Impact of exotic plant detritus on macrozoobenthic assemblages: evidence from a transitional aquatic ecosystem

We explored whether leaf detritus of an exotic species, Eucalyptus camaldulensis, affects the structural and functional processes of macrozoobenthic assemblages in four increased salt stress conditions of a transitional aquatic ecosystem (central Italy). We compared the dynamics of the weight loss of leaves of E. camaldulensis by leaching and feeding activities on palatable fraction, measuring the relative breakdown rates, with the weight loss of native Phragmites australis detritus. Highest values of abundance (481 and 245 individuals) were observed on native and exotic resources in 52.64 ± 6.01 psu due to Hydrobia complex acuta on the detritus leaves of Phragmites and Eucalyptus. While the two leaf detritus resources showed low breakdown rates by leaching, the donor-controlled community responded in terms of palatable fraction consumption better on native resources than on exotic plant detritus. Comparing the responses of macrozoobenthic assemblages to the different salt stress conditions and resources, we obtained two complex patterns, one denoting changes in structuring metrics (abundance and biomass) and another denoting a change in donor functionality (reduction of palatable fraction, increase of recalcitrant/leaching fractions). The macrozoobenthic assemblages responded simplifying their structure in stressed conditions of high salinity. Our results indicate that, when estimating the impact of exotic plant detritus on structuring processes of macrozoobenthic assemblages, we should include the breakdown rates of their trophic leaf resources, as well as the relevance of the relative fraction types (leaching, recalcitrant and palatable) influencing those processes. Eucalyptus trees, considered a foreign element in the Mediterranean landscapes, can play a paradoxically role in the detritus food webs.