Salinisation effects on leaf litter decomposition in fresh waters: Does the ionic composition of salt matter?

Salinisation (i.e. increased ion concentrations) in fresh waters is a growing threat worldwide that impacts freshwater communities. However, less is known about how increased salt concentrations affect key ecosystem processes such as leaf decomposition. We designed a laboratory experiment to assess the effects of a concentration gradient (1, 3, and 6 g/L) of three different salts (NaCl, CaCl2 and CH3CO2K), on leaf litter decomposition mediated by microbial decomposers and the larvae of a cased caddis fly (Schizopelex festiva, Trichoptera). Leaf discs of Quercus robur inoculated with microbial decomposers (a mixture of 5 fungal species) were incubated in microcosms under every possible salt x concentration combination and without salt addition (control), with a single individual of the cased caddis fly. Half of the leaf disks were not available for consumption by the trichopteran and represent a microbial only treatment when leaf mass loss was measured. Leaf decomposition driven by microbial decomposers was not affected by salinity despite the fact that all salt treatments depressed fungal biomass and microbial respiration compared to the control. However, the caddis flies were strongly affected by the high salt concentrations and consumed less leaf material at 6 g/L salt concentrations compared to control microcosms. The feeding activity of the trichopteran further depended on the salt type: CaCl2 had the most deleterious effects. Salinisation of fresh waters depresses leaf litter decomposition, mainly through deleterious effects on detritivores, the magnitude of the response being dependent on the ionic composition. Our results reiterate the need to reduce terrestrial run-off of salts into fresh waters because salinisation (especially increased levels of CaCl2) affects invertebrates and the key ecosystem processes they drive.