It's all about the fluxes: Temperature influences ion transport and toxicity in aquatic insects

Many freshwater ecosystems are becoming saltier and/or warmer, but our understanding of how these factors interact and affect the physiology and life history outcomes of most aquatic species remain unknown. We hypothesize that temperature modulates ion transport rates. Since ion transport is energetically expensive, increases in salinity and/or temperature may influence ion flux rates and ultimately, organismal performance. Radiotracer (Na-22(+), (SO4-2)-S-35, and Ca-45(2+)) experiments with lab-reared mayflies (N. triangulifer) and other field-collected insects showed that increasing temperature generally increased ion transport rates. For example, increasing temperature from 15 degrees C to 25 degrees C, increased Na-22(+) uptake rates by two-fold (p < 0.0001) and (SO4-2)-S-35 uptake rates by four-fold (p < 0.0001) in the caddisfly, Hydropsyche sparna. Smaller changes in Na-22(+) and (SO4-2)-S-35 uptake rates were observed in the mayflies, Isonychia sayi and Maccaffertium sp., suggesting species-specific differences in the thermal sensitivity of ion transport. Finally, we demonstrated that the toxicity of SO4 was influenced by temperature profoundly in a 96-h bioassay. Under the saltiest conditions (1500 mg L-1 SO4), mayfly survival was 78 % at 15 degrees C, but only 44 % at 25 degrees C (p < 0.0036). Conceivably, the energetic cost of osmoregulation in warmer, saltier environments may cause significant major ion toxicity in certain freshwater insects.