Internal Phosphorus Loading Research Articles

Mesocosm studies clarify effects of external and internal phosphorus (P) loads to treatment wetlands at lower limits of P removal

This study evaluates wetland phosphorus (P) removal where P concentrations can be extremely low (∼20 μg/L or less). Stormwater Treatment Area (STAs) are large water quality improvement projects constructed for Everglades restoration, where research is underway to understand the processes that control P removal at already low P concentrations, particularly in the lower reaches of the flow-ways, upstream of the discharge structures. One concept that is being researched consists of scraping the P-enriched muck soil from the discharge region of STAs to expose the underlying limerock (LR) substrate. This has been investigated in numerous subscale platforms and results in proliferation of periphyton and submerged aquatic vegetation (SAV) communities. In principle, it also reduces internal P loads to the water column, although this point has lacked direct data support. One 40-ha field-scale periphyton-based STA (PSTA) cell has operated since 2008 and has regularly reduced ∼15 μg/L inflow TP to ∼10 μg/L (long-term flow-weighted means). This study documents two mesocosm experiments aimed at better understanding PSTA systems. The first experiment investigated the response of outflow P to external P loading rates (PLR = 0.3–1.3 g/m2/yr). Together with the field data from full-scale STAs, the results establish that PSTA systems are responsive to upstream P inputs (a reduction in external P inputs results in lower outflow P), whereas adjacent muck-based STA treatment cells receiving similar P inputs are not. The second experiment investigated differences in discharge TP and plant tissue and sediment P. Seven substrate-vegetation combinations were used: 0, 5, and 15 cm of limerock (LR) “cap” over indigenous muck with and without SAV found in the STAs; and a LR-only system inoculated with SAV but heavily dominated by periphyton. Rooted SAV produced internal P loads to the water column at a magnitude that is dependent upon access to the soil-P pool (muck only > LR cap over muck > LR only). Thus, the likely reason why PSTA achieves lower P discharges and exhibits external PLR influence is very low internal P loading rates (iPLR), where more elevated iPLR in muck-based systems appears to mask the effects of reducing external loading under low-P conditions. These results improve our understanding of external and internal P load contributions in low-P wetlands.

Out of oxygen: Stratification and loading drove hypoxia during a warm, wet, and productive year in a Great Lakes estuary

Hypolimnetic hypoxia, or low oxygen in bottom waters, impairs ecosystem services of freshwater lakes and estuaries globally. Both hypoxia incidence and intensity are increasing around the world due to eutrophication and climate change. As the hypolimnion becomes hypoxic and ultimately anoxic, sediment-bound legacy phosphorus is released. Water column mixing due to large storm events or fall turnover entrains these nutrients to the surface, causing harmful algal blooms. To assess the dynamics of hypoxia throughout the growing season, we evaluated Muskegon Lake, where hypoxia recurs annually, utilizing high-frequency time-series data from the Muskegon Lake Observatory (MLO) buoy (https://www.gvsu.edu/wri/buoy/), biweekly nutrient sampling, and seasonal respiration experiments during 2021. While water-column stratification set the stage for hypolimnetic hypoxia, frequent wind-mixing events, and episodic intrusions of cold, oxygenated, upwelled Lake Michigan waters intermittently reduced the thickness or intensity of the hypoxic zone. Respiration experiments revealed that riverine and surface organic matter inputs contributed most to hypolimnetic hypoxia in the spring, whereas surface inputs did so during summer, and riverine inputs during fall, indicating seasonally variable sources drive hypoxia. Biweekly measurements indicated increased soluble reactive phosphorus in the hypolimnion during anoxia via internal phosphorus loading from the sediment with the potential for fueling surface blooms with net export of soluble reactive phosphorus and total phosphorus to nearshore Lake Michigan. Our findings on the role of seasonally changing temperature, loading, phytoplankton production, hypolimnetic respiration, and internal phosphorus loading in shaping hypoxia dynamics have relevance to similarly afflicted ecosystems in the Great Lakes Basin.

Controlled mechanism and removal performance of phosphorus in black and odorous water bodies by Fe-based layered double hydroxides

The effective approach for controlling internal phosphorus loading remains a longstanding challenge to the maintenance of remediated black and odorous water bodies. Here, three Fe-based LDHs materials were developed for alleviating the phosphorus pollution in black and odorous water through synergistic functions of phosphate adsorption, controlled release of phosphorus, and microbial metabolism. The mechanism of phosphate removal in overlying water is mainly specific electrostatic attraction and coordination. After the addition of Fe-based LDHs, the water quality indexes of black and odorous water bodies were significantly improved, including TP concentration (from 0.8 to 0.2 mg/L), the dissolved oxygen content (from 0.1 to 3.0 mg/L), COD, turbidity (from 34 to 18 FTU), and redox potential. Moreover, the morphology and physicochemical properties of sediments were regulated by Fe-based LDHs. Fluffier and smaller sediments particles due to the destruction of extracellular polymeric substances structure, which was beneficial to the release of endogenous phosphorus. Furthermore, the results of microbial community structure analysis illustrated that Proteobacteria and Dechloromonas played the most important role in phosphorus elimination in Fe-based LDHs system. That is, based on the biocompatibility of Fe-based LDHs, which act as a medium of the electron transfer in the microbial metabolism, combining with microorganisms for the synergistic remediation of black and odorous water bodies. Our study emphasizes the feasibility of Fe-based LDHs materials for the remediation of internal phosphorus loads in black and odorous water bodies, towards sustainable control.

Effect of capping mode on control of phosphorus release from sediment by lanthanum hydroxide.

The use of in situ active capping to control phosphorus release from sediment has attracted more and more attentions in recent years. It is important to identify the effect of capping mode on the control of phosphorus release from sediment by the in situ active capping method. In this study, the impact of capping mode on the restraint of phosphorus migration from sediment into overlying water (OW) by lanthanum hydroxide (LH) was studied. Under no suspended particulate matter (SPM) deposition condition, LH capping effectively restrained the liberation of endogenous phosphorus into OW during anoxia, and the inactivation of diffusive gradient in thin film-unstable phosphorus (UPDGT) and mobile phosphorus (PMobile) in the topmost sediment served as a significant role in the restraint of endogenous phosphorus migration into OW by LH capping. Under no SPM deposition, although the transformation of capping mode from the single high dose capping to the multiple smaller doses capping had a certain negative impact on the restraint efficiency of endogenous phosphorus liberation to OW by LH in the early period of application, it increased the stability of phosphorus in the static layer in the later period of application. Under SPM deposition condition, LH capping had the capability to mitigate the risk of endogenous phosphorus liberation into OW under anoxia conditions, and the inactivation of UPDGT and PMobile in the topmost sediment was a significant mechanism for the control of sediment phosphorus liberation into OW by LH capping. Under SPM deposition condition, the change in the covering mode from the one-time high dose covering to the multiple smaller doses covering decreased the efficiency of LH to limit the endogenous phosphorus transport into OW in the early period of application, but it increased the performance of LH to restrain the sedimentary P liberation during the later period of application. The results of this work suggest that the multiple LH capping is a promising approach for controlling the internal phosphorus loading in freshwater bodies where SPM deposition often occurs in the long run.

Introducing La(OH)3 nanoparticles into attapulgite for the control of sediments internal phosphorus release: Effectiveness, mechanisms, and microbial community response

In-situ passivation of engineering materials is an economical and effective method to control internal phosphorus load in aquatic systems. However, few studies have simulated aerobic and anaerobic environments to compare the engineering materials control sediment phosphorus release performance with or without sediment re-suspension. In this study, we prepared and optimized lanthanum modified thermally attapulgite (LMTA) for sediment phosphorus immobilization with a maximum adsorption capacity of 97.99 mg/g. An 88-day sediment core cultivation experiment revealed that LMTA reduced the concentration of soluble reactive phosphate (SRP) and total phosphorus (TP) by 89.3% and 79.4% in absence of re-suspension, and by 89.8% and 78.3% with re-suspension, respectively. LMTA also transformed sediment Mobile-P into HCl-P and Res-P, which are inert phosphorus forms. Moreover, high-throughput sequencing showed that Nitrospirae abundance was significantly increased with LMTA, which could promote the water nitrogen cycle. The results confirmed that LMTA effectively controls internal phosphorus release, even under sediment re-suspension conditions.

Sudden eutrophication of an aluminum sulphate treated lake due to abrupt increase of internal phosphorus loading after three decades of mesotrophy

Aluminum salts are widely used to immobilize phosphorus (P) in lakes suffering from internal loading. However, longevity of treatments varies among lakes; some lakes eutrophy faster than others. We conducted biogeochemical investigations of sediments of a closed artificial Lake Barleber, Germany that was successfully remediated with aluminum sulfate in 1986. The lake became mesotrophic for almost 30 years; a rather rapid re-eutrophication took place in 2016 leading to massive cyanobacterial blooms. We quantified internal loading from sediment and analyzed two environmental factors that might have contributed to the sudden shift in trophic state. Increase in lake P concentration started in 2016, reaching 0.3 mg L−1, and remained elevated into the spring of 2018. Reducible P fraction in the sediment was 37 - 58% of total P, indicating a high potential for mobilization of benthic P during anoxia. Estimated P release from sediments for 2017 was approximately 600 kg for the whole lake. This is consistent with sediment incubation results; higher temperature (20°C) and anoxia contributed to release of P (27.9 ± 7.1 mg m−2 d−1, 0.94 ± 0.23 mmol m−2 d−1) to the lake, triggering re-eutrophication. Loss of aluminum P adsorption capacity together with anoxia and high water temperatures (organic matter mineralization) are major drivers of re-eutrophication. Accordingly, treated lakes at some time require a repeated aluminum treatment for sustaining acceptable water quality and we recommend regular sediment monitoring in treated lakes. This is crucial given the effects of climate warming on duration of stratification in lakes which may result in the need for treatment of many lakes.

Decision Support for Lake Restoration: A Case Study in Swedish Freshwater Bodies

A considerable number of lakes in Sweden have high phosphorus internal loading from the sediments which cause cyanobacterial blooms every summer. Due to potential risks with such blooms for human health, drinking water supply, and ecosystem services, measures need to be taken to control the phosphorus content. Measures to control the phosphorus input from the surrounding land has been in focus. However, the measures have not been sufficient. This is because phosphorus deposited at the bottom of the lakes for many years are finally starting to leak to the water phase when the decomposition of sediments leads to anoxic conditions. In order to determine effective and efficient lake restoration measures, methods for lake restoration decision support by a multi-criteria analysis and the application of a decision analysis are developed. The multi-criteria analysis includes the determination of costs, longevity, and efficacy of six common lake restoration measures to reduce internal phosphorous loads in two lakes selected as a case study. The results show that aluminum treatment combines a highest efficacy with a high-cost efficiency being thus the optimal identified measure. The method involves adding an aluminum solution to the lakes’ sediment, which binds phosphorus, preventing it to be released to the water column. The multi-criteria model is integrated to a decision analytical model. The decision analytical model is used to identify the monetary socio-economic and environmental boundaries for the implementation of the optimal lake restoration measure.

Response of internal phosphorus loading from dredged and inactivated sediment under repeated resuspension in a eutrophic shallow lake

In this study, a laboratory sediment resuspension simulation system (RSS) was used to investigate the effect of wind-induced (5.3 and 8.7 m/s) repeated sediment resuspension on internal phosphorus (P) in sediment treated by dredging and La-modified clay (LMC) based inactivation in a shallow lake. The results indicated that the dredged sediment had a better capability to resist repeated wind disturbance than the LMC-inactivated sediment. The concentration of suspended solids (SS) in the inactivated treatment (70.7 mg/L) was 1.7 times that in the dredged treatment (41.7 mg/L) under moderate wind disturbance, and was similar for the two treatments under strong wind disturbance. Nevertheless, dredging performed better than inactivation in reducing 44 % total phosphorus (TP) in overlying water (43 % reduction by inactivation) and 31 % mobile P in sediment (27 % reduction by inactivation) under moderate wind disturbance (p < 0.01) compared with control treatment. Inactivation performed better in reducing 57 % P in porewater (52 % reduction by dredging) and 81 % P flux (13 % reduction by dredging) (p < 0.01) compared with control treatment. Surprisingly, under strong wind disturbance, LMC inactivation could still reduce 18 % P in porewater and 75 % P flux (p < 0.01), whereas dredging increased 25 % P in porewater and 13 % P flux compared with control treatment (p < 0.01). LMC inactivation can increase the sediment P adsorption capacity and decrease the equilibrium P concentration (EPC0) when compared with control treatment. The contrasting control effects of the two methods were probably due to the different P buffer mechanisms for the two treated sediment. The wind disturbance-induced sediment P release was controlled by the inactivation of Fe and co-inactivation of Fe and La at the surface of dredged and LMC-inactivated sediments, respectively. The results of this study indicated inactivation can be a better method to control sediment internal P loading with repeated strong wind disturbances in eutrophic lakes.

Water quality modeling of a eutrophic drinking water source: Impact of future climate on Cyanobacterial blooms

Cyanobacterial blooms are becoming more frequent in freshwater sources, causing concern throughout the world. Cyanobacterial blooms affect human health and the entire environment. Numerical modeling is an effective tool for investigating aquatic systems. In this study, a 3D hydrodynamic and water quality (ecological) model was used to simulate eutrophication of a drinking water source, Lake Vomb, in Sweden under present and future scenarios. The hydrodynamic model was set-up in MIKE 3 FM software based on meteorological, hydrological, and water quality data. The hydrodynamic model performance was satisfactory in terms of the water temperature simulation, with root-mean-square-error (RMSE) ranging from 0.38 to 1.2 °C. In the ecological model, Chlorophyll-a (Chl-a) was used as a proxy for Cyanobacteria, and the model proved acceptable in simulating the Chl-a concentrations, with a Nash-Sutcliffe efficiency (NSE) of 0.93 and 0.87 for calibration and validation respectively. The findings revealed that external nitrogen loading and internal phosphorus loading had significant impact on the nutrient concentrations in Lake Vomb. The findings also showed a correlation between Chl-a levels and total phosphorus levels in the lake. To simulate future water quality in the lake, two Representative Concentration Pathways (RCP) for the year 2050 were used to make projections for changes in air temperature and precipitation. Under the projected future climate, the simulations showed a considerable rise in Cyanobacteria biomass independent of the changes in external nutrient loading. The model findings can assist water managers in planning mitigation strategies by identifying major nutrient sources.

Advances in eutrophication science within Lake of the Woods, a complex transboundary lake – Forward to the special section

Outside of the five Great Lakes, Lake of the Woods (LoW) is the largest transboundary lake shared by Canada and the United States. Over the last two decades, an international consortium of researchers has advanced our understanding of eutrophication science in this complex watershed. This introductory paper outlines eleven research articles included in this special section, within three main themes pertinent to LoW: external phosphorus sources to LoW and internal phosphorus loading, lake and watershed modeling, and new insights into cyanobacterial and harmful algal blooms (cHABs). We conclude with a roadmap to guide future transboundary water quality management in LoW, including remaining research gaps and future monitoring needs.

Turning up the heat: Long‐term water quality responses to wildfires and climate change in a hypereutrophic lake

AbstractClear Lake (Lake County, CA, USA) is hypereutrophic and used for drinking water, tribal use, and supports a significant fishing economy. The Mendocino Complex (2018), one of the largest wildfires in California's post‐settlement history, burned 40% of the Clear Lake watershed, providing a timely opportunity to study the impacts of historical and current wildfires on this valuable aquatic resource. Using long‐term monthly monitoring data from 1968 to 2019, paired with historical watershed fire data, we found that for the three largest fire years in the watershed's history, 3‐year postfire median July–October epilimnetic total phosphorus (TP) concentrations were below or equal to 3‐year prefire TP concentrations. However, both median TP epilimnetic concentrations and deepwater temperature across the lake have increased since the late 1960s. Long‐term TP increases were more strongly correlated with monthly maximum air temperatures than precipitation, suggesting a potential role of warming‐induced water column stratification, dissolved oxygen (DO) depletion, and high potential for internal phosphorus loading. Hypoxic occurrences were correlated with higher hypolimnetic soluble reactive phosphorus and TP concentrations, but additional high‐frequency monitoring of DO will help determine the duration of anoxia and its contribution to internal phosphorus loading. These long‐term data suggest that for this large, hypereutrophic lake, wildfires did not significantly alter in‐lake TP concentrations based on long‐term, monthly monitoring and that other internal or external sources of TP may mask any wildfire effects. Nonetheless, our study underscores the value of synthesizing decades of water quality, watershed wildfire, and climate data to build a more comprehensive, nuanced picture of multiple long‐term threats to aquatic ecosystems under global change. Moreover, monitoring and studying fire effects across a wide range of lake types beyond this study will help promote more effective lake management during changing climates and increasingly frequent large wildfires.

Can the elimination of cyanobacteria by micro-sieving be an innovative lake purity improvement method?

Abstract The removal of harmful cyanobacteria has recently become an important target in water management strategies. Various methods have been developed to eliminate these microorganisms including: (1) biological methods, especially with floating treatment wetlands and riparian vegetation; (2) physical methods with aeration, mechanical circulation, and hypolimnetic oxygenation; (3) chemical methods with coagulation and flocculation processes; (4) barley straw. We propose that the in situ mechanical-based micro-sieving process provides an opportunity for this to succeed in practice. The appropriate, selective technical parameters and techniques can result in successful water quality improvement, which is essential to meet Water Framework Directive goals and especially for public health. Additionally, micro-sieving used for removing Gloeotrichia can contribute to a significant reduction in internal phosphorus loads, a necessary step in lake restoration. The theoretically probable mean cell-bound P-content transferred with G. echinulata colonies (during strong blooms) can be as high as 48 mg L−1, and this potential P-load is usually deposited on lake bottoms. The removal of cyanobacterium can result in significant limitations of internal P-sources. The method presented above could be a promising, practical, easy-to-use, and cost-effective method for managing and limiting cyanobacterial blooms.

Predicting anoxia in the wet and dry periods of tropical semiarid reservoirs

The dissolved oxygen (DO) level in the hypolimnion of lakes and reservoirs can reach anoxic conditions, which favor the release of phosphorus from the sediment bed to the water column. However, to estimate nutrient release from sediment is extremely important to quantify the duration of anoxia. In low latitude regions, the water-sediment layer is warmer than in temperate regions and eutrophication is usually more severe, potentially accelerating oxygen depletion and extending the anoxia period. Considering that the available equations to quantify the duration of anoxia were developed for temperate lakes, there is a need to effectively quantify this period in lakes and reservoirs located in other climate regions, such as the semiarid. In this study, the dynamics of thermal stratification was analyzed as a function of the Relative Water Column Stability coefficient (RWCS) and then correlated with DO dynamics for nineteen tropical semiarid reservoirs. RWCS values were higher during the rainy season, when anoxia duration was longer and the hypolimnion was thicker with respect to total water depth. Then, two new equations for quantification of anoxia duration, based on the equation originally developed for temperate climate, were adapted for the wet and dry seasons of the tropical semiarid region. The results showed that the proposed equations presented a better performance compared to the original one, which tends to underestimate anoxia in tropical semiarid reservoirs. This work intended to provide simple and locally adjusted tools to better quantify anoxic events and support the water quality and internal phosphorus load modeling for tropical semiarid reservoirs.

Benthivorous fish cause a shift from a clear water state established by combining​ phosphorus immobilization and submerged macrophytes to a turbid state: A mesocosm study

Both chemical phosphorus (P) immobilization and submerged macrophytes are widely used in lake restorations to reduce internal phosphorus loading. Their combined effects seem much stronger than used alone. However, benthivorous fish populations recover fast and they may thus counteract the restoration effects. We conducted a mesocosm experiment with three densities of crucian carp ( Carassius auratus gibelio ) to study the effects of benthivorous fish on the combined effects of lanthanum (La) modified bentonite and submerged macrophyte Vallisneria denseserrulata in lake restoration: control (fish-free), low-density (45 g m −2 ) and high-density (83 g m −2 ). Crucian carp increased the concentrations of total phosphorus, total nitrogen, total suspended solids, and chlorophyll a and deteriorated the light climate in the water column compared with the controls. The porewater soluble reactive phosphorus concentrations in the deep sediment were two times higher in the high-density treatment than in the control group and the low-density treatment. Crucian carp increased the mobile phosphorus (mainly the NaOH-OP) in the 0–2 cm sediment, likely due to the dilution of La in the surface sediment as caused by accelerated downward transportation of La, and at the same time La-bound P (part of the HCl-P fraction) was mixed deeper into the sediment with increasing fish density. In the high-density mesocosms, plant biomass and density were reduced by 37% and 51%, respectively, which is another possible mechanism for the water quality deterioration. We conclude that it is important to control benthivorous fish when conducting lake restoration by combining phosphorus immobilization and submerged macrophytes. • Benthivorous fish dampened the combined effects of Phoslock® and macrophytes. • High-density benthivorous fish increased porewater SRP concentrations. • Benthivorous fish increased the mobile P (mainly NaOH-OP) in the surface sediment. • Benthivorous fish hampered the growth of submerged macrophytes.

Decrease in macrofauna density increases the sediment phosphorus release and maintains the high phosphorus level of water column in Lake Taihu: A case study on Grandidierella taihuensis

Internal phosphorus (P) loading can increase the P level in the water column and further sustains cyanobacterial blooms. This study focused on the role of benthic fauna bioturbation in affecting the sediment P release and the P level of water column in a eutrophic lake, Lake Taihu. The macrofauna density decreased from 4766.56 ± 10541.80 ind/m2 in 2007 to 345 ± 447.63 ind/m2 in 2020 due to the frequent bottom-water hypoxia in Lake Taihu. The reduced macrofauna density majorly resulted from Grandidierella taihuensis, Limnodrilus hoffmeisteri, and Tanypus chinensis larvae, and their total density decreased by approximately 97% in 2020 compared to 2007. G. taihuensis, one of the major benthic faunas, was further used as a representative to investigate the effects of bioturbation on sediment P release using high-resolution sampling and imaging techniques. The results show that G. taihuensis can increase the O2 penetration depth by more than 20 mm through bio-irrigation, and causes the redox conditions in burrows and surrounding sediments to change dramatically within a few minutes due to the intermittent ventilation. Subsequent oxidation of the soluble Fe(II) led to the formation of Fe-oxide bound P in the surface sediments, thereby increasing the P retention in the sediments. When the G. taihuensis density was 1563 ind/m2 at the sampling site, approximately 0.12 g m−2 yr−1 P can be retained in sediments. As previous studies have shown that L. hoffmeisteri and T. chinensis played a similar role in increasing the P retention in sediments through their bioturbation activities, the sharp decline in benthic fauna density and burrowing activities in Lake Taihu should be an important reason for maintaining the high P level in the water column by decreasing the P retention in sediments.

Characterization of internal phosphorus loading in the sediment of a large eutrophic lake (Lake Taihu, China)

Lake Taihu suffers from severe algal blooms every year, which is attributed primarily to the release of sediment phosphorus (P), namely the internal P loading. However, the overall internal P loading and the P hotspots in sediment have not been fully studied. This paper presents several methods, including sequential P extraction, the use of diffusive gradient in thin film (DGT), and intact core incubation to give a detailed investigation of sediment internal P loading as well as its roles in algal dominated zones (ADZs) and grass dominated zones (GDZs) in Lake Taihu. Sediment microbial composition was also analyzed to investigate its relationship with P fractions. The results indicate that the total P and the mobile P fraction in the ADZ sediments are generally higher than those of the GDZ sediments. The percentage of sediment mobile P to TP is similar to the mobile P in their distributions. In contrast, calcium bound P accounts for most of the TP in GDZ, while mobile P contributes the most to TP in ADZ. Overall, sediment can release 256 tons of TP and 217 tons of soluble reactive phosphorus (SRP) over a period of six months in the warmer seasons. Similarly, a high concentration of DGT-measured P was observed in ADZs that are recognized as P hotspots in Lake Taihu. Sediments in ADZ and GDZ was dominated by the bacteria Firmicutes and Proteobacteria, respectively and which were closely related with mobile P and calcium bound P in sediment, respectively. GZD seems to be able to retain more P in sediments, thereby reducing its contribution to of internal P loading. These results indicate that the difference in sediment composition between ADZ and GDZ affects their roles in sediment internal P loading, therefore, different management strategies should be used to combat sediment internal P loads in the two zones.

Isolation of Inorganic-Phosphate-Solubilizing Bacteria and Analysis of their Effect on Phosphorus Release from Lagoon Sediments

Internal phosphorus (P) loading from sediments is an important cause of eutrophication, and inorganic-phosphate-solubilizing bacteria (IPB) are known to play a vital role in the process of sediment P release. In this study, 10 IPB strains were isolated from the sediments of a lagoon suffering from macroalgal blooms dominated by Chaetomorpha linum – Rongcheng Swan Lagoon, China – and their taxonomy was determined by 16S rRNA sequence analysis. The phosphate-solubilizing characteristics of the obtained IPB strains were evaluated using a liquid medium, and the ability of IPB strains to dissolve P from sediments and the transformation of inorganic phosphorus (IP) fractions were investigated by inoculating IPB strains into a sediment–water experimental system. The 10 isolated IPB strains belonged to one family, two genera, and four species. All IPB strains in the liquid medium culture dissolved tricalcium phosphate, but the phosphate-solubilizing ability of IPB varied greatly between strains and appeared to be promoted when the strains were grown in mixed-strain cultures. Bacillus megaterium showed the strongest phosphate-solubilizing ability. The IPB inoculation experiment showed a large decrease in the Ca-bound P (HCl-P) concentration of sediments in various IPB inoculation treatments, especially in sediments from the central lake. Our results indicate that, under the action of IPB, there is great potential for the release of HCl-P, which is the primary IP fraction of lagoon sediments, and more attention should be given to the endogenous P pollution in Swan Lagoon Wetland.

Improving oxygen conditions in periodically stagnant basins using sea-based measures - Illustrated by hypothetical applications to the By Fjord, Sweden

To improve the water quality of a habitat or to stop eutrophication driven by internal loading of phosphorus from anoxic bottoms, the deepwater in inshore and offshore sea basins may be oxygenated using sea-based measures. The minimum oxygen concentration DOmin in periodically stagnant basins is controlled by three factors, namely (i) the rate of oxygen depletion dDO/dt. (ii) The residence time Te of the stagnant water. (iii) The oxygen concentration DOstart in the basin water at the beginning of a stagnation period. A theoretical framework is presented, suitable for evaluation of sea-based measures to improve the oxygen conditions in periodically stagnant basins. Five different sea-based measures to improve the oxygen conditions are recognized and discussed. The application of sea-based measures might be particularly attractive for habitat improvements in basins with poor oxygen conditions despite land-based measures are fully implemented. The sea-based measures are illustrated by hypothetical applications to the By Fjord on the west coast of Sweden, a salt-stratified fjord with periodically stagnant anoxic basin water confined behind the shallow Sunninge Strait in the mouth. An interesting result of one of the hypothetical applications is that dredging of the Sunninge Strait in 1958 and 1976 reduced tidal currents by a factor of 4. This reduced the tidal power supply to vertical mixing in the basin water, which led to prolonged residence time Te, from 2-3 years to 3–5 years, and deteriorated oxygen conditions in the basin water.

Synergistic Effects and Ecological Responses of Combined In Situ Passivation and Macrophytes toward the Water Quality of a Macrophytes-Dominated Eutrophic Lake

Combined use of in situ passivation and macrophytes is a valuable technology that exerts remarkable effects on aquatic systems. However, the effectiveness and ecological functions of this combined technology for macrophytes-dominated eutrophic (MDE) lakes with organophosphorus-controlled internal phosphorus (P) loading were poorly understood. In this study, aquatic simulation experiments were performed to study the combination of La-modified materials (LMM; La-modified bentonite (LMB), and La/Al co-modified attapulgite (LAA)) with macrophytes (Myriophyllum verticillatum L. (MVL), Hydrilla verticillata (Linn. f.) royle (HVR), and Ceratophyllum demersum L. (CDL)) for the control of P mobility in the water column, and to investigate the passivator effects on the physiological characteristics of macrophytes. The mineralization of organophosphates (BD−Po, HCl−Po, and Res−Po) is an important factor for maintaining high internal P loadings and overlying water P concentrations in the experiments. Compared with individual treatment groups, the reduction of internal P release flux and porewater SRP concentrations was more obvious in the combined treatments. Moreover, the redox-sensitive P forms transformation is more pronounced in the surface sediments. In the LAA+M group, internal P release flux was reduced by 55% and 55% compared with individual passivators and macrophytes retreatment groups, respectively. In contrast, the LMB+M group decreased by 16% and 46%, respectively. Simultaneously, LMM had less effect on macrophytes traits compared with individual macrophytes group and enhanced the absorption of phosphate by macrophytes. The phosphate content of macrophytes in the LAA+M and LMB+M groups increased by 24% and 11%, respectively, in comparison with the individual macrophytes group. Results concluded that the combination of passivator and macrophytes enhanced the effect of ecological restoration and exerts a synergistic effect on internal P pollution with macrophytes.

Opportunities for combining data of Estonian and Russian monitoring to reflect on water quality in large transboundary Lake Peipsi

Lake Peipsi, one of the world’s largest lakes, is shared between Estonia and Russia. The water quality in different parts of the lake has so far been assessed independently. Here we explore opportunities for combining data of Estonian and Russian monitoring. For that, we 1) analysed the compatibility of data for some water quality variables; 2) estimated the potential effects of the differences in sampling frequency; 3) provided a few regression models to calculate the missing data for months not sampled by the Russian side. Data of the concurrent Estonian and Russian sampling indicated a good compatibility. Estonian data analysis suggested that water quality assessment results are sensitive to sampling frequency. For example, total phosphorus (TP) in the largest basin showed a long-term decreasing trend in three month data that disappeared when data for other months were added. Disregarding some months may lead to under- or overestimation of certain factors with no consistency in the response of different basins. Hence, data of the whole ice-free period are recommended for an adequate water quality assessment. Furthermore, we demonstrated that monthly values of the water quality variables of the same year are autocorrelated. Based on this, we filled the gaps in the long-term data and compiled a dataset for the whole lake that enables its most comprehensive use in water quality assessment and management. Long-term data revealed no water quality improvement of Lake Peipsi. Further reduction of the external nutrient load is needed. Eutrophication is sustained by high internal phosphorus load.