Impact of peri-urban landscape on the organic and mineral contamination of pond waters and related risk assessment

Ponds are important for their ecological value and for the ecosystem services they provide to human societies, but they are strongly affected by human activities. Peri-urban development, currently one of the most pervasive processes of land use change in Europe, exposes ponds to both urban and agricultural contaminants, causing a potential combination of adverse effects. This study, focused on 12 ponds located in a peri-urban area, has two main objectives: (1) to link the physico-chemical characteristics of the waters and the nature of their contaminants, either organic or mineral, with the human activities around ponds, and (2) to estimate the environmental risk caused by these contaminants. The ponds were sampled during two consecutive years in both spring and in autumn. Although the ponds were distributed over a limited geographical area, their contamination profiles were different and more correlated with the agricultural than the urban land use. In terms of aptitude for biology, half of the ponds were classified in degraded states due to their physico-chemical parameters, but without correlation with the endocrine disrupting activities and the levels of organic pollutants as indicators. The main quantified organic pollutants, however, were pesticides with sufficiently high levels in certain cases to induce an environmental risk exceeding the classical thresholds of risk quotient.

Manipulating the strength of organism-environment feedback increases nonlinearity and apparent hysteresis of ecosystem response to environmental change

Theory predicts that organism-environment feedbacks play a central role in how ecological communities respond to environmental change. Strong feedback causes greater nonlinearity between environmental change and ecosystem state, increases the likelihood of hysteresis in response to environmental change, and augments the possibility of alternative stable regimes. To illustrate these predictions and their dependence on a temporal scale, we simulated a minimal ecosystem model. To test the predictions, we manipulated the feedback strength between the metabolism and the dissolved oxygen concentration in an aquatic heterotrophic tri-trophic community in microecosystems. The manipulation consisted of five levels, ranging from low to high feedback strength by altering the oxygen diffusivity: free gas exchange between the microcosm atmosphere and the external air (metabolism not strongly affecting environmental oxygen), with the regular addition of 200, 100, or 50 ml of air and no gas exchange. To test for nonlinearity and hysteresis in response to environmental change, all microecosystems experienced a gradual temperature increase from 15 to 25°C and then back to 15°C. We regularly measured the dissolved oxygen concentration, total biomass, and species abundance. Nonlinearity and hysteresis were higher in treatments with stronger organism-environment feedbacks. There was no evidence that stronger feedback increased the number of observed ecosystem states. These empirical results are in broad agreement with the theory that stronger feedback increases nonlinearity and hysteresis. They therefore represent one of the first direct empirical tests of the importance of feedback strength. However, we discuss several limitations of the study, which weaken confidence in this interpretation. Research demonstrating the causal effects of feedback strength on ecosystem responses to environmental change should be placed at the core of efforts to plan for sustainable ecosystems.

Linking phytoplankton pigment composition and optical properties: A framework for developing remote-sensing metrics for monitoring cyanobacteria

This study has been performed in the framework of a research program aiming to develop a low-cost aerial sensor for the monitoring of cyanobacteria in freshwater ecosystems that could be used for early detection. Several empirical and mechanistic remote-sensing tools have been already developed and tested at large scales and have proven useful in monitoring cyanobacterial blooms. However, the effectiveness of these tools for early detection is hard to assess because such work requires the detection of low concentrations of characteristic pigments amid complex ecosystems exhibiting several confounding factors (turbidity, blooms of other species, etc.). We developed a framework for performing high-throughput measurements of the absorbance and reflectance of small volumes (∼ = 20 mL) of controlled mixtures of phytoplankton species and studied the potential of this framework to validate remote-sensing proxies of cyanobacteria concentration. The absorption and reflectance spectra of single and multiple cultures carried a specific signal that allowed for the quantitative analysis of culture mixes. This specific signal was shown to be related to known pigment absorbance spectra. The concentrations of chlorophyll-a and -b, phycocyanin and phycoerythrin could be obtained from direct absorbance measurements and were correlated with the concentration obtained after pigment extraction (R² ≥ 0.96 for all pigments). A systematic test of every possible two-band and three-band normalized difference between optical indices was then performed, and the coincidental correlation with chlorophyll-b (absent in cyanobacteria) was used as an indicator of non-specificity. Two-band indices were shown to suffer from non-specificity issues and could not yield strong and specific relationships with phycocyanin or phycoerythrin (maximum R² < 0.5). On the other hand, the three-band modified normalized difference indices yielded strong specific relationships (R² > 0.8).

Modeling the direct and indirect effects of copper on phytoplankton-zooplankton interactions

Predicting the effects of pollution at the community level is difficult because of the complex impacts of ecosystem dynamics and properties. To predict the effects of copper on a plant-herbivore interaction in a freshwater ecosystem, we built a model that focuses on the interaction between an alga, Scenedesmus sp., and a herbivore, Daphnia sp. The model assumes logistic growth for Scenedesmus and a type II functional response for Daphnia. Internal copper concentrations in Scenedesmus and Daphnia are calculated using a biodynamic model. We include two types of direct effects of copper on Scenedesmus and Daphnia that results from hormesis: a deficiency effect at low concentration and a toxic effect at high concentration. We perform a numerical analysis to predict the combined effects of copper and nutrient enrichment on the Scenedesmus-Daphnia interaction. Results show three types of outcomes depending on copper concentration. First, low (4 μg L(-1)) and high (50 μg L(-1)) copper concentrations cause deficiency and toxicity, respectively, leading to the extinction of all populations; for less extreme concentrations (between 4 and 5 μg L(-1) and between 16.5 and 50 μg L(-1)), only the consumer population becomes extinct. The two populations survive with intermediate concentrations. Second, when population dynamics present oscillations, copper has a stabilizing effect and reduces or suppresses oscillations. Third, copper, on account of its stabilizing effect, opposes the destabilizing effect of nutrient enrichment. Our model shows that (1) Daphnia is affected by copper at lower concentrations when community interactions are taken into account than when analyzed alone, and (2) counterintuitive effects may arise from the interaction between copper pollution and nutrient enrichment. Our model also suggests that single-value parameters such as NOEC and LOEC, which do not take community interactions into account to characterize pollutants effects, are unable to determine pollutant effects in complex ecosystems. More generally, our model underscores the importance of ecosystem-scale studies to predict the effects of pollutants.

CDOM fluorescence as a proxy of DOC concentration in natural waters: a comparison of four contrasting tropical systems

Chromophoric dissolved organic matter (CDOM) fluorescence or absorption is often proposed as a rapid alternative to chemical methods for the estimation of bulk dissolved organic carbon (DOC) concentration in natural waters. However, the robustness of this method across a wide range of systems remains to be shown. We measured CDOM fluorescence and DOC concentration in four tropical freshwater and coastal environments (estuary and coastal, tropical shallow lakes, water from the freshwater lens of two small islands, and soil leachates). We found that although this method can provide an estimation of DOC concentration in sites with low variability in DOC and CDOM sources in systems where the variability of DOC and CDOM sources are high, this method should not be used as it will lead to errors in the estimation of the bulk DOC concentration.

Ingredients for protist coexistence: competition, endosymbiosis and a pinch of biochemical interactions

1. The interaction between mutualism, facilitation or interference and exploitation competition is of major interest as it may govern species coexistence. However, the interplay of these mechanisms has received little attention. This issue dates back to Gause, who experimentally explored competition using protists as a model [Gause, G.F. (1935) Vérifications expérimentales de la théorie mathématique de la lutte pour la vie. Actualités Scientifiques et Industrielles, 277]. He showed the coexistence of Paramecium caudatum with a potentially allelopathic species, Paramecium bursaria. 2. Paramecium bursaria hosts the green algae Chlorella vulgaris. Therefore, P. bursaria may benefit from carbohydrates synthesised by the algae. Studying endosymbiosis with P. bursaria is possible as it can be freed of its endosymbiont. In addition, C. vulgaris is known to produce allelochemicals, and P. bursaria may benefit also from allelopathic compounds. 3. We designed an experiment to separate the effects of resource exploitation, endosymbiosis and allelopathy and to assess their relative importance for the coexistence of P. bursaria with a competitor that exploits the same resource, bacteria. The experiment was repeated with two competitors, Colpidium striatum or Tetrahymena pyriformis. 4. Results show that the presence of the endosymbiont enables the coexistence of competitors, while its loss leads to competitive exclusion. These results are in agreement with predictions based on resource equilibrium density of monocultures (R*) supporting the idea that P. bursaria's endosymbiont is a resource provider for its host. When P. bursaria and T. pyriformis coexist, the density of the latter shows large variation that match the effects of culture medium of P. bursaria. Our experiment suggests these effects are because of biochemicals produced in P. bursaria culture. 5. Our results expose the hidden diversity of mechanisms that underlie competitive interactions. They thus support Gauses's speculation (1935) that allelopathic effects might have been involved in his competition experiments. We discuss how a species engaged both in competition for a resource and in costly interference such as allelopathy may counterbalance these costs with a resource-provider endosymbiont.

Intra- and interspecific density-dependent dispersal in an aquatic prey-predator system

1. Dispersal intensity is a key process for the persistence of prey-predator metacommunities. Consequently, knowledge of the ecological mechanisms of dispersal is fundamental to understanding the dynamics of these communities. Dispersal is often considered to occur at a constant per capita rate; however, some experiments demonstrated that dispersal may be a function of local species density. 2. Here we use aquatic experimental microcosms under controlled conditions to explore intra- and interspecific density-dependent dispersal in two protists, a prey Tetrahymena pyriformis and its predator Dileptus sp. 3. We observed intraspecific density-dependent dispersal for the prey and interspecific density-dependent dispersal for both the prey and the predator. Decreased prey density lead to an increase in predator dispersal, while prey dispersal increased with predator density. 4. Additional experiments suggest that the prey is able to detect its predator through chemical cues and to modify its dispersal behaviour accordingly. 5. Density-dependent dispersal suggests that regional processes depend on local community dynamics. We discuss the potential consequences of density-dependent dispersal on metacommunity dynamics and stability.

Nutrient-limited food webs with up to three trophic levels: feasibility, stability, assembly rules, and effects of nutrient enrichment

Community structure is controlled, among multiple factors, by competition and predation. Using the R* rule and graphical analysis, we analyse here the feasibility, stability and assembly rules of resource-based food webs with up to three trophic levels. In particular, we show that (1) the stability of a food web with two plants and two generalist herbivores does not require that plants' resource exploitation abilities trade-off with resistance to the two herbivores, and (2) food webs with two plants and either one generalist herbivore and a carnivore or two generalist herbivores and two generalist carnivores are not feasible because of cascade competition between top consumers. The relative strength of species interactions and the relative impacts of plants and herbivores on factors which control their growth also play a critical role. We discuss how community structure constrains assembly rules and yields cascades of extinctions in food webs.

Functional diversity governs ecosystem response to nutrient enrichment

The relationship between species diversity and ecosystem functioning is a central topic in ecology today. Classical approaches to studying ecosystem responses to nutrient enrichment have considered linear food chains. To what extent ecosystem structure, that is, the network of species interactions, affects such responses is currently unknown. This severely limits our ability to predict which species or functional groups will benefit or suffer from nutrient enrichment and to understand the underlying mechanisms. Here our approach takes ecosystem complexity into account by considering functional diversity at each trophic level. We conducted a mesocosm experiment to test the effects of nutrient enrichment in a lake ecosystem. We developed a model of intermediate complexity, which separates trophic levels into functional groups according to size and diet. This model successfully predicted the experimental results, whereas linear food-chain models did not. Our model shows the importance of functional diversity and indirect interactions in the response of ecosystems to perturbations, and indicates that new approaches are needed for the management of freshwater ecosystems subject to eutrophication.