Fire transforms effects of terrestrial subsidies on aquatic ecosystem structure and function.

Fire can lead to transitions between forest and grassland ecosystems and trigger positive feedbacks to climate warming by releasing CO2 into the atmosphere. Climate change is projected to increase the prevalence and severity of wildfires. However, fire effects on the fate and impact of terrestrial organic matter (i.e., terrestrial subsidies) in aquatic ecosystems are unclear. Here, we performed a gradient design experiment in freshwater pond mesocosms adding 15 different amounts of burned or unburned plant detritus and tracking the chronology of detritus effects at 10, 31, 59, and 89 days. We show terrestrial subsidies had time- and mass-dependent, non-linear impacts on ecosystem function that influenced dissolved organic carbon (DOC), ecosystem metabolism (net primary production and respiration), greenhouse gas concentrations (carbon dioxide [CO2 ], methane [CH4 ]), and trophic transfer. These impacts were shifted by fire treatment. Burning increased the elemental concentration of detritus (increasing %N, %P, %K), with cascading effects on ecosystem function. Mesocosms receiving burned detritus had lower [DOC] and [CO2 ] and higher dissolved oxygen (DO) through Day 59. Fire magnified the effects of plant detritus on aquatic ecosystem metabolism by stimulating photosynthesis and respiration at intermediate detritus-loading through Day 89. The effect of loading on DO was similar for burned and unburned treatments (Day 10); however, burned-detritus in the highest loading treatments led to sustained hypoxia (through Day 31), and long-term destabilization of ecosystem metabolism through Day 89. In addition, fire affected trophic transfer by increasing autochthonous nitrogen source utilization and reducing the incorporation of 15 N-labeled detritus into plankton biomass, thereby reducing the flux of terrestrial subsidies to higher trophic levels. Our results indicate fire chemically transforms plant detritus and alters the role of aquatic ecosystems in processing and storing carbon. Wildfire may therefore induce shifts in ecosystem functions that cross the boundary between aquatic and terrestrial habitats.

Intraspecific variation in leaf litter alters fitness metrics and the gut microbiome of consumers.

Biodiversity can have cascading effects throughout ecosystems. While these effects are better understood at coarser taxonomic scales of biodiversity, there has been a resurgence in investigating how biodiversity within species may have cascading effects on communities and ecosystems. We investigate the broader trophic implications of intraspecific variation in the riparian tree, Alnus rubra, where immediately local or 'home' litter decomposes faster than 'away' litter in aquatic and terrestrial systems. With climate change shifting the distributions of plants across the globe, it is essential to understand how shifts in the intraspecific traits of leaf litter may have reverberating effects throughout ecosystems. Here, we find that intraspecific variation in leaf litter has fitness implications for invertebrate consumers, including the algivorous Dicosmoecus and detrivorous Psychoglypha caddisflies, which exhibited increased body size and muscle nitrogen content when incubated within in-situ river mesocosms supplied with local A. rubra litter. Litter source altered caddisfly gut microbiomes by increasing relative abundance of methanogens and methanotrophs among the non-local treatment group. Additionally, Dicosmoecus supplied with non-local litter may have shifted their diet towards a higher proportion of algae, as inferred from shifts in gut microbiome composition and isotopic ratios of muscle tissue. Overall, our study demonstrates that shifting distributions of plant genotypes across the globe may cause plant-microbe mismatches that will disrupt patterns of decomposition and may have consequences on the fitness and foraging behavior of consumers.