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Ecological memory (EM) recognizes the importance of previous stress encounters in promoting community tolerance and thereby enhances ecosystem stability, provided that gained tolerances are preserved during non-stress periods. Drawing from this concept, we hypothesized that the recruitment of tolerant species can be facilitated by imposing an initial sorting process (conditioning) during the early stages of community assembly, which should result in higher production (biomass development and photosynthetic efficiency) and stable community composition. To test this, phytoplankton resting stages were germinated from lake sediments originating from two catchments that differed in contamination history: one impacted by long-term herbicides and pesticides exposures (historically contaminated lake) from an agricultural catchment compared to a low-impacted one (near-pristine lake) from a forested catchment. Conditioning was achieved by adding an herbicide (Isoproturon, which was commonly used in the catchment of the historically contaminated lake) during germination. Afterward, the communities obtained from germination were exposed to an increasing gradient of Isoproturon. As hypothesized, upon conditioning, the phytoplankton assemblages from the historically contaminated lake were able to rapidly restore photosynthetic efficiency (p > 0.01) and became structurally (community composition) more resistant to Isoproturon. The communities of the near-pristine lake did not yield these positive effects regardless of conditioning, supporting that EM was a unique attribute of the historically stressed ecosystem. Moreover, assemblages that displayed higher structural resistance concurrently yielded lower biomass, indicating that benefits of EM in increasing structural stability may trade-off with production. Our results clearly indicate that EM can foster ecosystem stability to a recurring stressor. [ABSTRACT FROM AUTHOR]

Pollution-induced community tolerance (PICT) has been used to demonstrate effects of sediment contamination on microbes and meiofauna. Our study explored the potential to detect PICT in benthic macroinvertebrates of a lake with long-term mild lead (Pb) contamination. We collected macrobenthos from two areas in Caddo Lake, Texas, a control area (CO) with a mean sediment Pb level of 11 μg/g and Goose Prairie (GP) where sediment Pb levels averaged 74 μg/g. Upon return to the laboratory, we exposed macroinvertebrates to a lethal lead concentration and assessed 48-h mortality. Mortality of CO macrobenthos was significantly higher than that of GP macrobenthos, providing evidence that these communities differed in their tolerance to lead. A comparison of macrobenthos community composition between the areas showed that the GP macrobenthos lacked metal-sensitive taxa such as gastropods and amphipods (which were present at CO). Similarly, a higher proportion of the GP benthos belonged to metal-tolerant taxa such as isopods and chironomids. Thus, changes in community composition appeared to be at least partly responsible for differences in community tolerance. Our results showed that a sediment Pb concentration below effect-based sediment quality guidelines had a measurable impact on macrobenthos, thus demonstrating that results from single-species toxicity tests may underestimate impacts on communities. This study also confirms that the PICT approach with macroinvertebrates is a feasible and potentially powerful approach for detecting contaminant impacts. [ABSTRACT FROM AUTHOR]

Ecosystems are subject to a combination of recurring anthropogenic and natural disturbances, such as climate change and pesticide exposure. Biological communities are known to develop tolerance to recurring disturbances due to successive changes at both the community and organismal levels. However, information on how additional stressors may affect the development of such community tolerance is scarce to date. We studied the influence of hydrological disturbance on the reaction of zooplankton communities to repeated insecticide pulses in outdoor microcosms. The communities were exposed to three successive pulses of the insecticide esfenvalerate (0.03, 0.3, and 3 µg/L) and to the gradual removal of water and its subsequent replacement over three cycles or to a constant water level. Except at the highest esfenvalerate concentration, the communities developed tolerance to the toxicant, as indicated by their decreasing reaction to subsequent insecticide applications, and this development was enhanced by hydrological disturbance. The pronounced decline of the key taxa Daphnia spp. through the combined action of the two stressors was identified as the main mechanism responsible for the increase in community tolerance under a fluctuating water level. Under a constant water level, the abundance of Daphnia spp. did not decrease significantly without the insecticide treatment, indicating that other mechanisms were responsible for the observed community tolerance. The present study shows that additional stressors can facilitate the development of community tolerance and that such facilitation is propagated through community-level mechanisms. [ABSTRACT FROM AUTHOR]

The biological response of periphyton chronically exposed to metals of urban origin (Cd, Ni and Zn) was investigated with a Pollution-Induced Community Tolerance (PICT) approach using a previously developed short-term toxicity test based on β-glucosidase (heterotrophic) activity. Periphyton was grown on plastic membranes immersed in indoor aquaria contaminated with metals at realistic contamination levels (0.3, 3 μg/l for Cd, 5, 50 μg/l for Ni, 20, 200 μg/l for Zn). After 3 weeks of exposure, biofilms' parameters (dry-weight, chlorophyll a concentration, heterotrophic activity) were analyzed and tolerance acquisition of the heterotrophic communities was assessed using the toxicity test. Modifications of bacterial and eukaryotic community structure were assessed with Automated Ribosomal Intergenic Spacer Analysis (ARISA). Effects of metal exposure were observed on biofilms parameters in the Cd and Zn experiments. Tolerance levels increased for both Cd-exposed biofilms, and for the high metal treatment biofilms in the Ni and Zn experiments. Analysis of the ARISA profiles showed that metal exposure affected the structure of both bacterial and eukaryotic communities. Moreover, Cd tolerance of the Zn-exposed heterotrophic communities was evaluated, which showed that the Zn-tolerant community (high metal treatment in the Zn experiment) also became tolerant to Cd (co-tolerance). The study shows that tolerance acquisition can be detected after exposure to environmental metal concentrations using β-glucosidase activity as an endpoint in short-term toxicity tests. [ABSTRACT FROM AUTHOR]

A test was developed that measures in vivo chlorophyll afluorescence variables to assess the apparent sensitivity of freshwaterperiphytic algae to photosystem II inhibitors. Natural periphyticcommunities from rivers were collected on artificial substrata, and theeffects of short-term exposures to two PSII herbicides (atrazine andisoproturon) on the fluorescence parameters were measured with apulse-amplitude modulated fluorometer. The EC50 for each herbicide werecalculated from fluorescence yield indices, and these results were comparedto 14C-based primary production measurements on the samecommunities. The fluorescence-based method appears to give very reliableestimations of EC50 for each pesticide we tested, ranging from 0.46 to5.18 μM and 0.07 to 6.77 μM for atrazine and isoproturon,respectively. This method could be used in ecotoxicology monitoringprograms, to detect changes in natural periphyton populations sensitivity,following photosystem II herbicide contamination in rivers or lakes. [ABSTRACT FROM AUTHOR]