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Subsidy-stress responses can decouple density-dependent relationships between taxa within a community by increasing resources for some taxa while removing sensitive species. In this study, we examined the effects of a triad of subsidy-stress responses on predator-prey relationships in a benthic macroinvertebrate community under the action of insecticides, nutrients and predation pressure all at environmentally relevant doses., We stocked 72 outdoor artificial streams with benthic invertebrates to establish a fully factorial experiment to investigate the interactive effects of two anthropogenic factors: (i) three levels of nutrient enrichment (low, moderate and high); and (ii) three doses of imidacloprid, a common agricultural insecticide, at control, low (LOEC) and lethal doses (LC50). These factors were crossed with (iii) another, biotic stressor by stocking half of the streams with a predaceous perlid stonefly at near-natural densities., Fifty percent of the community-level responses were driven by the combined effect of nutrients and the insecticide. An additional 27% of the community variation was best explained by predation pressure, which we estimated as the gape width × density of Agnetina capitata at the end of the experiment., No negative effect of imidacloprid on A. capitata was detected ( P = 0.62). By the end of the 3-week experiment, the overall gape width and predation pressure of A. capitata had more than doubled (2.1-3.7x) in all moderate nutrient streams ( P < 0.01). Increased size and predation pressure of A. capitata covaried with the depressed density of all macroinvertebrate taxa as well as EPT (Ephemeroptera, Plecoptera, Trichoptera) taxa and the mayfly Baetis in particular., Structural equation models (SEM) highlighted that nutrient enrichment was the main driver of individual and community responses in the artificial streams. Additionally, SEM showed that density but not body size of Baetis was severely hampered by A. capitata predation, suggesting that these stoneflies selectively foraged on other species in the first 10 days of the experiment., Our findings highlight the difficulties of assessing the risk of contaminants in aquatic communities where interacting species respond differently to subsidies and stresses. [ABSTRACT FROM AUTHOR]

The vertical distribution within the substrate of three lotic mayflies (Rhithrogena sp., Paraleptophlebia sp. and Baetis tricaudatus Dodds) was examined in laboratory streams to determine if diel patterns of vertical movement across substrate surfaces existed and whether light intensity, temperature, substrate composition or food availability influenced the observed movement patterns. 2. Rhithrogena sp. and B. tricaudatus exhibited significant diel movement from lower substrate surfaces during the day to upper surfaces at night, although >64% of B. tricaudatus occupied the upper surfaces throughout the 24 h period. In contrast, Parakptophlebia sp. larvae did not change their vertical distribution over the 24 h period. 3. The proportion of mayflies occupying substrate surfaces was influenced by the amount of upper and lower surface area available in the substrate and, possibly, crevice sizes associated with lower surfaces. 4. The movement of Rhithrogena sp. from lower surfaces during the day to the upper surfaces during the night was influenced by changes in light intensity, but not by changes in food availability or water temperature. 5. The diel patterns of vertical movement exhibited by lotic mayflies in this investigation raise the possibility that at least some biotic interactions may vary over a diel cycle. These could include competitive interactions or temporal cycling of prey availability to predators. [ABSTRACT FROM AUTHOR]

Arctic landscapes are characterised by their numerous ponds and lakes. With increased climate warming and consequent changes in hydrological connectivity, the ecology of many of these waterbodies is expected to change.We sampled 13 ponds and 22 lakes for zooplankton and various limnological and physical environmental variables in the vicinity of Cambridge Bay, Nunavut (69.1169° N, 105.0597° W), with the aim of testing how well the degree of hydrological connectivity explained patterns in species composition and abundance and contributed to crustacean production. Ponds were hydrologically isolated while the lakes were arranged in four lake chains and ranged from the headwaters to highly inter‐connected lakes receiving water from one to 768 upstream lakes.In all sites combined, 77 zooplankton species were found, including 56 rotifers, six copepods, 11 cladocerans, two fairy shrimp species, a mysid, and a tadpole shrimp. We show that the zooplankton communities differed between hydrologically isolated (ponds) and connected (lakes) systems, with 17 species unique to ponds and 20 to lakes. Furthermore, the communities were more similar within lake chains and hence in lakes closer to each other. In ponds, distances between waterbodies had no impact on community similarity.Zooplankton abundance was higher in lakes (255 ind/L) than in ponds (65 ind/L) due to the higher number of rotifers that accounted for nearly 80% of the zooplankton abundance in lakes. In ponds, rotifers, cladocerans, and copepods were equally abundant. In terms of biomass, cladocerans represented 65% of total biomass in all waterbodies except for a chain of deep lakes that had abundant fish communities. In these lakes, cladoceran abundance and biomass were low and probably limited by fish predation.Zooplankton production was higher in ponds (4.6 mgC m−3 day−1) than in lakes (1.7 mgC m−3 day−1), and within lakes was lowest in the chain composed of large lakes (1.3 mgC m−3 day−1). The high production in ponds was closely linked to high zooplankton biomass and further explained by high gross primary production supported by relatively high nutrient concentrations in these shallow systems.Our study emphasises that hydrological connectivity is key to shaping zooplankton communities, although fish presence also appears to affect them. It shows that isolated ponds (that currently account for c83% of all waterbodies on southern Victoria Island) are hotspots of aquatic biological productivity that probably play an essential role in Arctic freshwater landscapes. Finally, our study provides critical baseline information on the current composition of Arctic zooplankton communities important for estimating climate change impacts. [ABSTRACT FROM AUTHOR]

The foraging and predator-avoidance behaviors of mayfly (Baetis tricaudatus) nymphs and predator--prey interactions were investigated under differing combinations of cadmium concentration and predation risk. Predators consisted of stonefly (Kogotus nonus) nymphs, juvenile brook trout (Salvelinus fontinalis), and longnose dace (Rhinichthys cataractae), either alone or in combination. Organisms were exposed to 0, 0.5, or 5.0 µg/L Cd for 7 d (invertebrates) or 30 d (vertebrates) prior to observation in artificial stream channels. Behavioral observations (20-min duration) were made both day and night over a 24-h period. Mayfly feeding rate was assessed by image analysis of grazed diatom mats cultured on ceramic tiles. Both predators and cadmium produced significant effects on the behavior of B. tricaudatus and K. nonus nymphs, with mayflies showing a hierarchical response in the presence of multiple predators. A significant increase in activity was observed in S. fontinalis exposed to 5.0 µg/L Cd, with significant decreases in capture efficiency of prey items apparent at 0.5 µg/L Cd. Conversely, the foraging behavior of R. cataractae was unaffected by cadmium, possibly as a result of species-specific cadmium tolerance. Such sublethal effects of a contaminant on the energy budgets of individuals and the interactions between species have implications for community structure in aquatic food webs [ABSTRACT FROM AUTHOR]

1. Laboratory experiments were performed to determine whether flightinitiation by lotic mayfly larvae of Baetis tricaudatus exposed to the longnose dace (Rhinichthys cataractae) is affected by an individual's size and its physiological state. 2. We used a three factorial ANOVA design to test whether flight initiation distances (FIDS) were affected by Baetis size (small, large), starvation level (low, high), and the length of a predator-free foraging period (short, long). 3. FIDs were significantly affected by the interaction between Baetis sizeand starvation level, and the main level effects of starvation and Baetis size. FIDs of small larvae were unaffected by starvation level and the length of the predator-free feeding period, whereas starvation reduced FIDs of large larvae 2-fold. 4. Subsequent experiments showed that size-dependent differences in FIDs could not be attributed tosize-specific differences in the economics of rates of food intake or gut evacuation. For both small and large larvae, food intake rates declined with increasing time spent foraging and both small and largelarvae consumed the majority (70-- 75 percent) of food within the first 3 h of the 12-h feeding period. Similarly, gut evacuation declined with increasing starvation time and rates of evacuation did not differ between small and large larvae. These data suggest that size-dependent differences in flight initiation by Baetis larvae do not involve the energetics of food intake or processing. [ABSTRACT FROM AUTHOR]

1. Laboratory experiments were completed to identify the mechanisms by which the predatory flatworm, Dugesia tigrina, imposes mortality on its Aedes aegypti and Daphnia magna prey. Feeding trials were completed in glass microcosms which contained one of three - nine densities of small or large individuals of each prey species. 2. Mortality by Dugesia on small and large Aedes followed a type II functional response, whereas the mortality of Daphnia resembled a type III functional response. Prey mortality imposed by Dugesia consisted of consumptive and non-consumptive elements. Non-consumptive mortality occurred when prey individuals trapped in mucus trails subsequently died but were not ingested. 3. Additional experiments were conducted to quantify consumptive (capture followed by ingestion) and non-consumptive mortality (death not followed by ingestion). 4. Consumptive mortality followed a type II functional response for small and large individuals of both prey species, whereas non-consumptive mortality increased with prey density, although the relationships differed with prey species and size. The nonconsumptive mortality of large Daphnia increased at an accelerating rate with prey density and exceeded consumptive mortality at intermediate prey abundances. In contrast, non-consumptive mortality of small Aedes and small Daphnia was lower than consumptive mortality and either increased with prey density at a decelerating (small Aedes) or accelerating (small Daphnia) rate. 5. These results suggest that the importance of consumptive and non-consumptive mortality to total prey mortality needs to be considered when modelling predator-prey dynamics. [ABSTRACT FROM AUTHOR]