Biomass loss and nutrient release from decomposing aquatic macrophytes: effects of detrital mixing

Aquatic plant decomposition is typically studied on species held separately, whereas diverse plant communities are usually found in lake littoral zones and decomposition occurs as mixtures of multiple species. Here, we examined whether detrital mixing affects the rate of aquatic macrophyte decomposition. Specifically, we measured decomposition rates of detritus from four species (Myriophyllum heterophyllum, Ceratophyllum demersum, Typha × glauca, and Potamogeton robinsii) in single, double, triple, and quadruple species mixtures held over two summer months in a mesotrophic lake in southern Ontario, Canada. We measured detrital mass loss after different time periods for all combinations. There were limited effects of mixing on decomposition rates with inhibitory effects observed in only two of the eleven multi-species mixtures. Decomposition rates of single and mixed species detritus varied with initial C:N and C:P ratios with faster rates seen for more nutrient-rich detritus. Overall, there was no effect of detrital species richness on macrophyte decomposition rates other than smaller differences among the averages of more species-rich mixtures. Our results were inconsistent with interactive effects of mixing on decomposition rates of multiple aquatic plant taxa. Instead, we found decomposition rates of mixed species communities were largely predicted by biomass composition and single-species decomposition estimates. There were also no apparent effects of species mixing on N- or P-specific fluxes or the ratio of these fluxes that resulted during decomposition during our experiment. Our results indicate that future changes in aquatic plant biodiversity may affect rates of decomposition in these ecosystems, but these should be largely predictable based on changes in plant communities and their biomass-weighted stoichiometry.