Riparian zone spiders as mercury sentinels
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Riparian zone spiders as mercury sentinels Dr. Christopher Pennuto 1,2 , Marley Smith 1 , and Dr. Alexander Nazarenko 3 1 Biology Department, 2 Center for Great Lakes Research, 3 Chemistry Department, Buffalo State College.

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Riparian zone spiders as mercury sentinelsDr. Christopher Pennuto1,2, Marley Smith1, and Dr. Alexander Nazarenko31Biology Department, 2Center for Great Lakes Research, 3Chemistry Department, Buffalo State College

Bridge orb-weaver spider (Larinioides sclopetarius) Photo: C. Pennuto

Abstract

Riparian communities can receive significant energy and material subsidies from adjacent aquatic ‘donor’ habitats. We investigated mercury flux from aquatic to terrestrial habitats via the emergence of aquatic insects and uptake by riparian spiders (Larinioides sclopetarius, Clerck 1757). Spiders and midges were collected from a site within the Buffalo River Area of Concern (AOC), a site with known sediment mercury contamination and an upstream site, outside of the AOC. Spiders at both sites contained significantly more mercury than their midge prey and, surprisingly, spiders at the upstream site held roughly 2x more Hg than those within the AOC. There was a significant negative size:total mercury relationship in spiders (P = 0.003). The largest spiders had lower mercury concentrations than the smallest spiders, indicating that mercury depuration is rapid in these riparian predators. The highest Hg concentration observed was just over 0.4 ppm (ww). Bioconcentration factors (BCF) differed between the AOC site and the upstream site, with the upstream site exhibiting a larger BCF (2.4 vs 1.5). These data indicate that spiders can be good mercury sentinels in urban environments and that emergent aquatic insects can be potential Hg contaminant vectors to riparian communities. Further research is needed to understand why the spiders upstream of the AOC showed higher Hg concentrations than spiders within the AOC

  • Results

  • There was a significant difference in spider density between the two sites (Table 1).

  • Mercury concentration declined with spider sample size at both sampling sites (Fig. 1), but there was no significant size/Hg relationship for spiders analyzed singly (upstream of AOC: r = -0.561, P = 0.439; within AOC: r = 0.593, P = 0.292; Fig. 2).

  • There was no significant difference in the size of single spiders analyzed for mercury (t = 0.11, df = 7, P = 0.916), but there was a significant difference in [Hg] between the sites (t = 4.91, df = 7, P = 0.002; Fig. 3, Table 2). Similarly, there was no significant difference in composite sample sizes (t = 1.18, df = 23, P = 0.252), but there was a significant difference in [Hg] between sites (t = 6.31, df = 23, P < 0.001).

  • Mercury levels in midges did not differ between sites (Fig. 3).

  • The upstream BCF was 2.19 and the BCF within the AOC was 1.49 (Table 1).

Table 2. Results of 2-way ANOVA examining site and species differences in total mercury (ppm ww) for spiders (L. sclopetarius) and midges (Chironomidae) near the Buffalo River AOC in summer 2004.

  • Introduction

  • Few studies have tracked the transfer of mercury from aquatic sediments to terrestrial food webs.

  • Mercury poses significant human and environmental health risks due is its neurotoxicological effects. The lowest effects level (LEL) benchmark for sediments in NY is 0.15 mg/kg whereas the severe effects level (SEL) benchmark is 1.3 mg/kg.

  • Diggins and Stewart (1998) reported sediment mercury levels at 0.882 mg/kg within the Buffalo River AOC.

  • This raises concerns that a portion of the mercury contained in these sediments may be transferred back to terrestrial food webs.

  • This study examined the transfer of mercury from emerging aquatic insects (Family Chironomidae) to riparian zone spiders (Larinioides sclopetarius) at a site within the Buffalo River AOC and at an upstream site outside the AOC. We quantified and compared mean mercury concentrations for spiders and midges between sites and calculated bioconcentration factors (BCFs) for each site.

Discussion

Surprisingly, upstream spiders had higher mercury levels compared to AOC spiders, and both sites were above expected background concentrations. These data suggest that other polluting activities are taking place upstream from the AOC. This upstream pollution may be flowing down the Buffalo River, further compounding the problem within the AOC. Due to the bioaccumulative nature of Hg in food webs, we can expect mercury levels in terrestrial predators to exceed safe exposure limits. Future studies should aim to investigate possible point sources for mercury pollution upstream and exchange patterns between areas within and outside the AOC. Also, since energy subsidies in riparian systems include significant cross-ecosystem transfer (Baxter et al. 2004), significantly different wildlife risks may occur between sites. This is further supported by differences in BCF between sites.

There was no significant relationship between mercury concentration and spider body size, but upstream spiders had significantly higher [Hg] than downstream spiders. Since there also was no significant difference in adult spider size, these stationary (i.e. high site fidelity) predators represent effective site monitors for mercury contamination.

References

Baxter, C.V., K.D. Fausch, M. Murakami, and P.L. Chapman. 2004. Ecology 85:2656-2663.

Diggins, T.P. and K.M. Stewart. 1998. J. N. Am. Benthol. Soc. 17:311-323.

Acknowledgements

This work was supported by a grant from the Great Lakes Protection Fund to CMP and distributed through the NY Great Lakes Research Consortium. Microbac Laboratories ran multiple samples and laboratory split analyses for which we are grateful.

Table 1: Spider (L. sclopetarius) sampling data from within and upstream from the AOC from summer 2004. Mean values are shown with standard error ranges.

  • Methods

  • Riparian spiders (L. sclopetarius) were collected using aspirators from a site within and a site upstream from the Buffalo River AOC. Adult midges were collected using sweep nets and aspirators near lanterns hung at the 2 sites.

  • Samples were freeze-dried, weighed and grouped into composite samples by mass. Samples were digested in 3.5 mL of a 3:4 HNO3:H2SO4 mixture at 85ºC for 45 minutes. The digestate was analyzed using a Perkin-Elmer MHS-15 Mercury Hydride System.

  • Spider mass/Hg concentration relationship was examined using correlation. Mercury trophic level relationship was examined using a 2-way ANOVA.


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