A common theme in this course has been the discussion of different contaminants and how they enter and persist in natural systems. Identifying the levels at which these contaminants are entering the food web and their method of transport are crucial to assessing their risk. As we discussed this topic in class I was reminded of a paper I read during my Freshwater Ecosystems course and am very glad I went back and re-read it. Published in Ecological Applications, the authors Walters et. al demonstrated how PCB's are transported from aquatic systems into terrestrial food webs through the capture and consumption of aquatic insects by Spiders and Herps. I thought this particularly appropriate for our classes interests.
One of the largest challenges in a study like this is to determine where the selected predators are obtaining most of their food from. To do this, the authors used a stable isotope analysis to identify the Carbon-13 and Nitrogen-15 ratios in both aquatic insect prey and terrestrial insect prey. Using this as a baseline, C-13 and N-15 ratios were then determined for a range of riparian predators including 4 groups of arachnids and 11 species of herptiles. Typically N increased in association with aquatic prey while increased C indicated more reliance on terrestrial prey. They found that riparian spiders such as Tetragnathidae and Dolomedes sp. (Pisauridae) had very similar C:N to aquatic insects while herptiles typically showed enrichment of C.
Next, the authors compared C and N values to PCB values from both their 4 sample sites and a reference site. PCB concentrations were significantly decreased with predator C enrichment indicating that higher reliance on terrestrial prey led to lower PCB concentrations. PCB values in Dolomedes and Tetragnathidae were an order of magnitude lower at reference sites compared with all the contaminated sites which demonstrates a high transfer of PCB's to predators from aquatic insects. The authors estimated that for this stretch of stream (25 km and heavily contaminated), aquatic insects transferred 6.1 g/yr of PCBs into the food web which is equivalent to the PCB mass returned by 50,000 returning Chinook salmon (Compton et al. 2006).
I found this paper absolutely fascinating because it highlights how poorly we understand the interconnectedness of aquatic and terrestrial systems. When we think about contaminated streams and rivers we typically think about vertical transport of contaminants (down-stream) but rarely do we consider the lateral transport into terrestrial systems. Aquatic insects provide a prey staple for a huge range of organisms including birds, bats, herps, spiders, fish, and more. Failure to consider the amount of pollutants they are absorbing and transporting into the trophic webs will lead us to gross under estimations of the short and long term effects of contaminants on our ecosystems. This paper also makes me think about how useful spiders would be as bioindicators of contamination, but I think I would like to expand on that in our class presentations.
Citations:
- Walters, D. M., K. M. Fritz, and R. R. Otter. 2008. The dark side of subsidies: Adult stream insects export organic contaminants to riparian predators. Ecological Applications 18:1835–1841.
- Compton, J. E., C. P. Andersen, D. L. Phillips, J. R. Brooks, M. G. Johnson, M. R. Church, W. E. Hogsett, M. A. Cairns, P. T. Rygiewicz, B. C. McComb, and C. D. Shaff. 2006. Ecological and water quality consequences of nutrient addition for salmon restoration in the Pacific Northwest. Frontiers in Ecology and the Environment 4:18–26.
I really enjoyed your comments on the paper. I actually took the same course last semester but completely forgot about that paper; thanks for the refresher! Something I've tried to imagine after reading your blog is a situation that involves recycling of pollutants, especially ones that are fat soluble. It seems like these pollutants just get passed vertically up the food web to the apex predator. Is there a situation where they can be passed "laterally" back and forth or all the way up the food chain and back down in the same environment? I thought about the possibility of some insect that decomposes dead predators (thus ingesting the pollutant), and then depositing those pollutants into a river through being preyed upon by some fish. The fish would then be consumed by another predator and the process starts again. I can't think of any specific examples, but that's something that ran through my head! Great read!
ReplyDeleteThat's a really cool thing to think about Trevor. One example of what your saying might be with crayfish. As decomposers and detritivores they would likely feed on the occasional terrestrial organism that washed into the stream. I have no idea as to the frequency of an occurrence like that though. A good deal of sediment, leaf litter, and other debris also washes into streams and acts as a food source for many insect detritivores and crayfish. That could be another method of transport back into aquatic systems. We know its all connected somehow, but we need to figure out the magnitude of the specific connections. Thanks for the response!
ReplyDeleteI have read a couple of papers like this and find the subject super fascinating as well. My study site for example, is really unique in that the main contamination (radiocesium) is in the terrestrial system versus the aquatic system. Last semester I worked on a literature review and found there was very little done on transfer back into aquatic systems from terrestrial systems, however this could potentially be very important at my study site. I would check out the book "Contaminants and ecological subsides" by Kraus, Walters and Mills, it has given me even more to think about.
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