Decomposition of red mangrove (Rhizophora mangle) leaf litter in litterbags

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Samuel H. Neely and Anne Raymond, Geology and Geophysics, Texas A&M University, College Station, TX

Background/Question/Methods
In south Florida mangrove swamps, peat accumulates from decomposed fallen leaves in the leaf litter layer. Detritivory may affect leaf mat thickness and peat accumulation rates because mangrove leaves experience high rates of consumption from macrodetritivores (>1 mm), such as the coffee bean snail. The goal of this project is to measure and compare surficial (0 cm depth) and subsurface (15 cm depth) decomposition rates of red mangrove (Rhizophora mangle) leaves without the influence of macrodetrivitores in three field locations within a mangrove swamp, Barnes Sound, Key Largo, Florida: (1) a tidally influenced fringe site dominated by R. mangle, (2) an inundated, intermediate basin site dominated by black mangroves (Avicennia germanins), and (3) a mixed forest inundated interior basin site. Fifty canopy R. mangle leaves were collected and placed in each litterbag. Mesh size of the litterbags was 1 mm, which excluded influences of macrodetritivores. Six litterbag replicates were deployed at each field site on January 1, 2020. Replicates were collected at set intervals throughout the year (1, 37, 69, 176, 275 and 353 days) and transported for in-lab analyses. Leaves extracted from each litterbag were individually rinsed with freshwater. Leaf litter (>1 mm) was dried at 50°C for 48 hours to obtain dry weights. These weights were fitted to a single exponential decay model: Wo=Wt/ e-kt. Regressions and ANCOVA were conducted to compare surficial and subsurface decomposition rates within and between sites.
Results/Conclusions
Rapid decomposition occurred within the subsurface of the intermediate basin site (F=15.08, p=0.004) when compared to its surficial decomposition, which suggests this site may be more taphonomically active than the interior basin and fringe sites. Because thick leaf mats form to overlay peat in basin sites where inundation shields the swamp surface from macrodetritivores, we expected there to be no difference in surficial decomposition between our basin sites; our data supports our hypothesis (F=3.73, p=0.09). Although there is no difference in surficial decomposition between fringe and interior basin sites (F=3.16, p<0.11), there is a significant difference between fringe and intermediate basin sites (F=9.37, p<0.01). The exclusion of macrodetrivitores from litterbags may have prolonged the surficial decomposition rate in the fringe site, which has a sparse leaf mat. This suggests that detritivory from macrodetritivores may increase decomposition rates and affect leaf mat thickness in non-inundated areas within south Florida mangrove swamps.