Phytoplankton is confronted with a variable assemblage of zooplankton grazers that create a strong selection pressure for traits that reduce mortality. Phytoplankton is, however, also challenged to ...remain suspended and to acquire sufficient resources for growth. Consequently, phytoplanktic organisms have evolved a variety of strategies to survive in a variable environment. An overview is presented of the various phytoplankton defense strategies, and costs and benefits of phytoplankton defenses with a zooming in on grazer-induced colony formation. The trade-off between phytoplankton competitive abilities and defenses against grazing favor adaptive trait changes—rapid evolution and phenotypic plasticity—that have the potential to influence population and community dynamics, as exemplified by controlled chemostat experiments. An interspecific defense–growth trade-off could explain seasonal shifts in the species composition of an in situ phytoplankton community yielding defense and growth rate as key traits of the phytoplankton. The importance of grazing and protection against grazing in shaping the phytoplankton community structure should not be underestimated. The trade-offs between nutrient acquisition, remaining suspended, and grazing resistance generate the dynamic phytoplankton community composition.
The amount of nano- and microplastic in the aquatic environment rises due to the industrial production of plastic and the degradation of plastic into smaller particles. Concerns have been raised ...about their incorporation into food webs. Little is known about the fate and effects of nanoplastic, especially for the freshwater environment. In this study, effects of nano-polystyrene (nano-PS) on the growth and photosynthesis of the green alga Scenedesmus obliquus and the growth, mortality, neonate production, and malformations of the zooplankter Daphnia magna were assessed. Nano-PS reduced population growth and reduced chlorophyll concentrations in the algae. Exposed Daphnia showed a reduced body size and severe alterations in reproduction. Numbers and body size of neonates were lower, while the number of neonate malformations among neonates rose to 68% of the individuals. These effects of nano-PS were observed between 0.22 and 103 mg nano-PS/L. Malformations occurred from 30 mg of nano-PS/L onward. Such plastic concentrations are much higher than presently reported for marine waters as well as freshwater, but may eventually occur in sediment pore waters. As far as we know, these results are the first to show that direct life history shifts in algae and Daphnia populations may occur as a result of exposure to nanoplastic.
In the Netherlands, legacies and diffuse nutrient pollution continue to fuel recurrent cyanobacterial blooms in mostly shallow and relatively small surface waters. A survey in peer-reviewed ...literature and Dutch grey-literature was performed to gain insight into the physical-, chemical- and biological in-lake interventions used to bring these waters towards their desired state. A critical overview is presented on efficacy of different measures to counteract cyanobacterial blooms directly via targeting the cyanobacteria or indirectly via reduction of nutrient availability. Many actions have no or limited effects on minimising cyanobacterial blooms (air-bubble- or oil screens, surface mixers, low-energy ultrasound, effective micro-organisms, fish introduction), while others are more effective, but may vary in longevity and costs (dams, excavation or dredging, hydrogen peroxide, phosphorus inactivation agents), meet legislation restrictions (copper-based algaecides, herbicides, dreissenids), or are not currently implemented (hypolimnetic withdrawal). The selection of promising interventions requires a proper diagnosis of each problem lake, based on water- and nutrient fluxes, the biology of the lake (plants, fish), the function of the lake and the characteristics of the method, such as efficacy, costs, safety and ease of implementation. In the Netherlands, ongoing diffuse loads and legacies necessitate repetitive in-lake interventions.
The hypothesis that the combination of the flocculent polyaluminium chloride (PAC) with the lanthanum-modified bentonite Phoslock® (Flock & Lock) could sink effectively a water bloom of cyanobacteria ...and could shift a turbid, cyanobacteria infested lake to a clear water lake was tested in a controlled laboratory experiment and a whole lake experiment. In the laboratory, a relatively low dose of the flocculent PAC (2.2 and 4.4 mg Al l−1) was insufficient to sediment positively buoyant cyanobacteria (Microcystis aeruginosa). Similarly, the lanthanum modified clay (dosed at 390 mg l−1) was insufficient to sediment the positively buoyant cyanobacteria. However, the combination of PAC and Phoslock® effectively sedimented cyanobacteria flocks. Likewise, a combined treatment of 2 tons PAC and 18 tons Phoslock® in Lake Rauwbraken in April 2008 effectively sedimented a developing cyanobacteria bloom of Aphanizomenon flos-aquae. The average chlorophyll-a concentration in the two years prior to this Flock & Lock treatment was 19.5 (±36.5) μg l−1, while it was as low as 3.7 (±4.5) μg l−1 in the years following the treatment. The combined treatment effectively reduced the amount of total phosphorus (TP) in the water column from on average 169 (±126) μg P l−1 before the application to 14 (±15) μg P l−1 after the treatment. Based on mean summer chlorophyll-a and TP concentrations, the lake was shifted from a eutrophic/hypertrophic state to an oligo/mesotrophic state. From directly after treatment in April 2008 until and including 2013, Lake Rauwbraken remained in an oligo-mesotrophic clear water state with TP reduced to less than 10% of the pre-treatment. This result shows that eutrophication in relatively small, isolated, stratifying lakes can be restored by targeting both water column and sediment P using a combination of flocculent and solid phase P-sorbent.
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•The combination of PAC and Phoslock® (Flock & Lock) precipitates cyanobacteria flocks.•Flock & Lock sedimented a developing cyanobacteria bloom in Lake Rauwbraken.•Flock & Lock reduced TP to less than 10% of the values before Flock & Lock.•Flock & Lock changed the lake from hypereutrophic to oligo-mesotrophic.•Lake Rauwbraken is already more than four years in the oligo-mesotrophic state.
Eutrophication is the primary worldwide water quality issue. Reducing excessive external nutrient loading is the most straightforward action in mitigating eutrophication, but lakes, ponds and ...reservoirs often show little, if any, signs of recovery in the years following external load reduction. This is due to internal cycling of phosphorus (P). Geo-engineering, which we can here define as activities intervening with biogeochemical cycles to control eutrophication in inland waters, represents a promising approach, under appropriate conditions, to reduce P release from bed sediments and cyanobacteria accumulation in surface waters, thereby speeding up recovery. In this overview, we draw on evidence from this special issue Geoengineering in Lakes, and on supporting literature to provide a critical perspective on the approach. We demonstrate that many of the strong P sorbents in the literature will not be applicable in the field because of costs and other constraints. Aluminium and lanthanum modified compounds are among the most effective compounds for targeting P. Flocculants and ballast compounds can be used to sink cyanobacteria, in the short term. We emphasize that the first step in managing eutrophication is a system analysis that will reveal the main water and P flows and the biological structure of the waterbody. These site specific traits can be significant confounding factors dictating successful eutrophication management. Geo-engineering techniques, considered collectively, as part of a tool kit, may ensure successful management of eutrophication through a range of target effects. In addition, novel developments in modified zeolites offer simultaneous P and nitrogen control. To facilitate research and reduce the delay from concept to market a multi-national centre of excellence is required.
Lanthanum-modified bentonite (LMB, commercially called Phoslock®) has been widely applied in freshwater systems to manage eutrophication. Little is known, however, about its behaviour and efficiency ...in binding filterable reactive phosphorus (FRP) in saline environments. We assessed if LMB would adsorb phosphate over a range of salinities (0–32 ppth) comparing the behaviour in seawater salts and equivalent concentrations of NaCl. Lanthanum release from the bentonite matrix was measured and the La species prevailing in saline environments were evaluated through chemical equilibrium modelling. We demonstrated that LMB was able to adsorb FRP in all the salinities tested. Filterable lanthanum (FLa) concentrations were similarly low (<5 μgL−1) at all seawater salinities but considerably elevated, on occasion >2000 times greater in equivalent NaCl salinities. Mineralogical analysis indicates that La present in the clay interlayer was (partially) replaced by Na/Ca/Mg present in the seawater and a possible secondary P-reactive phase was formed, such as kozoite (LaCO3OH) or lanthanite (La2(CO3)3·8H2O) that may be physically dissociated from the LMB. Geochemical modelling also indicates that most FLa dissociated from LMB would be precipitated as a carbonate complex. In light of the identification of reactive intermediate phases, further studies including ecotoxicologial assays are required to assess any deleterious effects from the application of LMB to saline waters.
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•La-modified bentonite (LMB) is able to adsorb FRP at all salinities tested.•Filterable La concentrations are similarly low in seawater and freshwater.•La dissociated from clay matrix and was replaced by cations in saline waters.•Free La3+ released in seawaters was removed by complexation with carbonate.•The use of LMB in saline environments must be further evaluated.
Phosphorus sorbents (PS) are viewed as a powerful tool to manage eutrophication. Here, we tested three commercially available PS - lanthanum-modified bentonite (LMB), aluminium-modified zeolite (AMZ) ...and aluminium salts (Al) on their capacity to chemically inactivate soluble reactive phosphorus (SRP) at six different temperatures (5 to 35 °C) and five pH values (6 to 10). We also evaluated if the SRP bound at a neutral pH would be released if pH increases to pH 10. Results showed that temperature affected the SRP binding behavior differently for each PS. For instance, the highest SRP binding capacities of LMB, AMZ and Al were 14.0, 29.9 and 251.1 mg P g−1 at 30 °C, 35 °C and 30 °C, respectively; and the lowest was at 35 °C for LMB, 25 °C for AMZ and 20 °C for Al (6.3, 4.0 and 205.2 mg P g−1, respectively). The pH also affected the SRP binding differently. When pH increased from pH 6 to pH 10, LMB and Al decreased their binding capacity from 10.0 to 4.9 and from 571.7 mg P g−1 to 21.3 mg P g−1, respectively. The SRP adsorption capacity of AMZ was similar at pH 7 and 10 (6.3 and 6.2 mg P g−1). We observed that in high pH, LMB did not release the SRP precipitated. In contrast, AMZ and Al desorbed around 39%, and 71% of the SRP adsorbed when pH changed from 7 to 10. Abiotic factors such as pH should be considered when selecting the most promising material in lake restoration.
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•Modified bentonite, zeolite and aluminium salts bound phosphate (SRP).•Langmuir isotherm model fitted the phosphate binding process adequately.•Phosphate binding onto P sorbents (PS) was exothermic and spontaneous.•Effects of pH and temperature on SRP binding performance differed among PS.
The novel lanthanum-modified clay water treatment technology (Phoslock®) seems very promising in remediation of eutrophied waters. Phoslock® is highly efficient in stripping dissolved phosphorous ...from the water column and in intercepting phosphorous released from the sediments. The active phosphorous-sorbent in Phoslock® is the Rare Earth Element lanthanum. A leachate experiment revealed that lanthanum could be released from the clay, but only in minute quantities of 0.13–2.13μgl−1 for a worst-case Phoslock® dosage of 250mgl−1. A life-history experiment with the zooplankton grazer Daphnia magna revealed that lanthanum, up to the 1000μgl−1 tested, had no toxic effect on the animals, but only in medium without phosphorous. In the presence of phosphorous, rhabdophane (LaPO4·nH2O) formation resulted in significant precipitation of the food algae and consequently affected life-history traits. With increasing amounts of lanthanum, in the presence of phosphate, animals remained smaller, matured later, and reproduced less, resulting in lower population growth rates. Growth rates were not affected at 33μgLal−1, but were 6% and 7% lower at 100 and 330μgl−1, respectively, and 20% lower at 1000μgl−1. A juvenile growth assay with Phoslock® tested in the range 0–5000mgl−1, yielded EC50 (NOEC) values of 871 (100) and 1557 (500)mg Phoslock®l−1 for weight and length based growth rates, respectively. The results of this study show that no major detrimental effects on Daphnia are to be expected from Phoslock® or its active ingredient lanthanum when applied in eutrophication control.
The lanthanum (La) modified bentonite Phoslock(®) has been proposed as dephosphatisation technique aiming at removing Filterable Reactive Phosphorus (FRP) from the water and blocking the release of ...FRP from the sediment. In the modified clay La is expected the active ingredient. We conducted controlled laboratory experiments to measure the FRP removal by Phoslock(®) in the presence and absence of humic substances, as La complexation with humic substances might lower the effectiveness of La (Phoslock(®)) to bind FRP. The results of our study support the hypothesis that the presence of humic substances can interfere with the FRP removal by the La-modified bentonite. Both a short-term (1 d) and long-term (42 d) experiment were in agreement with predictions derived from chemical equilibrium modelling and showed lower FRP removal in presence of humic substances. This implies that in DOC-rich inland waters the applicability of exclusively Phoslock(®) as FRP binder should be met critically. In addition, we observed a strong increase of filterable La in presence of humic substances reaching in a week more than 270 μg La l(-1) that would infer a violation of the Dutch La standard for surface water, which is 10.1 μg La l(-1). Hence, humic substances are an important factor that should be given attention when considering chemical FRP inactivation as they might play a substantial role in lowering the efficacy of metal-based FRP-sorbents, which makes measurements of humic substances (DOC) as well as controlled experiments vital.
Sediment dredging and Phoslock® addition were applied individually and in combination in an enclosure experiment in a Dutch hypertrophic urban pond. These measures were applied to control ...eutrophication and reduce the risk of exposure to cyanobacterial toxins. Over the 58 days course of the experiment, cyanobacteria (predominantly Microcystis aeruginosa) gradually decreased until they dropped below the level of detection in the combined treated enclosures, they were reduced in dredged enclosures, but remained flourishing in controls and Phoslock® treated enclosures. Cyanobacteria were, however, less abundant in the enclosures (medians chlorophyll-a 30–87 μg l−1) than in the pond (median chlorophyll-a 162 μg l−1), where also a thick surface scum covered one-third of the pond for many weeks.
Soluble reactive phosphorus (SRP), total phosphorus and total nitrogen concentrations were significantly lower in the combined dredged and Phoslock® treated enclosures than in controls. Median SRP concentrations were 24 μg P l−1 in the combined treatment, 58 μg P l−1 in dredged enclosures, and 90 μg P l−1 in controls and 95 μg P l−1 in Phoslock® treated enclosures. Hence, the combined treatment was most effective in decreasing SRP and TP, and in lowering cyanobacterial biomass.
Microcystin (MC) concentrations were analyzed by LC–MS/MS. MC concentrations and cyanobacterial biomass were positively correlated in all treatments. Mean MC concentrations in controls (71 μg l−1), Phoslock® treated enclosures (37 μg l−1) and dredged enclosures (25 μg l−1) exceeded the provisional guideline of 20 μg l−1, whereas mean MC concentrations were 13 μg l−1 in the combined treated enclosures. All samples contained the MC variants dmMC-RR, MC-RR, MC-YR, dmMC-LR and MC-LR; traces of MC-LY and nodularin were detected in few samples. The different treatments did not change the relative contribution of the variants to the MC pool; MC profiles in all treatments and the pond showed dominance of MC-RR followed by MC-LR. In the surface scum of the pond, total MC concentration was extremely high (64000 μg l−1 or 1300 μg g−1 DW), which poses a serious health hazard to children playing on the banks of the pond. Based on our results and pond characteristics, we propose combined sediment dredging and Phoslock® addition, fish removal and strong reduction of duck feeding by the neighborhood as most promising measures controlling cyanobacterial hazards in this pond.
► Sediment dredging and lanthanum-modified clay were tested in eutrophication control. ► Single treatments were less effective than a combination of both measures. ► The combination effectively reduced cyanobacteria, microcystin and SRP. ► The combination can be considered to restore the pond.
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