Miquel Lürling

Recovery rates reflect distance to a tipping point in a living system

Tipping points, at which complex systems can shift abruptly from one state to another, are notoriously difficult to predict. Theory proposes that early warning signals may be based on the phenomenon that recovery rates from small perturbations should tend to zero when approaching a tipping point; however, evidence that this happens in living systems is lacking. Here we test such ‘critical slowing down’ using a microcosm in which photo-inhibition drives a cyanobacterial population to a classical tipping point when a critical light level is exceeded. We show that over a large range of conditions, recovery from small perturbations becomes slower as the system comes closer to the critical point. In addition, autocorrelation in the subtle fluctuations of the system’s state rose towards the tipping point, supporting the idea that this metric can be used as an indirect indicator of slowing down. Although stochasticity prohibits prediction of the timing of critical transitions, our results suggest that indicators of slowing down may be used to rank complex systems on a broad scale from resilient to fragile.

Nanoplastic affects growth of S. obliquus and reproduction of D. magna

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.

The ecological stoichiometry of toxins produced by harmful cyanobacteria: an experimental test of the carbon-nutrient balance hypothesis

The elemental composition of primary producers reflects the availability of light, carbon and nutrients in their environment. According to the carbon-nutrient balance hypothesis, this has implications for the production of secondary metabolites. To test this hypothesis, we investigated a family of toxins, known as microcystins, produced by harmful cyanobacteria. The strain Microcystis aeruginosa HUB 5-2-4, which produces several microcystin variants of different N:C stoichiometry, was cultured in chemostats supplied with various combinations of nitrate and CO(2). Excess supply of both nitrogen and carbon yielded high cellular N:C ratios accompanied by high cellular contents of total microcystin and the nitrogen-rich variant microcystin-RR. Comparable patterns were found in Microcystis-dominated lakes, where the relative microcystin-RR content increased with the seston N:C ratio. In total, our results are largely consistent with the carbon-nutrient balance hypothesis, and warn that a combination of rising CO(2) and nitrogen enrichment will affect the microcystin composition of harmful cyanobacteria.

Controlling eutrophication by combined bloom precipitation and sediment phosphorus inactivation

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.

Determination of the neurotoxins BMAA (ß-N-methylamino-L-alanine) and DAB (a-,¿-diaminobutyric acid) by LC-MSMS in Dutch urban waters with cyanobacterial blooms

Abstract We aimed to determine concentrations of the neurotoxic amino acids β-N-methylamino-L-alanine (BMAA) and α-,γ-diaminobutyric acid (DAB) in mixed species scum material from Dutch urban waters that suffer from cyanobacterial blooms. BMAA and DAB were analysed in scum material without derivatization by LC-MSMS (liquid chromatography coupled to tandem mass spectrometry) using hydrophilic interaction chromatography (HILIC). Our method showed high selectivity, good recovery of added compounds after sample extraction (86% for BMAA and 85% for DAB), acceptable recovery after sample hydrolysation (70% for BMAA and 56% for DAB) and acceptable precision. BMAA and DAB could be detected at an injected amount of 0.34 pmol. Free BMAA was detected in nine of the 21 sampled locations with a maximum concentration of 42 μg/g DW. Free DAB was detected in two locations with a maximum concentration of 4 μg/g DW. No protein-associated forms were detected. This study is the first to detect underivatized BMAA in cyanobacterial scum material using LC-MSMS. Ubiquity of BMAA in cyanobacteria scums of Dutch urban waters could not be confirmed, where BMAA and DAB concentrations were relatively low; however, co-occurrence with other cyanobacterial neurotoxins might pose a serious health risk including chronic effects from low-level doses.

A comparative study on three analytical methods for the determination of the neurotoxin BMAA in cyanobacteria.

The cyanobacterial neurotoxin β-N-methylamino-L-alanine (BMAA) has been considered a serious health threat because of its putative role in multiple neurodegenerative diseases. First reports on BMAA concentrations in cyanobacteria were alarming: nearly all cyanobacteria were assumed to contain high BMAA concentrations, implying ubiquitous exposure. Recent studies however question this presence of high BMAA concentrations in cyanobacteria. To assess the real risk of BMAA to human health, this discrepancy must be resolved. We therefore tested whether the differences found could be caused by the analytical methods used in different studies. Eight cyanobacterial samples and two control samples were analyzed by three commonly used methods: HPLC-FLD analysis and LC-MS/MS analysis of both derivatized and underivatized samples. In line with published results, HPLC-FLD detected relatively high BMAA concentrations in some cyanobacterial samples, while both LC-MS/MS methods only detected BMAA in the positive control (cycad seed sarcotesta). Because we could eliminate the use of different samples and treatments as causal factors, we demonstrate that the observed differences were caused by the analytical methods. We conclude that HPLC-FLD overestimated BMAA concentrations in some cyanobacterial samples due to its low selectivity and propose that BMAA might be present in (some) cyanobacteria, but in the low µg/g or ng/g range instead of the high µg/g range as sometimes reported before. We therefore recommend to use only selective and sensitive analytical methods like LC-MS/MS for BMAA analysis. Although possibly present in low concentrations in cyanobacteria, BMAA can still form a health risk. Recent evidence on BMAA accumulation in aquatic food chains suggests human exposure through consumption of fish and shellfish which expectedly exceeds exposure through cyanobacteria.

Lake responses following lanthanum-modified bentonite clay (Phoslock®) application: an analysis of water column lanthanum data from 16 case study lakes.

Phoslock(®) is a lanthanum (La) modified bentonite clay that is being increasingly used as a geo-engineering tool for the control of legacy phosphorus (P) release from lake bed sediments to overlying waters. This study investigates the potential for negative ecological impacts from elevated La concentrations associated with the use of Phoslock(®) across 16 case study lakes. Impact-recovery trajectories associated with total lanthanum (TLa) and filterable La (FLa) concentrations in surface and bottom waters were quantified over a period of up to 60 months following Phoslock(®) application. Both surface and bottom water TLa and FLa concentrations were <0.001 mg L(-1) in all lakes prior to the application of Phoslock(®). The effects of Phoslock(®) application were evident in the post-application maximum TLa and FLa concentrations reported for surface waters between 0.026 mg L(-1)-2.30 mg L(-1) and 0.002 mg L(-1) to 0.14 mg L(-1), respectively. Results of generalised additive modelling indicated that recovery trajectories for TLa and FLa in surface and bottom waters in lakes were represented by 2nd order decay relationships, with time, and that recovery reached an end-point between 3 and 12 months post-application. Recovery in bottom water was slower (11-12 months) than surface waters (3-8 months), most probably as a result of variation in physicochemical conditions of the receiving waters and associated effects on product settling rates and processes relating to the disturbance of bed sediments. CHEAQS PRO modelling was also undertaken on 11 of the treated lakes in order to predict concentrations of La(3+) ions and the potential for negative ecological impacts. This modelling indicated that the concentrations of La(3+) ions will be very low (<0.0004 mg L(-1)) in lakes of moderately low to high alkalinity (>0.8 mEq L(-1)), but higher (up to 0.12 mg L(-1)) in lakes characterised by very low alkalinity. The effects of elevated La(3+) concentrations following Phoslock(®) applications in lakes of very low alkalinity requires further evaluation. The implications for the use of Phoslock(®) in eutrophication management are discussed.

Controlling toxic cyanobacteria: Effects of dredging and phosphorus-binding clay on cyanobacteria and microcystins

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.

Effects of lanthanum and lanthanum-modified clay on growth, survival and reproduction of Daphnia magna

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 0.13-2.13microgl(-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 1000microgl(-1) tested, had no toxic effect on the animals, but only in medium without phosphorous. In the presence of phosphorous, rhabdophane (LaPO(4).nH(2)O) 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 33microgLal(-1), but were 6% and 7% lower at 100 and 330microgl(-1), respectively, and 20% lower at 1000microgl(-1). A juvenile growth assay with Phoslock tested in the range 0-5000mgl(-1), yielded EC(50) (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.

Inducible colony formation within the Scenedesmaceae: adaptive responses to infochemicals from two different herbivore taxa

We studied the occurrence of colony formation within 40 different strains of Scenedesmaceae (Chlorococcales, Chlorophyta) in response to grazing‐released infochemicals from the herbivorous zooplankters Brachionus calyciflorus Pallas (Rotifera) and Daphnia magna Strauss (Cladocera). With the exception of two strains, all strains showed similar responses to both B. calyciflorus and D. magna infochemicals, either no response or inducible colony formation. Colony size was found to increase with B. calyciflorus infochemical concentration and could be described by a sigmoid function. The increase in colony size was more pronounced in the Scenedesmus species tested than in Desmodesmus species, which was probably due to higher threshold infochemical concentrations for colony induction in Desmodesmus. Therefore, the adaptivity of colony formation to the herbivory threat only holds above the threshold concentration for colony induction and as long as maximum colony size has not been attained. Taking this into account, our results suggest that inducible colony formation is a common adaptive response of many Scenedesmaceae to the threat of herbivory.