Assessing the effects of invasive and native leaf litter decomposition dynamics in agricultural water impoundments

Abstract

Leaf litter contributes to the functioning of aquatic ecosystems through allochthonous inputs of carbon, nitrogen and other elements. In many freshwater ecosystems, leaf litter inputs are among the most important cross-ecosystem nutrient contributions. However, native plant communities are under threat from invasive plant species, with largely unexplored consequences for recipient aquatic ecosystems. Broadly, ecological impacts of invasive alien species can be unpredictable and simultaneously span multiple habitat types and taxonomic groups. Invasive alien plants can have particularly severe ecological impacts, and plant inputs into aquatic environments can alter abiotic and biotic aquatic dynamics. Lakes and reservoir ecosystems are regarded as heterotrophic detritus-based habitats which are dependent upon allochthonous organic matter for the majority of energy inputs.

Allochthonous detritus is extremely important for the trophic dynamics of the microbial organisms, macroinvertebrates and benthic plants in lakes and reservoirs. In the present study, leaf litter nutrient inputs, decomposition and colonisation associated with four plant species was examined using a combination of mesocosm and field experimental approaches. Native sycamore fig Ficus sycomorus L., and silver cluster–leaf Terminalia sericea Burch. ex DC. decomposition dynamics were compared to invasive tickberry Lantana camara L and guava Psidium guajava L., whereby phosphate, nitrate, nitrite, silicate and ammonium releases were quantified over time. Leaf inputs significantly reduced pH, with reductions most marked by invasive L. camara. Conductivity was heightened by all leaf input treatments, excepting native T. sericea. Leaf inputs significantly affected all nutrient levels monitored in the water over time, except for silicate. In particular, leaf litter from invasive L. camara drove significantly increased nutrient concentrations compared to other native plant species, whilst effects of invasive P. guajava were less statistically clear.

The end weights of the leaf litter demonstrated decomposition differences among the species types, following a decreasing order of P. guajava > T. sericea > F. sycomorus > L. camara, further suggesting high organic inputs from invasive L. camara. Furthermore, ex-situ larval mosquito colonisation of with the above-mentioned native and invasive species leaves were assessed. Larval mosquito abundances differed significantly accordingly to leaf treatment, whilst no mosquitoes colonised leaf-free controls. Leaf litter from the invasive L. camara, invasive P. guajava and native F. sycomorus drove significant increases in mosquito abundances relative to native T. sericea. In situ macroinvertebrate colonisation, and quantify decomposition rates, of four species of native and invasive terrestrial plants was also assessed. Leaf treatments had a significant, group-specific effect on abundances and composition among focal macroinvertebrates. Invasive leaves reduced Physidae and Oligochaeta abundances, yet Ostracoda were significantly more abundant in the presence of invasive P. guajava. Chironomidae relative abundances increased under invasive L. camara treatments, whilst differences among leaf treatment effects on Coenogrionidae abundances were not statistically clear. In turn, macroinvertebrate diversity did not differ significantly among plant treatment groups, but the contributing taxa varied. The decomposition rate of the leaf litter demonstrated differences among the species types, following a decreasing order of L. camara > F. sycomorus > T. sericea > P. guajava. The study results highlight that differential leaf litter decomposition rates of invasive and native plant types play a significant role in nutrient release, thereby potentially supporting increased aquatic ecosystem productivity.

The study highlights that shifting terrestrial plant communities following invasion may alter aquatic nutrient availability and how insect communities may utilise such resources. In addition, the study highlights that even semi-aquatic organisms such as mosquitos are affected by differing leaf-litter inputs and this may have broader societal implications through vectoring of mosquitoborne disease. While the study showed that invasive leaf litter generally decomposes faster than native litter, the overall findings suggest that plant species-specific, rather than invasive versus native, considerations were important for colonization and nutrient release dynamics. As such, future studies should continue to assess characteristics of other dominant native and invasive plant species within the context of leaf litter allochthonous inputs into recipient aquatic ecosystems. Larger datasets will hopefully prove useful in developing a predictive framework for how riparian plant community shifts will impact on aquatic ecosystem functioning.