Landscapes Forest and Global Change - ESA - Escola Superior ...

P.C. Goebel et al. 2010. How important are riparian forests to aquatic foodwebs in agricultural watersheds
132
Isotope ratios are expressed as δ 13 C and δ 15 N values per mille [‰] relative to the international
reference standard VPDB using NBS 19, L-SVEC, IAEA-N-1 and IAEA-N-2. Standards
deviations of δ 13 C and δ 15 N replicate analyses were 0.2 ‰ and 0.2 ‰, respectively.
DOC samples were analyzed for δ 13 C using an O.I. Analytical Model 1010 TOC Analyzer (OI
Analytical, College Station, TX) interfaced to a PDZ Europa 20-20 isotope ratio mass
spectrometer (Sercon Ltd., Cheshire, UK). This analysis was conducted at the UC Davis Stable
Isotope Facility, in the Department of Plant Sciences. A 20-mL aliquot of sample was
transferred into a heated digestion vessel and reacted sequentially with phosphoric acid then
with sodium persulfate to convert DIC and DOC each into a pulse of CO 2 . The two sequential
CO 2 pulses liberated by the chemical treatments are carried in a helium flow to an infra-red gas
analyzer (IRGA), then to the isotope ratio mass spectrometer where the 13 C/ 12 C ratios are
measured and compared to ratios of laboratory standards calibrated against NIST Standard
Reference Materials.
We compared δ 13 C and δ 15 N values of dominant basal resources (autotrophs and detrital
materials) between forested and non-forested streams using a t-test. Likewise, to examine
differences in stable isotope signatures for macroinvertebrate functional feeding groups and
dominant fish species between forested and non-forested riparian areas, we also utilized a t-test.
3. Results
We detected no statistically significant differences in δ 13 C values of dominant autotrophs and
detrital material between forested and non-forested streams (Figure 1A). Algae δ 13 C values were
the most depleted, ranging from -40.5‰ to -22.3‰ while δ 13 C values of the epilithon were the
most enriched (ranging from -23.3‰ to -15.1‰), followed by FPOM δ 13 C values (ranging from
-23.0‰ to -11.3‰). With values ranging collectively from -32.5‰ to -23.6‰, δ 13 C values of
CPOM, riparian vegetation (tree and herbaceous), and macrophytes were similar (Figure 1A.).
We did detect significant differences in δ 15 N values of basal resources (Figure 1A).
Specifically, we found that macrophytes and CPOM were enriched in the non-forested sites
compared to the forested sites (t-test; P = 0.01 and 0.01, respectively). No differences in δ 15 N
values of the other autotrophs or detrital material were detected between forests and nonforested
riparian areas.
Overall, consumer δ 13 C values across all species are best characterized as enriched, with δ 13 C
values ranging between -28.0‰ and -20.2‰ and δ 15 N values between 3.2‰ to 8.5‰ for all
macroinvertebrate functional feeding groups (Figure 1). Both filterers and grazers had higher
δ 15 N values associated with non-forested riparian areas than forested riparian areas (t-test; P =
0.01 and 0.02, respectively). Similarly, δ 13 C values ranged from -22.0‰ to -26.2‰ and δ 15 N
values from 10.5‰ to 12.7‰ for selected fish species, with creek chubs having significantly
more enriched δ 13 C values in the non-forested riparian areas (t-test; P = 0.01). These values are
similar to the values for FPOM, CPOM, and riparian plants (Figure 1).
4. Discussion
Our results, which were only collected once and do not capture seasonal variation in isotopic
signatures, suggest that there are not marked differences in the primary sources of energy
supporting aquatic food webs among forested and non-forested headwater streams in this
agricultural watershed. Neither the primary consumers (e.g., grazers) nor higher-level
consumers (e.g., black-nose dace) showed differences in δ 13 C signatures among forested and
Forest Landscapes and Global Change -New Frontiers in Management, Conservation and Restoration. Proceedings of the IUFRO Landscape Ecology
Working Group International Conference, September 21-27, 2010, Bragança, Portugal. J.C. Azevedo, M. Feliciano, J. Castro & M.A. Pinto (eds.)
2010, Instituto Politécnico de Bragança, Bragança, Portugal.