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1. Despite the ubiquity and abundance of water striders (Hemiptera: Gerridae) in temperate streams and rivers and their potential usefulness as sentinels in contaminant studies, little is known about their feeding ecology and lipid dynamics. 2. In this study we used stable isotopes of carbon (d 13 C) and nitrogen (d 15 N) and elemental carbon to nitrogen ratios (C ⁄ N) to assess dietary habits and lipid content, respectively, for water striders. 3. To determine diet-tissue fractionation factors, nymphs of the most common species in New Brunswick, Canada, Aquarius remigis were reared in the laboratory for 73 days and exhibited rapid isotopic turnover in response to a switch in diet (C half-life = 1.5 days, N half-life = 7.8 days). Their lipid content increased towards the end of the growing season and resulted in lower d 13 C values. Diet-tissue fractionation factors were established after correction of d 13 C data for the confounding effect of de novo lipid synthesis (strider d 13 C adjdiet d 13 C adj = 0.1&, strider d 15 N -diet d 15 N = 2.7&). 4. Water striders from the majority of 45 stream sites (83%) in New Brunswick had less than 50% contribution of aquatic carbon to their diets but showed a gradual increase in the contribution of this carbon source to their diet with increasing stream size. 5. These data indicate that striders exhibit a strong connection to terrestrial carbon sources, making them important users of energy subsidies to streams from the surrounding catchment. However, this dependence on terrestrial organic matter may limit their utility as indicators of contamination of aquatic systems by heavy metals and other pollutants.

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