Benoît O. L. Demars

Impacts of Warming on the Structure and Functioning of Aquatic Communities : Individual-to Ecosystem-Level Responses

Environmental warming is predicted to rise dramatically over the next century, yet few studies have investigated its effects in natural, multi-species systems. We present data collated over an 8-year period from a catchment of geothermally heated streams in Iceland, which acts as a natural experiment on the effects of warming across different organisational levels and spatiotemporal scales. Body sizes and population biomasses of individual species responded strongly to temperature, with some providing evidence to support temperature size rules. Macroinvertebrate and meiofaunal community composition also changed dramatically across the thermal gradient. Interactions within the warm streams in particular were characterised by food chains linking algae to snails to the apex predator, brown trout These chains were missing from the colder systems, where snails were replaced by much smaller herbivores and invertebrate omnivores were the top predators. Trout were also subsidised by terrestrial invertebrate prey, which could have an effect analogous to apparent competition within the aquatic prey assemblage. Top-down effects by snails on diatoms were stronger in the warmer streams, which could account for a shallowing of mass-abundance slopes across the community. This may indicate reduced energy transfer efficiency from resources to consumers in the warmer systems and/or a change in predator-prey mass ratios. All the ecosystem process rates investigated increased with temperature, but with differing thermal sensitivities, with important implications for overall ecosystem functioning (e.g. creating potential imbalances in elemental fluxes). Ecosystem respiration rose rapidly with temperature, leading to increased heterotrophy. There were also indications that food web stability may be lower in the warmer streams.

Distribution of aquatic macrophytes in contrasting river systems : A critique of compositional-based assessment of water quality

A brief summary of the historical developments relating to plant distribution and aquatic macrophyte-nutrient indices provided a means of assessing the general context and validity of previous assumptions. This has particular current relevance because of the prominent use of bioindicators for defining nutrient enrichment. A survey of 161 sites distributed across two broadly contrasting groups of rivers (circum-neutral versus alkaline) recorded 110 species of aquatic macrophytes and these have been statistically analyzed to (i) rank and separate the individual effects of local environmental conditions and spatial isolation on species distribution in the two contrasting groups of sites; (ii) calculate a macrophyte index based on plant cover and species indicator values (Mean Trophic Rank, MTR); and finally (iii) investigate the implications for biomonitoring. Chemical, physical and hydrological site attributes together with spatial isolation, each explained a significant and at least partially independent influence over plant species distribution. It was extremely difficult, however, to separate the single effects of different site attributes on plant distribution. While some plant species are more restricted to certain environmental conditions, many appeared indifferent to the range of those being tested. The role played by nutrients (nitrogen (N) and phosphorus (P)) were either mostly indistinguishable from other site attributes (e.g., nitrate from conductivity) or subordinate (e.g., soluble reactive phosphorus, ammonium). It is therefore very unlikely that macrophyte species composition could provide a reliable bioindicator of the surrounding nutrient (N, P) status. The calculation of the plant index illustrated this unreliability by showing that strong correlations existed with many environmental variables, not just inorganic N and P.

Aquatic macrophytes as bioindicators of carbon dioxide in groundwater fed rivers.

Aquatic plants have been used as hydrological tracers in groundwater fed river systems. In nature, patterns in plant distribution have been attributed to ammonium (NH(4)) toxicity and phosphate (PO(4)) limitation, while some laboratory studies have focused on the role of the partial pressure of CO(2) (pCO(2)). The aims of this study were (i) to test whether plant distribution was more related to pCO(2) than NH(4) and PO(4) in nature, (ii) to develop and test the predictive power of new plant indices for pCO(2), NH(4) and PO(4), and (iii) to test the potential causality of the relationships using species eco-physiological traits. These tests were carried out with field data from the Rhine, Rhône and Danube river basins. Species composition was best related to the effect of pCO(2). The pCO(2) plant index was well calibrated (r(2)=0.73) and had the best predictive power (r(2)=0.47) of the three indices tested on independent datasets. The plant-pCO(2) relationship was supported by a biological mechanism: the ability of strictly submerged species of aquatic vascular plants to use HCO(3) under low pCO(2). This was not the whole story: the effects of pCO(2), NH(4) and PO(4) on plant distribution were partially confounded and interacted all together with temperature. However, neither NH(4) toxicity nor P limitation could be asserted using species eco-physiological traits. Moreover, the predictive power of the NH(4) and PO(4) plant indices was not as strong as pCO(2), at r(2)=0.24 and r(2)=0.27, respectively. Other potentially confounding variables such as spatial structure, biotic and physical factors were unlikely to confound the findings of this study.

Temperature effects on fish production across a natural thermal gradient

Abstract Global warming is widely predicted to reduce the biomass production of top predators, or even result in species loss. Several exceptions to this expectation have been identified, however, and it is vital that we understand the underlying mechanisms if we are to improve our ability to predict future trends. Here, we used a natural warming experiment in Iceland and quantitative theoretical predictions to investigate the success of brown trout as top predators across a stream temperature gradient (4–25 °C). Brown trout are at the northern limit of their geographic distribution in this system, with ambient stream temperatures below their optimum for maximal growth, and above it in the warmest streams. A five‐month mark‐recapture study revealed that population abundance, biomass, growth rate, and production of trout all increased with stream temperature. We identified two mechanisms that contributed to these responses: (1) trout became more selective in their diet as stream temperature increased, feeding higher in the food web and increasing in trophic position; and (2) trophic transfer through the food web was more efficient in the warmer streams. We found little evidence to support a third potential mechanism: that external subsidies would play a more important role in the diet of trout with increasing stream temperature. Resource availability was also amplified through the trophic levels with warming, as predicted by metabolic theory in nutrient‐replete systems. These results highlight circumstances in which top predators can thrive in warmer environments and contribute to our knowledge of warming impacts on natural communities and ecosystem functioning.

Integrated Experimental and Computational Hydraulic Science in a Unique Natural Laboratory

As part of a larger stream metabolism study, we report on field tracer experiments and subsequent modelling of stream transport for a unique set of streams in Iceland. Results indicate that: (1) intrinsic exchange rate between main stream and transient storage zones scales with flow velocity; (2) the fractional transient storage volume scales inversely with flow rate; (3) stream transient storage volume can change significantly over the growing season, especially if submerged vascular plants are present and (4) retention time within the transient storage zone can increase by up to 100% over the growing season.

On the Estimation of Solute Transport Parameters for Rivers

The modelling of solute transport in rivers is usually based on simulating the physical processes of advection, dispersion and transient storage, which requires the modeller to specify values of corresponding model parameters for the particular river reach under study. In recent years it has become popular to combine a numerical solution scheme of the governing transport equations with a parameter optimisation technique. However, there are several numerical schemes and optimisation techniques to choose from. The chapter addresses a very simple question, namely, do we get the same, or do we get different, parameter values from the application of two independent solute transport models/parameter optimisation techniques to the same data? Results from seven different cases of observed solute transport suggest the latter, which implies that parameter values cannot be transferred between modelling systems.

A Comparison of Three Solute Transport Models Using Mountain Stream Tracer Experiments

Stream ecology may be influenced by the temporary trapping of solutes in geomorphologic structures, which is usually quantified by fitting the Transient Storage Model to tracer data. This paper explores the relationships between the parameters of this model and those of two simpler models, namely the Advection-Dispersion Model and the Aggregated Dead Zone model. It is motivated by the possibility of obtaining more reliable transient storage parameter values by correlating them with the parameters of the other models instead of evaluating them directly. Results were obtained by fitting all three models to a set of tracer data from mountain streams, predominantly in Iceland. Some strong correlations were found between some of the parameters of the transient storage model and the advection-dispersion model, but no strong correlations were found between the parameters of the transient storage model and the aggregated dead zone model. For all three models, combinations of the optimized parameters correctly described the bulk movement of the solute cloud, giving confidence in the optimized parameters.

Reciprocal carbon subsidies between autotrophs and bacteria in stream food webs under stoichiometric constraints

Soils are currently leaching out their organic matter at an increasing pace and darkening aquatic ecosystems due to climate and land use change, or recovery from acidification. The implications for stream biogeochemistry and food webs remain largely unknown, notably the metabolic balance (biotic CO2 emissions), reciprocal subsidies between autotrophs and bacteria, and trophic transfer efficiencies. We use a flow food web approach to test how a small addition of labile dissolved organic matter affects the strength and dynamics of the autotrophs-bacteria interaction in streams. Our paired streams whole-ecosystem experimental approach combined with continuous whole-stream metabolism and stable isotope probing allowed to unravel carbon fluxes in the control and treatment streams. We increased the natural supply of dissolved organic matter for three weeks by only 12% by continuously adding 0.5 mg L−1 of sucrose with a δ13C signature different from the natural organic matter. Both photosynthesis and heterotrophic respiration increased rapidly following C addition, but this was short lived due to N and P stoichiometric constraints. The resulting peak in heterotrophic respiration was of similar magnitude to natural peaks in the control observed when soils were hydrologically connected to the streams and received soil derived carbon. Carbon reciprocal subsidies between autotrophs and bacteria in the control stream accounted for about 50% of net primary production and 75% of bacterial production, under low flow conditions when stream water was hydrologically disconnected from soil water. The reciprocal subsidies were weaker by 33% (autotrophs to bacteria) and 55% (bacteria to autotrophs) in the treatment relative to the control. Net primary production relied partly (11% in the control) on natural allochthonous dissolved organic carbon via the CO2 produced by bacterial respiration. Many large changes in ecosystem processes were observed in response to the sucrose addition. The light use efficiency of the autotrophs increased by 37%. Ecosystem respiration intensified by 70%, and the metabolic balance became relatively more negative, i.e. biotic CO2 emissions increased by 125%. Heterotrophic respiration and production increased by 89%, and this was reflected by a shorter (−40%) uptake length (SwOC) and faster (+92%) mineralisation velocity of organic carbon. The proportion of DOC flux respired and organic carbon use efficiency by bacteria increased by 112%. Macroinvertebrate consumer density increased by 72% due to sucrose addition and consumer production was 1.8 times higher in the treatment than in the control at the end of the experiment. The trophic transfer efficiencies from resources to consumers were similar between the control and the treatment (2-5%). Synthesis. Part of the carbon derived from natural allochthonous organic matter can feed the autotrophs via the CO2 produced by stream bacterial respiration, intermingling the green and brown webs. The interaction between autotrophs and bacteria shifted from mutualism to competition with carbon addition under nutrient limitation (N, P) increasing biotic CO2 emissions. Without nutrient limitation, mutualism could be reinforced by a positive feedback loop, maintaining the same biotic CO2 emissions. A small increase in dissolved organic carbon supply from climate and land use change could have large effects on stream food web and biogeochemistry with implications for the global C cycle under stoichiometric constraints.

Freshwater Conservation and Biomonitoring of Structure and Function: Genes to Ecosystems

Abstract Biomonitoring and conservation of freshwaters to date have fallen short of incorporating a fully ecological and evolutionary perspective. Due to this, the predictive capacity of current biomonitoring approaches is restricted and will have a limited ability to adapt in the face of rapid and global habitat modification and climate change. We briefly outline the present state of biomonitoring as well as some of its limitations. We then address how incorporating an ecological and evolutionary approach to biomonitoring and conservation will allow us to better understand interactions between the evolution and ecology of a species. This approach, alongside the incorporation of measures of ecosystem functioning and aided by new technologies such as novel molecular markers or the use of microbes, may facilitate the future development of a more comprehensive and effective biomonitoring framework.