Impacts of nutrient loading and fish grazing on the phytoplankton community and cyanotoxin production in a shallow tropical lake: Results from mesocosm experiments.

Given the increasing eutrophication of water bodies in Africa due to increasing anthropogenic pressures, data are needed to better understand the responses of phytoplankton communities to these changes in tropical lakes. These ecosystems are used by local human populations for multiple purposes, including fish and drinking water production, potentially exposing these populations to health threats if, for example, an increase in toxic cyanobacterial blooms is associated with increasing eutrophication. To test the short-term response of the phytoplankton community to the addition of nutrients (phosphorus and nitrogen, alone or in combination) and Nile tilapia, we developed an in situ mesocosm experiment in a freshwater lagoon located near Abidjan (Ivory Coast). We found that phytoplankton growth (estimated by chlorophyll-a quantification) was highly stimulated when both nitrogen and phosphorus were added, while there was no clear evidence for such colimitation by these two nutrients when considering their concentrations in the lagoon. Phytoplankton growth was accompanied by significant changes in the diversity and composition of this community and did not lead to an increase in the proportions of cyanobacteria. However, the addition of fish to some mesocosms resulted in a drastic decrease in phytoplankton biomass and a dominance of chlorophytes in this community. Finally, these experiments showed that the addition of nitrogen, alone or combined with phosphorus, stimulated microcystin production by cyanobacteria. In addition, no evidence of microcystin accumulation in the fish was found. Taken together, these data allow us to discuss strategies for controlling cyanobacterial blooms in this tropical ecosystem.

A review of the socioecological causes and consequences of cyanobacterial blooms in Lake Victoria.

Africa is experiencing high annual population growth in its major river basins. This growth has resulted in significant land use change and pollution pressures on the freshwater ecosystems. Among them, the Lake Victoria basin, with more than 42 million people, is a unique and vital resource that provides food and drinking water in East Africa. However, Lake Victoria (LV) has experienced a progressive eutrophication and substantial changes in the fish community leading to recurrent proliferation of water hyacinth and cyanobacteria. Based on an extensive literature review, we show that cyanobacterial biomasses and microcystin concentrations are higher in the bays and gulfs (B&Gs) than in the open lake (OL), with Microcystis and Dolichospermum as the dominant genera. These differences between the B&Gs and the OL are due to differences in their hydrological conditions and in the origins, type and quantities of nutrients. Using data from the literature, we describe the multiple ways in which the human population growth in the LV watershed is connected to the increasing occurrence of cyanobacterial blooms in the OL and B&Gs. We also described the consequences of cyanobacterial blooms on food resources and fishing and on direct water use and water supply of local populations, with their potential consequences on the human health. Finally, we discuss the actions that have been taken for the protection of LV. Although many projects have been implemented in the past years in order to improve the management of waste waters or to reduce deforestation and erosion, the huge challenge of the reduction of cyanobacterial blooms in LV by the control of eutrophication seems far from being achieved.

Separate treatment of hospital and urban wastewaters: A real scale comparison of effluents and their effect on microbial communities.

Hospital wastewaters (HWW) contain wider spectrum and higher quantity of pharmaceuticals than urban wastewaters (UWW), but they are generally discharged in sewers without pretreatment. Since traditional urban wastewater treatment plants (WWTP) are not designed to treat HWWs, treated effluents may still contain pollutants that could impair receiving aquatic environments. Hence, a better understanding of the effect of pharmaceuticals in the environment is required. Biofilms are effective "biological sensors" for assessing the environmental effects of pharmaceuticals due to their ability to respond rapidly to physical, chemical and biological fluctuations by changes in their structure and composition. This study evaluated the efficiency of biological treatment with conventional activated sludge system performed parallel on HWW and UWW. Furthermore, six successive monthly colonizations of biofilms were done on autoclaved stones, placed in grid-baskets in the hospital treated effluents (HTE) and urban treated effluents (UTE). The biomass of these biofilms as well as the structure and diversity of their bacterial communities were investigated. Results showed better treatment efficiency for phosphate and nitrite/nitrate during the treatment of UWW. Pharmaceuticals from all investigated therapeutic classes (beta-blockers, nonsteroidal anti-inflammatory drugs, antibiotics, analgesics and anticonvulsants) were efficiently removed, except for carbamazepine. The removal efficiency of the antibiotics, NSAIDs and beta-blockers was higher during the treatment of HWW. HTE and UTE shaped the bacterial communities in different ways. Higher concentrations of pharmaceuticals in the HTE caused adapted development of the microbial community, leading to less developed biomass and lower bacterial diversity. Seasonal changes in solar irradiance and temperature, caused changes in the community composition of biofilms in both effluents. According to the removal efficiency of pharmaceuticals, the separate treatment was beneficial. However, their high concentrations in the HTE and the following adaptations of biofilm communities identify the importance of adapting wastewater treatment to specific hospital pollutants.

Pollution-induced community tolerance (PICT) as a tool for monitoring Lake Geneva long-term in situ ecotoxic restoration from herbicide contamination.

Chemical monitoring revealed a regular decrease in herbicide concentration in Lake Geneva since last decades that may be linked to an ecotoxic restoration of nontarget phytoplanktonic communities. The Pollution-induced community tolerance (PICT) approach was tested as a tool to monitor the ecotoxic restoration of Lake Geneva for herbicides from 1999 to 2011. We conducted monthly assessments in 1999 and in 2011 for the tolerance of the phytoplankton communities to two herbicides (atrazine and copper), using PICT bioassays. The taxonomical composition of the communities was determined on the same collecting dates. The herbicide concentration decrease during the 12 years significantly influenced the composition of communities. The PICT monitoring indicated that a significant tolerance decrease in the community to both herbicides accompanied the herbicide concentration decrease. PICT measurements for atrazine and copper also changed at the intra-annual level. These variations were mainly due to community composition shifts linked to seasonal phosphorus and temperature changes. PICT monitoring on a seasonal basis is required to monitor the mean tolerance of communities. PICT appeared to be a powerful tool that reflected the toxic effects on environmental communities and to monitor ecotoxic ecosystem restoration.

Comparison of specific versus literature species sensitivity distributions for herbicides risk assessment.

Species sensitivity distributions (SSDs) are an important predictive tool in risk assessment. Usually, literature data are used to build SSDs that are mostly based on planktonic species. But, to get adequate protective thresholds for environmental communities, one could argue that SSD should be built on ecotoxicological data obtained from species found in the ecosystem that should be protected. This is particularly true when benthic algae are of concern. Due to the lack of literature data, building SSD on benthic microalgae is difficult. This paper aims in comparing SSDs, and thus protective thresholds (hazardous concentration that affects 5% of the species of a community, HC5), built on ecotoxicological data obtained (1) from literature and (2) with specific bioassays on benthic diatoms from a lake. Thresholds were derived for protection against four herbicides separately and for a mixture of them. Sensitivity data obtained from literature were statistically lower than the specific data for all herbicides: Species tested in the literature were usually more sensitive (mainly chlorophytes), leading to more protective lower HC5. The HC5 thresholds (literature or specific) derived for protection against the mixture were also compared to the observed sensitivity of an assemblage of benthic diatom species exposed to an increasing range of herbicide mixture concentrations. We observed that one species within the assemblage (Fragilaria rumpens) was affected at a concentration below both the literature and the specific HC5 thresholds.

Long-term relationships among pesticide applications, mobility, and soil erosion in a vineyard watershed.

Agricultural pesticide use has increased worldwide during the last several decades, but the long-term fate, storage, and transfer dynamics of pesticides in a changing environment are poorly understood. Many pesticides have been progressively banned, but in numerous cases, these molecules are stable and may persist in soils, sediments, and ice. Many studies have addressed the question of their possible remobilization as a result of global change. In this article, we present a retro-observation approach based on lake sediment records to monitor micropollutants and to evaluate the long-term succession and diffuse transfer of herbicides, fungicides, and insecticide treatments in a vineyard catchment in France. The sediment allows for a reliable reconstruction of past pesticide use through time, validated by the historical introduction, use, and banning of these organic and inorganic pesticides in local vineyards. Our results also revealed how changes in these practices affect storage conditions and, consequently, the pesticides' transfer dynamics. For example, the use of postemergence herbicides (glyphosate), which induce an increase in soil erosion, led to a release of a banned remnant pesticide (dichlorodiphenyltrichloroethane, DDT), which had been previously stored in vineyard soil, back into the environment. Management strategies of ecotoxicological risk would be well served by recognition of the diversity of compounds stored in various environmental sinks, such as agriculture soil, and their capability to become sources when environmental conditions change.

Seasonal shift in the sensitivity of a natural benthic microalgal community to a herbicide mixture: impact on the protective level of thresholds derived from species sensitivity distributions.

Seasonal changes in the structure and composition of a benthic microalgal community may lead to different responses to herbicide contamination during different seasons. Consequently, the thresholds derived from risk assessment tools such as species sensitivity distributions (SSDs) must allow for these changes. We built a single-substance SSD for each of four herbicides (atrazine, terbutryn, diuron and isoproturon), which was specific to the sensitivity of the benthic diatoms found in Lake Geneva, in order to derive protective thresholds for a mixture of these four herbicides using the concentration addition model. We then investigated (1) the structural parameters of a Lake Geneva benthic microalgal community during two contrasting seasons (summer 2012 and winter 2013), (2) the response of these communities to a herbicide mixture, and (3) the protective levels of the thresholds derived. The winter community was characterized by having greater biomass, diatom species richness, and diversity metrics, and lower non-diatom species richness than the summer community. The differences in the diatom communities composition in these seasons appeared to be primarily driven by the environmental nitrate concentrations and the temperature. Moreover, the species in the winter community were more resistant to herbicides than those found in the summer community. Consequently, the protective threshold for this herbicide mixture obtained in this study was in fact protective for the winter community, but not for the summer community based on their structural parameters. Thus, the protective level against herbicides of the threshold for the benthic microalgal community should take into account changes in the environmental physico-chemical conditions that strongly influence the structure and composition of the community. The fact that the succession of species over time (i.e., over the seasons) is difficult to predict introduces uncertainties into the estimation of protective thresholds and questions their applicability year round.

Linking diatom sensitivity to herbicides to phylogeny: a step forward for biomonitoring?

Phylogeny has not yet been fully accepted in the field of ecotoxicology, despite studies demonstrating its potential for developing environmental biomonitoring tools, as it can provide an a priori assessment of the sensitivity of several indicator organisms. We therefore investigated the relationship between phylogeny and sensitivity to herbicides in freshwater diatom species. This study was performed on four photosystem II inhibitor herbicides (atrazine, terbutryn, diuron, and isoproturon) and 14 diatom species representative of Lake Geneva biofilm diversity. Using recent statistical tools provided by phylogenetics, we observed a strong phylogenetic signal for diatom sensitivity to herbicides. There was a major division in sensitivity to herbicides within the phylogenetic tree. The most sensitive species were mainly centrics and araphid diatoms (in this study, Thalassiosirales and Fragilariales), whereas the most resistant species were mainly pennates (in this study, Cymbellales, Naviculales, and Bacillariales). However, there was considerable variability in diatom sensitivity within the raphid clade, which could be explained by differences in trophic preferences (autotrophy or heterotrophy). These traits appeared to be complementary in explaining the differences in sensitivity observed at a refined phylogenetic level. Using phylogeny together with complementary traits, as trophic preferences, may help to predict the sensitivity of communities with a view to protecting their ecosystem.

The effect of temperature and a herbicide mixture on freshwater periphytic algae.

Temperature is a strong driver of biofilm formation and of the dynamics of microalgae in freshwater. Moreover, exposure to herbicides is a well-known stressor of periphytic communities in anthropized aquatic environments. We tested these two environmental factors on periphytic communities that had been sampled from the littoral zone of Lake Geneva and acclimatized in the lab for 3 weeks at 18, 21, 24 and 28 °C. After this acclimation period, differences in the composition of the diatom community and decreases in cell density were observed corresponding to the temperature gradient. These acclimated communities were then exposed to 23 and 140 nM of a mixture composed of equitoxic quantities of atrazine, terbutryn, diuron and isoproturon. The periphytic community was more sensitive to the herbicide mixture at 18 °C than at higher temperatures, suggesting that higher temperature reduced its toxicity. Small and pioneer diatom species known to be promoted by contamination also appeared to benefit from higher temperatures. Temperature therefore appears to condition the herbicide sensitivity of periphytic communities.

Assessment of toxicity thresholds in aquatic environments: does benthic growth of diatoms affect their exposure and sensitivity to herbicides?

Benthic diatoms evolved in a biofilm structure, at the interface between water and substrata. Biofilms can adsorb toxicants, such as herbicides, but little is known about the exposure of biofilm organisms, such as benthic diatoms, to these adsorbed herbicides. We assessed the sensitivity of 11 benthic diatoms species to 6 herbicides under both planktonic and benthic conditions using single-species bioassays. The concentration that reduced the growth rate of the population by 10% (EC10) and 50% (EC50), respectively, varied depending on the species, the herbicides, and the growth forms involved. As a general trend, the more hydrophobic the herbicide, the more species were found to be sensitive under benthic growth conditions. Statistical differences (alpha<5%) were observed between the sensitivities under planktonic and benthic growth conditions for many hydrophobic herbicides. A protective effect of the biofilm against herbicides was observed, and this tended to decrease (at both the EC10 and EC50 levels) with increasing hydrophobicity. The biofilm matrix appeared to control exposure to herbicides, and consequently their toxicity towards benthic diatoms. For metolachlor, terbutryn and irgarol, benthic thresholds derived from species sensitivity distributions were more protective than planktonic thresholds. For hydrophobic herbicides, deriving sensitivity thresholds from data obtained under benthic growth seems to offer a promising alternative.

The use of antioxidant enzymes in freshwater biofilms: temporal variability vs. toxicological responses.

This study aims to investigate the potential of antioxidant enzyme activities (AEA) as biomarkers of oxidative stress in freshwater biofilms. Therefore, biofilms were grown in channels for 38 days and then exposed to different concentrations (0-150 µg L(-1)) of the herbicide oxyfluorfen for 5 more weeks. Under control conditions, the AEA of biofilms were found to change throughout time with a significant increase in ascorbate peroxidase (APX) activity during the exponential growth and a more important role of catalase (CAT) and glutathione reductase (GR) activities during the slow growth phase. Chronic exposure to oxyfluorfen led to slight variations in AEA, however, the ranges of variability of AEA in controls and exposed communities were similar, highlighting the difficulty of a direct interpretation of AEA values. After 5 weeks of exposure to oxyfluorfen, no clear effects were observed on chl-a concentration or on the composition of other pigments suggesting that algal group composition was not affected. Eukaryotic communities were structured clearly by toxicant concentration and both eukaryotic and bacterial richness were reduced in communities exposed to the highest concentration. In addition, during acute exposure tests performed at the end of the chronic exposure, biofilms chronically exposed to 75 and 150 µg L(-1) oxyfluorfen showed a higher CAT activity than controls. Chronic exposure to oxyfluorfen provoked then structural changes but also functional changes in the capacity of biofilm CAT activity to respond to a sudden increase in concentration, suggesting a selection of species with higher antioxidant capacity. This study highlighted the difficulty of interpretation of AEA values due to their temporal variation and to the absence of absolute threshold value indicative of oxidative stress induced by contaminants. Nevertheless, the determination of AEA pattern throughout acute exposure test is of high interest to compare oxidative stress levels undergone by different biofilm communities and thus determine their antioxidant capacity.

Long-term impact of hydrological regime on structure and functions of microbial communities in riverine wetland sediments.

In a context of global change, alterations in the water cycle may impact the structure and function of terrestrial and aquatic ecosystems. Wetlands are particularly at risk because hydrological regime has a major influence on microbially mediated biogeochemical processes in sediments. While the influence of water availability on wetland biogeochemical processes has been comprehensively studied, the influence of hydrological regime on microbial community structure has been overlooked. We tested for the effect of hydrological regime on the structure and functions of microbial communities by comparing sediments collected at multiple sites in the Ain département (Eastern France). Each site consisted of two plots, one permanently and one seasonally inundated. At the time of sampling, all plots were continuously inundated for more than 6 months but still harboured distinct bacterial communities. This change in community structure was not associated with marked modifications in the rates of microbial activities involved in the C and N cycles. These results suggest that the observed structural change could be related to bacterial taxa responding to the environmental variations associated with different hydrological regimes, but not strongly associated with the biogeochemical processes monitored here.

Using bioassays and species sensitivity distributions to assess herbicide toxicity towards benthic diatoms.

Although benthic diatoms are widely used in ecological studies of aquatic systems, there is still a dearth of data concerning species sensitivities towards several contaminants. Within the same community, different species may respond differently depending on their physiological and ecological characteristics. This lack of knowledge makes specific appropriate risk assessment impossible. To find out whether species sensitivity distribution (SSD) could be used to estimate the risk of herbicide toxicity for diatoms, we need to know whether their sensitivity depends on their physiological and ecological characteristics. We carried out single-species bioassays on 11 diatom species exposed to 8 herbicides. Dose-responses relationships were used to extrapolate the Effective Concentration 5 (EC(5)) and the Effective Concentration 50 (EC(50)) for each exposure. These data were used to fit a SSD curve for each herbicide, and to determine the Hazardous concentration 5 (HC(5)) and 50 (HC(50)). Our results revealed a high level of variability of the sensitivity in the set of species tested. For photosystem-II inhibitor (PSII) herbicides, diatoms species displayed a typical grouping of sensitivity levels consistent with their trophic mode and their ecological guild. N-heterotroph and "motile" guild species were more tolerant of PSII inhibitors, while N-autotroph and "low profile" guild species were more sensitive. Comprehensive SSD curves were obtained for 5 herbicides, but not for sulfonylurea herbicides or for dimetachlor, which had toxicity levels that were below the range of concentration tested. The SSD curves provided the following ranking of toxicity: diuron> terbutryn> isoproturon> atrazine> metolachlor. The HC that affected 5% of the species revealed that, even at the usual environmental concentrations of herbicides, diatom assemblages could be affected, especially by isoproturon, terbutryn, and diuron.

Light History Influences the Response of Fluvial Biofilms to Zn Exposure.

Fluvial biofilms are subject to multistress situations in natural ecosystems, such as the co-occurrence of light intensity changes and metal toxicity. However, studies simultaneously addressing both factors are rare. This study evaluated in microcosm conditions the relationship between short-term light intensity changes and Zn toxicity on fluvial biofilms with long-term photoacclimation to different light conditions. Biofilms that had long-term photoacclimation to 25 µmol photons · m(-2)  · s(-1) (low light [LL] biofilms), 100 µmol photons · m(-2)  · s(-1) (medium light [ML] biofilms), and 500 µmol photons · m(-2)  · s(-1) (high light [HL] biofilms) were characterized by different structural (Chlorophyll-a [Chl-a], total biomass-AFDW, EPS, algal groups, and diatom taxonomy) and physiological attributes (ETR-I curves and photosynthetic pigments). HL biofilms showed higher light saturation intensity and a higher production of xanthophylls than LL biofilms. In contrast, LL biofilms had many structural differences; a higher proportion of diatoms and lower AFDW and EPS contents than ML and HL biofilms. A clear effect of light intensity changes on Zn toxicity was also demonstrated. Zn toxicity was enhanced when a sudden increase in light intensity also occurred, mainly with LL biofilms, causing higher inhibition of both the Φ'PSII and the ΦPSII . A decoupling of NPQ from de-epoxidation reaction (DR) processes was also observed, indicating substantial damage to photoprotective mechanisms functioning in biofilms (i.e., xanthophyll cycle of diatoms) due to Zn toxicity. This study highlights the need to take into account environmental stress (e.g., light intensity changes) to better assess the environmental risks of chemicals (e.g., metals).

Effects of organic herbicides on phototrophic microbial communities in freshwater ecosystems.

Over the past 15 years, significant research efforts have been channeled into assessing the effects of organic herbicides on freshwater phototrophic microbial communities. The results of this research are reviewed herein. The main conclusions we have reached after performing this review can be summarized into five points: · Most relevant assessments have dealt with the effects of triazine and phenylurea herbicides. Herbicides from these chemical classes are often considered to be model compounds when photosystem-II inhibitors are studied. · Until the early 2000s, the vast majority of investigations conducted to evaluate herbicide effects on phototropic microbes were performed in microcosms or mesocosms. In such studies, herbicides were usually applied alone, and often at concentrations much higher than those detected in the environment. More recently, the trend has been toward more realistic and relevant studies, in which lower herbicide concentrations were considered, and compound mixtures or successive treatments were tested. Increasingly, in situ studies are being designed to directly evaluate microbial community responses, following chemical exposures in contaminated aquatic environments. · Several biological end points are used to evaluate how organisms in the phototrophic microbial community respond to herbicide exposure. These end points allow the detection of quantitative changes, such as chl a concentrations, total cell counts or periphytic biomass, qualitative changes such as community structure to algal diversity, or functional changes such as photosynthesis and respiration, among others. They may give different and complementary information concerning the responses of microbial communities. · PICT approaches, which have generally combined functional and structural measurements, may prove to be valuable for assessing both an immediate impact, and for factoring in the contamination history of an ecosystem at the community level. · Finally, any relevant assessment of pesticide effects should incorporate a detailed environmental characterization that would include abiotic parameters (light, flow speed, nutrient content), or biotic parameters (diversity and structure of biofilms), because these control the bioavailability of pesticides, and thereby the exposure of microbial communities. To improve the value of ecotoxicological risk assessments, future research is needed in two key areas: first, more information on the effects of pollutants at the community level must be obtained (new tools and new end points), and second, more effort must be directed to reinforce the ecological relevance of toxicological investigations.

Enhanced co-tolerance and co-sensitivity from long-term metal exposures of heterotrophic and autotrophic components of fluvial biofilms.

Understanding the interactive effects of multiple stressors on ecosystems has started to become a major concern. The aim of our study was therefore to evaluate the consequences of a long-term exposure to environmental concentrations of Cu, Zn and As on the pollution-induced community tolerance (PICT) of lotic biofilm communities in artificial indoor channels. Moreover, the specificity of the PICT was assessed by evaluating the positive and negative co-tolerance between these metals. Photosynthetic efficiency and substrate-induced respiration (SIR), targeting the autotrophic and heterotrophic communities respectively were used in short-term inhibition bioassays with Cu, Zn and As to assess sensitivities of pre-exposed biofilms to the metals tested. Diversity profiles of a phototrophic, eukaryotic and prokaryotic community in biofilms following the different treatments were determined and analyzed with principal component analysis. The results demonstrated that pre-exposure to metals induced structural shifts in the community and led to tolerance enhancements in the phototrophic and heterotrophic communities. On the other hand, whatever the functional parameter used (i.e. photosynthesis and SIR), communities exposed to Cu were more tolerant to Zn and vice versa. Furthermore, only phototrophic communities pre-exposed to As developed tolerance to Cu but not to Zn, whereas no co-tolerance between Cu and As was observed in the heterotrophic communities. Finally, phototrophic and heterotrophic communities exposed to Cu and Zn became more sensitive to As, reflecting a negative co-tolerance between these metals. Overall, our findings support the fact that although the mode of action of the different metals is an important driver for the structure and thus the tolerance of the communities, it appears that the detoxification modes are the most important factors for the occurrence of positive or negative co-tolerance.

Leaf litter recycling in benthic and hyporheic layers in agricultural streams with different types of land use.

Changes in land use and intensification of agricultural pressure have greatly accelerated the alteration of the landscape in most developed countries. These changes may greatly disturb the adjacent ecosystems, particularly streams, where the effects of pollution are amplified. In this study, we used the leaf litter breakdown rate to assess the functional integrity of stream ecosystems and river sediments along a gradient of either traditional extensive farming or a gradient of vineyard area. In the benthic layer, the total litter breakdown process integrates the temporal variability of the anthropogenic disturbances and is strongly influenced by land use changes in the catchment even though a low concentration of toxics was measured during the study period. This study also confirmed the essential role played by amphipods in the litter breakdown process. In contrast, microbial processes may have integrated the variations in available nutrients and dissolved oxygen concentrations, but failed to respond to the disturbances induced by vineyard production (the increase in pesticides and metal concentrations) during the study period. The response of microbes may not be sensitive enough for assessing the global effect of seasonal agricultural practices. Finally, the leaf litter breakdown measured in the hyporheic zone seemed mainly driven by microbial activities and was hence more affected by vertical exchanges with surface water than by land use practices. However, the breakdown rate of leaf litter in the hyporheic zone may constitute a relevant way to evaluate the impact on river functioning of any human activities that induce massive soil erosion and sediment clogging.

In situ spatio-temporal changes in pollution-induced community tolerance to zinc in autotrophic and heterotrophic biofilm communities.

Pollution-induced community tolerance (PICT) uses increased tolerance in populations at contaminated sites as an indicator of contaminant effects. However, given the broad structural and functional complexity that characterizes biological communities, the acquisition of PICT could vary with (i) target community, (ii) intensity of toxicant exposure, (iii) the species succession stage, and (iv) the physicochemical characteristics of the studied site. To assess the spatio-temporal changes of zinc-induced tolerance in fluvial biofilm communities, we conducted an in situ study in Osor River (North-East Catalonia, Spain), which has zinc contamination. Biofilms were developed for 5 weeks in a non-metal-polluted site, and were then transferred to different sites in Osor River with different levels of zinc contamination. The spatio-temporal changes of biofilm PICT to zinc was determined using photosynthetic activity bioassays and respiration-induced aerobic bioassays at T(0), and at 1, 3 and 5 weeks of exposure. We also performed physicochemical characterization of the sites, taxonomic analysis of diatoms, bacterial and fungal diversity and profiled pigments of phototrophic communities. We used multivariate ordination to analyze results. In addition to natural species succession, the intensity of metal pollution exerted structural pressure by selecting the most metal-tolerant species, but differently depending on the type of biofilm. Zn-tolerance values indicated that exposure to high levels of zinc had effects that were similar to a longer exposure to lower levels of zinc.

Impact of chronic and acute pesticide exposures on periphyton communities.

Aquatic ecosystems face variable exposure to pesticides, especially during floodings which are associated with short bursts of high contaminant concentrations that influence biological systems. A study was undertaken to highlight the impact of the herbicide diuron applied in mixture with the fungicide tebuconazole on natural periphyton during flooding events. Periphyton were grown in two series of two lotic outdoor mesocosms: one series was non-contaminated while the other was exposed to chronic contamination. After 4weeks, one channel of each series was exposed to three successive pulses, with each pulse followed by one week of recovery. Impacts on periphyton were assessed by using Denaturing Gel Gradient Electrophoresis to characterize eukaryotic community structure. At a functional scale, photosynthetic efficiency was quantified during each pulse, and the induced tolerance to diuron was estimated by performing short-term inhibition tests based on photosynthetic efficiency. Moreover, pesticide concentrations in the water column and periphyton matrix were measured. Diuron was adsorbed in the periphyton during each pulse and desorbed 13h after pulse end. The different pulses affected the eukaryotic community structures of the control biofilms, but not of the chronically exposed ones. During the first pulse, photosynthetic efficiency was correlated with pesticide concentration in the water phase, and there was no difference between periphyton from chronically contaminated channels and control channels. However, during the second and third pulses, the photosynthetic efficiency of periphyton chronically exposed to pesticides appeared to be less impacted by the acute pulsed exposure of pesticide. These changes were consistent with the acquisition of induced tolerance to diuron since only after the third pulse that periphyton from chronic channel became tolerant to diuron. Our experimental study indicates that the effects of pulsed acute exposures to pesticides on periphyton depended on whether the communities had previously been exposed to the same stressors or not.