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stream ecology

When a tree falls in a stream, there's always something around to make use of it.

Cross-posted at Highly Allochthonous (for obvious reasons)

ResearchBlogging.org Allochthonous may have some obscure usage related to rocks, but in ecology, allochthonous material is a major concept that underpins thinking about nutrient cycling and food web dynamics. In its most general definition, allochthonous material is something imported into an ecosystem from outside of it. Usually, ecologists are thinking about organic matter and the nutrients (C, N, and P) that come with it.

Allochthonous material in the form of coarse particulate organic matter in a mountain stream in Oregon.

Allochthonous material in the form of coarse particulate organic matter in a mountain stream in Oregon.

In streams, allochthonous material includes leaves that fall or are washed into the water and branches and trees that topple into the stream. These would both be called “coarse particulate organic matter” or “CPOM” in the lingo of stream ecologists. In headwater streams, especially in forested areas, there is a lot of CPOM, and the community of aquatic organisms has a high proportion of “shredders” – the critters that that feed on CPOM and break it up into tinier bits called “fine particulate organic matter” or FPOM. In turn, organisms called “collectors” make use of the FPOM by filtering it from the water or accessing it in the sediments. [Allochthonous material can also include dissolved organic matter (DOM) carried into the stream by overland or subsurface flow.]

Schematic illustration of the River Continuum Concept, as modified from Vannote et al. (1980)

Schematic illustration of the River Continuum Concept, as modified from Vannote et al. (1980)

As you move downstream from the headwaters toward medium-sized rivers, the stream channel becomes wider and allochthonous input from overhanging forest and riparian vegetation decreases in abundance and importance relative to primary production (or autochthonous organic mattter) driven by available sunlight. In other words, algae and aquatic plants become the most important food producers. Organisms called “grazers” who scrape algae from surfaces become an important component of the aquatic food web, and grazers become less abundant.

Farther downstream, the ecosystem shifts again, as there is so much FPOM moving with the water and sediment, that collecters far outnumber either shredders or grazers. There’s still allochthonous input from the banks and being carried in by tributaries, and there’s still primary production occurring in the stream, but upstream “system inefficiency” or “leakage” in the processing of nutrients and organic material lets large river aquatic communities be based on material washing in from upstream.

The adjustment of river ecosystems in a downstream fashion that I’ve described above is part of the “river continuum concept”, described by Vannote and colleagues in 1980 in the Canadian Journal of Fisheries and Aquatic Science, and it is one of the unifying principles of modern stream ecology. At its root, the river continuum concept is driven by the relative proportion of allochthonous to autochthonous organic matter inputs to the stream.

While I’m not an ecologist, I was raised by one and I work with them, so when I hear the word allochthonous, I pictures leaves and logs in streams, rather than anything to do with rocks. So, I’ll end this post with some nice pictures of allochthonous material.

An overwhelming amount of allochthonous material in a headwater stream, Gaston County, North Carolina

An overwhelming amount of allochthonous material in a headwater stream, Gaston County, North Carolina. One of my MS students showed that debris jams like this were the biggest driver of groundwater-stream interactions, variations in sediment size, and changes in water chemistry in these tiny streams.

Allochthonous organic material in Clark Creek, Charlotte. High water has washed branches and leaves into the creek, where they got hung up on the riffle (or riprap).

Allochthonous organic material in Clark Creek, Charlotte. High water has washed branches and leaves into the creek, where they got hung up on the riffle (or riprap). What role do natural and artificial geomorphic structures (with their FPOM trapping abilities) play in promoting ecosystem health in urban streams? My colleagues and I are trying to find out.

Large wood jam on Mallard Creek, near Harrisburg, NC

Large wood jam on Mallard Creek, near Harrisburg, NC. For several years, I've taken my Fluvial Processes class to this spot, in part so that they can observe the geomorphic effects of wood in streams.

Wood in streams is utilitarian. During my PhD, I used stable large logs to cross streams and attach equipment.

I use large logs to cross streams and attach equipment. Here, in a spring-fed stream in Oregon, with extremely stable water levels and no floods, allochthonous material that falls into the stream stays where it falls and forms a substrate for a fabulous community of mosses and ferns.

Not a stream. Allochthonous input onto the surface of a lava flow, from the edge of a forest.

Not a stream. Here we are looking at allochthonous input onto the edge of a lava flow, from the forest beyond. On this young lava flow (in the Oregon Cascades), I found substantially greater soil depth near the edge of the flow, where organic acids from decaying allochthonous organic matter had probably sped up the weathering process, as well as contributing directly to the soil. In my PhD dissertation, one subsection had "allochthonous inputs" for a title.

Vannote, R., Minshall, G., Cummins, K., Sedell, J., & Cushing, C. (1980). The River Continuum Concept Canadian Journal of Fisheries and Aquatic Sciences, 37 (1), 130-137 DOI: 10.1139/f80-017

Graduate Assistantships: Biogeochemistry, Stream Ecology, and Hydrology at UNC Charlotte, NC

Come work with me!

Research assistantships are available at the MS or Ph.D. level at the University of North Carolina at Charlotte to participate in a recently funded NSF project investigating the effects of stormwater management on ecosystem function in urban watersheds.  The overall goal is to better understand and predict the impacts of stormwater BMPs on receiving streams over a range of spatial and temporal scales through a combination of field based research and watershed scale ecological modeling.  This interdisciplinary project will link (1) mass-balance based monitoring of individual BMPs, (2) ecosystem processes (nutrient uptake, metabolism, temperature and biological indices) in the receiving stream and (3) monitored and modeled watershed outputs of flow, nitrogen, and carbon.

Applicants interested in aquatic biogeochemistry, hydrology, stream ecology and/or watershed modeling are encouraged to apply.  Students will have flexibility to develop independent research questions within the context of this project that broadly address the interactions among hydrology, biogeochemistry and ecology in aquatic ecosystems.

Qualifications:  degree in biology, ecology, environmental engineering, hydrology or related field is required.  Successful applicants should have a strong interest in working in an interdisciplinary research environment, be creative, motivated and capable of working well both independently and cooperatively and possess strong communication and quantitative skills. Competitive stipends and tuition waivers are available for highly motivated students.  For more information on admission requirements and deadlines, visit http://graduateschool.uncc.edu.  Additional information about the McMillan Lab can be found at http://www.coe.uncc.edu/~smcmil10.  Opportunities exist for collaboration with the labs of Sandra Clinton and Anne Jefferson at UNC Charlotte who are collaborators on the project.

Interested students with strong motivation to succeed in research should contact Sara McMillan via email (smcmillan@uncc.edu).  Please submit a statement of career goals and research interests, full CV, unofficial transcripts and GRE scores, and contact information for three potential references.  Review of applications will begin immediately and continue until suitable candidates are found. The anticipated start date is flexible, but should be sometime between January and August 2011.