Dissolved Inorganic Phosphorus Concentrations Research Articles

Traditional land use effects on nutrient export from watersheds to coastal seas

Land use features can have a significant impact on the flux and stoichiometry of nutrients from the watershed to the sea along the freshwater continuum. Traditional agricultural watersheds designated as a Globally Important Agricultural Heritage System (GIAHS) may have unique features on nutrient dynamics. Here, we conducted seasonal, high-resolution spatial surveys for concentrations of dissolved inorganic phosphorus (DIP) and nitrogen (DIN) and stable isotopes of water and nitrate of the GIAHS in Japan (Kunisaki Peninsula). The highest upstream DIN concentrations (mean = 83 μM) were observed in the non-irrigation period in autumn. 17O isotopes of nitrate suggested that microbial nitrification in forest soil was the primary source (> 95%) of total nitrate. Spatial differences in DIP and DIN concentrations among the upstream, midstream, and downstream waters were small, with high DIN/DIP ratios (> 50) were exported from the watershed to coastal seas. Nutrient concentrations exhibited different dynamics during summer when irrigation was taking place with average DIP concentrations increasing from upstream (0.5 μM) to downstream (2.1 μM) waters. A generalized linear model showed that the proportion of paddy fields, cultivated land, residential area as well as forest were the primary drivers of DIP concentrations. DIN concentrations decreased moving downstream likely due to denitrification in the anaerobic irrigation ponds and paddy fields. This study showed that the traditional agricultural landscapes mitigated the excess DIN loading from forested areas while supplying the essential DIP for coastal production, suggesting an important forest-river-sea connection system that manages nutrient inputs to coastal waters.

Influences of phosphorus concentration and porewater advection on phosphorus dynamics in carbonate sands around the Weizhou Island, northern South China Sea

A series of flow-through reactor experiments were undertaken to assess the potential effect of porewater advection and dissolved inorganic phosphorus (DIP) concentration on benthic DIP dynamics in permeable sediments collected from the Weizhou Island, northern South China Sea. The flux of DIP ranged from −0.13 to 0.05 mmol m−2 h−1, and the reversal from DIP efflux to influx occurred when the DIP concentration reached a threshold. DIP release from the sediment into the seawater peaked at intermediate advection rate, which perhaps provide optimum conditions for DIP release related to CaCO3 dissolution. Phosphorus limitation in seawater could be relieved by DIP release from the sediment, and CaCO3-bound P in carbonate sands may play a major role in benthic DIP release and decrease in the molar nitrogen/phosphorus ratio in seawater around the Weizhou Island.

Spatiotemporal river flux and composition of nutrients affecting adjacent coastal water quality in Hainan Island, China

Human activities have changed global nutrients cycling and appreciably altered marine ecosystems in riverine-estuarine systems. This is evident by the significant shifts in the supply of nitrogen and phosphorus. However, very little is known about the concentrations and fluxes and of these nutrients and their seasonal changes in Hainan Island, China. We examined Hainan Island, which is the second largest island in China, to assess the spatiotemporal concentrations and fluxes of nutrients using seasonal data on a whole-of-system scale covering river estuaries and adjacent coastal water (2017). The annual average total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) concentrations were 77.23 ± 31.36 μmol/L and 3.25 ± 1.67 μmol/L, respectively. The annual fluxes of pollutants from the discharge of coastal rivers on Hainan Island into coastal waters were 1.8 × 104 t for TDN and 1.1 × 103 t for TDP, respectively. The month with the largest contribution to the annual pollutants load was October, which contributed 13.60% of TDN and 17.79% of TDP. In addition, the largest river fluxes were Nandu River, which contributed 7.5 × 103 t of TDN and 364 t of TDP. The annual average concentrations of pollutants in the surface coastal water were 7.60 ± 10.71 μmol/L for dissolved inorganic nitrogen (DIN) and 0.24 ± 0.31 μmol/L for dissolved inorganic phosphorus (DIP). The annual average N/P ratios in coastal rivers and seawater were 30 and 65, which were significantly higher than the Redfield ratio (N/P = 16). The DIN and DIP concentrations of most monitoring stations met the national first-class seawater standards, but the problem of nutrient pollution in some local coastal water areas was still outstanding. The coastal water quality monitoring should consider the influence of tropical storms influence on Hainan Island. The effective improvement of coastal water quality should be based on land–ocean coordination monitoring of coastal water quality and the riverine nutrient flux of Hainan Island.

Groundwater‐controlled phosphorus release and transport from sandy aquifer into lake

AbstractAwareness of groundwater‐borne dissolved inorganic phosphorus (DIP) loadings into lakes and its role in lake eutrophication is increasing, albeit DIP of natural origin is often ignored. Release of geogenic DIP from an adjacent aquifer and its transport with groundwater into a eutrophic lake is described by combining hydrogeochemical data collected in this study (sampling of piezometers, hydrogeochemical profiles, and seepage meters) with groundwater flow and discharge rates from earlier studies. The major part of the DIP that entered the lake with discharging groundwater was mobilized from iron hydroxides reduced by organic matter buried in the sediments of the old lake bottom. This is indicated by the correlation between DIP and ferrous iron (Fe2+) concentrations, with a DIP/Fe2+ molar ratio of 0.06, and an increase in pH. One‐dimensional reactive transport modeling indicated that high discharge rates (> 0.1 m d−1) of anoxic groundwater upwelling in areas adjacent to the lakeshore prevent downward diffusion of oxygen into the aquifer and do not leave enough time for DIP to become rebounding to the mineral phases at the sediment‐water interface. The groundwater‐controlled DIP input into the lake calculated along a two‐dimensional cross‐section averaged 0.01 mol DIP m−2 yr−1. A 2 m wide offshore high discharge zone delivered approximately 13% of the DIP into the lake. The continuous, external loading of geogenic DIP sustains lake eutrophication and explains the failure of two previous lake restoration attempts.

Exponential decay of between-month spatial dissimilarity congruence of phytoplankton communities in relation to phosphorus in a highland eutrophic lake.

Phytoplankton species composition has long been recognized to be structured by environmental filtering, but our knowledge of patterns of spatial dissimilarity congruence between the phytoplankton community and environmental divers is rather limited. Specifically, a study on whether there are specific temporal properties that could be more related to spatial dissimilarity remains to be seen. We examined the extent to how spatial dissimilarity changed with seasonal succession by measuring β-diversity in phytoplankton communities in Lake Erhai (from January 2012 to December 2014 at 15 sampling sites) as a function of different period conditions (high-density period and low-density period). We found that congruences of spatial dissimilarity in algal communities over time were neither stable in time nor showed a seasonal pattern. The spatial dissimilarity congruence between the phytoplankton community and dissolved inorganic phosphorus (DIP) concentration followed exponential decay patterns, and this congruence was led by algal cell density. This result implies that species and functions of phytoplankton are specialized, and DIP concentration drastically increases in high-density periods than in low-density periods. This means that DIP enrichment is related to the loss of algal diversity and functions and the increase of algal biomass in eutrophic lakes.

Formation mechanism of the Microcystis aeruginosa bloom in the water with low dissolved phosphorus.

The utilization of phosphorus by algae in the low-phosphorus state has drawn wide concerns due to the high risk of forming algal blooms. The cyanobacteria Microcystis aeruginosa (M. aeruginosa) grew well under low-phosphorus condition by hydrolyzing dissolved organic phosphorus (DOP) to dissolved inorganic phosphorus (DIP) through alkaline phosphatase (AP). There was a negative correlation between DIP concentration and AP activity of algae. AP activity significantly increased at 0-3 d (p < 0.05), and reached the peak values of 43.06 and 49.11 King unit/gprot on day 5 for DIP (0.1 mg/L) and DOP (4.0 mg/L), respectively. The relative expression of phosphate transporter gene increased with decreasing phosphorus concentrations. The catalase activity under low-phosphorus condition increased significantly (p < 0.05) after one week, and was generally higher than 0.15 U/mgprot on day 14. Understanding the utilization efficiency and mechanism of DIP and DOP in the low-phosphorus state would help to inhibit the formation of algal blooms.

Use of the high-affinity phosphate transporter gene, pstS, as an indicator for phosphorus stress in the marine diazotroph Crocosphaera watsonii (Chroococcales, Cyanobacteria).

The marine diazotroph Crocosphaera watsonii provides fixed carbon (C) and nitrogen (N) to open-ocean regimes, where nutrient deficiency controls productivity. The growth of Crocosphaera can be limited by low phosphorus (P) concentrations in these oligotrophic environments. Biomarkers such as the high-affinity ABC transporter phosphate-binding gene, pstS, are commonly used to monitor when such organisms are under P stress; however, transcriptional regulation of these markers is often complex and not well-understood. In this study, we interrogated changes in pstS transcript levels in C.watsonii cells under P starvation, and in response to added dissolved inorganic phosphorus (DIP), dissolved organic phosphorus (DOP), and changing light levels. We observed elevated relative pstS transcript levels in C.watsonii WH8501 at DIP concentrations below 60 and above 20nmol·L-1 . Transcript levels were suppressed by both inorganic and bioavailable organic phosphorus; however, the P stress response was more sensitive to DIP than DOP sources. Increasing light intensity resulted in increased relative pstS transcript abundances independently of low external P, and seemed to exacerbate the physiological effects of P stress. The variable response to different P compounds and rapid and transient influence of high light on pstS transcript abundances suggests that pstS is an indicator of internal P status in Crocosphaera.

Diazotrophy as the main driver of the oligotrophy gradient in the western tropical South Pacific Ocean: results from a one-dimensional biogeochemical–physical coupled model

Abstract. The Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE) cruise took place in the western tropical South Pacific (WTSP) during the austral summer (March–April 2015). The aim of the OUTPACE project was to investigate a longitudinal gradient of biological and biogeochemical features in the WTSP, and especially the role of N2 fixation in the C, N, and P cycles. Two contrasted regions were considered in this study: the Western Melanesian Archipelago (WMA), characterized by high N2 fixation rates, significant surface production and low dissolved inorganic phosphorus (DIP) concentrations, and the South Pacific Gyre (WGY), characterized by very low N2 fixation rates, surface production and high DIP concentrations. Since physical forcings and mixed layer dynamics in both regions were similar, it was considered that the gradient of oligotrophy observed in situ between the WMA and WGY was not explained by differences in physical processes, but rather by differences in biogeochemical processes. A one-dimensional physical–biogeochemical coupled model was used to investigate the role of N2 fixation in the WTSP by running two identical simulations, only differing by the presence (simWMA) or absence (simWGY) of diazotrophs. We showed that the nitracline and the phosphacline had to be, respectively, deeper and shallower than the mixed layer depth (MLD) to bring N-depleted and P-repleted waters to the surface during winter mixing, thereby creating favorable conditions for the development of diazotrophs. We also concluded that a preferential regeneration of the detrital phosphorus (P) matter was necessary to obtain this gap between the nitracline and phosphacline depths, as the nutricline depths significantly depend on the regeneration of organic matter in the water column. Moreover, the model enabled us to highlight the presence of seasonal variations in primary production and P availability in the upper surface waters in simWMA, where diazotrophs provided a new source of nitrogen (N) to the ecosystem, whereas no seasonal variations were obtained in simWGY, in the absence of diazotrophs. These main results emphasized the fact that surface production dynamics in the WTSP is based on a complex and sensitive system which depends on the one hand on physical processes (vertical mixing, sinking of detrital particles), and on the other hand on biogeochemical processes (N2 fixation, remineralization).

Phosphorus Fluxes from Three Coastal Watersheds under Varied Agriculture Intensities to the Northern Gulf of Mexico

This study aims to evaluate recent total phosphorus (TP) and dissolved inorganic phosphorus (DIP) transport from three coastal rivers—the Calcasieu, Mermentau, and Vermilion Rivers—that drain watersheds with varied agriculture intensities (21%, 67%, and 61%, respectively) into the northern Gulf of Mexico, one of the world’s largest summer hypoxic zones. The study also examined the spatial trends of TP and DIP from freshwater to saltwater along an 88-km estuarine reach with salinity increasing from 0.02 to 29.50. The results showed that from 1990–2009 to 2010–2017, the TP fluxes for one of the agriculture-intensive rivers increased while no significant change was found for the other two rivers. Change in river discharge was the main reason for this TP flux trend. The two more agriculture-intensive river basins showed consistently higher TP and DIP concentrations and fluxes, as well as higher DIP:TP ratios than the river draining less agriculture-intensive land, confirming the strong effect of land uses on phosphorus input and speciation. Longitudinal profiles of DIP along the salinity gradient of the estuarine reach displayed characteristic input behavior. Desorption of DIP from suspended solids and river bed sediments, urban inputs, as well as stronger calcium carbonate and phosphorus co-precipitation at the marine endmember could be the reasons for such mixing dynamics.

Distributions of particulate and dissolved phosphorus in aquatic habitats of Peninsular Malaysia

Particulate phosphorus was the dominant phosphorus species and accounted for 72 ± 5% of total phosphorus in coastal habitats, 63 ± 4% in estuaries, 58 ± 6% in lakes and 80 ± 7% in aquaculture farms whereas dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP) were minor components. Correlation analyses (DIP vs Chl a; R2 = 0.407, df = 31, p < 0.001) suggested phosphorus limiting conditions in lakes, which was corroborated with the highest alkaline phosphatase activity (APA) that fluctuated from 0.38 to 41.14 nmol L−1 min−1. In contrast, APA was elevated in coastal habitats and estuaries only when DIP concentration decreased below 0.9 μM. Moreover size-fractionation experiment showed that the highest APA was detected in the 0.2–2 μm pico-size fraction. Our results suggested that the main APA in coastal habitats and estuaries was from phototrophic pico-eukaryotes and heterotrophic bacteria, and regulated largely by DIP availability.

Uptake kinetics and storage capacity of dissolved inorganic phosphorus and corresponding N:P dynamics in Ulva lactuca (Chlorophyta).

Dissolved inorganic phosphorus (DIP) is an essential macronutrient for maintaining metabolism and growth in autotrophs. Little is known about DIP uptake kinetics and internal P-storage capacity in seaweeds, such as Ulva lactuca (Chlorophyta). Ulva lactuca is a promising candidate for biofiltration purposes and mass commercial cultivation. We exposed U.lactuca to a wide range of DIP concentrations (1-50μmol·L-1 ) and a nonlimiting concentration of dissolved inorganic nitrogen (DIN; 5,000μmol·L-1 ) under fully controlled laboratory conditions in a "pulse-and-chase" assay over 10d. Uptake kinetics were standardized per surface area of U.lactuca fronds. Two phases of responses to DIP-pulses were measured: (i) a surge uptake (VS ) of 0.67±0.10μmol·cm-2 ·d-1 and (ii) a steady state uptake (VM ) of 0.07±0.03μmol·cm-2 ·d-1 . Mean internal storage capacity (ISCP ) of 0.73± 0.13μmol·cm-2 was calculated for DIP. DIP uptake did not affect DIN uptake. Parameters of DIN uptake were also calculated: VS =12.54± 1.90μmol·cm-2 ·d-1 , VM =2.26±0.86μmol· cm-2 ·d-1 , and ISCN =22.90±6.99μmol·cm-2 . Combining ISC and VM values of P and N, nutrient storage capacity of U.lactuca was estimated to be sufficient for ~10d. Both P and N storage capacities were filled within 2d when exposed to saturating nutrient concentrations, and uptake rates declined thereafter at 90% for DIP and at 80% for DIN. Our results contribute to understanding the ecological aspects of nutrient uptake kinetics in U.lactuca and quantitatively evaluating its potential for bioremediation and/or biomass production for food, feed, and energy.

Concentrations and Compositions of Different Forms of Nitrogen and Phosphorus in Atmospheric Aerosols in the Qingdao Coastal Region and over the Yellow and Bohai Sea

The total suspended particulate (TSP) samples were collected in the Qingdao coastal region and over the Yellow and Bohai Sea from June to July in 2016. The diurnal and nightly TSP samples were also continuously collected in the Qingdao coastal region from August 6 to 15. The concentrations of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), dissolved total nitrogen (DTN), dissolved total phosphorus (DTP), total nitrogen (TN), and total phosphorus (TP) in the TSP samples were analyzed. Results showed that the concentrations of different forms of nitrogen and phosphorus in Qingdao were higher than those over the Yellow and Bohai Sea during the same sampling period. The contribution of dissolved N was similar to that of insoluble N to TN in Qingdao, with the ratio of DTN to TN of 56%. However the DTN was the dominant contributor of TN over the Yellow and Bohai Sea, accounting for 72% of TN on average. The inorganic nitrogen was the dominant species of DTN in Qingdao and over the Yellow and Bohai Sea (YBS), accounting for 67% and 75% of DTN, respectively. The contribution of dissolved P to TP was similar to that of insoluble P to TP in Qingdao and over the Yellow and Bohai Sea, and the DTP accounted for 49% and 58% of TP in Qingdao and over YBS, respectively. The ratio of IP to DTP was slightly higher than that of OP, with values of 56% and 59% in Qingdao and over the YBS, respectively. The origin of the air mass affected the concentrations and compositions of nitrogen and phosphorus in the aerosols. The concentrations of DIN, dissolved organic nitrogen (DON), TN, DIP, and dissolved organic phosphorus (DOP) in the aerosols from southern air mass were higher than those from northern and marine air masses. The concentration of DON in the diurnal aerosol samples was similar to that in the nightly samples; however, the concentrations of DIN and TN were higher in the diurnal aerosols than those in the nightly aerosols. The DTN was the dominant species of TN in the diurnal and nightly aerosol samples, accounting for 79% of TN on average. Inorganic nitrogen was the dominant species of DTN in the diurnal and nightly aerosols. The ratio of DIN to DTN decreased from 70% in diurnal samples to 61% in nightly samples. The concentrations of DIP were close to that of DOP in the diurnal and nightly aerosols; however, the concentrations of TP were higher in the diurnal aerosols than in the nightly ones. The insoluble P was the dominant form of TP in the aerosols, accounting for 83% and 62% of TP during the day and night, respectively. The contribution of DTP to TP in the nightly aerosols samples was much higher than that in the diurnal aerosols. For both day and night samples, inorganic phosphorus was the dominant species of DTP in aerosols, with a ratio of 71%-77%.

Distribution Characteristics of Chromophoric Dissolved Organic Matter and Nutrients from the Yellow Sea and Bohai Sea in Autumn

Water samples were collected from the Yellow and Bohai seas during November 2013 and the chromophoric dissolved organic matter (CDOM) and nutrients concentrations were investigated, including their composition, source and distribution characteristics. CDOM was analyzed by excitation-emission matrix spectroscopy (EEMS) in combination with a parallel factor analysis (PARAFAC). Three terrestrial humic-like substances (C1, C2, and C3) and one protein-like (C4) substances were identified. The three terrestrial humic-like components had a similar horizontal distribution, decreasing from inshore to offshore. The protein-like component, showed higher values for both inshore and offshore areas in the Yellow Sea at the surface layer and the middle layer of the Bohai Sea, while in other layers it had a similar distribution pattern as the terrestrial humic-like components. In the Bohai Sea, the dissolved inorganic nitrogen (DIN) concentration showed a decreasing trend from inshore to offshore areas and the concentration of dissolved inorganic phosphorus (DIP) gradually decreased from the Caofeidian coastal and central areas to other adjacent areas. The dissolved organic nitrogen (DON) showed an increasing from inshore to the central area of the Bohai Sea. DIN and DON were higher in surface layer than in the bottom layer and for DIP this was the opposite in the Bohai Sea. In the Yellow Sea, DON concentrations showed a decreasing trend from inshore to offshore areas. DIN and DIP had higher concentrations at inshore areas in the surface and middle layers, while in the bottom layer they had higher concentrations in offshore areas. The vertical distribution of the DIN and DIP showed higher values in the bottom layer than surface layers and for DON this was the opposite. These results also showed that the concentrations of DIN, DON, and DIP in the Bohai sea were overall higher than the Yellow Sea. A discriminant analysis was performed through redundancy analysis (RAD) of these water quality parameters, including the four fluorescent components, an absorption coefficient (a355), chlorophyll a concentration, conductivity, dissolved oxygen (DO), dissolved organic carbon (DOC), DIN, DON, and DIP. The RDA indicated that the four fluorescent components are mainly affected by terrestrial inputs. DOC was affected by both terrestrial and marine sources but terrestrial inputs were the major contributor. It was also indicated that the DIN concentration was affected by terrestrial inputs in the Bohai Sea area and by both terrestrial and marine sources in the Yellow Sea. DON concentrations were mainly affected by marine sources in the Bohai Sea and terrestrial inputs in the Yellow Sea. DIP concentrations were affected by both terrestrial and marine sources in the studied areas.

Impact of Water-Sediment Regulation Scheme on seasonal and spatial variations of biogeochemical factors in the Yellow River estuary

Seasonal and spatial distributions of nutrients and chlorophyll-a (Chl-a), together with temperature, salinity and total suspended matter (TSM), were investigated in the Yellow River estuary (China) to examine the biogeochemical influence of the “Water and Sediment Regulation Scheme (WSRS)” that is used to manage outflows from the river. Four cruises in April, June (early phase of WSRS), July (late phase of WSRS) and September were conducted in 2013 (WSRS from 19th June to 12th July). The results showed that nutrient species could be divided into two major groups according to their seasonal and spatial distributions. One group included NO3−, dissolved organic nitrogen (DON) and Si(OH)4, primarily from freshwater discharge. NO3− and DON related to anthropogenic sources were also separated from Si(OH)4, which was related to weather. The other group included dissolved inorganic phosphorus (DIP), dissolved organic phosphorus (DOP), NO2−, and NH4+. Along with freshwater inputs, sediment absorption/desorption showed impacts on DIP and DOP concentration and distribution. Nitrification was a dominant factor controlling NO2− concentrations. NH4+ was influenced by both sediment absorption/desorption and nitrification. The WSRS not only shifted the seasonal patterns of nutrients in the estuary, with high concentrations moved from autumn to June and July, but also promoted the nutrient spread to the south central part of the Bohai Sea. Spatial distribution of Chlorophyll-a (Chl-a) was influenced by the WSRS, with high concentrations being found in the river mouth in June and September, flanking the river mouth in July, and in the south central part of the Bohai Sea in September. Although Chl-a concentrations increased in June and July, the seasonal patterns did not change. The highest concentrations were found in September. Nutrient loadings during the WSRS relieved DIP and Si(OH)4 limitation in the estuary and south central Bohai Sea, causing an excess of DIN and disrupting the balance of DIN/DIP in the estuary and Bohai Sea. High turbidity and freshwater flushing depressed the growth of phytoplankton during the WSRS. The growth of phytoplankton was nutrient limited in June (DIP) when the WSRS started and in September after DIP and Si(OH)4 had been consumed by phytoplankton.

Change characteristics of DSi and nutrition structure at the Yangtze River Estuary after Three Gorges Project impounding and their ecological effect

Abstract The variation law of dissolved silica (DSi), dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and nutrition structure after the Three Gorges Project (TGP) impounding as well as their ecological effect were analyzed according to monitoring survey of the Yangtze River Estuary in spring (May) and summer (August) from 2004-2009. The results showed that after impounding, DSi and DIN concentration decreased and increased, respectively. During the study period, DSi decreased by about 63%, while DIN almost tripled. DIP concentration fluctuated slightly. With respect to nutrition structure, N:P increased, whereas Si:P and Si:N declined. According to chemometry standard of nutrient limits, nutrition structure tended to be imbalanced and the limiting factor of phytoplankton growth (P) was studied. Changes of nutrition structure have largely decreased diatom and caused different composition of dominant phytoplankton species. This may change ecosystem structure of the Yangtze River Estuary.

Large temporal changes in contributions of groundwater-borne nutrients to coastal waters off a volcanic island

We examined the contribution of submarine groundwater discharge (SGD) to nutrient budgets in Hwasun Bay, Jeju Island, Korea in August 2009, October 2014, and May 2015. The concentrations of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) in fresh groundwater were in the range of 285−716 μM and 2.3−3.2 μM, respectively, which were each 1−2 orders of magnitude higher than those in the bay seawater. The outer-bay seawater flowing into the bay was oligotrophic (2.9 ± 1.9 μM for DIN and 0.2 ± 0.3 μM for DIP). Nutrient budget calculations were performed for each season by accounting for submarine fresh groundwater discharge (SFGD) and water residence times. In August 2009 (DIN = 1.8 μM and DIN:DIP ratio = 4.6 for the outerbay water), DIN inputs from SFGD accounted for approximately 40% of the DIN inventory in the bay seawater. In October 2014 (DIN = 1.1 μM and DIP < 0.05 μM for the outer-bay water), DIP from SFGD accounted for approximately 100% of the DIP inventory in the bay seawater. In May 2015, mean concentrations of DIN and DIP in the bay seawater were 8.6 ± 12 μM and 0.11 ± 0.04 μM, respectively, with conservative behaviors in the bay seawater in association with excessive groundwater inputs. These results imply that SGD plays a critical but different role in nutrient budgets and stoichiometry in coastal waters off a volcanic island depending on open-ocean nutrient conditions.

Distribution, relationship and significance of phytoplankton, chlorophyll-a and environment variables in spring season of the Zhelin Reservoir, Jiangxi Province

江西柘林水库是长江中游的大型峡谷型旅游性水库，库容79.2×10⁸ m³，长115 km.2015年4月在56 km的中心库区走航和定点测量了水库表层水体浮游藻类、叶绿素a（Chl.a）浓度与溶解无机氮（DIN）浓度、溶解无机磷（DIP）浓度、溶解硅（DSi）浓度、水温、浊度和溶解氧浓度等环境因子的分布特征.结果表明：1）水库属于中营养水体，表层主要浮游藻类（细胞丰度>1000 cells/L）有34种，平均生物量为0.41 mg/L.主要优势藻类（优势度≥0.02）为硅藻和蓝藻，藻类组成与DIN浓度、DIP浓度、DSi浓度和水温等环境因子关系密切，4种因子对藻类结构的解释水平达60%以上.2）水库水体Chl.a浓度具有显著的次表层叶绿素最大值（SCM）现象，SCM层深度为3~8 m，厚度为2~7 m，SCM层占整个水体的25.2%~74.1%.SCM层的藻类对营养盐吸收消耗致使DIN、DIP和DSi浓度下降，同时藻类的产氧使溶解氧浓度增加.3）水库对DSi具有显著的生物过滤器效应，中、上层约有11%~12%的DSi被生物吸收利用，从上游至下游，累积约有21%的DSi被藻类吸收沉降于库底.4）人类氮、磷排放对水库生态和水质有严重影响，毗邻县城区域水体的Chl.a和DIP浓度分别是自然河段的2.9倍和3倍左右.;Zhelin Reservoir is a large canyon-reservoir in the midstream of the Yangtze River, and the storage capacity is 79.2×10⁸ m³ and length is 115 km. Through measurements on a moving vessel and at fixed-point sites in the Zhelin Reservoir in April, 2015, the distribution of phytoplankton, chlorophyll-a (Chl.a) concentration and main environment variables (including dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), dissolved silicon (DSi), water temperature, turbidity, dissolved oxygen (DO)) were analyzed. The redundancy relationship of phytoplankton taxa and environmental variables was analyzed using the software CANOCO 4.5. The results showed that, 1) the reservoir water was categorized as a middle-status in nutrients. There were 34 main phytoplankton species in the surface (the cell density of which exceeding 1000 cells/L), and the average biomass of reservoir was 0.41 mg/L. The dominant algae (dominance be equal or greater than 0.02) were the diatoms and cyanobacteria. DIN, DIP, DSi and water temperature can impact on the structure of algae, and the four factors were explained for more than 60% variation of the algal structure. 2) The reservoir had a significant phenomenon of subsurface chlorophyll maximum (SCM). The depth of SCM appears at the water depth from 3 to 8 m, and the thickness is about 2-7 m. The Chl.a in the SCM layer is 25.2%-74.1% among the total in the vertical. The algae in the SCM layer absorbed the nutrients, resulting in decreased concentrations of DIN, DIP and DSi and the increased DO concentration. 3) The reservoir had significant biological and biochemical filtering effect for the DSi. About 11% to 12% DSi were absorbed by organisms in the middle and upper area of in the reservoir, and accumulating about 21% DSi was absorbed by algae from upstream to downstream. 4) Nitrogen and phosphorus emissions by human activities have a serious impact on the ecology and water quality of the reservoir and the adjacent county region. The concentrations of Chl.a and DIP in the region is about 2.9 times and 3 times higher than that in the natural region of the reservoir, respectively.

Effect of Nutrient Cycles in Tributaries on the Transport of Nutrient in the Three Gorge Reservoirs:A Case Study of Caotang River

Backwater regions have been formed in the downstream of tributaries since the Three Gorge Reservoir (TGR) was impounded, as the most influential area to the aquatic environment within the reservoir. In order to study the effects and significance of the backwater regions on the nutrient cycling and substance fluxes, dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), dissolved silicon (DSi) were monitored in situ in Caotang tributary backwater region, the adjacent mainstream and Caotang tributaries' upstream every month from August 2012 to July 2013. The results showed that the concentrations of DIN, DIP and DSi in Caotang tributary backwater region were similar to those in the mainstream, and the monthly variation tendency was very consistent with the mainstream Changjiang. DIN, DIP and DSi in the Caotang tributary were mainly originated from the mainstream. The influence of the mainstream water could extend to the upmost of the backwater region due to the good connection and rapid, sufficient water exchange between mainstream and tributary. Nutrients from the mainstream were retained and consumed by primary producers in the tributary during the growing seasons in spring and summer, while released back to the water during late autumn and winter. Nutrient cycling in the tributary could change the seasonal pattern of nutrient transportation. The mainstream nutrient fluxes could receive significant seasonal modification from the large numbers (more than 40) of tributaries within the TGR domain.

Dynamics of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fixation and fate of diazotroph-derived nitrogen in a low-nutrient, low-chlorophyll ecosystem: results from the VAHINE mesocosm experiment (New Caledonia)

Abstract. N2 fixation rates were measured daily in large (∼ 50 m3) mesocosms deployed in the tropical southwest Pacific coastal ocean (New Caledonia) to investigate the temporal variability in N2 fixation rates in relation with environmental parameters and study the fate of diazotroph-derived nitrogen (DDN) in a low-nutrient, low-chlorophyll ecosystem. The mesocosms were fertilized with ∼ 0.8 µM dissolved inorganic phosphorus (DIP) to stimulate diazotrophy. Bulk N2 fixation rates were replicable between the three mesocosms, averaged 18.5 ± 1.1 nmol N L−1 d−1 over the 23 days, and increased by a factor of 2 during the second half of the experiment (days 15 to 23) to reach 27.3 ± 1.0 nmol N L−1 d−1. These later rates measured after the DIP fertilization are higher than the upper range reported for the global ocean. During the 23 days of the experiment, N2 fixation rates were positively correlated with seawater temperature, primary production, bacterial production, standing stocks of particulate organic carbon (POC), nitrogen (PON) and phosphorus (POP), and alkaline phosphatase activity, and negatively correlated with DIP concentrations, DIP turnover time, nitrate, and dissolved organic nitrogen and phosphorus concentrations. The fate of DDN was investigated during a bloom of the unicellular diazotroph UCYN-C that occurred during the second half of the experiment. Quantification of diazotrophs in the sediment traps indicates that ∼ 10 % of UCYN-C from the water column was exported daily to the traps, representing as much as 22.4 ± 5.5 % of the total POC exported at the height of the UCYN-C bloom. This export was mainly due to the aggregation of small (5.7 ± 0.8 µm) UCYN-C cells into large (100–500 µm) aggregates. During the same time period, a DDN transfer experiment based on high-resolution nanometer-scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N2 isotopic labeling revealed that 16 ± 6 % of the DDN was released to the dissolved pool and 21 ± 4 % was transferred to non-diazotrophic plankton, mainly picoplankton (18 ± 4 %) followed by diatoms (3 ± 2 %). This is consistent with the observed dramatic increase in picoplankton and diatom abundances, primary production, bacterial production, and standing stocks of POC, PON, and POP in the mesocosms during the second half of the experiment. These results offer insights into the fate of DDN during a bloom of UCYN-C in low-nutrient, low-chlorophyll ecosystems.

Evaluation of organic pollution and eutrophication status of Kaohsiung Harbor, Taiwan

The pollution level and eutrophication conditions of bottom water in Kaohsiung Harbor (Taiwan) were evaluated using organic pollution index (OPI) and eutrophication index (EI). Water samples were seasonally collected (February, May, August, and October) from 20 sites of the harbor in 2012. Distribution of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), chemical oxygen demand (COD), and dissolved oxygen (DO) in the water samples were investigated. Results showed that the average concentrations with standard deviations of DIN, DIP, COD, and DO for all water samples (n = 80) were 0.16 ± 0.11, 0.18 ± 0.07, 3.06 ± 1.09, and 5.49 ± 0.84 mg L−1, respectively. The distribution of DIN, DIP, and COD concentrations presents a trend that estuary > channel > harbor entrance. These observations clearly indicate that upstream industrial and municipal wastewater discharges along the river bank are major sources of contamination in the harbor water. Based on the analyses using the OPI and EI, results show that pollution was much heavily in the wet season (OPI = 25.9 and EI = 38) than dry season (OPI = 13.4 and EI = 18.0). Moreover, it is worth to notice that all of water collected from Kaohsiung Harbor were classified as the heavily polluted and eutrophication situation.