Aquatic organic matter decomposition in the terrestrial environments of an intermittent headwater stream

Rivers and their riparian zones are linked by reciprocal subsidies such as leaf fall or the emergence of biphasic aquatic organisms. Transfers of subsidies from freshwater to terrestrial ecosystems have been broadly studied, yet few studies have explored the transfer of aquatic organic matter (AOM) to surrounding terrestrial ecosystems as a response of hydrological variability. When rivers dry or flood, AOM can be transferred to terrestrial ecosystems and decomposed by terrestrial organisms; however, this process remains poorly investigated. In this study, we monitored the decomposition rate of several types of AOM (algae, macroinvertebrate and fish) exposed to different drying intensity, on the gravel bars and in the riparian zone of an intermittent headwater stream. The contribution of different terrestrial organisms to this decomposition rate was also explored. We showed that decomposition rates did not differ between the gravel bars and riparian zone although the invertebrate assemblages, which colonized the AOM, did. The decomposition rates depended mainly on the type of organic matter, with AOM of animal origin being decomposed more rapidly than that of vegetal origin. Microorganisms and vertebrates contributed most to the decomposition. Our results suggest that stranded AOM is consumed by terrestrial organisms; however, environmental conditions such as temperature and humidity can affect its decomposition. As extreme hydrological events are becoming more frequent, we need further research to explore how stranded AOM decomposition changes across seasons, river types and climates to improve our understanding of this process and its importance for terrestrial food webs.