Dissolved Inorganic Nitrogen Ratio Research Articles

The underestimated role of manganese in modulating the nutrient structure in a eutrophic seasonally-stratified reservoir

Accumulation and subsequent release of nutrients have great potential to trigger algal blooms in lakes and reservoirs. We conducted high vertical resolution (2 m interval) monitoring at ∼monthly intervals over a year for hydrological parameters, Chl-a, ammonium (NH4+), nitrate (NO3−) and different species of phosphorus (P) and manganese (Mn) in a 40-meter-deep subtropical reservoir (Shanmei Reservoir) in Fujian, southern China. In this seasonally stratified reservoir featured with high nutrient loading, the consistent trend in the ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) between the euphotic zone and the hypolimnion, coupled with its mirrored correlation with Chl-a concentration indicates that upward flux from the hypolimnion affects phytoplankton growth in the euphotic zone. The monthly variation of the depth-integrated multiple species of N and P indicates that during the stratification period in the hypoxic hypolimnion, approximately 80% of the DIP is removed, leading to a remarkable decoupling phenomenon between NH4+ and DIP. This process effectively increases the ratio of DIN to DIP in the hypolimnion, thereby significantly reducing the potential of algal blooms caused by the upward flux. A robust positive linear correlation between iron-manganese bound phosphorus (CBD-P) and particulate Mn was observed during stratification period implying that DIP was scavenged by sediment-released Mn throughout the water column. Vertical profiles during stratification showed that upward diffusion of Mn2+ facilitated the formation of Mn oxide zones near the oxycline. The most significant decrease in DIP inventory occurs when Mn oxide zones migrate either upwards from the bottom or downwards from the oxycline, indicating that the migration of Mn oxides on the vertical profile is a key factor in the decoupling of NH4+and DIP.Our findings underscore the importance of Mn cycling as an underappreciated DIP self-immobilization process in the water column of reservoirs characterized by high nutrient loading. Furthermore, we propose that denitrification and Mn cycling establish a consecutive feedback mechanism, preventing excessive nutrient accumulation in low oxygen bottom water. In the context of global changes, we anticipate a heightened prominence of this feedback mechanism.

Trends in nutrients in the Changjiang River

Better documentation and understanding of long-term temporal dynamics of nutrients in watersheds are necessary to support effective water quality management. We examined the hypothesis that the recent management of fertilizer use and pollution control in the Changjiang River Basin could govern the fluxes of nutrients from the river to the sea. Results based on historical data since 1962 and surveys in recent years show that concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) in the mid- and downstream reaches were higher than those in the upper reaches due to intensive anthropogenic activities, while dissolved silicate (DSi) was distributed evenly from the up- to downstream reaches. Fluxes of DIN and DIP increased rapidly, and DSi declined during the 1962–1980 and 1980–2000. After the 2000s, concentrations and fluxes of DIN and DSi remained almost unchanged; those of DIP remained stable until the 2010s and slightly decreased afterward. The decline in fertilizer use explains 45 % of the variance in the decline of DIP flux, followed by pollution control, groundwater and water discharge. As a result, the molar ratio of DIN:DIP, DSi:DIP and ammonia:nitrate varied largely during 1962–2020, and the excess DIN relative to DIP and DSi lead to increased limitations of silicon and phosphorus. A turning point probably occurred for nutrient fluxes in the Changjiang River in the 2010s, with the pattern of DIN from continuous increase to stability and DIP from increase to decrease. This decline in phosphorus in the Changjiang River has many similarities with the rivers worldwide. The continued basin nutrient management is likely to have a major influence on river nutrient delivery and therefore may control coastal nutrient budget and ecosystem stability.

Effects of wastewater effluent-borne nutrients on phytoplankton off the coast of Jeju Island.

The spatiotemporal distributions of nutrients in coastal waters surrounding eight wastewater treatment plants (WWTPs) in four seasons were investigated to determine the effects of WWTP effluents on seawater off Jeju Island, Korea. The highest concentrations of nutrients were observed in the outlets of WWTPs with relatively high ammonium concentrations among dissolved inorganic nitrogen (DIN). The reduced DIN (NO2- and NH4+)/total DIN ratios are used as a potential short-term index for marine environmental conditions. In seawater surrounding the WWTPs, relatively low nutrient concentrations were observed in spring and fall, due to enhanced biological production, which is closely linked to decreased N/P ratios. Because the highest WWTP effluent fluxes of ammonium in this study were similar to the fluxes of nutrients from submarine groundwater discharge, diffusion from bottom sediments, and discharge from land-based fish farm wastewater, WWTP effluent-derived nutrients are potentially important in oligotrophic environments and can be readily utilized by phytoplankton.

Impacts of Mesoscale Eddies on Biogeochemical Variables in the Northwest Pacific

Mesoscale eddies play an important role in regulating biogeochemical cycles. However, the response of biogeochemical variables to cold and warm eddies has not been well elucidated, mainly due to most previous studies relying on remote sensing techniques and lacking in situ observations below the surface water. Here, we used hydrographic and biochemical data from one survey in the northwestern Pacific to document the vertical biogeochemical structure of one cold and two warm eddies. We first compared the changes of key variables in the eddy core relative to eddy outside, explained the role of key layers (the mixing depth, pycnocline, nutricline, euphotic) in causing these changes, and then analyzed the main environmental factors affecting chlorophyll a (Chla) and phytoplankton communities. Finally we focused on the response mechanisms of key biogeochemical variables to the cold and warm eddies. The results showed that biological variables (Chla, microphytoplankton, picophytoplankton), salinity, dissolved inorganic nitrogen (DIN), dissolved inorganic phosphate (DIP), and dissolved inorganic silicate (DSi) in the cold eddy core increased by 0.2–134%, while in the warm eddy core, they decreased by 0.2–70% relative to the eddy outside. The cold and warm eddies were able to force the deep chlorophyll maximum (DCM), which rose or fell with the pycnocline, nutricline and euphotic depth (Zeu) as a whole. Cold eddies with a raised thermocline could lead to about 20 m elevated DCM and enhanced phytoplankton biomass when the nutricline and thermocline were coincident. In contrast, warm eddies drove isopycnals downward, resulting in a 10–25 m drop in DCM and a decrease in nutrient and Chla concentrations at the center of the eddies. The significant difference in the vertical structure of the phytoplankton community between the center and the outside of the eddy might be explained by the direct influence of both nutrient concentrations and stoichiometry changes. The contribution of microphytoplankton to total biomass was much smaller than that of picophytoplankton in oligotrophic waters where the DIN:DIP and DSi:DIN ratios are significantly low. Compared to nutrients, photosynthetically active radiation (PAR) might not be the main factor controlling phytoplankton biomass and abundance attributed to Zeu being consistently deeper than the mixed depth (Zm), whereas it was likely to be the key limiting factor affecting the vertical distribution of the phytoplankton community.

Noctiluca blooms intensify when northwesterly winds complement northeasterlies in the northern Arabian Sea: Possible implications

Wind systems are known as nutrient sources playing significant roles in the oceanic realm and global climate oscillations. This study explores, for the first time, the effect of winds on the winter blooms of the mixotrophic dinoflagellate, the green variant of Noctiluca scintillans (NSG) in the northern Arabian Sea. When the NSG abundance was lower (i.e., <∼10000 cells l−1), it was coupled to silicic acid (H4SiO4), on which diatoms (phytoplankton) in turn depended. At higher abundance (i.e., NSG>∼10000 cells l−1), H4SiO4 and H4SiO4:DIN (dissolved inorganic nitrogen) ratio fell. The NSG was then intensely green and chlorophyll-a richer, attributed to a change in the mode of NSG's nutrition from heterotrophy to autotrophy-dominance. The back-trajectory model revealed that the winds were mostly northeasterly (NE) initially (during February) and were north-westerly (NW) towards the end of winter (March). Separately for the NE and NW winds, the NSG abundance was 10655±18628 and 28896±46225 cells l−1, respectively. The H4SiO4:DIN ratio correspondingly reached <0.2 and ≥0.4. The NSG was modelled with high significance (p<0.001, N=33) versus the NE and NW wind speeds. Thus, while the NE winds deepened the mixed layer and caused nutrient enrichment and phytoplankton production, the NW winds facilitated the recovery of the H4SiO4:DIN ratio and economical use of H4SiO4 for phytoplankton production. It is hypothesized that this process is helped by iron input from NW desert winds during the latter part of winter when the NSG blooms intensify.

Nutrient Budget and Biogeochemical Dynamics in Sansha Bay, China: A Coastal Bay Affected by Intensive Mariculture

AbstractThis study examines the spatiotemporal distribution of dissolved and particulate nutrients in Sansha Bay, a coastal bay characterized by complex hydrodynamics (monsoon‐driven river inputs and alongshore coastal waters), and intensive mariculture. However, nutrient stoichiometry and underlying biological responses in this ecosystem remain unclear. High nutrient (39.6–87.5 μmol L−1 dissolved inorganic nitrogen (DIN), 1.79–3.77 μmol L−1 dissolved inorganic phosphorus (DIP)) in winter suggested entrainment of the nutrient‐enriched China Coastal Current superimposed with mariculture inputs; lower nutrient concentrations (9.6–47.1 μmol L−1 DIN, 0.31–1.57 μmol L−1 DIP) in summer, and high Chlorophyll a (0.8–17.0 mg m−3) and dissolved oxygen (4.48–8.78 mg L−1), were driven by the exchange of nutrient‐poor coastal waters and enhanced biological consumption. Using the Land‐Ocean Interaction Coastal Zone (LOICZ) mass balance model, field observations of nutrient sources/sinks allowed construction of a robust nutrient budget, in which Δ indicated non‐conservative dissolved nutrient fluxes. In both winter and summer, the external DIN and DIP additions ratio ΔDIN:ΔDIP was ∼21.8–20.4:1, while Si(OH)4 addition/removal occurred seasonally. Potential N biogeochemical processes were also explored. Using a three end‐member mixing model, nutrient biogeochemistry (regeneration/uptake, denoted by δ) within the aquaculture ecosystem was estimated separately from complex physical mixing effects. In winter, nutrient regeneration dominated (δDIN:δDIP ∼20:1; δSi(OH)4:δDIN &lt;1:2), likely due to remineralization of particles (phytoplankton with a 16:1 Redfield ratio and/or fish feed/feces with a non‐Redfield ratio). In contrast, nutrient consumption dominated in summer (δDIN:δDIP ∼13.5:1; δSi(OH)4:δDIN ∼2.4:1). Results of this study are relevant to other coastal bays affected by intensive aquaculture, riverine inputs and coastal waters.

Seasonal variations of nutrient concentrations and their ratios in the central Bohai Sea

The Bohai Sea is a typical semi-enclosed sea in the northwest Pacific, which is subject to serious eutrophication due to human activities. Similar to hydrographic variables such as water temperature and salinity, the nutrient concentrations and ratios in the Bohai Sea also exhibit seasonal variations. However, the effects of external inputs, biological processes, and benthic processes on these seasonal variations have not been quantified to date. To address this issue, a physical–biological coupled model was developed to capture the seasonal nutrient cycling in the central Bohai Sea. The simulation results revealed apparent seasonal variations in the concentrations of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silicate (DSi). The N/P ratio (as the molar ratio of DIN/DIP) also exhibited an apparent seasonal variation, with the maximum and minimum values in surface water occurring in summer (>100) and winter (<30), respectively. The Si/N ratio (as the molar ratio of DSi/DIN) was slightly higher in summer than in other seasons. The budget for three types of nutrient indicated that the biological processes determined the seasonal variations in nutrient concentrations and the N/P ratio. The external inputs of nutrients via river input, atmospheric deposition, and sediment release were probably important in summer when they could alleviate the reduced nutrient concentrations due to biological processes. To maintain a reasonable nitrogen budget, it was necessary to include benthic nitrogen loss, which removed a large amount of inorganic nitrogen in summer and autumn. In addition, the variation of N/P ratio of phytoplankton uptake can reduce the ratio of DIN to DIP in surface water by ~20 in summer as compared to the calculation with a fixed N/P ratio (16:1) in phytoplankton uptake.

Effects of nutrient-rich submarine groundwater discharge on marine aquaculture: A case in Lianjiang, East China Sea

Submarine groundwater discharge (SGD) and associated nutrient have long been received insufficient attention in the aquaculture areas of Lianjiang, East China Sea. In this study, we used 224Ra mass balance model to evaluate the importance of SGD in the aquaculture areas of Lianjiang in different seasons. The results showed that SGD fluxes were 0.14 ± 0.070 m3 m−2 d−1 in July and 0.077 ± 0.040 m3 m−2 d−1 in October 2019, which corresponded to approximately 22% and 32% of the river discharge, respectively. Nevertheless, the dissolved inorganic nitrogen (DIN) and phosphorus (DIP) fluxes contributed by SGD over total nutrient inputs were approximately 58% and 73%, respectively in July, 11% and 33%, respectively in October 2019. The ratio of DIN to DIP in seawater was slightly higher than that derived from SGD in July and significantly lower than that derived from the river and SGD in October, which was prone to profound modulated by shellfish (e.g., clam Ruditapes philippinarum and Sinonovacula constricta) and nori (e.g., Pyropia haitanensis) that selectively acquire nitrogen and phosphorous, suggesting that SGD could affect the nutrient structure and aquaculture activities in Lianjiang. Moreover, the new primary production via SGD yielded (2.47 ± 1.32) × 103 mg C m−2 d−1 in July and (3.52 ± 1.76) × 102 mg C m−2 d−1 in October 2019, which were approximately 8–25% and 10–31% of the production in the entire study area. These meaningful discoveries indicated that SGD and associated nutrient are the predominant regulator of nutrient and aquaculture structure under significant seasonal differences.

Wildfires lead to decreased carbon and increased nitrogen concentrations in upland arctic streams

The Central Siberian Plateau is undergoing rapid climate change that has resulted in increased frequency of forest fires and subsequent alteration of watershed carbon and nutrient dynamics. Across a watershed chronosequence (3 to >100 years since wildfire) we quantified the effects of fire on quantity and composition of dissolved organic matter (DOM), stream water nutrient concentrations, as well as in-stream nutrient uptake. Wildfires increased concentrations of nitrate for a decade, while decreasing concentrations of dissolved organic carbon and nitrogen (DOC and DON) and aliphatic DOM contribution for five decades. These post-wildfire changes in stream DOM result in lower uptake efficiency of in-stream nitrate in recently burned watersheds. Nitrate uptake (as uptake velocity) is strongly dependent on DOM composition (e.g. polyphenolics), ambient dissolved inorganic nitrogen (DIN), and DOC to DIN ratios. Our observations and experiments suggest that a decade-long pulse of inorganic nitrogen and a reduction of DOC export occur following wildfires in streams draining the Central Siberian Plateau. Increased fire frequency in the region is thus likely to both decrease DOM and increase nitrate delivery to the main stem Yenisei River, and ultimately the Arctic Ocean, in the coming decades.

Estimating submarine groundwater discharge and associated nutrient inputs into Daya Bay during spring using radium isotopes

Daya Bay, a semi-enclosed bay in the South China Sea, is well known for its aquaculture, agriculture, and tourism. In recent years, many environmental problems have emerged, such as the frequent (almost yearly) occurrence of harmful algal blooms and red tides. Therefore, investigations of submarine groundwater discharge (SGD) and associated nutrient inputs to this bay have important theoretical and practical significance to the protection of the ecological system. Such a study was conducted using short-lived radium isotopes 223Ra and 224Ra. The estimated SGD fluxes were 2.89 × 107 m3/d and 3.05 × 107 m3/d based on 223Ra and 224Ra, respectively. The average SGD flux was about 35 times greater than that of all the local rivers. The SGD-associated dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) fluxes ranged from 1.95 × 106 to 2.06 × 106 mol/d and from 5.72 × 104 to 6.04 × 104 mol/d, respectively. The average ratio of DIN to DIP fluxes in SGD was 34, much higher than that in local rivers (about 6.46), and about twice as large as the Redfield ratio (16). Our results indicate that SGD is a significant source of nutrients to the bay and may cause frequent occurrence of harmful algal blooms. This study provides baseline data for evaluating potential environmental effects due to urbanization and economic growth in this region.

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%.

Comparison of land-based sources with ambient estuarine concentrations of total dissolved nitrogen in Jiaozhou Bay (China)

Seasonal, land-sea synchronous surveys were conducted from 2012 to 2013 to characterize the relationship between the composition of land-based total dissolved nitrogen (TDN) and the concentration of dissolved inorganic nitrogen (DIN) in Jiaozhou Bay (JZB). A total of 11 freshwater riverine sampling sites were selected at the river mouths and at waste water outfalls around JZB, while a total 23 Bay stations were established in JZB. Among them, 11 Bay stations were located near the 11 outfalls. Each land-sea sampling was conducted synchronously during a semi-tidal cycle. The contribution of NO3N, NO2N, NH4N, and dissolved organic nitrogen (DON) to TDN in land-based freshwater were similar to those in JZB seawater, while the contribution of the sum of NO3N and NO2N to TDN and the contribution of DON to TDN were about 3.2 and 4.1 times higher than the contribution of NH4N to TDN, respectively. These results showed that inputs of all land-based forms of nitrogen impact the DIN in seawater. Spatial distributions of DIN and DON, showing a gradual decrease from inner bay to the mouth of the bay, were negatively correlated with S in different seasons. In summer and winter, the ratio of DIN to DON in seawater (Rs) gradually decreased from the inner bay to the center of the bay, and the ratio of land-based DIN to DON (RL) was less than RS, indicating net transformation from land-based DON into marine DIN. However, in spring and autumn, the distribution of Rs was opposite to that in summer and winter, and RL was greater than RS, indicating net conversion from land-based DIN into marine DON. Throughout the whole year, net land-based DON was transformed into marine DIN. We provided direct evidence that the variation in DIN concentration in JZB was affected both by land-based TDN inputs and by their hydrodynamic transport and biogeochemical transformation processes.

Variability of the dissolved nutrient (N, P, Si) concentrations in the Bay of Annaba in relation to the inputs of the Seybouse and Mafragh estuaries

Dissolved inorganic nitrogen (DIN), phosphate (PO4) and silicic acid (Si(OH)4) loads from the Seybouse and the Mafragh estuaries into the Bay of Annaba, Algeria, were assessed at three stations of the Bay over three years. The Seybouse inputs had high levels of DIN and PO4, in contrast to the Mafragh estuary’s near-pristine inputs; Si(OH)4 levels were low in both estuaries. The DIN:PO4 molar ratios were over 30 in most samples and the Si(OH)4:DIN ratio was less than 0.5 in the Seybouse waters, but nearly balanced in the Mafragh. The specific fluxes of Si–Si(OH)4 (400–540kgSikm−2yr−1) were comparable in the two catchments, but those of DIN were several-fold higher in the Seybouse (373kgNkm−2yr−1). The inner Bay affected by the Seybouse inputs had high levels of all nutrients, while the Mafragh plume and the outer marine station were less enriched.

Factors Affecting Dissolved Silica Concentrations, and DSi and DIN Stoichiometry in a Human Impacted Watershed (Po River, Italy)

Data on dissolved (DSi) and particulate (PSi) silica concentrations, along with DSi and DIN (dissolved inorganic nitrogen) stoichiometry in surface waters were reviewed for the Po river watershed. DSi in the cascade river and lake ecosystems followed clear upstream-downstream gradients. The DSi (77–178 kt Si y − 1) and PSi (879–1,486 kt Si y − 1) loadings from the Po river to the Adriatic Sea display strong inter-annual variability, related with the river discharge. In the lowland river reaches, the DSi to DIN ratio highlighted frequent potential Si limitation, especially in summer during diatom blooms.

Long-term changes in the fishery ecosystem structure of Laizhou Bay, China

Laizhou Bay provides a critical spawning and nursery habitat for many fishery species, including commercially important species, such as Fenneropenaeus chinensis and Larimichthys polyactis. The bay is severely stressed due to high fishing pressure and environmental changes. Based on the long-term ecosystem surveys in Laizhou Bay during the main spawning period (May) of most fishery species from 1959 to 2008, the responses of the Laizhou Bay fishery ecosystem were analyzed here, including regime shifts in species composition, biomass, species diversity, zooplankton, phytoplankton, and environmental variables. The dominant species of large-size and high economic value (e.g. Trichiurus haumela, L. polyactis) have been replaced by the short-lived, low-trophic-level planktivorous pelagic species (e.g. Setipinna taty, Engraulis japonicus) since the 1980s, and it is probable that the small-sized pelagic fishes have been recently replaced by invertebrates (e.g. Oratosquilla oratoria, Crangon affinis). The biomass of fishery resources declined continuously from 423.6 kg haul−1 h−1 in 1959 to 164.6 kg haul−1 h−1 in 1982, 37.7 kg haul−1 h−1 in 1993, and less than 8 kg haul−1 h−1 in 1998–2008. Moreover, the biomass of zooplankton showed an increasing trend during 1959–2006, but showed a slight decline in 2008. The abundance of phytoplankton increased from 1959 through 1982, decreased substantially in 1993, and increased again until 2004. More recently, however, the phytoplankton abundance was very low. The sea surface temperature (SST) and sea bottom temperature (SBT) in May increased by 0.23°C a−1 and 0.16°C a−1, respectively, during 1982–2008. The salinity in May showed large fluctuations and reached its lowest values in 2004 and 2006. The ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphate (DIP) increased. However, the dissolved silicon (DSi):DIP and DSi:DIN ratios decreased to a low level during 1959–2008. These changes seriously impacted primary production, and cascade effects then changed the structure and function of the fishery ecosystem. Further analysis indicated that multiple stresses caused the alterations in the structure of the Laizhou Bay fishery ecosystem. The top-down effect was identified as the main influence on the fishery species (at the top of the food chain) over the past five decades due to the increasing fishing pressure, whereas the bottom-up effect increased over the past three decades due to the strong variations in the environment.

Biogeochemistry of nutrients in an estuary affected by human activities: The Wanquan River estuary, eastern Hainan Island, China

Nutrient dynamics were studied in the estuary of the Wanquan River, a tropical mountainous river system of Hainan Island, China, during 2006–2009. The nutrients measured included NO3−, NO2−, NH4+, PO43−, Si(OH)4, total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), dissolved organic nitrogen (DON), and dissolved organic phosphorus (DOP). The Wanquan River showed great variation in nutrient levels, was enriched in dissolved inorganic nitrogen (DIN) and dissolved silicate, and depleted in PO43−. The levels of dissolved silicate were higher than average for tropical systems. As a consequence the DIN:PO43− and Si(OH)4:DIN ratios were higher than the Redfield ratio. DON accounted for 18% of TDN in the Wanquan River, and DOP represented approximately 61% of TDP.Nutrients in the Wanquan River estuary behave either conservatively or nonconservatively. Nutrient biogeochemistry in the estuary is affected by human activities in adjacent areas and heavy rainfall associated with typhoons. Phosphorus may be the potential limiting nutrient for phytoplankton growth. A simple steady-state box model showed that riverine input was the major source of nutrients to the estuary, which acted as a source of all nutrients except dissolved silicate, TDP, and DOP. The results indicate that substantial quantities of nitrogen and PO43− are transported to the coastal system, and suggest that dissolved silicate accumulates in the sediment or is transformed into other forms.

N/P ratio of nutrient uptake in the Baltic Sea

Abstract. The N/P ratio of nutrient uptake, the change of dissolved inorganic nitrogen (DIN) relative to the change of dissolved inorganic phosphorus (DIP), is a key parameter for many ecological models. In the Baltic Sea ecosystem, the N/P ratio of nutrient uptake varies among different basins and different seasons. The N/P ratio of nutrient alteration, i.e., the ratio of DIN to DIP altered before and after spring blooms, is not the same as the N/P ratio of nutrient uptake, but the former can be regarded as an indicator of the latter in the Baltic Sea. Based on the observed N/P ratio of nutrient alteration, we hypothesize a non-Redfield N/P ratio of nutrient uptake. The 3-D-ecosystem model ERGOM coupled with the circulation model DMI-BSHcmod was used to test this hypothesis. When the Redfield ratio was used in the model, the DIP surplus after spring blooms was too high and resulted in excessive growth of cyanobacteria and too much nitrogen fixation. When the non-Redfield ratio was used in the model, these problems tended to disappear. In summary, we show that: (1) the Redfield N/P ratio of nutrient uptake in the Baltic Sea tends to be too high; (2) a N/P ratio of 10:1 appears to work better than the Redfield value; and (3) the N/P ratio of nutrient uptake in the Baltic Proper during spring blooms is around 6:1. As the model limitation using one identical value for two N/P ratios for nutrient uptake and remineralization, the quantitative conclusions are only convincing as a model parameter even though it obviously improves model predictions. Whether this model parameter is consistent with the biological nutrient uptake is worth being further verified with some laboratory investigations or simulations using a more sophisticated model with independent N/P ratios for nutrient uptake and remineralization.

Long-term ecological interactions between nutrient and phytoplankton community in the Changjiang estuary

Both nitrate (NO3−) and soluble reactive phosphate (PO43−) concentration in the freshwater end-member at the mouth of the Changjiang River have increased dramatically since the 1960s. Within the same period in the sea area, with surface salinity>30, NO3− concentration has shown an obvious increase, PO43− has not changed greatly and dissolved reactive silica (SiO32−) has deceased dramatically. An examination of the elemental ratio of NO3− to PO43− at the mouth of the Changjiang River did not show a systematic trend from the 1960s to 2000s largely because both nutrients increased simultaneously. In comparison, the elemental ratio of dissolved inorganic nitrogen (DIN) to PO43− in surface seawater, with salinity>22, has shown a clearly increasing trend. Furthermore, an overall historical change of the SiO32−:PO43− ratio has undergone a reverse trend in this area. Based on the changes of SiO32−:PO43− and DIN:PO43− ratios, we can conclude that an overall historical change of SiO32−:DIN ratio has decreased in this area from the 1950–1960s to 2000s. The argument that phytoplankton productivity in the Changjiang estuary has been enhanced by increasing nutrient input from the riverine transport was supported by these results. A comparative study analyzing the shift of phytoplankton composition from the mid-1980s to 2000s was also made. The results indicated that the average yearly percentage of diatom species in the Changjiang estuary has decreased from 84.6% during 1985–1986 to 69.8% during 2004–2005. Furthermore, the average yearly percentage of diatom abundance in the Changjiang estuary decreased from 99.5% during to 75.5% over the same time period, while the abundance of dinoflagellates has increased dramatically, from 0.7% to 25.4%.

Effect of wind on continental shelf carbon fluxes off southeast Australia: A numerical model

A coupled physical‐biological‐chemical model is used to study the effect of upwelling‐favorable and downwelling‐favorable winds on carbon biogeochemistry on the continental shelf off the southeast Australian mainland. Along the continental shelf, from 30°S to 34°S, upwelling‐favorable winds, with the aid of bottom Ekman transport, bring dissolved‐inorganic‐carbon (DIC)‐rich slope waters onto the shelf, increasing the carbon held in shelf waters. For downwelling‐favorable winds, bottom Ekman transport still lifts slope waters onto the shelf, but the slope water transport, and therefore carbon held, is reduced compared with the upwelling scenario. Under upwelling‐favorable winds, filaments of DIC and dissolved‐inorganic‐nitrogen (DIN)‐rich water reaching the surface produce an outgassing near the site of upwelling and absorption downstream due to primary productivity. In a region of the ocean that is generally absorbing, the net effect of upwelling is a reduced absorption of atmospheric CO2 as a result of the ratio of deep DIC and DIN (12.2:1 mol C:mol N) being greater than the Redfield ratio (6.625). Carbon fluxes in the waters off the southeast Australian mainland are variable in space, with the transport of continental shelf waters to deep waters occurring mainly where alongshore currents separate from the coast and flow over the 200‐m isobath.

Nutrients and Other Abiotic Factors Affecting Bacterial Communities in an Ohio River (USA)

Nitrogen and phosphorus additions from anthropogenic sources can alter the nutrient pool of aquatic systems, both through increased nutrient concentrations and changes in stoichiometry. Because bacteria are important in nutrient cycling and aquatic food webs, information about how nutrients affect bacterial communities enhances our understanding of how changes in nutrient concentrations and stoichiometry potentially affect aquatic ecosystems as a whole. In this study, bacterial communities were examined in biofilms from cobbles collected across seasons at three sites along the Mahoning River (Ohio) with differing levels of inorganic nutrient inputs. Members of the alpha-, beta-, and gamma-proteobacteria, the Cytophaga-Flavobacteria cluster, and the Domain Bacteria were enumerated using fluorescent in situ hybridization. Detrended canonical correspondence analysis (DCCA) revealed that stoichiometric ratios, especially the dissolved inorganic nitrogen (DIN):soluble reactive phosphorus (SRP) molar ratio (NO(2)/NO(3) + NH(4):soluble reactive phosphorus), were correlated with abundance of the various bacterial taxa. However, the patterns were complicated by correlations with single nutrient concentrations and seasonal changes in temperature. Seasonal cycles appeared to play an important role in structuring the community, as there were distinct winter communities and temperature was negatively correlated with abundance of both alpha-proteobacteria and Cytophaga-Flavobacteria. However, nutrients and stoichiometry also appeared to affect the community. Numbers of cells hybridizing the Domain Bacteria probe were correlated with the DOC:DIN ratio, the beta-proteobacteria had a negative correlation with soluble reactive phosphorus concentrations and a positive correlation with the DIN:SRP ratio, and the Cytophaga-Flavobacteria had a significant negative partial correlation with the DIN:SRP ratio. Abundances of the alpha- or gamma-proteobacteria were not directly correlated to nutrient concentrations or stoichiometry. It appears that nutrient stoichiometry may be an important factor structuring bacterial communities; however, it is one of many factors, such as temperature, that are interlinked and must be considered together when studying environmental bacteria.