Reactive Phosphorus Concentrations Research Articles

Performance of vertical flow wetland in the treatment of medium organic strength synthetic wastewater

Treatment wetlands have been used for decades to treat wastewaters with low organic strength, but little information is available on the treatment of wastewaters with high organic strength. The objective of this study was to evaluate the efficiency of a vertical flow wetland in the primary treatment of synthetic wastewater with medium organic strength (1976 mg COD/L). Alternatives in the reuse of the resulting treated wastewater were also considered. The wetland consisted of several layers of sand and gravel of different granulometry and was planted with the helophyte Eryngium horridum, a species rarely used in treatment wetlands. The wetland was fed with synthetic wastewater by a single feed pulse, with a hydraulic retention time of 24 h. The treatment removed 81% of TSS and 70% of COD. The concentrations of ammonium nitrogen and soluble reactive phosphorus increased due to partial mineralization of organic matter. Total phosphorus and total nitrogen removal were 28% and 5%, respectively. Wastewater suitability for reuse in irrigation increased after treatment. However, a later stage is recommended to further reduce COD and nutrient concentration and disinfect the water.

Quantifying the relative importance of agricultural land use as a predictor of catchment nitrogen and phosphorus concentrations

Agriculture is a major source of nitrogen (N) and phosphorus (P) in freshwater ecosystems, and different management strategies exist to reduce farmland nutrient losses and thus mitigate freshwater eutrophication. The importance of agricultural sources of N and P as drivers of water quality is known to vary spatially, but quantification of the relative importance of the nutrient sources shaping this variability remains challenging, especially with reference to inputs from waste water treatment works. Addressing this knowledge gap is key for targeting management strategies to where they are likely to have the greatest effect. To advance our understanding in this area, this study assesses the impact of population density as a driver of the relative importance of agricultural land use for predicting mean Total Oxidised Nitrogen (TON) and Reactive Phosphorus (RP) concentrations in rivers in England, using two different data-driven, statistical approaches: a generalised linear model and random forest. Our results show that agricultural N and P sources dominate in catchments with low population density, where stream water concentrations are lower and waste water treatment works are numerous, but smaller in terms of the population equivalent served. Agricultural N and P sources are not important predictors of N and P in catchments with high population density, where contributions from waste water treatment works dominate. These results require cautious interpretation, as model validation outcomes show that high TON and RP concentrations are consistently underpredicted. Altogether, our results suggest that the relative contribution of agricultural sources may be overestimated in densely populated catchments, relative to point sources from waste water treatment works, and that management strategies to reduce the contribution of agriculture to N and P in rivers may be better targeted towards catchments with lower population density, as this is where agricultural land use is the primary source of N and P.

A landscape limnology approach to assessing controls on soluble reactive phosphorus in sediment porewater and internal loading risk

Sediment nutrients can be mobilized to overlying water via internal loading, which can be important to aquatic productivity. Using data from 143 Canadian lakes, we show high (~2400-fold) variation of soluble reactive phosphorus (SRP) concentrations in surficial sediment porewater, with results suggesting internal phosphorus loading (IPL) is also likely to vary widely. Consistent with past work at smaller scales, we show that lake depth, pH, trophic status, and bulk sediment Al:P and Fe:P influence porewater SRP, and IPL. Median porewater SRP concentration in lakes with high Al:P (molar ratios >10) were 4.8-fold smaller than in lakes with lower Al:P. In lakes where bulk sedimentary Fe:P molar ratios were >10 porewater SRP was 3.9-fold lower than in lakes with lower Fe:P. High pH (>7.8), along with hyper-eutrophic lakes were associated with higher porewater SRP. Finally, shallow lakes (<4 m depth) had median porewater SRP concentration 6-fold higher than deep lakes (>9 m depth). Important regional differences emerged, linked to regional variation in pH, soils, lake depth and trophic status, and paralleling areas of poor water quality. For example, median porewater SRP in lakes from the Boreal Plains and Prairies ecozones (dominated by Chernozems/Mollisols) was 64-fold and 44-fold higher than in the Boreal Shield (dominated by Podzols/Spodosols) (respectively), although we note that IPL risk is likely important across many ecozones. Using national data, we found in-lake measurements (particularly pH, and salinity) showed strong capacity in predicting porewater SRP (explaining 60–72 % of the variance in the data). Importantly, watershed predictors showed good predictive power, explaining ~50 % of variance in porewater SRP using variables including soil types, and % agriculture. Porewater SRP can be predicted with reasonable accuracy using easily measured variables, as can estimates of internal phosphorus loading, suggesting that landscape limnology holds strong potential in helping to inform lake management by informing understanding of in-lake nutrient sources.

Spatiotemporal investigation of geochemical and hydrological controls on release of soluble reactive phosphorus from the shallow aquifer of a riparian zone.

Phosphorus (P) that accumulates in agricultural riparian zones can be released under certain hydrological and biogeochemical conditions, thereby limiting the effectiveness of these zones in reducing P loads from field to stream. The study objective was to explore factors that may be contributing to, or limiting, high soluble reactive phosphorus (SRP) concentrations in the shallow aquifer of an alluvial upland riparian zone located in a continental climate. Field investigations including porewater sampling from six vertical nests, soil sampling, and continuous soil moisture, groundwater table, and redox measurements were conducted over 19 months. Porewater SRP concentrations were generally low in the aquifer considering all sampling times (median=14.7µg/L; interquartile range [IQR]=11.1µg/L, 287 samples). The overall low SRP may be due to low reducible labile soil P (median=21.1 µgP/g dw, IQR=10.9 µgP/g dw, 21 samples). However, high SRP concentrations(>52µg/L, 95% quartile) did occur intermittently in space and time with no clear spatial or temporal patterns. Analyses indicate that most high concentrations were likely not associated with factors previously reported to influence SRP release in riparian aquifers, including redox conditions, pH, and soil drying and wetting. Further, data indicate that internally released or externally supplied SRP may undergo rapid (re-) sequestration within the aquifer, limiting its vertical or horizontal transport. The study findings highlight the complexity of P behavior in riparian zones and the need for caution when assessing the effectiveness of conservation practices and in interpreting potential impacts of subsurface water quality on stream water quality when monitoring locations are distant from the stream.

Immobilization of phosphorus in sediment-water system by hydrous iron oxide and hydrous iron oxide/calcite mixture under feed input condition

The efficiency and mechanism of hydrous iron oxide (HFO) and HFO/calcite mixture to inactivate the phosphorus in the overlying water (OW)/sediment system under the feed adding condition were explored, and the effect of HFO and HFO/calcite mixture addition on the diversity, composition and function of bacterial communities in the sediment was examined. HFO and HFO/calcite mixture direct addition can effectively lower the concentration of soluble reactive phosphorus (RSP) and diffusion gradient in thin film-unstable phosphorus (PDGT) in OW and inactivate the PDGT in the upper sediment. The elimination efficiencies of RSP by the direct HFO and HFO/calcite mixture addition were 48.9–97.0 % and 42.4–95.4 %, respectively. The alteration in the addition mode from the one-time to multiple direct addition was beneficial to the immobilization of RSP and PDGT in OW and PDGT in the upper sediment by HFO and HFO/calcite mixture under the feed input condition in the long run. Permeable fabric wrapping reduced the inactivation efficiency of RSP in OW by HFO and HFO/calcite mixture, but it made the recycling of these materials possible. Most of P immobilized by HFO and HFO/calcite mixture was relatively or very stable. After the HFO and HFO/calcite mixture addition, the composition of bacterial communities in the surface sediment changed. However, the bacterial communities in the amended sediments still can perform good ecological function. Our findings suggest that HFO and HFO/calcite mixture are promising phosphorus-immobilization materials for the inactivation of RSP and PDGT in OW and PDGT in the upper sediment under the feed inputting condition.

Predicting river phytoplankton blooms and community succession using ecological niche modeling

AbstractExcessive phytoplankton concentrations in rivers can result in the loss of plant and invertebrate communities, and threaten drinking water supplies. Whilst the physicochemical controls on algal blooms have been identified previously, how these factors combine to control the initiation, size, and cessation of blooms in rivers is not well understood. We applied flow cytometry to quantify diatom, chlorophyte, and cyanobacterial group abundances in the River Thames (UK) at weekly intervals from 2011 to 2022, alongside physicochemical data. A niche modeling approach was used to identify thresholds in water temperature, flow, solar radiation, and soluble reactive phosphorus (SRP) concentrations required to produce periods of phytoplankton growth, with blooms only occurring when all thresholds were met. The thresholds derived from the 2011 to 2018 dataset were applied to a test data set (2019–2022), which predicted the timing and duration of blooms at accuracies of &gt; 80%. Diatoms and nano‐chlorophyte blooms were initiated by flow and water temperature, and usually terminated due to temperature and flow going out of the threshold range, or SRP and Si becoming limiting. Cyanobacterial bloom dynamics were primarily controlled by water temperature and solar radiation. This simple methodology provides a key understanding of phytoplankton community succession and inter‐annual variation and can be applied to any river with similar water quality and phytoplankton data. It provides early warnings of algal and cyanobacterial bloom timings, which support future catchment management decisions to safeguard water resources, and provides a basis for modeling changing phytoplankton bloom risk due to future climate change.

Spring applied phosphorus loss with cover crops in no-till terraced field

Cover crops (CC) can improve phosphorus (P) cycling by reducing water related P losses and contributing to P nutrition of a rotational crop. This is particularly important in claypan soils with freeze-thaw cycles in early spring in the Midwest U.S. This 4-year study (2019–2022) examined the impact of CC monoculture and mix of CC species on P losses from a fertilizer application, and determined the P balance in soil compared to no cover crop (noCC). The CC mix consisted of wheat (Triticum aestivum L.), radish (Raphanus raphanistrum subsp. Sativus), and turnip (Brassica rapa subsp. Rapa) (3xCCmix) in 2019 and 2021 before corn, and cereal rye (Secale cereale L.) was planted as monoculture before soybean in 2020 and 2022. The 3xCCmix had no effect on total phosphorus (TP) and dissolved reactive phosphorus (PO4–P) concentration or load in 2019 and 2021. Cereal rye reduced TP and PO4–P load 70% and 73%, respectively, compared to noCC. The variation in soil moisture, temperature, and net precipitation from fertilizer application until CC termination affected available soil P pools due to variability in CC species P uptake, residue decomposition, and P loss in surface water runoff. Overall, the P budget calculations showed cereal rye had 2.4 kg ha−1 greater P uptake compared to the 3xCCmix species which also reduced P loss in water and had greater differences in soil P status compared to noCC. This study highlights the benefit of CCs in reducing P loss in surface runoff and immobilizing P through plant uptake. However, these effects were minimal with 3xCCmix species and variability in crop residue decomposition from different CC species could affect overall P-soil balance.

Nutrient structure dynamics and microbial communities at the water-sediment interface in an extremely acidic lake in northern Patagonia.

Lake Caviahue (37° 50 'S and 71° 06' W; Patagonia, Argentina) is an extreme case of a glacial, naturally acidic, aquatic environment (pH ~ 3). Knowledge of the bacterial communities in the water column of this lake, is incipient, with a basal quantification of the bacterioplankton abundance distribution in the North and South Basins of Lake Caviahue, and the described the presence of sulfur and iron oxidizing bacteria in the lake sediments. The role that bacterioplankton plays in nutrient utilization and recycling in this environment, especially in the phosphorus cycle, has not been studied. In this work, we explore this aspect in further depth by assessing the diversity of pelagic, littoral and sediment bacteria, using state of the art molecular methods and identifying the differences and commonalties in the composition of the cognate communities. Also, we investigate the interactions between the sediments of Lake Caviahue and the microbial communities present in both sediments, pore water and the water column, to comprehend the ecological relationships driving nutrient structure and fluxes, with a special focus on carbon, nitrogen, and phosphorus. Two major environmental patterns were observed: (a) one distinguishing the surface water samples due to temperature, Fe2+, and electrical conductivity, and (b) another distinguishing winter and summer samples due to the high pH and increasing concentrations of N-NH4+, DOC and SO42-, from autumn and spring samples with high soluble reactive phosphorus (SRP) and iron concentrations. The largest bacterial abundance was found in autumn, alongside higher levels of dissolved phosphorus, iron forms, and increased conductivity. The highest values of bacterial biomass were found in the bottom strata of the lake, which is also where the greatest diversity in microbial communities was found. The experiments using continuous flow column microcosms showed that microbial growth over time, in both the test and control columns, was accompanied by a decrease in the concentration of dissolved nutrients (SRP and N-NH4+), providing proof that sediment microorganisms are active and contribute significantly to nutrient utilization/mobilization.

Godzilla mineral dust and La Soufrière volcanic ash fallout immediately stimulate marine microbial phosphate uptake

During the “Godzilla” dust storm of June 2020, unusually high fluxes of mineral dust traveled across the Atlantic from the Sahara Desert, reaching the Caribbean Basin, Gulf Coast, and southeastern United States. Additionally, an eruption of the La Soufrière volcano on St. Vincent in April 2021 generated substantial ashfall in the southeastern Caribbean. While many studies have analyzed mineral dust’s ability to relieve nutrient limitation of phosphorus (P) in the P-stressed North Atlantic, less is known about the impact of extreme events and other natural aerosols on fluxes of P into seawater and from seawater into marine microbial cells. We quantified P and iron (Fe) content in mineral dust from the Godzilla dust storm and volcanic ash from the La Soufrière eruption collected at Ragged Point, Barbados. We also performed seawater incubations to assess the marine microbial response to aerosol deposition. Using environmentally-relevant concentrations of atmospheric particles for within the ocean’s mixed layer allowed us to draw realistic conclusions about how these deposition events impacted P cycling in situ. Volcanic ash has lower P content than mineral dust, and P in volcanic ash is far less soluble (~1%) than assumed in current atmospheric deposition models. Adding mineral dust and the volcanic ash leachate in concentrations representing different deposition scenarios increased soluble reactive phosphorus (SRP) concentrations in coastal seawater by ~7-32 nM. Phosphate uptake rate was stimulated in coastal seawater after either mineral dust or volcanic ash deposition at aerosol concentrations relevant to the Godzilla dust event, with ash eliciting the fastest uptake rate. Furthermore, high concentrations of both the mineral dust and volcanic ash led to slightly elevated alkaline phosphatase activity (APA) compared to the relevant controls, indicating higher potential for use of dissolved organic phosphorus (DOP) as a P source. Quantifying these aerosols’ impacts on P cycling is a significant step towards achieving a better understanding of their potential roles in relieving nutrient limitation and fueling the biological carbon pump.

Environmental implications of salinity and flooding induced changes in porewater nitrogen and phosphorous dynamics

ABSTRACT This study aimed to investigate the effects of fresh water and seawater flooding on phosphorous (P) and nitrogen (N) dynamics in two calcareous soils (Biscayne and Krome) of South Florida using a 28-day greenhouse flooding experiment. Porewater samples were collected at three depths over four periods (1, 7, 14, and 28 days) of flooding and analyzed for selected water quality parameters. Results showed that seawater-flooded Biscayne soil had a 4.4 fold higher porewater soluble reactive phosphorus (SRP) concentration than freshwater. Additionally, porewater SRP concentration and total phosphorus (TP) in Biscayne soil increased with prolonged flooding duration and column depth. Furthermore, both freshwater and seawater flooding increased porewater NH4 + - N concentration; however, the increase was more pronounced in seawater flooded samples. NH4 + - N concentration also increased with flooding duration. Overall, the study highlights the risks of nutrient leaching from seawater flooded coastal soils that could contribute to the deterioration of surface and groundwater quality.

Establishing riverine nutrient criteria using individual taxa thresholds

Nutrient enrichment is one of the most pervasive impacts on aquatic ecosystems globally. Approaches to establish nutrient criteria that safeguard aquatic ecosystem health are highly variable and, in many instances, criteria are derived from correlations between in-situ nutrient concentrations and biological indices. Summarising entire assemblages with a single index can result in a substantial loss of information and potentially weaker relationships. In this study, we compared the derivation of nutrient criteria using biological indices and those from individual taxa for rivers and streams in New Zealand. Random forest models, including nutrient concentrations, were built to predict two biological indices and individual taxa across New Zealand's river monitoring network. For all acceptable models, the response of the biological indices and individual taxa to increasing Dissolved Inorganic Nitrogen (DIN) and Dissolved Reactive Phosphorus (DRP) were then predicted for every river reach across the nation, and nutrient concentrations that protected 80% of taxa were then identified. Models for the biological indices were poor but were good for most of the taxa, with nutrient concentrations almost always being the most influential factor. To ensure persistence of at least 80% of the taxa within a river reach, we estimated that DIN (Dissolved Inorganic Nitrogen) concentrations would need to be below 0.57–1.32 mg/L, and DRP (Dissolved Reactive Phosphorus) concentrations below 0.019–0.033 mg/L, depending on the river type. In general, high order, low slope rivers and streams required more stringent nutrient criteria than steep, low order streams. The link between nutrient concentrations and biological indices were weak and likely suffer from the loss of information from summarising an entire assemblage into a single numeric. We consider that the derivation of nutrient criteria for waterways should also examine the individual relationships with the taxa in a river system to establish protection for a desired proportion of taxa.

Microbial interactions strengthen deterministic processes during community assembly in a subtropical estuary

Systematic studies on the assembly process and driving mechanisms of microbial communities in estuaries with diverse seasonal and spatial scales are still limited. In this study, high-throughput sequencing, and microbial network analysis were combined to decipher the impact of environmental changes and biological interactions on the maintenance of microbial diversity patterns in the Jiulong River Estuary (JRE). The results showed that overall, stochastic processes dominated the bacterioplankton community assembly in the estuary, accounting for 49.66–74.78 % of the total. Additionally, bacterioplankton community diversity varied significantly across seasons and subzones. Specifically, the concentration of soluble reactive phosphorus (SRP) in the estuary steadily reduced from winter to summer, and the corresponding bacterioplankton community interactions gradually shifted from the weakest interaction in winter to the strongest in summer. The deterministic processes contributed more than half (50.34 %) to microbial assembly in the summer, but only 25.22 % in winter. Deterministic processes prevailed in the seaward with low SRP concentrations and strong bacterioplankton community interactions, while stochastic processes contributed 70.14 % to the assembly of microbial communities riverward. Biotic and abiotic factors, such as nutrients and microbial interactions, jointly drove the seasonal and spatial patterns of bacterioplankton community assembly, but overall, nutrients played a dominant role. Nevertheless, the contributions of nutrients and microbial interactions were equivalent in spatial assembly processes, albeit nutrients were the primary seasonal driver of the bacterioplankton community assembly process. This study emphasizes the significance of microbial interactions in the bacterioplankton community assemblage. These findings provide new and comprehensive insights into the microbial communities' organization in estuaries.

Application of classification machine learning algorithms for characterizing nutrient transport in a clay plain agricultural watershed

Excess nutrients in surface water and groundwater can lead to water quality deterioration in available water resources. Thus, the classification of nutrient concentrations in water resources has gained significant attention during recent decades. Machine learning (ML) algorithms are considered an efficient tool to describe nutrient loss from agricultural land to surface water and groundwater. Previous studies have applied regression and classification ML algorithms to predict nutrient concentrations in surface water and/or groundwater, or to categorize an output variable using a limited number of input variables. However, there have been no studies that examined the application of different ML classification algorithms in agricultural settings to classify various output variables using a wide range of input variables. In this study, twenty-four ML classification algorithms were implemented on a dataset from three locations within the Upper Parkhill watershed, an agricultural watershed in southern Ontario, Canada. Nutrient concentrations in surface water were classified using geochemical and physical water parameters of surface water and groundwater (e.g., pH), climate and field conditions as the input variables. The performance of these algorithms was evaluated using four evaluation metrics (e.g., classification accuracy) to identify the optimal algorithm for classifying the output variables. Ensemble bagged trees was found to be the optimal ML algorithm for classifying nitrate concentration in surface water (accuracy of 90.9%), while the weighted KNN was the most appropriate algorithm for categorizing the total phosphorus concentration (accuracy of 87%). The ensemble subspace discriminant algorithm gave the highest overall classification accuracy for the concentration of soluble reactive phosphorus and total dissolved phosphorus in surface water with an accuracy of 79.2% and 77.9%, respectively. This study exemplifies that ML algorithms can be used to signify exceedance of recommended concentrations of nutrients in surface waters in agricultural watersheds. Results are useful for decision makers to develop nutrient management strategies.

Nutrient uptake in tropical rivers receiving wastewater treatment plant discharge: High mass removal but low nutrient uptake efficiencies

Wastewater treatment plants (WWTPs) have eliminated many problems related to sewage inputs to aquatic systems. However, the treated effluents still affect river and stream ecosystems. Yet, very limited information is available about the fate of treated effluents in tropical receiving water bodies. Here, we examined the longitudinal changes in ammonium (NH4-N), nitrate (NO3-N), and soluble reactive phosphorus (SRP) concentrations in reaches downstream from chronic WWTP inputs in three receiving rivers in São Paulo state (Southeastern Brazil). The studied WWTPs were not designed or operated to have specific tertiary treatment for nutrient removal at the time of the study. We conducted a total of five sampling campaigns between 2019 and 2021. We applied the nutrient spiraling approach to these high-magnitude nutrient additions to examine net nutrient uptake. Changes in nutrient loads along the reaches were also examined. Overall, nutrient concentrations and loads were considerably increased by WWTP discharges. Net uptake lengths (SW-net) for NO3-N, NH4-N, and SRP ranged from 1.2 to 13.6, 1.5–infinity (uptake coefficient < 0), and 1.2–7.5 km, respectively. Net uptake velocities (Vf-net) varied between 0.7–2.5, −0.4 to 4.1, and 0.2–10.2 mm/min, respectively. Areal net uptake rates (Unet) ranged from 0.78 to 10.7, −20.08–45.8, and 0.18–4.4 g/m2/min for NO3-N, NH4-N, and SRP, respectively. High export (long SW-net) and limited net uptake along the reaches (low Vf-net) indicated that nutrients were transported downstream for long distances without efficient removal (nutrient availability higher than demand) despite the high mass removal (high Unet). These results suggest that the export of nutrients is substantial, potentially creating water quality impairments to downstream ecosystems. Our study highlighted that WWTPs with limited nutrient removal have considerable effects on water chemistry, nutrient cycling and loading in tropical receiving freshwaters.

Innovative receiving phase for Chemcatcher® passive sampler for phosphorus in the water environment: Calibration of sampling rate by water temperature and pH

Passive sampling is a technique for monitoring orthophosphate (PO4-P) in the water environment. Compared with traditional grab sampling followed by PO4-P quantification, kinetic-type passive samplers such as Chemcatcher® express representative concentrations of PO4-P as time-weighted average concentrations (CTWA). They can also potentially evaluate much lower PO4-P concentrations, but the available receiving phases of Chemcatcher® used for PO4-P were extremely limited. We developed a new receiving phase, the PSfZS sheet, comprising a zirconium sulfate–surfactant micelle mesostructure and polysulfone matrix. We examined its performance in terms of PO4-P sorption characteristics, PO4-P selectivity, and PO4-P sampling rate (Rs). Its capacity was adequate (12.0 μg-P/cm2) and selectivity for PO4-P uptake was good. The Rs for PO4-P increased with increasing water temperature (8.1–29.1 °C) and decreasing pH (4.1–9.7) in a laboratory calibration, and ranged from 5.27 × 10−2 L/d to 1.66 × 10−1 L/d. We placed the samplers in a municipal wastewater treatment plant, a shallow eutrophic lake, and an oligotrophic caldera lake. The Rs in the deployment sites was calibrated by monitored water temperature and pH. The estimated CTWA of PO4-P in the municipal wastewater treatment plant was similar to the averaged concentration of soluble reactive phosphorus determined by multiple grab samplings. In the lake deployments, we found that the new sampler can quantify CTWA values of PO4-P below 10 μg/L, and thus it provides more technical monitoring options and contributes to the conservation and management of the water environment.

Promoting effect and mechanism of residual feed organic matter on the formation of cyanobacterial blooms in aquaculture waters

Cyanobacterial blooms and eutrophication often occur in aquaculture waters where feeds are used. We hypothesize that residual feed organic matter (RFOM) can contribute to increased dissolved reactive phosphorus (SRP) concentrations in the water column by altering sediment phosphorus release characteristics (PRCS), including phosphorus fractions (i.e., Fe–P) and phosphorus adsorption (i.e., Qmax, EPC0), which is ultimately conducive to the formation of cyanobacteria blooms. Three types of aquaculture waters that were never feeding (NF), long-term feeding (LF), and stopped feeding (SF) for several years were studied to verify the above hypothesis. The results showed that the concentrations of SRP and chlorophyll-a in LF were significantly higher than those in SF and NF (P < 0.05). RFOM significantly affects PRCS with a power function relationship with EPC0 and a linear relationship with Qmax. Similarly, SRP in the water column was positively correlated with PRCS (i.e., Fe–P, EPC0), cyanobacterial biomass, and trophic state index (P < 0.01). This suggested that SRP may be an essential link between sediment and algal community structure. Simulation experiments further confirmed the above hypothesis, With the increase in RFOM (94.36 g/kg), PRCS (i.e., Fe–P, EPC0, and Qmax) increased significantly, which were 6.60, 282.36, and 2.47 times that of the control group, respectively. The sediment bacterial community and the partial least squares path model indicated that RFOM could regulate the SRP concentration in the water body and promote the formation of cyanobacterial blooms through PRCS, in which Firmicutes played an important role. This study revealed the driving process and mechanism of feeding behavior on the formation of cyanobacterial blooms and eutrophication in aquaculture waters, which provided a basis for the health management and cleaning strategy of aquaculture waters.

Cyanobacterial organic matter (COM) positive feedback aggravates lake eutrophication by changing the phosphorus release characteristics of sediments

Phosphorus is a key nutrient that causes eutrophication in lakes. Our investigation of 11 eutrophic lakes found that the concentrations of soluble reactive phosphorus (SRP) in the water column and EPC0 in sediments decreased with aggravated eutrophication. There was a significant negative correlation between the SRP concentrations and eutrophication parameters such as chlorophyll a (Chl-a), total phosphorus (TP) and algal biomass (P < 0.001). In addition, SRP concentrations were significantly affected by EPC0 (P < 0.001), while EPC0 was significantly affected by the content of cyanobacterial organic matter (COM) in sediments (P < 0.001). Based on these findings, we hypothesized that COM can alter the phosphorus release characteristics of sediments, including the phosphorus adsorption parameters of sediment (PAPS) and the phosphorus release rate of sediment (PRRS), thereby stabilizing SRP concentrations at lower levels and rapidly replenishing them when depleted by phytoplankton, which in turn benefits cyanobacteria due to their low SRP adaptation strategies. Simulation experiments were conducted to confirm this hypothesis by adding higher plant OM and COM to sediments. The results showed that all types of OM could significantly increase the maximum phosphorus adsorption capacity (Qmax), but only COM could reduce sediment EPC0 and promote PRRS (P < 0.001). Changes in these parameters (i.e., Qmax, EPC0, and PRRS) resulted in a larger SRP adsorption quantity and faster SRP release rate at low SRP concentrations. This promotes the competitive edge of cyanobacteria due to they have a higher affinity for phosphorus than other algae. As an important component of cyanobacteria, EPS can change the phosphorus release characteristics (i.e., PAPS and PRRS) by reducing sediment particle size and increasing sediment surface functional groups. This study revealed the positive feedback effect of COM accumulation in sediments on lake eutrophication from the perspective of phosphorus release characteristics of sediments, which provides a basic reference for the risk assessment of lake eutrophication.

The development of deep-ocean anoxia in a comprehensive ocean phosphorus model

We analyse a model of the phosphorus cycle in the ocean given by Slomp and Van Cappellen (Biogeosciences 4:155–171, 2007. https://doi.org/10.5194/bg-4-155-2007). This model contains four distinct oceanic boxes and includes relevant parts of the water, carbon and oxygen cycles. We show that the model can essentially be solved analytically, and its behaviour completely understood without recourse to numerical methods. In particular, we show that, in the model, the carbon and phosphorus concentrations in the different ocean reservoirs are all slaved to the concentration of soluble reactive phosphorus in the deep ocean, which relaxes to an equilibrium on a time scale of 180,000 y, and we show that the deep ocean is either oxic or anoxic, depending on a critical parameter which we can determine explicitly. Finally, we examine how the value of this critical parameter depends on the physical parameters contained in the model. The presented methodology is based on tools from applied mathematics and can be used to reduce the complexity of other large, biogeochemical models.

Quantification of Nutrient Fluxes from Sediments of Lake Hulun, China: Implications for Plateau Lake Management

Dramatic changes in lake water conditions may induce sensitive feedback in sediments, such as the release of phosphate and ammonia. There is a lack of the understanding of sediment nutrient release fluxes and their responses to various environmental factors in plateau lakes. In this study, nutrient contents in the water and sediment, sediment nutrient release rates, and their responses to various influential factors in a steppe shallow plateau lake, Lake Hulun, China, were investigated. Sediment is a large store of nutrients, with bioavailable phosphorus of 555 mg kg−1 in surface lake sediments. If water became anoxic, sediments will release appreciable amounts of phosphate (2.1 mg P m−2 d−1) and ammonia (40.0 mg N m−2 d−1), three times greater than those in oxic conditions. Raised temperatures greatly increased sediment phosphate release rates, while the highest temperature (23 °C) slightly decreased ammonia release rates due to strong nitrification. Assuming a whole year of internal loading, ammonia and dissolved reactive phosphorus concentrations in water column would increase by an estimated 0.22 mg N L−1 and 0.01 mg P L−1, respectively, which is equal to 2 and 0.5 times of their ambient concentrations. A pH increase from 8.4 to 9.4 significantly decreased internal nitrogen and phosphorus release rates by around three times. Overall, dissolved oxygen (DO) and pH had a greater impact on sediment nutrient release from lake sediments compared to temperature, and further increased the eutrophic status in the overlying water when these three parameters varied within the normal seasonal range (temperature: 1.9–23.1 °C, DO: oxic and anoxic; pH: 7.5–9.4). With the periodic low DO and frequency of drought in the future, lake management strategies that mitigate water eutrophication, such as water diversion, may need to be considered.

Seasonal variation and release of soluble reactive phosphorus in an agricultural upland headwater in central Germany

Abstract. Soluble reactive phosphorus (SRP) concentrations in agricultural headwaters can display pronounced seasonal variability at low flow, often with the highest concentrations occurring in summer. These SRP concentrations often exceed eutrophication levels, but their main sources, spatial distribution, and temporal dynamics are often unknown. The purpose of this study is therefore to differentiate between potential SRP losses and releases from soil drainage, anoxic riparian wetlands, and stream sediments in an agricultural headwater catchment. To identify the dominant SRP sources, we carried out three longitudinal stream sampling campaigns for SRP concentrations and fluxes. We used salt dilution tests and natural 222Rn to determine water fluxes in different sections of the stream, and we sampled for SRP, Fe, and 14C dissolved organic carbon (DOC) to examine possible redox-mediated mobilization from riparian wetlands and stream sediments. The results indicate that a single short section in the upper headwater reach was responsible for most of the SRP fluxes to the stream. Analysis of samples taken under summer low-flow conditions revealed that the stream water SRP concentrations, the fraction of SRP within total dissolved P (TDP), and DOC radiocarbon ages matched those in the groundwater entering the gaining section. Pore water from the stream sediment showed evidence of reductive mobilization of SRP, but the exchange fluxes were probably too small to contribute substantially to SRP stream concentrations. We also found no evidence that shallow flow paths from riparian wetlands contributed to the observed SRP loads in the stream. Combined, the results of this campaign and previous monitoring suggest that groundwater is the main long-term contributor of SRP at low flow, and agricultural phosphorus is largely buffered in the soil zone. We argue that the seasonal variation of SRP concentrations was mainly caused by variations in the proportion of groundwater present in the streamflow, which was highest during summer low-flow periods. Accurate knowledge of the various input pathways is important for choosing effective management measures in a given catchment, as it is also possible that observations of seasonal SRP dilution patterns stem from increased mobilization in riparian zones or from point sources.