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

Abstract

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 (²²Na⁺, ³⁵SO₄^-2, and ⁴⁵Ca²⁺) 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 °C to 25 °C, increased ²²Na⁺ uptake rates by two-fold (p < 0.0001) and ³⁵SO₄^-2 uptake rates by four-fold (p < 0.0001) in the caddisfly, Hydropsyche sparna. Smaller changes in ²²Na⁺ and ³⁵SO₄^-2 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 °C, but only 44 % at 25 °C (p < 0.0036). Conceivably, the energetic cost of osmoregulation in warmer, saltier environments may cause significant major ion toxicity in certain freshwater insects.

Keywords: Ion transport; Osmoregulation; Salinity; Temperature; Toxicity.

Fig. 1.

Example of the approach taken to assess ion uptake rates in aquatic insect larvae. In this case, each point represents the acquisition of ²²Na in an individual N. triangulifer larva at 20 °C. Linear regression analysis is based on the mean values for each time point. These particular data are presented in Fig. 2B and indicated by an arrow.

Fig. 2.

Uptake rates (mean ± SE) of sulfate, sodium, and calcium across two or three temperatures in N. triangulifer, I. sayi. Maccaffertium sp., and H. sparna, depending on number of animals available. Each point represents the slope of a time-course of 8 individuals per time point (n = 8). The arrow in plot B refers to the data shown in Fig. 1.

Fig. 3.

(a) N. triangulifer standard metabolic rates (SMR) across three temperatures. Each point represents the mean of 8 individuals. (b) Sulfate uptake rate plotted against SMR (R² = 0.997). (c) Sodium uptake rate plotted against SMR (R² = 0.998).

Fig. 4.

(a) The percent survival of N. triangulifer across 6 concentrations of sulfate and three temperatures (15, 20, and 25 °C). Each point represents the mean of 5 wells with 10 mayflies/well. (b) Development time of N. triangulifer across 3 concentrations of sulfate and three temperatures (20, 23, and 26 °C). Each bar represents the mean of 5 jars with 15 mayflies/jar. Brackets indicate the percent increase from control to high sulfate-exposed mayflies.