Combining Organic Matter Source and Relative Trophic Position Determinations to Explore Trophic Structure |
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Authors: | Rachel Marie Wilson Jeffrey Chanton Graham Lewis Douglas Nowacek |
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Institution: | (1) Department of Oceanography, Florida State University, 117 N. Woodward Ave., Tallahassee, FL 32306-4320, USA;(2) Northwest Florida Water Management District, 81 Water Management Dr., Havana, FL 32333, USA;(3) Present address: Nicholas School of the Environment and Pratt School of Engineering, Duke University—Marine Laboratory, 135 Duke Marine Lab Rd., Beaufort, NC 28516, USA |
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Abstract: | Stable isotope ratios of carbon and sulfur were used to assess organic matter utilization of numerically abundant consumers
present in Apalachicola Bay, FL, USA. These results were used to infer nitrogen isotopic enrichment of organic matter sources
in an effort to establish baseline δ
15N enrichment for trophic evaluations. We compared results from concentration-independent and concentration-dependent mixing
models and found that the two methods resulted in widely different conclusions about the importance of organic matter sources
that varied ninefold in sulfur concentrations. Nitrogen isotopic enrichment was used to determine relative trophic positions
of consumer organisms. Source elemental concentrations of nitrogen were also considered in the calculation of relative trophic
levels in a concentration-dependent approach. Concentration-independent and concentration-dependent methods of calculating
trophic results were compared. While relative trophic levels of individual species varied continuously from approximately
1.7 to 3.5, comparisons of trophic level among consumers indicated four possible trophic groupings. Filter feeders (mussels
and oysters) made up the lowest trophic tier while teleost fishes made up the highest trophic tier. Invertebrates sampled
were assigned intermediate nondiscrete trophic levels. Because δ
15N values of important organic matter sources in the system were similar, the concentration-independent and concentration-dependent
methods did not result in significantly different conclusions about trophic level for any of the consumers examined. However,
a comparison of the two methods applied to a hypothetical case found that differences in base δ
15N values ranging approximately 4‰ resulted in significantly different trophic-level assignments when comparing the concentration-dependent
and concentration-independent methods of trophic-level calculations. Our results confirm that consideration of the elemental
concentrations of the base organisms is an important factor in determining source contributions and may affect trophic-level
calculations in systems with a sufficient range of base nitrogen enrichment. However, this result depended on the relative
isotopic signatures of the chosen sources and their elemental concentrations and should be considered individually for each
system. |
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