Significant and conservative long-range transport of dissolved organic nutrients in the Changjiang diluted water

The Changjiang River is one of the main nutrient sources in the northwestern Pacific marginal seas. However, most of the previous studies have neglected the long-range transport (>200 km) of riverine nutrients since they are rapidly consumed. In this study, we examined the long-range transport (200–800 km) of nutrients in the surface layer during the summer of 2017. The plots of nutrients against salinity display that dissolved organic nitrogen (DON) was conservative over ~800 km, while more than 99% of the dissolved inorganic nitrogen (DIN) was removed within 200 km. As a result, in the study region, DON concentrations (avg. 7.0 ± 1.3 µM), which are minor in the river water, were much higher than DIN concentrations (avg. 0.28 ± 0.26 µM). Both nutrients, N and P, showed a similar pattern. Our results suggest that dissolved organic nutrients play a critical role on the long-range transport of riverine nutrients in surface waters and subsequent ecosystem changes.

Sampling and analyses. Surface seawater samples were collected during two periods in the East China Sea ( Fig. 1) from August 16 to 19, 2017 on the R/V Ara of Jeju National University, Korea, and from August 22 to September 6, 2017 on the R/V Tamgu-3 and the R/V Tamgu-8 of National Institute of Fisheries Science (NIFS), Korea. Seawater samples were collected using Niskin bottles mounted on a CTD rosette sampler. Salinity was measured using a portable sensor (Orion star A329, Thermo Scientific).
Seawater samples for nutrients were filtered through pre-combusted (450 °C, 4 h) glass-fiber filters (Whatman GF/F, 47 mm in diameter, 0.7 µm in pore size) and stored frozen (−20 °C) until analysis. Dissolved inorganic nutrients, including NH 4 + , NO 2 − , NO 3 − , and PO 4 3− , were analyzed using a nutrient auto-analyzer (New QuAAtro39, SEAL Analytical). Reference seawater materials of nutrients (KANSO Technos, Tsukuba, Japan) were run for verification of analyses. The detection limits for NH 4 + , NO 2 − , NO 3 − , and PO 4 3− were 0.03 µM, 0.01 µM, 0.02 µM, and 0.01 µM, respectively. In this study, the sum of NH 4 + , NO 2 − , and NO 3 − is considered as DIN, and PO 4 3− is considered as DIP. Total dissolved nitrogen (TDN) and phosphorus (TDP) analyses were determined using the method described by Grasshoff et al. 23 . Briefly, an acid potassium persulfate (N and P analysis grade, Wako) solution was added to a filtered seawater sample, and then autoclaved at 120 °C for 30 min. The resulting NO 3 − and PO 4 3− were measured with the auto-analyzer as described above. The accuracy of the TDN concentrations was verified using the deep-seawater reference material (32-33 µM; University of Miami, USA). The concentrations of DON and DOP were calculated by subtracting the measured DIN and DIP concentrations from the measured TDN and TDP concentrations, respectively.

Results and Discussion
Distributions of salinity and dissolved nutrients in the East China Sea. The salinity ranged from 25.33 to 33.99 (avg. 29.79 ± 1.99) in the surface waters of the study region (Supplementary Table S1). The salinity distribution in the surface waters shows that the low-salinity (<31) water is widely distributed over the East China Sea (Fig. 1), due to the influence of CDW 24 . A patch of low-salinity water (<28) was observed in the southwestern region of Jeju Island, which seems to be detached from the Changjiang plume 21,25 . In general, higher concentrations of nutrients were observed in the lower salinity areas, including the low-salinity patch area (Fig. 2).
The concentrations of nutrients in the study region, including all previous data observed in this region, were plotted against the distance from the estuary (Fig. 3). It is clear that DIN and DIP concentrations decreased  Table S1). This trend suggests that the contributions of DON and DOP to the TDN and TDP were dominant (about 96 ± 3% for TDN and about 78 ± 10% for TDP) in the study region, although they were minor in the source river water. Behaviors of dissolved nutrients during the long-range transport. In order to understand the behaviors of nutrients over the long-range transport, the concentrations of DIN, DIP, DON, and DOP were plotted against salinities (Fig. 4). The concentrations of DIN and DIP show conservative mixing patterns for low-salinity waters (salinity < 20). They were then sharply removed and depleted in the study region, depending on the distance from the estuary, mainly between 100 km and 200 km (Figs. 3 and 4). The conservative mixing patterns in the low-salinity water have been reported by Yao et al. 16 and Chen et al. 17 . Zhang 29 attributed the conservative behavior of DIN and DIP in this zone to low primary production (chlorophyll a < 5 µg L −1 ) owing to high total suspended matter (up to 1300 mg L −1 ), causing limited light penetration, up to a distance of ~50 km off the river mouth. Zhang 29 also showed a sharp increase in chlorophyll a concentration up to 20 µg L −1 at a distance of approximately 100 km off the Changjiang estuary, followed by a rapid decrease toward the open ocean. As such, the concentrations of DIN (0.28 ± 0.26 µM) and DIP (0.07 ± 0.04 µM) in the study region were much lower the expected concentrations of DIN (11.2 ± 6.7 µM) and DIP (0.16 ± 0.09 µM) from conservative mixing (Fig. 4). This seems to be associated with rapid consumption of DIN and DIP by phytoplankton. In general, the consumed nutrients are known to be mostly re-mineralized in the subsurface layer in this region 13 . In contrast to DIN and DIP, the concentrations of DON (7.0 ± 1.3 µM) and DOP (0.24 ± 0.08 µM) measured in the study region were similar to the expected concentrations of DON (6.7 ± 0.8 µM) and DOP (0.38 ± 0.02 µM) from conservative mixing (Fig. 4). This result indicates that organic nutrients are conservatively mixed from the river mouth water to the offshore water up to 800 km (Figs. 3 and 4). The arrival time of CDW from the Changjiang River mouth to the vicinity of Jeju Island (~450 km) was estimated to be 20-35 days, based on the horizontal distribution of 223 Ra activities 24 . During this transport time, the degradation of dissolved organic nutrients should be insignificant considering the fact that only approximately 7-11% of the riverine dissolved organic carbon (DOC) in seven river estuaries of the southeastern USA is decomposable in a 100 day timeframe due to  19 . The blue filled symbols denote DON and DOP concentrations in the Changjiang River obtained from Liu et al. 26,27 and Shen and Liu 28 . The distances were calculated between Chongming Island (located in the Changjiang estuary) and each sampling station using the latitude and longitude of those points.  30,31 . Similarly, the production of dissolved organic nutrients might be insignificant (~10% relative to the DON change of ~14 µM) if the known production rate of DON (~2 nmol N L −1 h −1 ) in eutrophic oceans (i.e., Southern California Bight, shelf break of Atlantic off Spain, and Chilean upwelling region) 32-34 is applied. In addition, the atmospheric deposition of DON by precipitation or vertical mixing seems to be negligible considering that the concentrations of DON in the surface water in the study region, precipitation 35 , and the subsurface water in this region 13 are similar under a strong vertical stratification in summer. Thus, we conclude that the other sources and sinks cannot influence significantly the distributions of DON and DOP in the study region and perhaps are included in the scattering of the plots (Fig. 4).
In order to estimate the removal of nutrients in the mixing zone (0-800 km), we calculated the conservative mixing proportions between river water and pristine open-ocean seawater in the study region. The end-member concentrations of DIN, DIP, DON, and DOP were assumed to be 153 µM, 2.6 µM, 20 µM, and 0.54 µM for river waters 19,[26][27][28]   The proportions of DON and DOP against TDN and TDP observed in the study region were comparable to those observed in major oligotrophic oceans, such as the eastern subtropical North Pacific Ocean (DON: 5-6 µM, 96-99% of TDN, DOP: 0.1-0.35 µM, 75-90% of TDP) 36 , South Pacific Ocean (DON: 5.5-6 µM, 100% of TDN, DOP: 0.1-0.25 µM, 20-50% of TDP) 37 , and Atlantic Ocean (DON: 3.5-6.5 µM, 55-100% of TDN, DOP: 0.07-0.43 µM, 67-100% of TDP) 38 . Thus, the role of DON and DOP concentrations, together with their ratios, in the study region may be similar to that in other major oceans. For example, diatoms dominate in the inshore waters (salinity < 31 and DIN > 10 µM) due to the strong influence of the Changjiang plume in the East China Sea, while the density of dinoflagellates increases gradually with distance away from the diatom dominated zone 20 . This could happen because many dinoflagellate species are capable of taking up organic nutrients 39,40 , although this process is limited for diatoms 39,41 .

Conclusions
The concentrations of riverine DIN and DIP decreased sharply from the Changjiang River estuary and depleted within 200 km from the river mouth, and the concentrations of DIN and DIP were about 0.28 µM and 0.07 µM, respectively, between 200 and 800 km. This seems to be associated with rapid consumption of inorganic nutrients by phytoplankton. In contrast, DON and DOP exhibited conservative mixing behaviors up to 800 km, and their concentrations were about 7.0 µM and 0.24 µM, respectively, between 200 and 800 km. Therefore, DON and DOP account for about 96% and 78% of TDN and TDP, respectively, between 200 and 800 km, although their proportions were minor in the river water. Our results suggest that dissolved organic nutrients play a critical role on riverine nutrient transport to the surface waters of the open ocean. Further studies are necessary to determine the role of riverine nutrient composition changes on ecosystem changes in the remote oceans. Data Availability. The datasets analysed during the current study are available from the corresponding author on reasonable request.