Biomass decaying and elemental release of aquatic macrophyte detritus in waterways of the Indian River Lagoon basin, South Florida, USA

Highlights: • The decomposition rates (k) of detritus varied with plant species and decaying time. • Effects of plant species were greater than decaying time on mass loss. • Concentrations of nutrients and metals showed temporal changes. • Water lettuce contributed most to the release of nutrient and Zn, Pb and Mn. • Cattail contributed most to the release of Cr and Cu. Decaying experiments of four major aquatic macrophyte detritus, namely cattail (Typha orientalis), water lettuce (Pistia stratiotes), hydrilla (Hydrilla verticillata) and maidencane (Panicum hemitomon), were conducted using the litterbag technique in the stormwater detention pond of South Florida, USA. Dry weight and chemical composition of remaining biomass were dynamically determined during the 185-day decay experiment. The results showed that decomposition rates (k), and the derived turnover (t_50% and t_95%) were species specific. The k values decreased in the order of hydrilla (0.0123 g day⁻¹) > water lettuce (0.0082 g day⁻¹) > maidencane (0.0049 g day⁻¹) > cattail (0.0031 g day⁻¹), whereas t_50% and t_95% varied in the reverse way. Biomass properties including concentrations of C, N, P, lignin, cellulose, hemicellulose, and the ratios of C/N, C/P, N/P and lignin/N affected decaying rate of the studied aquatic plants. The dry mass loss and concentrations of C, N, P, lead (Pb), chromium (Cr), copper (Cu), manganese (Mn), zinc (Zn), lignin, cellulose, hemicellulose and ratios C/N, C/P, N/P and Lignin/N of plant detritus were significantly affected by species, decaying time, and their interactions. However, the influence of species differences was greater than that of decaying time on those indexes. The estimated amounts (kg) of nutrients and metals released based on k values for the waterways of the IRL basin (water surface area 15.6 km²) were N 126.85 × 10³, P 8.89 × 10³, Zn 408.20, Pb 97.95, Cr 128.99, Mn 313.03, and Cu 82.40. Water lettuce contributed most, accounting for 52.13% N, 56.81% P, 74.95% Zn, 59.58% Pb, and 74.65% Mn, followed by hydrilla, cattail and maidencane. For Cr and Cu, cattail had the greatest contribution of 65.77% and 54.15%, respectively, followed by water lettuce, hydrilla and maidencane.