrently, they found the emergency of P deficiency in the vicinity of the Changjiang estuary, which progressively expanded beyond the estuary. Given the significance of the P-limitation condition in this work, including a paragraph outlining this nutrient regime shift would not only justify your work but also emphasize the importance of your finding. Response: Thanks for your helpful suggestion. We have implemented your idea as follows (copied from the revised version of our manuscript): “Hence, coastal seas are likely more affected than the open ocean in terms of an increased input of N, which can lead to elevated N:P ratios39,40 and therefore potentially a gradual shift from N deficiency to P deficiency41.” “In contrast to the increased atmospheric N deposition, DIP concentrations have been continuously decreasing since the mid-1990s in the Yellow Sea, hence causing P deficiency41,56” “Increasing atmospheric and riverine N input in recent years at a global level has elevated the N:P ratios in coastal seawater, and progressively expanded beyond the coastal regions24,35,40,41. In the context of decreased prevalence of N deficiency over time, the impact of N deposition is gradually shifting from alleviating N limitation to increasing DIP limitation, especially in coastal regions35,41,71. Over the past four decades, the disproportionate and sustained input of N relative to P into the China Coastal Seas has led to the transition from N limitation to P limitation in extensive areas41.” 2. I challenge use of log (Chl a/DIP): The use of log (Chl a/DIP) = 1.2 appears to be subjectively chosen. To me this ratio simply indicates the conditions of low DIP concentration and high chl a concentration. I suggest using N* = [N] – 16 [P] as an alternative approach. N* is direct measure of either N excess (P deficit) or N deficit (P excess). Can you recalculate all data used in Figure 5a? N and P concentration data in most locations are likely available. I like to see how the results based on N* differ from those based on log (Chl a/DIP) = 1.2. Response: We implemented the idea to calculate N* but found no significant correlation between the relative change of APA and DIP deficiency (Fig. R1), even if we excluded site E1, which was sampled at the end of the spring bloom. Although N* has been widely used in various studies (Browning et al., 2017; Kim et al., 2014; Liang et al., 2022), as we mentioned in this study, Chl a is equally important in terms of affecting APA because it is an indicator or phytoplankton biomass. The same N* is likely corresponding to different Chl a concentrations likely resulting in different levels of P stress and thus varying APAs. For instance, when high N* is accompanied by low Chl a, the small-sized phytoplankton are less affected by P stress (Marañón, 2015), as the residual P nutrient is likely matching their low P requirements. Nevertheless, to address your suggestion in our manuscript, a new figure has been added to the supplementary information (Fig. S10). This figure has also been discussed in the main manuscript as follows: “In addition, the relative change of APA was not significantly correlated with N* (DIN/16-DIP)37 (Fig. S10), suggesting the important role of phytoplankton (Chl a) in affecting DOP utilization in coastal seas.” 3. Addition of river water (rich in N relative to P) to seawater samples can be problematic in microcosm experiments: The introduction of river water into the seawater samples resulted in a decrease in salinity. This alteration in salinity subsequently impacted the growth of phytoplankton How did you take this fact into consideration when you interpret the results? Response: Because of your comment, we have estimated the impact of adding riverine water on the salinity in our experiments as good as we could. According to the result reported by Qing et al. (2013), the salinity of the surface seawater in the central Bohai Sea usually is between 30 - 33 Practical Salinity Units (PSU) across all sampled seasons. In our study, 30.25 mL and 121 mL of riverine water were added to River-low and River-high groups, respectively. The salinity of riverine water used in the microcosm experiments was 4.38 PSU. Therefore, the salinity of our 20 L incubation systems decreased by about 0.05 PSU for River-low group and by about 0.2 PSU for River-high group, which likely has a very weak impact on phytoplankton growth (Pilkaitytë et al., 2004) and APA (Fu and Bell, 2003). Nevertheless, we have addressed this in the supplementary methods as follows: “In addition, the salinity of the 20 L incubation systems decreased by about 0.05 PSU for the River-low group and 0.2 PSU for the River-high group, which likely had negligible impacts on phytoplankton growth1.” 4. Delta DOP* is not entirely clear to me relative to delta DOP: Please provide more detailed explanation in comparison with delta DOP. Response: We have split the original Fig. 3 into two figures (Fig. 3 for APAs, Fig. 4 for P utilization) and updated them to make the parameters of P utilization much clearer. We have also tried to improve the explanation of △DOP* in the manuscript and in the figure legend of Fig. 4 as follows: “Note that △DOP as net DOP utilization did not show a significant response either to aerosol or riverine water additions in most experiments (Figs. 4, S5c-d, and S8), which was likely ascribed to the high turnover rate of easily degradable DOP in coastal seas60,61. We therefore defined △DOP* (△DIN/16-△DIP, Eq. 4) on the basis of DIN (△DIN) and DIP (△DIP) utilization (Eq. 3), and the Redfield ratio for N:P requirement (16) of phytoplankton.” “Fig. 4 | Phosphorus utilization in response to aerosol additions. DIP utilization (△DIP) and net DOP utilization (△DOP) were calculated during the incubations from the change of DIP and DOP concentrations, respectively. Potential maximum DOP utilization (△DOP* = △DIN/16-△DIP) was calculated by the DIN (△DIN) and DIP (△DIP) utilization during the incubations using the Redfield ratio of 16:1 for N:P. Significant differences (p < 0.05) between different P utilization (△DIP, △DOP, △DOP*) in the same group are indicated by different letters and NS = not significant, and significant differences (p < 0.05) for P utilization between control and treatment groups are linked by coloured lines and indicated by coloured stars (one-way ANOVA and Turkey HSD post hoc test).” 5. Figures 2 and 3 are very hard to follow: I am extremely unhappy with Figures 2 and 3, as they lack intuitive appeal. Considering that these figures encompass the key findings of this work, I strongly encourage the authors tom invest considerable effort in revising them. Upon reading the figure legends, I encountered considerable difficulty in interpreting the figures. Ideally, the figures should be self-explanatory by visual examination along. Response: We agree and therefore have completely revised Figures 2 and 3 (see below). To make Fig. 2 clearer, we have moved N+Fe and N+P+Fe and the riverine water experimental groups to the supplement because they were just used for determining the nutrients that were mainly limiting the phytoplankton biomass. Furthermore, we have split Fig. 3 into two figures (Fig. 3 for APAs, Fig. 4 for P utilization) and re-designed them to make them clearer. We hope this has increased their visual appeal. We have also added a new figure to the supplement for illustrating P utilization at different stations (Fig. S8) and we moved the riverine data to the supplement as well (Fig. S5). Please see below for our new figures. Reviewer #2: General comments: 1. My reservations on recommending the paper for publication are mainly how much this comprehensive dataset advances knowledge as the idea that low DIP or an increased N:P ratio will induce APA and utilisation of DOP is well established and accepted, though as I have noted there are also reports of constitutional uptake of DOP. Response: We agree that low DIP or an increased N:P ratio will induce APA and the utilisation of DOP, and this is very established knowledge indeed and it was also confirmed by our work, which is reassuring. However, the novelty of our work is to show the significance of atmospheric deposition in driving this biogeochemical process in coastal seas based on a case study in the China Coastal Seas, which we extended to the global level. This has not been shown before as most previous studies have focussed on open oligotrophic oceans when it comes to the role of atmospheric deposition in driving biogeochemical cycles. To make this clearer, we have amended our text as follows: “Over the past four decades, the disproportionate and sustained input of N relative to P into the China Coastal Seas has led to the transition from N to P limitation in extensive coastal areas41, which has also been observed around the globe wherever coastal seas receive significant input of dissolved inorganic nitrogen either by rivers or through atmospheric deposition. If this N deposition continues under a ‘business-as-usual’ scenario, coastal biogeochemical processes require re-examination because our data suggest that DOP utilization by phytoplankton will become increasingly important with consequences for P-cycling in coastal seas.” For constitutional uptake of DOP, please see our response to your comment 2.0 down below. 2. Lines 52–54, There is also reported constitutional uptake of DOP (e.g. refs in Fitzsimons et al 2020. DOI: 10.1016/j.jembe.2020.151434 ; Mahaffey et al. 2014. doi: 10.3389/fmars.2014.00073 Response: The term ‘constitutional uptake’ of DOP appears not to be widely used according to our literature search and conversations we have had with colleagues. However, the definition we found was used in the revised version of our manuscript (Dyhrman and Ruttenberg, 2006; Fitzsimons et al., 2020; Mahaffey et al., 2014; Sebastián et al., 2004). According to this literature (see previous sentence), APA encompasses inducible APA that is sensitive to changes in environmental conditions (such as changing DIP concentrations) and constitutive APA that appears to represent the background level of APA (Sebastián et al., 2004) if we are not mistaken. If correct indeed, constitutive APA will not be sensitive to changing DIP concentrations, and therefore can be also detected under high DIP concentrations (Sebastián et al., 2004). However, APA under these conditions (e.g., eutrophic environments) usually is very low and therefore would not have a significant influence on our data (Extended Data Fig. 2) (Dyhrman and Ruttenberg, 2006; Mahaffey et al., 2014). Nevertheless, to acknowledge this uncertainty, we have revised as follows: “The activity of AP (APA) is mainly composed of inducible APA and constitutive APA. Although there is evidence of background APA at relatively low levels (constitutive APA) independent of DIP concentrations12-14, AP usually is induced by low concentrations of DIP. Thus, the APA is usually inversely correlated with DIP concentrations, and inducible AP therefore becomes activated for the utilization of DOP when DIP concentration falls below a certain DIP threshold15-17.” 3. Line 69, Delete “furthermore” Response: Agreed. We have revised accordingly. 4. Lines 81–82, Delete sentence. Paragraph should start with ”Recent studies in oligotrophic open oceans have revealed that….” Response: Agreed. We have revised accordingly. 5. Line 86, “Our previous work…” Response: Agreed. We have revised accordingly. 6. Line 88, Change “although” to “even though” Response: Agreed. We have revised accordingly. 7. Lines 96-97, “Consequently, P has now become the main limiting nutrient. Response: Agreed. We have revised accordingly. 8. Lines 109-110, “…in biogeochemical cycles of coastal seas.” Response: Agreed. We have revised accordingly. 9. Line 129, Rewrite to correct sentence structure. Response: Agreed. We have revised as follows: “At Y3 and B1 , the Chl a concentrations were also high (3.07 – 4.47 μg·L-1), even though DIN Spr (< 1 μM) and Si(OH) (0.20 – 0.79 μM)concentrations were relatively low. This is likely because 4 the seawater was collected during a spring bloom.” 10. Line 132, corresponded Response: Agreed. We have revised accordingly. 11. Line 133, Use the indefinite article. Response: Agreed. We have revised accordingly. 12. Line 152, “…irrespective of the limiting nutrient,…” Response: Agreed. We have revised accordingly. 13. Lines 175-176, “…respectively, during the incubations.” Response: Agreed. We have revised accordingly. We have also found similar mistakes elsewhere and changed them as follows: “where Chl a and Chl a stand for the average Chl a concentration in the control and N, P, c-avg t-avg N+P, or aerosol treatment groups, respectively, over the time course during the microcosm experiments (N = 3)” “where the △DIN and △DIP represent the change of DIN and DIP concentrations, respectively, during the incubations.” “DIP utilization (△DIP) and net DOP utilization (△DOP) are calculated during the incubations from the change of DIP and DOP concentrations, respectively.” 14. Line 234, “…characterized as…” Response: Agreed. We have revised accordingly. 15. Line 250, This is interesting but what about constitutional uptake of DOP, which has been reported. This ties in with my first comment above. Response: Through the method of measuring APA, both the constitutive and inducible APA are captured. According to the general positive linear relationship between RC and Log (Chl a/DIP) (Fig. 6a), APA 10 as well as the increasing extent of APA (2% - 228%) induced by aerosol additions (Fig. 3), we conclude that the inducible uptake of DOP likely plays a dominant role in our study and not the uptake of DOP driven by constitutive APA. 16. Lines 263-267, I’m not clear about the relevance of novelty of this text; phytoplankton bloom in temperate waters and this study implies induced uptake of DOP. Response: What we have tried here to explain is that temperature can affect the overall rate of DOP utilization, but not so much its induction. Hence, temperature has an effect on the overall activity of the enzyme (e.g., Arrhenius equation) but DIP is responsible for its induction (e.g., initiation of overall activity above background level), which is the main subject of our work. To make this clearer, we have amended our text as follows: “The seawater temperature, although an important factor determining enzyme activity20, is likely not the main determining factor for DOP utilization although it has an influence due to the temperature-dependence of enzyme activity. For example, in the subtropical Atlantic at relatively high seawater temperatures (21 – 29 ℃) and off the coast of northern Oregon at relatively low seawater temperatures (11 – 12 ℃), AP was activated regardless of the water temperature when phytoplankton experienced P stress13,17.” 17. Line 317, Report the purity of the water; Milli-Q is a brand name. Response: Agreed. It was deionized water (18.2 MΩ cm). 18. Line 322, metal Response: Agreed. We have amended is accordingly. 19. Line 323 and throughout, Check that tenses are correct and consistent in the manuscript. Response: The tenses have been rectified throughout the manuscript. 20. Rees at al (2009) doi:10.1016/j.ecss.2008.12.005 observed an increase in APA in the Western English Channel and this corresponded with a salinity decrease related to increased riverine inputs, increasing the N:P ratio in the sampling area. If these inputs are riverine or atmospheric it stands to reason that DOP utilisation could be stimulated. Response: We have referred to this paper in our revised version and amended our text as follow: “These results corroborate a study in the Western English Channel where the increase in APA was associated with increased river runoff that resulted in elevated N:P ratios18. Thus, apart from the atmospheric deposition, river runoff likely will also enhance DOP utilization in coastal seas through a similar mechanism. However, air pollutants relative to river runoff will have an impact over larger geographical areas because of atmospheric long-range transport38.” 21. The last sentence on curbing nitrogen inputs is vague and could be constrained using estimates of anthropgenic inputs, such as Jickells et al (2017) doi:10.1002/2016GB005586. This could strengthen Figure 5, where N deposition is key. Response: We have referred to this paper in our revised version and amended our text as follow: “Atmospheric deposition is considered an important source of nutrients22-24 for the surface ocean, as it usually contains significant amounts of N and trace metals (e.g., Fe) but relatively low concentrations of P23,25,26.” “Our results suggest that DOP utilization by microorganisms such as phytoplankton will become more important in the future. Atmospheric N input to the ocean has significantly increased from 1850 (10 Tg N yr-1) to 2005 (39 Tg N yr-1) and is expected to remain elevated until at least 2100 (-17% - +8%)24 at a business-as-usual scenario. Thus, the Atmospheric Nitrogen Pump is at least expected to have a continued impact on the biogeochemistry of coastal seas globally, and its role might even become more prominent under increasing N deposition22 unless we reduce the input of terrigenous N-rich material such as aerosol and riverine water.” Reviewer #3: General comments: 1. In my opinion, the authors fail to convincingly demonstrate 1) that DOP utilization is stimulated by aerosol addition, 2) that APA is stimulated by aerosol addition which would be required for DOP utilization, and 3) that the driving mechanism leading to enhanced DOP utilization by phytoplankton is the alteration of the ambient DIN:DIP by aerosol addition. Response: With all due respect, our data show that APA and DOP uptake are enhanced after the addition of aerosols (Fig.3, Fig. 5a, S8). As APA is responsible for DOP uptake, we have provided a causative relationship with our work driven by the addition of N-rich aerosols depleting the pool of DIP before APA is induced to utilize DOP. The latter is possible under these circumstances because there is sufficient DIN in the system (>> DIN:DIP) due to “The Atmospheric Nitrogen Pump”. To make this clearer, we have added the following text: “These results corroborate a study in the Western English Channel where the increase in APA was associated with increased river runoff that resulted in elevated N:P ratio18. Thus, apart from the atmospheric deposition, river runoff likely will also enhance DOP utilization in the coastal seas through a similar mechanism. However, air pollutants relative to river runoff will have an impact over larger geographical areas because of atmospheric long-range transport38.” 2. River water amendment microcosms were conducted at B1 with the magnitude of the DOP utilization effect comparable to that from aerosol addition, weakening the authors conclusion that aerosols alone are important for stimulating DOP utilization in coastal seas. Response: Both atmospheric deposition and riverine water have elevated N:P nutrient ratios and are therefore regarded as important terrestrial nutrient sources for coastal seas (Jickells et al., 2017; Kim et al., 2011; Peñuelas and Sardans, 2022). The impact of atmospheric deposition covers a broader range through long-range transport, while that of rivers is more concentrated in nearshore areas (Galloway et al., 2008; Li et al., 2022). In this study, however, we tested the role of riverine water in DOP because it is a known source of DIN. Hence, it served as a control for which we already knew the outcome, but it was not the focus of our work because there is much more literature on the role of riverine N-input in coastal seas compared to the role of atmospheric deposition. Furthermore, we did not say anywhere in our manuscript that aerosols ‘alone’ are important for stimulating DOP utilization in coastal seas. In fact, our work provides first evidence that they are an additional source of N with significant consequences for our understanding of P-cycling in coastal seas around the globe. 3. As acknowledged by the authors, due to the weak support from interpreting enhanced DOP utilization in the microcosms by differences in measured DOP concentrations, the authors present a derived metric, DOP*, to support the notion that DOP utilization is enhanced by aerosol addition. DOP* represents the theoretical potential maximum in DOP utilization that might have occurred in the microcosm after computing the residual between the expected phytoplankton P demand (computed from the observed DIN utilization and Redfield N:P stoichiometry) and the observed DIP utilization. Using DOP* as the metric to gauge stimulation of DOP utilization by aerosol addition, raises the number of stations exhibiting a positive effect from 2 out of 8 to 4 out of 8, still not a very conclusive result in my opinion. The APA data presented in Figure 3 generally support the interpretations from using delta DOP or delta DOP*; i.e. in microcosms where a stimulation effect was observed for aerosol addition on DOP utilization, APA is higher and vice versa. Response: Our microcosm experiments can be divided up into three types according to the main source of phosphorous: Ⅰ. DIP was the main source of phosphorous. At E1 and Y1-DCM, DIP was the main P nutrient in control and treatment groups. Ⅱ. Equal contribution of DIP and DOP (i.e., the transition from P repletion to deficiency). At Y3 and Y2, DIP was the main P nutrient in the control group, and DOP utilization was induced and even became the main P nutrient in our aerosol treatments. Ⅲ. DOP was the main source of phosphorous. At B1 , B1 , Y1, and Y3 , DOP was the main P nutrient in our control and treatment groups. In our study, △DOP* can be activated by aerosol additions corresponding to II and III (see above) (i.e., Y3 , B1 , Y2, Y3 , and B1 ), where P deficiency occurred (Figs. 4, S8). To make this clearer, we amended the manuscript as follows: “Ⅰ. DIP was the main source of phosphorous. In experiments using eutrophic seawater with relatively high DIP concentrations (0.07 – 0.11 μM) including E1 and Y1-DCM, DIP was the main bioavailable P nutrient, and 0.04 - 0.05 μM DIP was consumed by phytoplankton in the control groups (Figs. 4a and S8a, f).” “Ⅱ. DIP and DOP were equally used as a source of phosphorous. DIP was also the main P nutrient at Y3 and Y2 (Fig. S8b, g).” “Ⅲ. DOP was the main source of phosphorous. At B1 , B1 , Y1, and Y3 , where DIP was highly deficient (0.01 – 0.03 μM), DOP contributed 82.4% - 92.5% of P and became the primary P nutrient for phytoplankton growth in the control groups (Fig. 4c). However, although DOP was still the main P nutrient in the treatment groups, no significant enhancement of △DOP was found after aerosol additions (Fig. S8c, d, e, h).” “At E1 and Y1-DCM, where DIP was the main P nutrient, △DOP* was low (-0.06 – 0.02 μM) (Fig. 4a).” “In addition, △DOP* significantly responded to aerosol additions at Y3 , B1 , Y2, Y3 , and B1 where DOP was the main P nutrient or might have become the main P nutrient after aerosol additions, of which the response was positively correlated with the amount added (Figs. 4b-c and S8).” 4. As for the conclusion that elevated DIN:DIP is responsible for elevated DOP utilization, the data in Figure 4b largely supports this by identifying a significant positive correlation between the relative APA stimulation (expressed as a percentage relative to the controls) versus DIN:DIP as the predictor variable. However, dissolved iron has also been identified as an important co-control on DOP utilization in marine surface waters (see Browning et al. 2017 Nat. Comm., Liang et al., 2022 Nat. Geo.). The authors need to demonstrate data and/or other arguments to include or rule out the potential control of enhanced DOP utilization by Fe addition from aerosols and possibly river water. Response: Dissolved iron controlling DOP utilization is primarily important in oligotrophic open oceans (Browning et al. 2017; Liang et al. 2022), and we also referred to these two papers in our manuscript. To provide further evidence, we have conducted an additional microcosm experiment while we revised our manuscript using surface seawater collected in the Yellow Sea (37.00 °N, 123.83 °E). These additional results corroborate our finding that iron additions will not enhance APA in these coastal waters but nitrogen (Fig. S4a – see below). These new results have been added to the supplementary information (Fig. S4). Additional text has been added to the main manuscript as follow: “Although the role of essential trace metals such as Zn and Co for the APA was not tested in our study, the APA did not significantly respond to Fe or N+Fe additions (Extend Data Fig. 1 and Fig. S4), and the APA in the aerosol treatment was comparable to those of the riverine water treatment (Figs. 3b-c and S5a-b), suggesting that other trace metals usually enriched in aerosols and riverine water were likely not having a substantial impact on our results.” 5. The authors should report the limit of detection and analytical uncertainty for DOP determinations after considering the additive uncertainties from the separate determinations of TDP and DIP. The precision on TDP analyses is unreported so it is impossible as a reviewer to determine if the delta DOP values reported in Figure 3 on the order of -30 to +90 nM are even larger than the analytical uncertainty on DOP determinations. Response: We have added the error margins of DIP (1.39% of the average value of 0.06 μM) and TDP (0.77% of the average value of 0.20 μM) analyses which were calculated by the relative standard deviation of five replicates. The detection limit for DOP analysis (0.025 μM) was calculated by the sum of those of DIP and TDP (DL + DL ). The error margins for DOP analysis (1.27% of the average value of 0.14 μM) were calculated using the standard deviations (SD) and average values (avg) of five replicates for TDP and DIP analysis. Detailed modifies are showed as follows: “The error margin for the DIP analysis was 1.39% of the average value of 0.06 μM, which represents the relative standard deviation of five replicates.” “The detection limit of TDP was 0.02 μM, and the error margins for TDP analysis was 0.77% of the average value of 0.20 μM through the same calculating method as that of DIP” “DOP was estimated by subtracting DIP from TDP. The detection limit (0.025 μM) for DOP analysis was calculated by the sum of those of DIP and TDP (DL + DL ). The error margin for DOP analysis (1.27% of the average value of 0.14 μM) was calculated using the standard deviations (SD) and average values (avg) of five replicates for TDP and DIP analysis.” Lastly, I am excited by the identified log10(chl/DIP) metric for predicting APA and potentially DOP utilization (Fig. 5). I believe many marine biogeochemists will find its predictions at the global scale to be useful in helping predict the role of DOP in the local to regional biogeochemistry across the global ocean. Response: Thank you very much. We are also excited about such relationship, because data about DOP utilization in the global coastal ocean is relatively scarce (Lin et al., 2016; Burt et al., 2018; Shao et al., 2023). Thus, we hope our study is a valuable contribution shedding new light on the role of atmospheric N deposition in biogeochemical cycles of coastal seas."}, {"round": 2, "reviewer_comments": {"reviewer_1": "RE-REVIEW of a manuscript enfitled \"Atmospheric aerosols sfimulate the ufilizafion of dissolved organic phosphorus in coastal seas\" by Haoyu Jin et al. In the revised manuscript by Jin and his colleagues demonstrates comprehensive response to the concerns raised by both myself and another referee. I am safisfied with the revisions they have implemented., and I no longer have any reservafions that would impede the publicafion of this paper. This manuscript offers valuable insights into the P cycle, with far-reaching implicafions for the carbon cycle in the northwestern Pacific.", "reviewer_3": "I appreciate the effort the authors have taken to consider my earlier concerns about the study. I am especially appreciafive of the considerable extra effort involved to rapidly conduct another field study to specifically address the role of Fe nutrifional status on APA and DOP ufilizafion in their study area. I am now generally supporfive of publicafion but not addifion issues that need to be addressed by the authors. 1) Why do the authors compare the APA data to N* in Fig. S10 and elsewhere and not P*? P* as defined in Deutsch et al., 2007 Nature represents ‘phosphorus excess’ relafive to nitrogen and Redfield phytoplankton requirements. Marine algae should increasingly turn to DOP to safisfy P demand as P* decreases, which should drive up APA. Framing in terms of N* or ‘nitrogen excess’ seems unintuifive to me. P* has been used to infer DOP accumulafion versus consumpfion across the open ocean before in this manner (Liang et al., 2022 Nature Geoscience). Also I don’t believe that the authors have computed N* correctly in the revised manuscript. N* is defined as N* = [NO3] – 16*[PO4] when ignoring the scaling constants as introduced in Gruber & Sarmiento, 1997 GBC. The authors compute N* as ([NO3]/16) – [PO4]. I encourage revising of the use of N* as a predictor variable in diagnosing APA or delta DOP pafterns. 2) The new Figure 2 capfion is lacking a descripfion of subpanel b. It is also unclear to me from reading the capfion and looking at the figure what the colored boxes represent. Are you presenfing a box and whiskers plot? Why are the data points and the colored bars different colors within a stafion? Please consider revising the figure or its capfion for clarity. 3) I am supporfive of the reframing of the results into 3 groups of stafions based on contribufions of DIP and DOP to overall P demand and the associated Fig. 4. In fact it is this reframing that has helped be befter interpret the study results and befter convince me of the study’s conclusions. I think reworking Fig. 3 to also group by P demand would also aid in interpretafion. 4) Thank you for providing the key informafion regarding the measurement accuracy and precision for TDP, SRP, and DOP determinafions. 5) My earlier comment concerning river vs aerosols addifions as controls on sfimulafing DOP ufilizafion and APA was mofivated by the study’s fitle. My interpretafion of the fitle is that aerosol addifions alone are the driving mechanism that alter the DIN:DIP rafio of coastal waters driving the system towards P limitafion and thus DOP ufilizafion. But your river amendment results and associated discussion in the text suggest that river inputs act in the same manner to alter the DIN:DIP. Thus I am supporfive of a more inclusion fitle that highlights the role of aerosols and river inputs working in tandem in the coastal ocean to alter the DIN:DIP towards P limitafion and DOP ufilizafion."}, "author_response": "Response to reviewers’ comments. Text in italics (copied from revised manuscript) represents how we have addressed the reviewers’ suggestions in our revised manuscript. Our response in blue. Reviewer #1 (Remarks to the Author): RE-REVIEW of a manuscript entitled \"Atmospheric aerosols stimulate the utilization of dissolved organic phosphorus in coastal seas\" by Haoyu Jin et al. In the revised manuscript by Jin and his colleagues demonstrates comprehensive response to the concerns raised by both myself and another referee. I am satisfied with the revisions they have implemented., and I no longer have any reservations that would impede the publication of this paper. This manuscript offers valuable insights into the P cycle, with far-reaching implications for the carbon cycle in the northwestern Pacific. Response: Thank you very much for your positive response. Reviewer #3 (Remarks to the Author): I appreciate the effort the authors have taken to consider my earlier concerns about the study. I am especially appreciative of the considerable extra effort involved to rapidly conduct another field study to specifically address the role of Fe nutritional status on APA and DOP utilization in their study area. I am now generally supportive of publication but not addition issues that need to be addressed by the authors. 1) Why do the authors compare the APA data to N* in Fig. S10 and elsewhere and not P*? P* as defined in Deutsch et al., 2007 Nature represents ‘phosphorus excess’ relative to nitrogen and Redfield phytoplankton requirements. Marine algae should increasingly turn to DOP to satisfy P demand as P* decreases, which should drive up APA. Framing in terms of N* or ‘nitrogen excess’ seems unintuitive to me. P* has been used to infer DOP accumulation versus consumption across the open ocean before in this manner (Liang et al., 2022 Nature Geoscience). Also I don’t believe that the authors have computed N* correctly in the revised manuscript. N* is defined as N* = [NO3] – 16*[PO4] when ignoring the scaling constants as introduced in Gruber & Sarmiento, 1997 GBC. The authors compute N* as ([NO3]/16) – [PO4]. I encourage revising of the use of N* as a predictor variable in diagnosing APA or delta DOP patterns. Response: Thank you for your additional comments to improve our work further. We calculated N* because reviewer #1 suggested that in the first-round of revision. It tries to shed light on the impact of P deficiency on APA, based on the inverse relationship between APA and DIP concentrations. Upon recalculating N* following your advice, we found that it still exhibits a weak correlation with the relative change of APA after aerosol addition (Fig. S10a). Similarly, we implemented the reviewer’s advice to calculate P* in terms of DOP accumulation or consumption, but found no significant correlation between the relative change of APA and P*. Considering the well-established relationship between relative change of APA and Log10(Chl a/DIP) in this study, the above results also support the important role of biomass (Chl a) in affecting DOP utilization. Therefore, we combined N* and P* in Fig. S10 and added the explanation as follows: “In addition, the relative change of APA was not significantly correlated with N* (DIN-16×DIP) in terms of P deficiency or P* (DIP-DIN/16) in terms of DOP consumption or accumulation42,65 (Fig. S10), suggesting the important role of phytoplankton biomass (Chl a) in affecting DOP utilization in coastal seas” 2) The new Figure 2 caption is lacking a description of subpanel b. It is also unclear to me from reading the caption and looking at the figure what the colored boxes represent. Are you presenting a box and whiskers plot? Why are the data points and the colored bars different colors within a station? Please consider revising the figure or its caption for clarity. Response: We are presenting a box plot to show the limiting nutrient and the distribution of △R across different nutrient additions. The coloured dots indicate △R in different nutrient treatment groups, and the coloured boxes indicate the limiting nutrients (blue = N limitation, yellow = P limitation, red = N+P colimitation, and white = not significant). To make it clearer, we have modified Fig. 2 and its caption as follows: Fig. 2 | Response of Chl a concentration to nutrient, aerosol and riverine water additions expressed by △R. △R (log (Chl a /Chl a )/t) is the Log ratio of the 10 t-avg c-avg 10 average Chl a concentration during the incubations in treatment relative to control groups. A) △R in nutrient addition experiments. The coloured dots indicate △R in different treatment groups. The colour in boxes indicates the limiting nutrients (blue = N limitation, yellow = P limitation, red = N+P colimitation, and white = not significant) (p < 0.05). The lines across the boxes represent 25th (bottom) and 75th (top) percentiles. The whisker caps represent minimum (bottom) and maximum (top) values. Significant level was calculated using one-way ANOVA and Turkey HSD post hoc test. B) △R in aerosol and riverine water addition experiments. The lines across the boxes represent 25th (bottom), 50th (middle), and 75th (top) percentiles. The whisker caps represent minimum (bottom) and maximum (top) values. Significant differences (p < 0.05) between △R in spring and summer are indicated with a connecting line and p value (unpaired t-test). 3) I am supportive of the reframing of the results into 3 groups of stations based on contributions of DIP and DOP to overall P demand and the associated Fig. 4. In fact it is this reframing that has helped be better interpret the study results and better convince me of the study’s conclusions. I think reworking Fig. 3 to also group by P demand would also aid in interpretation. Response: We have revised Fig. 3 as follows: Fig. 3 | The activity of alkaline phosphatase (APA) in response to aerosol addition. Significant differences between different groups at the same day of the incubations are indicated by different letters and NS = not significant (one-way ANOVA and Turkey HSD post hoc test). Error bars indicate standard deviations; N=3. 4) My earlier comment concerning river vs aerosols additions as controls on stimulating DOP utilization and APA was motivated by the study’s title. My interpretation of the title is that aerosol additions alone are the driving mechanism that alter the DIN:DIP ratio of coastal waters driving the system towards P limitation and thus DOP utilization. But your river amendment results and associated discussion in the text suggest that river inputs act in the same manner to alter the DIN:DIP. Thus I am supportive of a more inclusion title that highlights the role of aerosols and river inputs working in tandem in the coastal ocean to alter the DIN:DIP towards P limitation and DOP utilization. Response: We have changed “atmospheric nitrogen pump” to “anthropogenic nitrogen pump” and revised the manuscript wherever we did not mention the role of rivers. This includes the title, abstract, introduction, results, discussion, and references. We have also amended figure 7 to include the importance of rivers. Please see our version with tracked changes for all details and amendments. Many thanks for this suggestion. It makes sense and it broadens the reach of our paper."}]

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