Movement of mercury from streams to terrestrial consumers by aquatic insects across a gradient of land cover types and dissolved organic carbon

Background/Question/Methods

Streams contribute energy, nutrients and pollutants to terrestrial systems, with important implications for terrestrial food webs and ecosystems. Aquatic insects bioaccumulate mercury as stream-dwelling larvae and transport it to land as winged adults. The importance of aquatic insects as a food source for terrestrial consumers makes them potentially important vectors of mercury to terrestrial consumers. Land cover type is known to affect streamwater dissolved organic carbon (DOC), and DOC is mechanistically linked to increases in mercury concentrations in fresh water. Recent studies have found elevated mercury in terrestrial insectivorous predators, such as spiders, that may be consuming large amounts of adult aquatic insects. The aim of this study was to evaluate mercury concentrations in terrestrial consumers that feed on aquatic insects and to explore the  relationships between land cover type, streamwater DOC, streamwater mercury, and mercury concentrations as drivers. To examine the effects of DOC on mercury in terrestrial consumers, we sampled 11 streams that varied in land cover type and DOC concentration, and measured mercury concentrations in streamwater and adult Tetragnatha elongata (Araneae: Tetragnathidae), an orb-weaving spider that captures prey along streams. 

Results/Conclusions

Approximately 95% of the prey collected from webs of T. elongata were aquatic insects. Methylmercury (MeHg), a potent neurotoxin, accounted for around 71% of the total mercury in T. elongata. The percent of wetland in a catchment was associated with higher DOC concentrations, and streamwater MeHg increased with DOC concentration. Paradoxically, there was no significant correlation between either streamwater DOC or MeHg concentration and the concentration of MeHg in T. elongata, indicating that higher concentrations of streamwater MeHg do not necessarily result in higher spider MeHg. However, we found a significant negative correlation between streamwater DOC and the ratio of MeHg in T. elongata to methylmercury in stream water, indicating that elevated stream DOC is associated with lower bioavailability of methylmercury. Our study highlights the dual role of DOC in mercury cycling in streams -- both as a transporter and as a limiting factor in uptake by biota -- and shows that this dual role may extend to higher trophic levels and across ecosystems (i.e. streams to forests) via emerging aquatic insects.