Impacts of seasonality on mercury concentrations, polyunsaturated fatty acids, and stable isotopes: implications for the use of tetragnathid spiders as sentinels.

Abstract

Riparian spiders are used in ecotoxicology as sentinels of bioavailable contaminants that are transferred from aquatic to terrestrial habitats via emergent aquatic insects. Spiders in the family Tetragnathidae are particularly of interest because a high proportion of their diet consists of emergent aquatic insects and their contaminant loads reflect the amount transferred through the food web to riparian predators. The transfer of contaminants can be determined through food web tracers such as stable isotopes and polyunsaturated fatty acids; however, it is unclear how contaminants and tracers vary over the course of a year. The objective of this study was to determine whether seasonality affected size, carbon and nitrogen stable isotopes, polyunsaturated fatty acid biomarkers, mercury, and other trace metal concentrations in tetragnathid spiders. Spiders were sampled fortnightly from a single site on the Stones River in Tennessee, USA, for an entire active season (April through October). Spider mass and length steadily increased from April to September to a maximum average value of 0.078 ± 0.03 g, then decreased in October. Seasonal trends were observed for carbon and nitrogen stable isotopes, with significantly decreased signatures occurring late in the active season. Overall, methyl mercury concentrations (range: 12.1-134.4 ng/g) and the methyl-total mercury ratio (range: 49%-98%) increased throughout the active season, with higher variability observed at the end of the active season. Collectively, our results indicate that seasonality affected several important endpoints and that spiders collected during the end of the active season may not be representative of spiders during the entire active season.