Seasonal and spatial variations of stream insect emergence in an intensive agricultural landscape.

A growing amount of literature exists on reciprocal fluxes of matter and energy between ecosystems. Aquatic subsidies of winged aquatic insects can affect terrestrial ecosystems significantly, but this issue is rarely addressed in agroecosystems. By altering the production of benthic macroinvertebrates, agricultural practices could increase or decrease the strength of aquatic subsidies and subsequently the provision of several ecosystem services to agriculture. Effects of seasons and environmental variables on aquatic insect emergence were investigated in third-order agricultural streams in northwestern France. Most emerging dry mass (DM) of caught insects belonged to Trichoptera (56%), Chironomidae (25%) and Ephemeroptera (19%). We estimated that annual emerging dry mass of aquatic insects ranged between 1445 and 7374 mg/m2/y depending on the stream. Seasonal variations were taxon-specific, with Ephemeroptera emerging only in spring, Trichoptera emerging in spring and early summer, and Chironomidae emerging throughout the year. The percentage of watershed area covered by agriculture, ammonium concentration and hypoxia positively influenced emerging DM of Chironomidae but negatively influenced Ephemeroptera. Emerging DM of Trichoptera and Ephemeroptera increased significantly as water conductivity and temperature increased. Channel openness increased the emerging DM of all taxonomic groups, but Chironomidae were more abundant in narrow, incised streams. Assuming that the biomass of aquatic invertebrates ultimately disperse toward terrestrial habitats, nutrient accumulations on land near streams were estimated to reach 0.5-2.3 kg C ha-1 y-1, 0.1-0.5 kg N ha-1 y-1 and 0.005-0.03 kg P ha-1 y-1, depending on the stream. This suggests a significant flux of aquatic nutrients to agroecosystems and the need for future studies of its potential influence on the ecosystem services provided to agriculture.

Combined effect of temperature and ammonia on molecular response and survival of the freshwater crustacean Gammarus pulex.

Freshwater ecosystems are experiencing mounting pressures from agriculture, urbanization, and climate change, which could drastically impair aquatic biodiversity. As nutrient inputs increase and temperatures rise, ammonia (NH3) concentration is likely to be associated with stressful temperatures. To investigate the interaction between NH3 and temperature on aquatic invertebrate survival, we performed a factorial experiment on the survival and molecular response of Gammarus pulex, with temperature (10, 15, 20, and 25°C) and NH3 (0, 0.5, 1, 2, 3, and 4mg NH3/L) treatments. We observed an unexpected antagonistic interaction between temperature and NH3 concentration, meaning survival in the 4mg NH3/L treatment was higher at 25°C than at the control temperature of 10°C. A toxicokinetic-toxicodynamic (TK-TD) model was built to describe this antagonistic interaction. While the No Effect Concentration showed no significant variation across temperatures, the 50% lethal concentration at the end of the experiment increased from 2.7 (2.1-3.6) at 10°C to 5.5 (3.5- 23.4) mg NH3/L at 25°C. Based on qPCR data, we associated these survival patterns to variations in the expression of the hsp70 gene, a generic biomarker of stress. However, though there was a 14-fold increase in hsp70 mRNA expression for gammarids exposed to 25°C compared to controls, NH3 concentration had no effect on hsp70 mRNA synthesis across temperatures. Our results demonstrate that the effects of combined environmental stressors, like temperature and NH3, may strongly differ from simple additive effects, and that stress response to temperature can actually increase resilience to nutrient pollution in some circumstances.

Individual and joint toxicity of the herbicide S-metolachlor and a metabolite, deethylatrazine on aquatic crustaceans: Difference between ecological groups.

We studied the individual and joint acute toxicity of S-metolachlor (SMOC) and deethylatrazine (DEA - a metabolite of atrazine) on different non-target freshwater crustaceans. We used animals from different ecological groups: two amphipods from surface running water (Gammarus pulex and Gammarus cf. orinos), an isopod from surface stagnant water (Asellus aquaticus) and an amphipod living in groundwater (Niphargus rhenorhodanensis). Organisms were exposed to different levels of SMOC and DEA, alone or in binary mixture. Temperature effect on SMOC toxicity was assessed by exposing G. pulex and N. rhenorhodanensis to SMOC at 11 °C and 15 °C. Studying mortality as the biological endpoint, N. rhenorhodanensis was more resistant than surface water species towards SMOC and DEA. Among surface water species, G. pulex was the most sensitive while Gammarus cf. orinos and A. aquaticus showed similar responses to both compounds. Temperature increase did not change SMOC toxicity but modify the shape and steepness of the dose-response curve. We used a Model Deviation Ratio (MDR) approach to evaluate the predictability of Concentration Addition (CA) and Independent Action (IA) models to mixture toxicity. Results indicated either an additive or an antagonistic or a synergistic interaction depending on the concentrations combination and the test species. Our finding conclusively show the suitability of CA and IA in predicting mixture toxicities but results should be interpreted with caution according to ecological group of exposed species in risk assessment procedures.

Physiological and metabolic responses to rising temperature in Gammarus pulex (Crustacea) populations living under continental or Mediterranean climates.

Latitudinal thermal gradients offer the possibility of comparing the current performance of populations of a single species living in contrasting thermal conditions. The Rhône River Valley (France) presents a 5°C thermal gradient corresponding to the increase in temperature predicted by climatic models (IPCC, 2007). We studied the thermal tolerance to rising temperature (from 15 to 30°C) of five populations of the key species Gammarus pulex living either in the North (i.e. the cold part) or in the South (i.e. the warm part) of the river Valley. Individuals were acclimated at 18, 21, 24, 27 or 30°C during 10days. After this period, we here measured experimentally the populations' survival, ventilatory rate, oxygen consumption, and glycogen and triglyceride contents. Southern populations have a higher survival rate and higher oxygen consumption at higher temperatures (27 and 30°C) in comparison with northern populations. Southern individuals also presented a hyperventilation, and higher energy stores compared to northern individuals whatever the acclimation temperature considered. In a global change context, the rising temperatures during the next decades may differently impair the metabolism and the survival of populations of G. pulex from different geographical origins. These differences in ecophysiological responses of organisms must be taken into account to predict the consequences of climate change.

Ecophysiological responses to temperature of the "killer shrimp" Dikerogammarus villosus: is the invader really stronger than the native Gammarus pulex?

With global climate changes, biological invasions are considered to be one of the main causes of the decline of freshwater biodiversity. In this context, predicted increases in global temperature may alter the geographical distributions of native and invasive species. The purpose of our study was to examine the metabolic, behavioral and physiological responses to short-term temperature acclimation of two widely distributed species (the most successful European invader, Dikerogammarus villosus, and its main victim, Gammarus pulex), in order to estimate the potential effect of global warming on its invasion of freshwater ecosystems. Our results show that D. villosus is more vulnerable to high temperatures than G. pulex. The native species seems to be best adapted to intermediate temperatures (10-20°C) with a possibility of adjustment to "extreme" temperatures (5-27°C), whereas the "killer shrimp" D. villosus seems best adapted to lower temperatures (5-10°C) with a limited possibility of adjustment above 20°C. In the light of our results, global warming is likely to be less favorable to the invasive species. However, D. villosus showed reduced metabolic and activity rates, associated with higher glycogen content. This adaptive strategy was interpreted as having functional advantages, allowing D. villosus to successfully invade harsh and/or unpredictable biotopes. In addition, our results show that glycogen stores may be used as a powerful indicator of the optimal thermal window for aquatic ectotherms.

Parasite-induced changes in the diet of a freshwater amphipod: field and laboratory evidence.

Trophically transmitted parasites are likely to strongly influence food web-structure. The extent to which they change the trophic ecology of their host remains nevertheless poorly investigated and field evidence is lacking. This is particularly true for acanthocephalan parasites whose invertebrate hosts can prey on other invertebrates and contribute to leaf-litter breakdown. We used a multiple approach combining feeding experiments, neutral lipids and stable isotopes to investigate the trophic ecology of the freshwater amphipod Gammarus roeseli parasitized by the bird acanthocephalan Polymorphus minutus. Infected compared to uninfected amphipods consumed as many dead isopods, but fewer live isopods and less leaf material. Infection had no influence on the total concentration of neutral lipids. Contrary to what we expected based on laboratory findings, the nitrogen isotope signature, which allows for the estimation of consumer's trophic position, was not influenced by infection status. Conversely, the carbon isotope signature, which is used to identify food sources, changed with infection and suggested that the diet of infected G. roeseli includes less perilithon (i.e. fixed algae on rocks, stones) but more terrestrial inputs (e.g. leaf material) than that of uninfected conspecifics. This study shows evidence of changes in the trophic ecology of P. minutus-infected G. roeseli and we stress the need to complement feeding experiments with field data when investigating top-down effects of infection in an opportunistic feeder which adapts its diet to the available food sources.