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•A nitrifying bioreactor was continuously operated for 61 days.•Drastic changes in stable isotope ratios were observed in the bioreactor.•δ15N and δ18O of NO3− changed spatially with ...wastewater flow.•Sludge δ13C and δ15N spatially increased from −26‰ to −18‰ and from 5‰ to 30‰•Isotopic signature of sludge and nitrate was related to process performance.
This study determined the spatial and temporal changes in natural abundance of stable isotopes (δ13C, δ15N, and δ18O) with regard to nitrate (NO3−) and retained sludge in a nitrifying bioreactor. The bioreactor was continuously fed with synthetic wastewater including ammonium for 61 days at 20 °C. After the start-up period of the bioreactor, the NO3− concentration in the effluent gradually increased. The stable isotopes (δ15N and δ18O) of NO3− in the effluent also increased in a phase of incomplete nitrification. The profile experiments showed that the concentration and stable isotopes of NO3− changed simultaneously along the wastewater flow in the bioreactor. The stable isotope analysis revealed that nitrification efficiency seems to be strongly related to the δ15N of NO3−. Moreover, the δ13C and δ15N of the retained sludge drastically changed along the reactor length, from −26‰ to −18‰ and from 5‰ to 30‰, respectively, after 61 days of operation. The isotopic composition of the retained sludge might be affected by the isotope ratios (δ15N and δ18O) of NO3− in the bioreactor. Therefore, the isotope signatures of the retained sludge seem to closely reflect process performance such as nitrification efficiency throughout the operational period. Our findings suggest that the spatial distribution of the isotopic composition of the retained sludge can be used to detect process occurrence within the bioreactor over time.
Abstract
Nitrogen-fixing activity has been observed in the rhizosphere of mangrove ecosystems, suggesting a close mangrove–diazotroph relationship. In regularly flooded soil, however, the pathway by ...which atmospheric nitrogen reaches the diazotrophs in the rhizosphere is unknown. This study provides evidence that mangrove aerial roots serve as pathways that supply nitrogen gas to the diazotrophs colonizing buried roots. A plastic chamber was attached on the exposed part of a Rhizophora stylosa Griff prop root, and 15N2 tracer gas was injected into it. The entire root, including the below-ground part, was collected for analysis of 15N labelling and nitrogenase activity. We detected 15N labelling in buried root materials 2 h after gas injection. Compared with the δ15N contents in root material from an untreated tree, the increment was >10‰ in lateral roots. The nitrogenase activity measured on the other R. stylosa roots was highest in lateral roots, matching well with the results of 15N labelling. Our results indicate that atmospheric nitrogen is taken into aerial mangrove roots through lenticels, diffuses into the buried root system and is fixed by diazotrophs. The unusual appearance of mangrove aerial roots, which has intrigued researchers for many years, could be a key to the high productivity of mangrove ecosystems.
Purpose
There is increased recognition of the importance of mangroves worldwide, with efforts being made to sustainably manage these ecosystems for forestry and fishery use. Although successive ...monitoring of mangrove growth after planting has been conducted in some afforestation stands, measurements of soil environmental changes accompanying plant growth have not been made in most stands. In this study, we observed the interactive relationship between the underground root biomass of mangrove,
Rhizophora stylosa
, and soil chemical properties at an afforestation stand on Tarawa atoll, Kiribati.
Methods
We first estimated underground root biomass in the stand. Next, we measured the concentrations of dissolved phosphorus, nitrogen, and other ions (Br
−
, Ca
2+
, K
+
, Na
+
, Cl
−
, and SO
4
2−
) in soil pore water, as well as the isotopic ratio of leaf carbon and nitrogen in mangrove patches of different ages.
Results
Estimated underground root biomass was positively related with phosphate and nitrate concentrations in soil pore water, indicating the formation of a rhizosphere environment. Leaf δ
15
N analysis suggested that the discrimination of nitrogen isotopes during nitrification and/or uptake of NH
4
+
and NO
3
−
occurs in accordance with plant growth. Differences in salt stress among the patches were reflected in leaf δ
13
C, suggesting it would be a good indicator of the physiological response of mangrove plants to salinity.
Conclusions
Our findings revealed the changes that occur on a yearly basis in the chemical properties of mangrove leaves and soil pore water after mangrove plantation. These data help to improve our understanding of environmental succession during the formation of mangrove ecosystems.
Stable isotope ratios of nitrate are a powerful tool to evaluate aquatic environment stress from treated and untreated sewage. However, there is generally a lack of knowledge on the change in stable ...isotope ratios within wastewater treatment plants. We investigated nitrogen and oxygen stable isotope ratios (δ15N and δ18O) of nitrate in four types of advanced treatment processes operated in parallel; (A) extended aeration activated sludge, (B) anaerobic-anoxic-aerobic (A2O), (C) recycled nitrification-denitrification, and (D) modified Bardenpho. The results exhibited spatial variation of δ15N and δ18O for nitrate within the treatment steps. The changes in δ15N and δ18O may result from the reactor conditions (aerobic, anoxic, and anaerobic) and the order of these processes. As decreasing nitrate concentration in the anoxic stages, the δ15N/δ18O ratio for nitrate increased at a rate of 1.3 to 1.6 coupling with the reduction in the nitrate concentration in the anoxic stages. The δ15N and δ18O signatures were attributed to process performance in regard to nitrogen removal. In particular, the modified Bardenpho process has higher nitrogen removal efficiency over other processes, producing effluent with lower nitrate concentration and higher stable isotopes (δ15N: 23.6 to 25.5‰, δ18O: 2.8 to 4.5‰). We concluded that the stable isotope signatures mirrored the treatment efficiency and effluent characteristics.
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•Isotopic ratios of NO3− were determined in municipal wastewater treatment processes.•Spatial change in stable isotope ratios was found within treatment steps.•Isotope signatures rely on treatment types and steps.•Treatment performance and characteristics may influence isotopic fluctuation.
Heat Waves Can Cause Hypoxia in Shallow Lakes Shinohara, Ryuichiro; Matsuzaki, Shin‐Ichiro S.; Watanabe, Mirai ...
Geophysical research letters,
28 April 2023, Letnik:
50, Številka:
8
Journal Article
Recenzirano
Odprti dostop
We assessed how warm air temperatures, high solar radiation, and weak wind speeds might induce hypoxia in a shallow lake during a heat wave. We simulated bottom‐water dissolved oxygen concentrations ...and compared concentrations in 2022 with the average for the previous 30 years. We found that hypoxia was most sensitive to wind speeds. When the wind speed was low, convection was insufficient to prevent hypoxia, but there was no hypoxia if the wind speed equaled the average speed during the previous 30 years. However, if solar radiation and air temperatures equaled the respective averages during the previous 30 years, hypoxia did not occur, even if wind speeds were low. We conclude that the combined effects of weak winds and either high solar radiation or air temperatures induced hypoxia during the heat wave of 2022.
Plain Language Summary
The concentration of dissolved oxygen (DO) is one of the most important characteristics of lake ecosystems. However, a mechanistic understanding of the formation of bottom‐water DO in shallow water during heat waves is still limited by the paucity of relevant data. We combined high‐frequency monitoring via a new buoy system and numerical simulation to analyze how meteorological changes during a heat wave affected bottom‐water DO concentration in a shallow lake. We found that slackening of wind speeds and increases of either air temperatures or solar radiation induced bottom‐water hypoxia by constraining convective mixing. Wind speeds during the night were low, and the supply of DO by convective mixing during the night was insufficient to prevent hypoxia.
Key Points
Hypoxia was observed in Lake Kasumigaura during a heat wave in 2022
We observed higher solar radiation and air temperatures, and weaker winds in 2022 than the average for the previous 30 years
Insufficient convection led to hypoxia because of the combined effects of weak winds, high solar radiation, and high air temperatures
The oxygenated hypolimnion accounts for a volumetrically significant part of the global freshwater systems. Previous studies have proposed the presence of hypolimnion-specific bacterioplankton ...lineages that are distinct from those inhabiting the epilimnion. To date, however, no consensus exists regarding their ubiquity and abundance, which is necessary to evaluate their ecological importance. The present study investigated the bacterioplankton community in the oxygenated hypolimnia of 10 deep freshwater lakes. Despite the broad geochemical characteristics of the lakes, 16S rRNA gene sequencing demonstrated that the communities in the oxygenated hypolimnia were distinct from those in the epilimnia and identified several predominant lineages inhabiting multiple lakes. Catalyzed reporter deposition fluorescence in situ hybridization revealed that abundant hypolimnion-specific lineages, CL500-11 (Chloroflexi), CL500-3, CL500-37, CL500-15 (Planctomycetes) and Marine Group I (Thaumarchaeota), together accounted for 1.5-32.9% of all bacterioplankton in the hypolimnion of the lakes. Furthermore, an analysis of single-nucleotide variation in the partial 16S rRNA gene sequence (oligotyping) suggested the presence of different sub-populations between lakes and water layers among the lineages occurring in the entire water layer (for example, acI-B1 and acI-A7). Collectively, these results provide the first comprehensive overview of the bacterioplankton community in the oxygenated hypolimnion of deep freshwater lakes.
The study region was the Sakura River, a tributary of Lake Kasumigaura, Japan.
We asked whether particulate organic phosphorus (POP) contributed to the P cycle in rivers. Previous studies have ...focused on the interaction between PO43– and particulate inorganic P (PIP), but our study focused on whether POP could also be the source of PO43–.
After rain events during irrigation, concentrations of suspended particles and particulate P (PP) increased. POP accounted for 42.4 % of PP concentrations. The PIP and POP concentrations were both positively correlated with suspended particle concentrations. The PO43– concentrations were significantly higher during decomposition experiments when suspended particles were present in the water. Decomposition experiments revealed that the P composition changed and the proportion of organic P decreased as DNA-P disappeared from the PP. The fact that the DOP concentration was significantly lower in the treatment with suspended particles than without suspended particles suggested that DOP mineralization rates were increased by the presence of suspended particles. In addition to mineral-sorbed P, the POP derived from the irrigation of paddy fields may also be bioavailable.
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•Organic phosphorus composition in a river was investigated.•Organic P composition and proportion were changed by decomposition.•Suspended particles promoted mineralization of dissolved organic phosphorus.
Abstract
We tested the hypothesis that long-term brightening of solar radiation and changes in the underwater light climate would increase the primary production of phytoplankton in the springtime in ...Lake Kasumigaura, Japan. Global solar radiation fluctuates on a decadal scale, and the solar radiation in May has been increasing since the 1990s in Japan. We developed a model of depth-integrated primary production (gC m−2 h−1) based on long-term monitoring data. The model based on photosynthesis-irradiance curve clarified that primary production increased by 13.2%, from 0.093 gC m−2 h−1 in 1992 to 0.105 gC m−2 h−1 in 2019, because of the combined effects of solar radiation and water temperature increases. Sensitivity analysis of the model showed that primary production was most sensitive to water quality parameters, i.e. the light attenuation coefficient and nutrient effects on Pmax. Nutrient effects were significantly correlated with the particulate organic carbon to nitrogen ratio, indicating that nitrogen shortage in phytoplankton cells may affect primary production. These results suggest that long-term springtime increases in solar radiation and water temperature increase primary production, but that water quality also controls primary production.
Purpose
Sediment oxygen demand (SOD) measurement currently requires a long preparation time and bulky experimental equipment, which represent major obstacles to conducting SOD measurements. We ...developed a new SOD measurement method that can be easily conducted in a shorter time than the existing laboratory method.
Materials and methods
The undisturbed sediment core was transferred into the custom-made small glass vial (inner diameter 12.8 mm, length 125 mm). The SOD sample was incubated in the dark with or without stirring the overlying water inside the vial. The change in DO concentration was measured using a non-contact fluorescent oxygen sensor, and then the SOD was calculated. For comparison and verification of the new method, SOD was also measured using a large undisturbed sediment core (inner diameter 11 cm, length 50 cm).
Results and discussion
The SOD measurements using the new method were positively correlated with the SOD measurements using the large undisturbed sediment core, suggesting that the new method is a feasible alternative to the conventional method. In the new method, many SOD measurements can be taken from a single sediment core sample. This makes it possible to reduce the uncertainty associated with the highly heterogeneous structure of the sediments, and thus the new method has great advantages in obtaining reliable SOD data.
Conclusions
A novel high-throughput method for SOD was developed that shortens sample preparation time and improves convenience of measurement. The new method is expected to contribute substantially to the accumulation of SOD monitoring data and to make great progress in elucidating the dynamics of dissolved oxygen in lakes.
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