Using stable isotopes to determine how the geomorphology of watersheds affects the carbon and nitrogen signatures of stream fish
Beth Vanden Heuvel
University of Washington (School of Environmental and Forest Resources)
Abstract
Methods
Stable isotopes of carbon and nitrogen are commonly used to trace the flow of
nutrients through food webs. These isotopes, in combination with fatty acid analysis,
have shown that heterotrophic carbon pathways play a significant role in benthic lake
habitats, but the role they play in streams remains uncertain. Flat watersheds are often
dominated by wetlands that accumulate large amounts of organic matter that become a
potential source of energy for stream consumers. We hypothesize that streams draining
flat watersheds should be dominated by heterotrophic carbon flows compared to
streams draining steep watersheds where algal production is more important. We
compared the δ13C and δ15N of sculpins collected from streams draining flat and steep
watersheds to test this hypothesis. We found little correlation between the slope of the
watershed and the δ13C, though there was a tendency for δ15N to become more enriched
in flatter watersheds. Instead, it appears that extremely high variation in the isotope
signature among sculpins reflected a high degree of individual foraging specialization
that appears to mask the effects of watershed geomorphology on energy flows to the
sculpin populations.
• Field work in the Wood River Basin of Alaska: Selected contrasting
watersheds and froze sculpin samples from multiple streams of each type.
• Lab work: Measured the fork length of each sculpin, dried them in a freeze
dryer and ground to a homogenized powder for stable isotope analysis.
• Isotope analysis: Processed the samples at the Isolab to determine the δ13C
and δ15N. Samples were combusted in a Thermally-Coupled Elemental
Analyzer (TC-EA) and sent through a Thermo-Finnigan MAT Mass
Spectrometer.
Results
Introduction
The source of carbon supporting food webs can flow from autotrophic or
heterotrophic pathways. Heterotrophic pathways, typically of terrestrially-derived
matter, involve significant fractionation of carbon, enabling us to track the carbon
pathway with isotope analysis (ex. methane-derived carbon (MDC) generally
measuring around -60‰ to -80‰, compared to -27.8‰ for C3 photosynthetic
pathways and -13‰ for C4 pathways) (Fry and Peterson 1987, Jones and Grey
2011). In flatter watersheds that have high levels of organic matter that fuels
bacterial metabolism of carbon, the scene is set for carbon to enter the food web
through heterotrophic pathways, therefore showing a more depleted δ13C
signature than from photosynthetic pathways. Thus, we used stable isotopes of C
and N to estimate the degree to which watershed features affect the dominant
flows of organic matter to stream consumers.
Figure 3. CN Biplot of the isotope signatures of sculpins with each stream noted with a different color. Potential food sources are overlaid on the sculpin data.
Figure 1. Correlation between sculpin fork length and the d15N and d13C of their tissue.
Since larger sculpin would be expected to have a more varied diet and possibly a residual
isotope signature from previous salmon spawning events, the first step of analysis was to
determine if the relationship between sculpin size and their fork length was significant. No
significant relationship was detected for the d15N, and little change was seen in the trends after
correcting for the effect on d13C.
Objective: To quantify how the nitrogen and carbon
isotope signatures of Alaskan sculpins are controlled by
watershed geomorphic features.
Lisi et al. 2013
Top: A map of the Wood River watershed in SW Alaska, with photos of sculpin. Bottom: Three stream watersheds displaying the range of geomorphic features in the basin.
The C/N biplot of all of the pre-spawning data shows a large range of variation in the isotope
values of both carbon and nitrogen. The variation is not only between streams, but also within
streams as seen by the color-coded points for each stream (Figure 3). The data fall within the
range of a number of food sources that the sculpins would be expected to prey upon.
Discussion
The data did not support our hypothesis that streams draining flatter
watersheds would yield the depleted C signatures that are indicative of
heterotrophic carbon flows, compared to the signatures of autotrophically based
watersheds. The high variation in sculpin signatures, not only between the
watersheds, but also within populations of the same stream, suggest that the fish
are exhibiting a high level of individual specialization. Individuals are likely
claiming their own territory within a stream and only feeding within that
microhabitat. Therefore, their signatures only reflect a small subset of the stream
conditions, rather than the watershed as a whole. This specialization within the
streams appears to be swamping out any trend that would be seen between the
watersheds.
References
Jones and Grey. (2011). Biogenic methane in freshwater food webs. Freshwater Biology, 56:
213-229.
Lisi et al. (2013). Association between geomorphic attributes of watersheds, water
temperature and salmon spawn timing in Alaskan streams. Geomorphology, 185: 78-86.
Fry, B. & Peterson, B. (1987). Stable isotopes in ecosystem studies. Annual Review of
Ecology, Evolution, and Systematics Online, 18: 293-320.
Acknowledgements
Figure 2. Relationships between watershed slope and temperature, and the isotope signature of sculpins
The relationships between the geomorphology and the isotope values supported our
hypothesis, but demonstrated very weak trends. The carbon was more depleted in flatter
watersheds where methanogenic carbon with depleted δ13C would be present. The nitrogen was
more enriched in flat, warm watersheds, where denitrification results in higher δ15N.
RESEARCH POSTER PRESENTATION DESIGN © 2012
www.PosterPresentations.com
This work was done through the UW Alaska Salmon Program with financial support of the
Harriet Bullitt Professorship. I’m thankful for the guidance of Daniel Schindler and Adrianne
Smits throughout this project. I am also grateful for the fieldwork and laboratory help of Kyle
Samek, Adrianne Smits, Sydney Clark, and McKenzie Conseur.