Study system and organisms:

Organisms were collected from the Hengill geothermal valley in Iceland, which has been extensively studied over the past decade (Friberg et al. 2009; Woodward et al. 2010a; O'Gorman et al. 2017; 2019). The system includes the river Hengladalsá and several of its tributaries, which are groundwater fed (Friberg et al. 2009). Due to geothermal activity in the Hengill region, the streams experience a temperature gradient from around 4–25 °C, driven by indirect heating of bedrock, rather than direct upwelling of chemically altered water (O'Gorman et al. 2017). As a result, the streams are very similar in all other physiochemical characteristics (Friberg et al. 2009; O'Gorman et al. 2017), facilitating the study of temperature effects on organisms and communities without other confounding factors.

Previous research in the system has shown that the dipteran larva Limnophora riparia is the most abundant invertebrate predator, while blackfly larvae from the Simuliidae family are the most abundant prey (O'Gorman et al. 2017). Both predator and prey are distributed across the entire temperature gradient in the system, although they are least common in the coldest streams and their population abundances increase log-linearly with temperature up to 25 °C (Archer et al. 2019). Simuliidae prey (7.09 ± 1.39 mm; mean ± SD body length) were hand-collected from the river Hengladalsá (7.7 ± 1.9 °C) as a single source common to all experiments, while L. riparia predators (10.43 ± 1.39 mm; mean ± SD body length) were hand-collected from three streams of different temperature between May and July 2015. We collected only third instar larvae of a similar size from all three streams to standardise the predator size as much as possible in the experiments (Fig. S1). The streams were categorised as cold (IS11; 4.5 ± 1.5 °C), tepid (IS5; 13.8 ± 0.9 °C), and warm (IS8; 18.0 ± 1.0 °C) using Maxim Integrated DS1921G Thermochron iButtons, which logged temperature every four hours from 1st May to 3rd July.

Quantifying metabolic rates:

To quantify the effect of thermal acclimation on the temperature dependence of the predator’s metabolic rate, we measured the oxygen consumption rate of individual L. riparia from the cold, tepid, and warm streams at 5, 10, 15, 20, and 25 °C. Before each experiment, individuals were confined in glass chambers immersed in a water bath to allow them to adjust to the experimental temperature for 15 minutes. The glass chambers were completely filled (i.e. no headspace) with water from the river Hengladalsá, which was filtered through a 0.45 µm Whatman membrane filter and bubbled to reach 100% oxygen saturation. Note that by using stream water from a common source, the starting concentration of oxygen in the experiments was always approximately the same. A magnetic stir bar was placed at the bottom of each chamber but separated from the organism by a mesh screen. In each trial, one individual L. riparia was placed in each of seven chambers and the eighth chamber was used as an animal-free control to account for sensor drift and any background microbial respiration (which was minimal throughout).

Oxygen consumption was measured with an oxygen microelectrode (MicroResp, Unisense, Denmark) fitted through a capillary in the gas-tight stopper of each chamber. Three measurement periods were recorded for each individual predator (10-15 seconds each, where oxygen concentration was measured every second). Oxygen concentrations were not allowed to drop below 70% of the starting value to avoid stressing the predators, measuring anaerobic metabolism, or quantifying nonlinear oxygen depletion. Metabolic rates [µmol O2 h-1] were calculated as the best fitting line through all the data points measured in each chamber, corrected for background rates in the animal-free control chamber, then converted to energetic equivalents [J h-1] using atomic weight (1 mol O2 = 31.9988 g), density (1.429 g l-1), and a standard conversion (1 ml O2 = 20.1 J; Peters 1983). Metabolic rate was measured for 5-10 individuals of the cold, tepid, and warm populations of L. riparia at each experimental temperature, with a new individual used in every trial. The body length of L. riparia was measured after each trial to estimate individual dry mass, ML [mg], from length–weight relationships established for the system (Archer et al. 2019).

Quantifying feeding rates:

Functional response experiments were conducted in a climate chamber (GRAM K400LE, type 3011-1F4B) at the University of Iceland. Plastic cylindrical containers (7.3 cm diameter, 11.5 cm height) filled with 100 ml water collected from the river Hengladalsá served as experimental arenas. Each experimental unit encompassed one predator individual and prey individuals varying in their initial densities (1, 2, 3, 4, 8, 16, 32, or 48 individuals). Note that these densities are representative of the natural environment, where Simuliidae can reach up to 8,500 individuals m-2 (or 36 individuals per unit area of our experimental arenas; Archer et al. 2019). Predators were starved for at least 24 hours prior to the beginning of each experiment. All prey individuals were placed in the arenas first to help them become accustomed to the new environment before predators were added. Experimental arenas were placed randomly in the climate chamber so there was no systematic pattern in the treatment combinations. Experiments were run at four temperatures (4, 6, 10, and 18 °C, which was the maximum temperature of the climate chamber) for precisely 24 hours, during which time aeration was not provided to avoid any physical disturbance during the experiment. At the end of each experiment, predators were removed, and the remaining living prey were counted. Predator and prey larvae occasionally pupated during the experiments, which prevents the predator from feeding, but leaves the prey vulnerable to predation. As such, experimental units where the predator had pupated were discounted, but pupated prey individuals were still counted as living prey. In order to assess natural mortality of the prey, one predator-free control was added for every prey density in each experimental block. The experiments consisted of all possible combinations of the three predator populations (i.e. cold, tepid, and warm acclimation), eight prey densities, four experimental temperatures, and at least three replicates, resulting in 297 experimental units and 58 predator-free controls.