Changes in nutrient content and stable isotope ratios of C and N during decomposition of seagrasses and mangrove leaves along a nutrient availability gradient in Florida Bay, USA

Abstract

The decomposition of the mangrove Rhizophora mangle and the seagrass Thalassia testudinum was examined using litterbags along a natural gradient in nutrient availability. Seagrass leaves had a higher fraction of their biomass in the labile pool (57%), compared to mangrove leaves (36%) and seagrass rhizomes (29%); the overall decomposition rates of the starting material reflected the fractionation into labile and refractory components. There was no relationship between the N or P content of the starting material and the decomposition rate.

Nutrient availability had no influence on decomposition rate, and mass was lost at the same rate from litterbags that were buried in the sediment and litterbags that were left on the sediment surface. The dynamics of N and P content during decomposition varied as a function of starting material and burial state. N content of decomposing mangrove leaves increased, but seagrass rhizomes decreased in N content during decomposition while there was no change in seagrass leaf N content. These same general patterns held for P content, but buried seagrass leaves increased in P content while surficial leaves decreased. δ¹³C and δ¹⁵N changed by as much as 2‰ during decomposition.

Keywords:

• Detritus
• Decomposition
• Stable carbon isotopes
• Stable nitrogen isotopes
• Stoichiometry

This work was funded by the National Science Foundation under Grant No. 9910514 (FCE-LTER) as well as an NSF-REU supplement. The research was performed under Permit 2000078 issued by Everglades National Park. Bill Anderson and the Stable Isotope Laboratory at FIU analyzed our stable isotope samples. Susie Escorcia, Kevin Cunniff, Dottie Byron and Brad Peterson provided invaluable field and lab assistance. Marianne Holmer and an anonymous reviewer made suggestions that improved the clarity of this paper. This is contribution #206 of the Southeast Environmental Research Center at FIU.