Interactions between nutrients and organic micro-pollutants in shallow freshwater model ecosystems

Nutrients versus emerging contaminants-Or a dynamic match between subsidy and stress effects on stream biofilms

Freshwater ecosystems are threatened by multiple anthropogenic stressors, which might be differentiated into two types: those that reduce biological activity at all concentrations (toxic contaminants), and those that subsidize biological activity at low concentrations and reduce it at high concentrations (assimilable contaminants). When occurring in mixtures, these contaminants can have either antagonistic, neutral or synergistic effects; but little is known on their joint effects. We assessed the interaction effects of a mixture of assimilable and toxic contaminants on stream biofilms in a manipulative experiment using artificial streams, and following a factorial design with three nutrient levels (low, medium or high) and either presence or absence of a mixture of emerging contaminants (ciprofloxacin, erythromycin, diclofenac, methylparaben, and sulfamethoxazole). We measured biofilm biomass, basal fluorescence, gross primary production and community respiration. Our initial hypotheses were that biofilm biomass and activity would: increase with medium nutrient concentrations (subsidy effect), but decrease with high nutrient concentrations (stress effect) (i); decrease with emerging contaminants, with the minimum decrease at medium nutrient concentrations (antagonistic interaction between nutrients subsidy and stress by emerging contaminants) and the maximum decrease at high nutrient concentrations (synergistic interaction between nutrients and emerging contaminants stress) (ii). All the measured variables responded linearly to the available nutrients, with no toxic effect at high nutrient concentrations. Emerging contaminants only caused weak toxic effects in some of the measured variables, and only after 3-4 weeks of exposure. Therefore, only antagonistic interactions were observed between nutrients and emerging contaminants, as medium and high nutrient concentrations partly compensated the harmful effects of emerging contaminants during the first weeks of the experiment. Our results show that contaminants with a subsidy effect can alleviate the effects of toxic contaminants, and that long-term experiments are required to detect stress effects of emerging contaminants at environmentally relevant concentrations.

Different effects of copper (II), cadmium (II) and phosphate on the sorption of phenanthrene on the biomass of cyanobacteria

Due to the large surface area and high organic carbon content of cyanobacteria, organic contaminants can be readily sorbed on cyanobacteria during algal blooms, and then be transferred to the food web. This process is likely to be affected by the coexisting metals and nutrients, however, the possible impacts remain unclear. Effects of Cu(2+), Cd(2+), and phosphate on the sorption of phenanthrene on cyanobacterial biomass collected from an algal bloom were therefore studied. Continuous decrease in phenanthrene sorption was observed in the presence of low concentrations of Cu(2+), and Cd(2+) (<0.04 mmol L(-1)), because Cu(2+) and Cd(2+) were coadsorbed with phenanthrene on the surface of cyanobacteria as suggested by scanning electron microscopy-energy dispersive X-ray (SEM-EDX) and Fourier transform infrared (FTIR) analyses. Phenanthrene sorption began to increase with the further increase in Cu(2+) concentration, but remained lower than that in the absence of Cu(2+). This increase in sorption was ascribed to the cation-π interaction between Cu(2+) and phenanthrene, as suggested by the enhanced ultraviolet absorbance at 251 nm. In contrast, sorption rebounding of phenanthrene did not occur in the presence of higher concentrations of Cd(2+). The different effects of Cu(2+) and Cd(2+) on phenanthrene sorption were attributed to that Cd(2+) required much more energy than Cu(2+) to form cation-π complexes with phenanthrene in the solutions. Phenanthrene sorption decreased continuously with the increase in phosphate concentration. Phosphate blocked the binding sites, modified the cell morphology, and increased the negative charge as well as the hydrophilicity of the cyanobacterial surface, thereby suppressing phenanthrene sorption. This study indicates that sorption of aromatic organic compounds by cyanobacteria could be significantly alerted by concentrations and properties of the coexisting transition metals and phosphates, which may subsequently affect their transfer to the food web in eutrophic waters.

Priorities to improve the ecological risk assessment and management for pesticides in surface water

This article deals with prospective and retrospective ecological risk assessment (ERA) procedures for pesticides in surface waters as carried out under European legislation (Regulation 1107/2009/EC; Directive 2009/128/EC; Directive 2000/60/EC). Priorities to improve the aquatic risk assessment and management of pesticides are discussed on basis of the following 5 theses: 1) the management of the environmental risks of pesticides in surface water requires an appropriate implementation of feedback mechanisms between prospective and retrospective ERA, 2) an appropriate ERA cannot be carried out without well-defined specific protection goals, described in terms of focal vulnerable populations and related exposure assessment goals, 3) the interaction between the assessment of exposure and eco(toxico)logical effects in ERA is at a lower level of sophistication than either assessment of exposure or assessment of effects in the field, 4) there is insufficient experimental proof that, in prospective ERA, the chronic effect assessment procedures accurately predict long-term population- and community-level impacts, and 5) multiple stress by pesticides in aquatic ecosystems cannot be ignored in ERA, but in individual water bodies, toxicity usually is dominated by a limited number of substances.

Bioconcentration and intracellular storage of hexachlorobenzene in charophytes and their potential role in monitoring and remediation actions

It has been hypothesized that highly hydrophobic substances (LogK(OW) > 5) including many persistent organic pollutants cannot overtake protective tissues and diffuse inside the body of plants due to steric hindrance or very slow diffusion. We investigated the bioaccumulation of hexachlorobenzene (HCB, LogK(OW) = 5.5) in a benthic charophycean macro-alga: Chara rudis. Chara species are a group of common freshwater algae with a complex body structure encompassing a protective layer of cortex cells surrounding large internode cells. The charophyte cell wall has many features in common with that of higher plants; therefore, they are useful models to investigate bioaccumulation mechanisms in general. We found that HCB diffused through the cortex and reached the cytoplam of internode cells. More than 90% of the HCB mass found in the organism was in the cortex and 10% in the internode cell cytoplasm. The cortex reached a pseudoequilibrium partitioning with water, and the bioconcentration factor was in the same range as that of lower aquatic organisms such as phytoplankton. Charophytes are therefore very efficient accumulators of hydrophobic compounds. Based on the structural and ecological features of charophytes, we speculated on their possible use as biomonitors and bioremediation tools.

Hydrological and environmental conditions as key drivers for spatial and seasonal changes in PCDD/PCDF concentrations, transport and deposition along urban cascade reservoirs

To investigate the drivers for transport and deposition of 17 2,3,7,8-substituted PCDDs/PCDFs along an urban river, water samples from five reservoirs located along the river course were collected in January and July 2008. The concentrations of 17 congeners of PCDD/PCDF were determined and compared to environmental - physical, chemical and biological - conditions. The obtained data revealed that the concentration of the sum of toxic PCDDs/PCDFs in water samples differ between reservoirs as well as between seasons, ranging from 12.04 pg L(-1) in UP (first in the cascade) to 1327.94 pg L(-1) in PR (last in the cascade) during winter of 2008; and from 34.94 pg L(-1) in UP to 1352.50 pg L(-1) in TR (next to last) in summer 2008. In comparison, water samples collected from the river had a concentration several times lower at the first two sites (sites no. 1 and 4) and no detectable values at the last three stations (sites no. 7, 8, 10). The obtained data demonstrated strong or moderate correlations between the sum of 17 PCDDs/PCDFs and TEQ in reservoir water samples and physical, chemical and biological conditions, such as: Mg(2+) (R=0.82; R=0.80, respectively), SO(4)(2-) (R=0.80; R=0.80, respectively), K(+) (R=0.80; R=0.80, respectively), Ca(2+) (R=0.67, R=0.70, respectively), OSM (R=0.63, R=0.70, respectively). In addition, the positive strong correlation between TEQ concentrations and the water temperature (R=0.63) and chlorophyll a content (R=0.90) was noted. The violent weather conditions occurred during the research season with periods of intensive storm events (up to 32 mm in mid July), and thus the increased river flow velocity (up to 0.45 m(3)s(-1)) could have a direct and indirect influence on PCDDs/PCDFs concentration through changes in the sedimentation/resuspension ratio and consequently in transport, deposition and degradation processes along the river/reservoirs.

Responses of Hyalella azteca and phytoplankton to a simulated agricultural runoff event in a managed backwater wetland

We assessed the aqueous toxicity mitigation capacity of a hydrologically managed floodplain wetland following a synthetic runoff event amended with a mixture of sediments, nutrients (nitrogen and phosphorus), and pesticides (atrazine, S-metolachlor, and permethrin) using 48-h Hyalella azteca survival and phytoplankton pigment, chlorophyll a. The runoff event simulated a 1h, 1.27 cm rainfall event from a 16 ha agricultural field. Water (1L) was collected every 30 min within the first 4h, every 4h until 48 h, and on days 5, 7, 14, 21, and 28 post-amendment at distances of 0, 10, 40, 300 and 500 m from the amendment point for chlorophyll a, suspended sediment, nutrient, and pesticide analyses. H. azteca 48-h laboratory survival was assessed in water collected at each site at 0, 4, 24, 48 h, 5 d and 7 d. Greatest sediment, nutrient, and pesticide concentrations occurred within 3h of amendment at 0m, 10 m, 40 m, and 300 m downstream. Sediments and nutrients showed little variation at 500 m whereas pesticides peaked within 48 h but at <15% of upstream peak concentrations. After 28 d, all mixture components were near or below pre-amendment concentrations. H. azteca survival significantly decreased within 48 h of amendment up to 300 m in association with permethrin concentrations. Chlorophyll a decreased within the first 24h of amendment up to 40m primarily in conjunction with herbicide concentrations. Variations in chlorophyll a at 300 and 500 m were associated with nutrients. Managed floodplain wetlands can rapidly and effectively trap and process agricultural runoff during moderate rainfall events, mitigating impacts to aquatic invertebrates and algae in receiving aquatic systems.

Seasonal variation of total and biochemically available concentrations of PAHs in a floodplain lake sediment has no effect on the benthic invertebrate community

Floodplain areas in the Netherlands are moderately to heavily contaminated with trace metals and organic contaminants. The impact of this sediment pollution on benthic invertebrates is best described with so-called 'bioavailable' concentrations instead of total concentrations. Studies into the effect of contaminants on biota are usually a snapshot in time, with one or two samples in a year. In such studies, effect of seasonal variation cannot be detected. Objective of this study was to investigate the temporal variation of sediment PAH contamination in one floodplain lake, and possible consequences for the benthic invertebrate community. This was done by sampling biota, abiotic variables and sediment contaminants every four weeks during one year. We observed a limited seasonal variation in total and bioavailable concentrations of PAHs. Multivariate analysis revealed that the PAH contamination is not a significant factor for the benthic invertebrate community in this floodplain lake. Bioavailable concentrations of PAHs were low and no effects could be expected based on the measured values. No increase of available PAHs was observed in periods where conditions for degradation were absent (low temperature, anaerobic). These results also indicate that in such floodplain lake systems, with historical pollution, the timing of sampling to assess sediment contamination with organic pollutants is of limited importance. This can be useful for planning sampling in risk assessment studies.

Interactions between nutrients and organic micro-pollutants in shallow freshwater model ecosystems

Effects of nutrients and toxicants in aquatic ecosystems may interact in several ways. Here, we (a) present an overview of reported mechanisms that may play a role in these interactions, and (b) compare these reported mechanisms against the results of a suite of experiments performed with organic micro-pollutants in outdoor and indoor microcosms representing shallow lakes dominated either by phytoplankton or by macrophytes. The presence of macrophytes affected the fate of the persistent hydrophobic organic chemicals (triphenyltin, polycyclic aromatic hydrocarbons, and polychlorinated biphenyls) in particular. Differences in community structure affected indirect treatment-related effects and recoverability of affected populations in the lambda-cyhalothrin and TPT experiments high nutrient status. Four major interaction categories reported in the literature are I) dilution of pollutants by biomass, II) impact on transport and cycling of pollutants, III) direct toxicant-food interactions, and IV) indirect toxicant-food interactions. Results of our experiments showed interactions in all four categories (I, II, III, and IV). However, it is concluded that risks of organic micro-pollutants cannot be adapted to (or compensated for?) these four mechanism because of time and space dependent feedback in shallow water communities.