Christophe Piscart

Salinisation of rivers: An urgent ecological issue

Secondary salinisation of rivers and streams is a global and growing threat that might be amplified by climate change. It can have many different causes, like irrigation, mining activity or the use of salts as de-icing agents for roads. Freshwater organisms only tolerate certain ranges of water salinity. Therefore secondary salinisation has an impact at the individual, population, community and ecosystem levels, which ultimately leads to a reduction in aquatic biodiversity and compromises the goods and services that rivers and streams provide. Management of secondary salinization should be directed towards integrated catchment strategies (e.g. benefiting from the dilution capacity of the rivers) and identifying threshold salt concentrations to preserve the ecosystem integrity. Future research on the interaction of salinity with other stressors and the impact of salinization on trophic interactions and ecosystem properties is needed and the implications of this issue for human society need to be seriously considered.

Effects of intense agricultural practices on heterotrophic processes in streams.

In developed countries, changes in agriculture practices have greatly accelerated the degradation of the landscape and the functioning of adjacent aquatic ecosystems. Such alteration can in turn impair the services provided by aquatic ecosystems, namely the decomposition of organic matter, a key process in most small streams. To study this alteration, we recorded three measures of heterotrophic activity corresponding to microbial hydrolasic activity (FDA hydrolysis) and leaf litter breakdown rates with (k(c)) and without invertebrates (k(f)) along a gradient of contrasted agricultural pressures. Hydrolasic activity and k(f) reflect local/microhabitat conditions (i.e. nutrient concentrations and organic matter content of the sediment) but not land use while k(c) reflects land-use conditions. k(c), which is positively correlated with the biomass of Gammaridae, significantly decreased with increasing agricultural pressure, contrary to the taxonomic richness and biomass of Trichoptera and Plecoptera. Gammaridae may thus be considered a key species for organic matter recycling in agriculture-impacted streams.

Saving freshwater from salts

Ion-specific standards are needed to protect biodiversity Many human activities—like agriculture and resource extraction—are increasing the total concentration of dissolved inorganic salts (i.e., salinity) in freshwaters. Increasing salinity can have adverse effects on human health (1); increase the costs of water treatment for human consumption; and damage infrastructure [e.g., amounting to

Global scale variation in the salinity sensitivity of riverine macroinvertebrates: Eastern Australia, France, Israel and South Africa

700 million per year in the Border Rivers catchment, Australia (2)]. It can also reduce freshwater biodiversity (3); alter ecosystem functions (4); and affect economic well-being by altering ecosystem goods and services (e.g., fisheries collapse). Yet water-quality legislation and regulations that target salinity typically focus on drinking water and irrigation water, which does not automatically protect biodiversity. For example, specific electrical conductivities (a proxy for salinity) of 2 mS/cm can be acceptable for drinking and irrigation but could extirpate many freshwater insect species (3). We argue that salinity standards for specific ions and ion mixtures, not just for total salinity, should be developed and legally enforced to protect freshwater life and ecosystem services. We identify barriers to setting such standards and recommend management guidelines.

Potential impact of invasive amphipods on leaf litter recycling in aquatic ecosystems

Salinity is a key abiotic property of inland waters; it has a major influence on biotic communities and is affected by many natural and anthropogenic processes. Salinity of inland waters tends to increase with aridity, and biota of inland waters may have evolved greater salt tolerance in more arid regions. Here we compare the sensitivity of stream macroinvertebrate species to salinity from a relatively wet region in France (Lorraine and Brittany) to that in three relatively arid regions eastern Australia (Victoria, Queensland and Tasmania), South Africa (south-east of the Eastern Cape Province) and Israel using the identical experimental method in all locations. The species whose salinity tolerance was tested, were somewhat more salt tolerant in eastern Australia and South Africa than France, with those in Israel being intermediate. However, by far the greatest source of variation in species sensitivity was between taxonomic groups (Order and Class) and not between the regions. We used a Bayesian statistical model to estimate the species sensitivity distributions (SSDs) for salinity in eastern Australia and France adjusting for the assemblages of species in these regions. The assemblage in France was slightly more salinity sensitive than that in eastern Australia. We therefore suggest that regional salinity sensitivity is therefore likely to depend most on the taxonomic composition of respective macroinvertebrate assemblages. On this basis it would be possible to screen rivers globally for risk from salinisation.

Environmental mediation of intraguild predation between the freshwater invader Gammarus pulex and the native G. duebeni celticus

The impact of biological invasions on local biodiversity is well established, but their impact on ecosystem functioning has only been sketchily documented. However, biological invasions may impede services provided by aquatic ecosystems, such as, for example, the decomposition of organic matter, a key process in most small streams. To address this question, we experimentally quantified the leaf litter breakdown activity of native and invasive amphipod species, which are keystone species in aquatic ecosystems. The breakdown rate of each species was used to estimate the potential leaf litter recycling in the Rhône and Meurthe Rivers in sites occupied solely by native species and sites dominated by invasive species. We found that invaders were not able to compensate for the activity of native species and that the replacement of native species led to a decrease of at least 66% in the rate of leaf litter recycling. Our approach provides empirical evidence of the functional impact of non-indigenous species on leaf litter recycling, using standard protocols and literature data.

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

Invasive species and environmental change often occur simultaneously across a habitat and therefore our understanding of their relative roles in the decline of native species is often poor. Here, the environmental mediation of a critical interspecific interaction, intraguild predation (IGP), was examined between invasive (Gammarus pulex) and native (G. d. celticus) freshwater amphipods. In the laboratory, IGP asymmetries (males preying on congeneric females) were examined in river water sourced from zones where: (1) the invader has completely displaced the native; (2) the two species currently co-exist, and (3) the native currently persists uninvaded. The invader was always a more effective IG predator, but this asymmetry was significantly weaker moving from ‘invader-only water’ through ‘co-existence water’ to ‘native-only water’. The constituent of the water that drives this mediation of IGP was not identified. However, balancing the rigour of laboratory experiments with field derived ‘environment’ has advanced understanding of known patterns in a native species decline, and its co-existence and persistence in the face of an invader.

Effect of habitat-related resources on fatty acid composition and body weight of the invasive Dikerogammarus villosus in an artificial reservoir.

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.

Seasonal variations of the effect of temperature on lethal and sublethal toxicities of ammonia for three common freshwater shredders

Successful invasions are attributed to a combination of adaptations to biotic and abiotic conditions and many studies have highlighted the role of biological traits of invaders in their success. Among the most useful traits for successful establishment in a new ecosystem are the size of the invader and its diet and feeding habits. During the last twenty years, Dikerogammarus villosus has been one of the most successful invaders of large European rivers and lakes. However, regarding feeding habits, results were confusing and sometimes contradictory. In this study, we attempt to define the feeding strategy of D. villosus using their FA content according to sex and the seasonal and the spatial variation in its feeding habit. Our results indicated that D. villosus is rather unspecialized with plasticity in its feeding behaviour, switching between different functional feeding groups depending on seasons, habitats and available resources. The selection of high quality resources may hence provide a strong advantage during the invasion process by optimizing the growth and the reproduction of the invader.

The combined effects of water level reduction and an increase in ammonia concentration on organic matter processing by key freshwater shredders in alluvial wetlands

In a context of global change, increases in temperature and in ammonia concentration should strongly affect the crustaceans of wetlands. We experimentally examined, at three different seasons (i.e. winter, spring, and summer), the effect of temperature (12, 18, and 24°C) on the lethal (survival rates) and sublethal (oxygen consumption) toxicity of unionized ammonia (NH(3)) on the amphipods Gammarus pulex and Gammarus roeselii and the isopod Asellus aquaticus. Our results demonstrate (1) a gradient of increasing tolerance and survival from G. roeselii to G. pulex and A. aquaticus, (2) an increasing toxicity of ammonia with temperature, and (3) a strong seasonal variation of the tolerance to ammonia, with a higher tolerance of individuals in winter than in summer. However, the sub-lethal effect of ammonia on the oxygen consumption rate was species dependant and changed according to temperature or season. Global change and resulting variations in crustacean densities will potentially affect the ecosystem functioning (e.g. organic matter recycling).