Addressing algal blooms by bio-pumps to reduce greenhouse gas production and emissions with multi-path

Taihu Lake, the third largest freshwater lake, is typical of many eutrophic lakes in China. In recent years, Taihu Lake is frequently suffering from extensive algal blooms which dramatically change the environmental conditions (e.g., oxygen concentration and pH) and thereafter may influence phosphorus (P) exchanges at the sediment–water interface. Here, we used the repeated ANOVA to compare the water P concentration either with or without algae addition by a mimic experiment. Over a period of 20 days of incubation, chlorophyll a (Chl-a), water temperature, dissolved oxygen (DO), oxidation–reduction potential (ORP), pH, nitrate (NO3 −), as well as the P fractions of overlying water and sediments were monitored. Results showed that compared with the Control treatment algal bloom significantly decreased DO, ORP, and pH and increased P concentration of overlying water. The first 7 days of incubation could be considered as a period of algal decomposition for the Algae treatment based on the decreased Chl-a concentration, as well as the black and smelly water. The increased total P (TP), dissolved total P (DTP), and soluble reactive P (SRP) during the initial 7 days mainly resulted from the P release from the decomposing algae when DO concentration was lower than 0.5 mg L−1, and NO3 − having a potential to suppress iron (Fe) reduction was up to 1.1 mg L−1. Subsequently, Chl-a concentration dramatically increased and reached at a maximum value on September 17, which indicated that algal growth became the dominant process. The increased P level during algal growth might result from the sediment Fe-bound P release when DO and NO3 − concentration was kept at low levels, which could be further confirmed by the lowest NaOH-extractable P (NaOH-P) in the sediments on September 19. The results indicate that influence of algal blooms on sediments P release cannot be neglected during algal decomposition as well as the followed period of algal growth in Meiliang Bay, the highly polluted region of Taihu Lake.

Algal bloom could drastically influence the nutrient cycling in lakes. To understand how the internal nutrient release responds to algal bloom decay, water and sediment columns were sampled at 22 sites from four distinct regions of China's eutrophic Lake Taihu and incubated in the laboratory to examine the influence of massive algal bloom decay on nutrient release from sediment. The column experiment involved three treatments: (1) water and sediment (WS); (2) water and algal bloom (WA); and (3) water, sediment, and algal bloom (WSA). Concentrations of dissolved oxygen (DO), total nitrogen (TN), total phosphorus (TP), ammonium (NH (4) (+) -N), and orthophosphate (PO (4) (3-) -P) were recorded during incubation. The decay of algal material caused a more rapid decrease in DO than in the algae-free controls and led to significant increases in NH (4) (+) -N and PO (4) (3-) -P in the water. The presence of algae during the incubation had a regionally variable effect on sediment nutrient profiles. In the absence of decaying algae (treatment WS), sediment nutrient concentrations decreased during the incubation. In the presence of blooms (WSA), sediments from the river mouth released P to the overlying water, while sediments from other regions absorbed surplus P from the water. This experiment showed that large-scale algal decay will dramatically affect nutrient cycling at the sediment-water interface and would potentially transfer the function of sediment as "container" or "supplier" in Taihu, although oxygen exchange with atmosphere in lake water was stronger than in columns. The magnitude of the effect depends on the physical-chemical character of the sediments.

Turn the potential greenhouse gases into biomass in harmful algal blooms waters: A microcosm study

Neglected methane production and toxicity risk in low-frequency ultrasound for controlling harmful algal blooms

To accurately identify the dynamic distribution of heavy metals （Ni, Cu, Zn, Cd, and Pb） in eutrophic lakes and their response mechanism to environmental changes, the distribution levels of heavy metals in the overlying water and surface sediments of Meiliang Bay in Taihu Lake were analyzed during summer and winter. Additionally, the change characteristics of heavy metals at the sediment-water interface in particular were observed using film diffusion gradient technology （DGT） and high-resolution dialysis technology （HR-Peeper）. The results showed that the average concentrations of five heavy metals both in overlying water and surface sediments generally decreased in the order of Zn>Ni>Cu>Pb>Cd. The concentrations of heavy metals in the overlying water, as well as the bioavailable and dissolved concentrations of heavy metals （cDGT and cPeeper） at the sediment-water interface, were generally higher in summer than in winter. The Mantel test showed significant correlations between cDGT at the sediment-water interface and heavy metal concentrations in overlying water, confirming that heavy metal migration from sediment directly affected the levels of heavy metals in overlying water. Redundancy analysis （RDA） was further used to identify the key environmental factors regulating the dynamic changes of heavy metals at the sediment-water interface, including dissolved oxygen （DO）, chlorophyll-a （Chla）, pH, sediment pH （S_pH）, and sediment redox potential （S_Eh）. Among them, cDGT-Ni, cDGT-Cu, cDGT-Cd, and cDGT-Pb were mainly positively correlated with Chla and pH, and negatively correlated with S_Eh, S_pH, and DO. This reflects that algal blooms during summer significantly promoted the heavy metal migration from sediment to water.

Algal blooms are increasing in coastal waters worldwide. The study on the features of algal pollution in water bodies and the ways to eliminate them is of vital importance. Preventing, treating, and monitoring algal blooms can be an unanticipated cost for a water system. To tame algal bloom in a lake, the government provides funds through budget allocation. In this paper, we propose a mathematical model to investigate the effect of budget allocation on the control of algal bloom in a lake. We assume that the growth of budget follows logistic law and also increases in proportion to the algal density in the lake. A part of the budget is utilized for the control of inflow of nutrients, while the remaining is used in the removal of algae from the lake. Our results show that algal bloom can be mitigated from the lake by reducing the inflow rate of nutrients to a very low value, which can be achieved for very high efficacy of budget allocation for the control of nutrients inflow from outside sources. Also, increasing the efficacy of budget allocation for the removal of algae helps to control the algal bloom. Further, more budget should be used on the control of nutrient’s inflow than on the removal of algae, as the presence of nutrients in high concentration will immediately proliferate the growth of algae. Moreover, the combined effects of controlling the inflow of nutrients and removing algae at high rates will result in nutrients and algae-free aquatic environment. Further, we modify the model by considering a discrete time delay involved in the increment of budget due to increased density of algae in the lake. We observe that chaotic oscillations may arise via equilibrium destabilization on increasing the values of time delay. We apply basic tools of nonlinear dynamics such as Poincare section and maximum Lyapunov exponent to confirm the chaotic behavior of the system.

To investigate the effects of emergent plants on CH4 efflux and elucidate the key factors responsible for these effects, annual monitoring of CH4 emissions and methanogen community dynamics in a full-scale constructed wetland (CW) was conducted. Five emergent plants (Typha orientalis, Cyperus alternifolius, Arundo domax, Iris pseudacorus, and Thalia dealbata) commonly used in CWs were selected for investigation. The greatest CH4 flux (annual mean 19.4mgm-2h-1) was observed from I. pseudacorus, while the lowest CH4 flux (7.1mgm-2h-1) was observed from Thalia dealbata. The CH4 flux from five emergent plants showed marked seasonal variation. Total nitrogen (TN) and total phosphorous (TP) were weakly correlated with CH4 emissions, whereas total carbon (TC) and root biomass of plants were positively correlated with CH4 emissions. Quantitative real-time PCR (q-PCR) analysis indicated that the gene abundance of eubacterial 16S rRNA, particulate methane monooxygenase (pmoA) and methyl coenzyme M reductase (mcrA) significantly differed among plant species. Differences in TC, root biomass, and dissolved oxygen (DO) caused by plant species were potential factors responsible for differences in methanogens, methanotrophs, and CH4 emissions. Methanobacteriaceae, Methanoregulaceae, Methanomicrobiaceae, and Methanosarcinaceae were the dominant families of methanogens. The pathways of methanogenesis from the five emergent plants differed, with the main pathway being hydrogenotrophic, while both hydrogenotrophic and acetotrophic methanogens were involved in A. domax. Redundancy analysis (RDA) further indicated that emergent plant types had a profound influence on the methanogenic communities. Taken together, these results suggest emergent plant species can significantly influence CH4 fluxes in CW through microbial communities, biochemical pathways for methanogenesis, TC, and DO. Furthermore, plant species in CWs should be considered an important factor in evaluating greenhouse gases emission. Finally, it is necessary to effectively manage CWs vegetation to maximize their environmental benefits. Graphical abstract ᅟ.

High-resolution diurnal variation mechanism of oxygen and acid environments at the water–sediment interface during cyanobacterial decomposition

In order to control the cyanobacterial blooms in eutrophic water, an Ag/AgCl@ZIF-8 floating coating was prepared by a dip-coating method with a sponge, innovatively employed as a carrier for the removal of algae in natural water samples. The as-prepared photocatalyst was characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (SEM). The effects of this Ag/AgCl@ZIF-8 coating on algal removal and phytoplankton community structure in natural water samples with cyanobacterial blooms were investigated under sunlight. Results showed that Ag/AgCl@ZIF-8 distributed uniformly on the surface of the coating with good stability and algae removal efficiency in water bodies. After 6 h of exposure under sunlight, the chlorophyll a in the natural water samples was degraded by 99.9%, the densities of Microcystis aeruginosa were reduced by 92.6% and the densities and biomass of the other algae decreased by about 80%. Meanwhile, the content of colored dissolved organic matter (CDOM) in the samples was decreased, effectively controlling the cyanobacterial blooms. It was found that O2•− played the main role in the photocatalytic inactivation. In conclusion, the Ag/AgCl@ZIF-8 coating has a promising application potential for the removal of harmful cyanobacteria, and provides a new idea for the control of cyanobacterial blooms in water bodies.

This study investigates the decomposition process of algal blooms (ABs) in eutrophic lakes and its impact on the labile endogenous nitrogen (N) cycle. In situ techniques such as diffusive gradients in thin films (DGT) and high-resolution dialysis (HR-Peeper) were employed to decipher the vertical distribution of N fractions within the sediment–water interface (SWI) in Taihu, China. Additionally, an annular flume was used to simulate regional differences in lake conditions and understand labile nitrogen transformation during AB decomposition. This study reveals that the NH4+-N fraction exuded from algae is subsequently converted into NO3-N and NO2-N through nitrification, resulting in a significant increase in the concentrations of NO3−-N and NO2−-N at the SWI. The decomposition of algae also induces a significant increase in dissolved organic matter (DOM) concentration, referring to humic acid and humus-like components; a seven-millimeter decrease in dissolved oxygen (DO) penetration depth; as well as a significant decrease in the pH value near the SWI, which consequently promotes denitrification processes in the sediment. Moreover, the decomposition process influences nitrogen distribution patterns and the role conversion of sediments between a “source” and a “sink” of nitrogen. This investigation provides evidence on the migration and/or transformation of N fractions and offers insights into the dynamic processes across the SWI in eutrophic lakes.

The release of algal toxins in algae-containing water sources poses a serious threat to drinking water safety and human health. The conventional water treatment processes of water plants have a limited ability to remove algae and algal toxins, especially algal toxins with a molecular weight (MW) of less than 1000 Da. To eliminate algal pollution from a water source, a two-stage ultrafiltration (UF) process with a large polysulfone hollow fiber membrane with a MW cut-off of 200 kDa and a small aromatic polyamide roll membrane with a MW cut-off of 1 kDa were applied after a traditional sand filter in a water treatment plant. UF operation conditions, including the operating time, pressure, and membrane flux, were investigated. With an operating pressure of 0.05-0.08 MPa, the polysulfone hollow fiber membrane removed algae effectively, as the influent algal cell concentration ranged from 1-30 cells/mL but exhibited a limited removal of algal toxins. With an operating pressure of 0.3-0.4 MPa, the elimination of microcystins (MCs) reached 96.3% with the aromatic polyamide roll membrane. The operating pressure, membrane flux, and operating time were selected as the experimental factors, and the effects on the UF efficiency to remove algal toxins and biodegradable dissolved organic carbon were investigated by the response surface methodology. The model showed that the order of influence on the membrane operating efficiency was operating pressure > membrane flux > running time. The optimal UF operating conditions were an operating pressure of 0.3 MPa, a membrane flux of 17.5 L/(m2·h), and a running time of 80 min.

Modeling the influence of benthic primary production on oxygen transport through the water–sediment interface

One of the most significant problems in eutrophic lakes is the presence of algal blooms, which may affect phosphorus (P) inactivation agents (PIAs) in the control of P release from sediment. Therefore, in this study, lanthanum/aluminium co-modified thermally treated calcium-rich attapulgite (LA@TCAP) is used to analyse the influence and mechanisms of algal blooms on P inactivation effect to provide technical support for the application of PIAs. In August 2020, lake water, sediments, and algae were collected from Zhushan Bay in Lake Taihu and LA@TCAP was prepared in the laboratory. These samples were used to establish three groups of microscopic simulation experiments. Through sequential extraction and 31P nuclear magnetic resonance, the mobile forms of P (Mobile-P) of sediment in each experimental group were measured and analysed, and the influence of algal blooms on the inactivation effect of the P immobilisation of LA@TCAP was determined. Physical and chemical properties of overlying water (DO, pH, algae organic matter (AOM), etc) and microbial community structure of capping layers were used to understand the influence mechanisms of algal blooms on the inactivation effect of PIAs. The concentration of dissolved oxygen (DO) concentration in the overlying water was reduced due to the decomposition of algal blooms, which increased the relative abundance of P-solubilising bacteria, and transformed more inert forms of P (Inert-P) into Mobile-P in the capping layer. Simultaneously, the algal blooms released OP, which passed through the capping layer and increased the OP content in the sediment. Under this dual effect, the Mobile-P content in the sediment increased, making LA@TCAP unable to inactivate the increased mobile-P, which was bound to affect the inactivation effect of LA@TCAP. Besides, the AOM released from the algal blooms combined with the metal ions in LA@TCAP, resulting in the reduction of adsorption sites of LA@TCAP for P. The algal blooms significantly decreased the DO concentration in the overlying water, thereby affecting the microbial community and transforming more Inert-P into Mobile-P. In addition, the adsorption performance of LA@TCAP for P also reduced owing to AOM competed with P for the metal ions in the LA@TCAP. Thus, algal blooms reduced the ability of LA@TCAP to control the P release from sediment.

Algal blooms have increased in frequency, intensity, and duration in response to nitrogen (N) cycling in freshwater ecosystems. We conducted a high-resolution sedimentary study of N transformation and its associated microbial activity in Lake Taihu to assess the accumulation rates of the different N fractions in response to algal blooms, aiming to understand the mechanisms of N cycling in lacustrine environments. Downcore nitrification and denitrification processes were measured simultaneously in situ via diffusive gradients in thin-films technique, peeper, and microelectrode devices in a region of intensified algal blooms of shallow lake. The decomposition of different biomasses of algal blooms did not change the main controlling factor on different N fractions in profundal sediment. However, the decomposition of different algal biomasses led to significant differences in the nitrification and denitrification processes at the sediment–water interface (SWI). Low algal biomasses facilitated the classic process of N cycling, with the balanced interaction between nitrification and denitrification. However, the extreme hypoxia under high algal biomasses significantly limited nitrification at the SWI, which in turn, restricted denitrification due to the lack of available substrates. Our high-resolution results combined with estimates of apparent diffusion fluxes of the different N fractions inferred that the lack of substrates for denitrification was the main factor influencing the positive feedback loop between N and eutrophication in freshwater ecosystems. Moreover, this positive feedback can become irreversible without technological intervention.

The production of phytoplankton (algal) toxins and their control is of concern because of the need to reduce their negative impacts on water quality and facilitate effective management of algal blooms. The present study was conducted between September 2017 to May 2018, focusing on Kisumu Bay in the Kenyan portion of Lake Victoria, in order to establish the magnitude of potential impacts on phytoplankton composition and microcystin following a prolonged presence of water hyacinth coverage between 2013 and 2018 within the gulf, with an estimated coverage range varying between 644 and 1224 ha. Triplicate samples of physico‐chemical parameters, nutrients, phytoplankton, chlorophyll‐a and algal toxins (N = 88) were collected at eleven sampling sites to determine their spatio‐temporal variability. The main identified algal taxa comprised Cyanophyceae, Bacillariophyceae, Chlorophyceae, Euglenophyceae, Zygnematophyceae and Dinophyceae. The most dominant algal species were Microcystis aeruginosa (25%), Merismopedia spp. (23%) and Anabaena flos‐aquae (16%). Enzyme‐linked immunosorbent assay (ELISA) technique was used to determine microcystin (MC) toxins in the water. Mean MC‐LR and MC‐YR concentrations were significantly correlated (R2 = 0.972), exceeding WHO standards at three sampling sites (Coca Cola, 2.84 ± 4.76; Kisumu pier, 1.78 ± 1.87; Midpoint, 1.44 ± 2.71 μg/L MC–LR). There were significant temporal variations (p < .05) in the SRP, TN, NO3‐N, NO2‐N, NH4‐N, SiO2‐Si, MC‐LR, MC‐YR, dissolved oxygen (DO), total dissolved and suspended solids (TDS; TSS), turbidity, electrical conductivity, Secchi depth, temperature and pH levels. The water depth, TP and DO also vary spatially. The nitrogen to phosphorus concentration ratios differed from the expected N:P ratio of 16:1, indicating a highly eutrophic status. The disproportionate ratio of total phosphorus and total nitrogen in the bay may be responsible for the enhanced cyanobacterial blooms it exhibits. The results of the present study provide useful information and data for formulating regulations for water quality management.