Tuesday, May 27, 2008
327

Variation of periphyton carbon stable isotopic ratio (δ13C) with autotrophic index and current velocity in two atlantic forest streams, Southeast Brazil

Fausto Machado Silva and Timothy Peter Moulton. Departamento de Ecologia, Universidade Estadual do Rio de Janeiro, Depto. de Ecologia, UERJ, Rua São Francisco Xavier 524, Maracanã, Rio de Janeiro, CEP 20550-013, Brazil, Rio de Janeiro, Brazil

Our previous work showed that algae were the prevalent source of carbon in the foodweb of a small-order stream. This evidence comes from carbon and nitrogen stable isotope analyses and jives with other tropical research. A difficulty occurs with the separation of the algal component of the periphyton matrix. In addition, the algal isotope ratios vary with environmental conditions. Here we tested a mathematical separation of the component signatures of algae and other material – we regressed delta13C against AFDM/chlorophyll ratio (“autotrophic index”). We used site as a fixed factor and current and canopy cover as covariates to remove their effects and to investigate their influence on algal delta13C. We found significant mathematical separation for sites Cap and MDA (p=0.005 and 0.016), with the constant indicating the algal delta13C as -23.9 and -21.5. Site PBP gave a poor regression and delta13C of -18.6. The general model had a significant r2 (0.82) but low explanatory power of the variables; canopy (p= 0,095) and the interaction site and canopy (p=0,066). This result showed no effect of current on delta13C (p= 0,3). Other evidence from herbivore signatures implied effects of current and we are loathe to reject this effect without further investigation.


Web Page: Stable isotopes, carbon, periphyton

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