Estimating dissolved organic carbon concentrations with CDOM measurements on the AquaFluor Handheld
The Cary Institute of Ecosystem Studies is a non-profit research institute located in Millbrook, NY housing over 100 scientists and support staff. One of their work units, the Hubbard Brook Ecosystem Study, is assigned to the USDA Forest Service field laboratory at the Hubbard Brook Experimental Forest (HBEF) in N. Woodstock, New Hampshire. Don Buso, Manager of Field Research working in close conjunction with US Forest Service personnel, is responsible for the longest running small watershed ecosystem monitoring study in North America (50 years!). The HBEF has been set aside for research on all aspects of ecological study including ground-breaking work on forestry, water quality, and migratory birds. This is the site where Acid Rain was first identified as an environmental problem and the data from this relatively remote site has been used extensively to formulate national legislative direction regarding environmental pollution, such as the Clean Air Act Amendments of 1990.
Given the increasing loading and retention of carbon in the earth's atmosphere, and resulting changes in global climate, it is critical to understand the sources and sinks of carbon in terrestrial ecosystems. In 2001 we conducted a chemical survey of the entire 3000 ha HBEF valley to map dissolved organic carbon (DOC) concentrations in streamwater. This unique study of sample sites at 100m intervals revealed that DOC could vary between sample sites on the same stream as much as between separate streams (Likens and Buso: Biogeochemistry 2006). However, the huge amount of work needed to collect and process DOC samples at even finer scales precluded an even more intense survey. The AquaFluor looked like the perfect instrument to provide field data for estimates of DOC concentrations at any necessary scale, interval, location in the forest.
We set our AquaFluor (with a CDOM channel) to read 1000 Relative Fluorescense Units (RFUs) with the secondary standard and took measurements in 2 HBEF watersheds, W3 and W9 (see inset boxes on map above), but at intervals of 50m or less (using GPS). We used a small syringe to obtain the sample from the stream and rinsed the sample cuvette once before filling and reading the AquaFluor. We checked the secondary standard every 10 measurements (it varied by about 2% over 8 hours). To calibrate theAquaFluor, we took about 20 samples from each stream for later DOC analysis (filtered and run on a Shimadzu 5000A TOC detector). The AquaFluor behaved in a highly predictable manner with a highly correlated curvelinear function (r-sq 0.85 or better) between the in-situ RFU measurements and the laboratory DOCs (see chart on right). There was some 'quenching' of RFU sensitivity in highly colored waters, but that was expected from the information in the manual.
Using the AquaFluor, we were able to survey streamwater DOC in each watershed at less than 50m intervals with 2 people in just two 8-hour days, an impossible feat with the standard sample-bottle approach. And, our laboratory was able to turn-around the small number of DOC calibration samples (~40) in just a few days, rather than weeks. Further, in the field we were able to track down a number of sub-watersheds to various DOC sources, such as small bogs and poorly drained conifer stands. Lastly, a second survey of W9, following torrential rains from Hurrican Irene demonstrated clearly that the RFU to DOC relationship had changed, strongly suggesting the quality of DOC depends on the degree of hydration of humic soils in the catchment.
Portability, simplicity and reliability. We made it a point to use this instrument in the simplest possible way to measure CDOM and it did a great job predicting DOC. On-the-fly field chemistry is simply the best way to understand aquatic ecosystems when in new and unfamiliar terrain and this is one instrument we do not go out in the woods without. Anyone can use it: we farmed the AquaFluor out to undergraduates working on summer projects, to graduate students working on Masters degrees, and even had our Water Quality technician's 8-yr old daughter use it to measure CDOM and demonstrate the results. The most difficult thing about using the AquaFluor is not being able to say 'yes' to all the researchers who wanted to try it out.
Author: Don Buso
Institution: Cary Institute of Ecosystem Studies, Millbrook, NY, USA