FRESHWATER SALINIZATION INCREASES CARBON DIOXIDE FLUXES FROM STREAM LEAF LITTER

Salinization of freshwater ecosystems, such as streams and rivers, from anthropogenic sources affects ~80% of watersheds in the United States. Freshwaters are critical sources of greenhouse gases (GHG) to the atmosphere, as microbial communities consume carbon from leaf litter that falls into streams and release carbon dioxide. Freshwaters have electrical conductivity (EC) that ranges from near 0 to 1,500 μS cm^-1. Extreme salinity changes from fresh to brackish and marine alters greenhouse gas (GHG) emissions; however, small changes in EC within the freshwater range is poorly understood. Shifts in stream microbiomes and their impact of leaf litter degradation may change GHG production. Using lab microcosms with senesced or green leaf litter to simulate freshwater streams, we exposed submerged leaf litter salinity to increasing sodium chloride (NaCl) concentrations and measured carbon dioxide (CO₂) losses at 24, 48, and 96 hours. We collected stream water from a 100% forested headwater stream in Southwest Virginia, USA and added NaCl to generate an EC range from 45 to 2000 μS cm^-1. Microcosms were aerated continuously and kept in a dark location to prevent photosynthesis. Results show that salinity and leaf litter type had a strong non-linear effect on CO₂ losses from microcosms. After 24 hours, leaf litter exposed to minimal salinization (100 to 500 μS cm^-1) produced the largest CO₂ fluxes. Smallest fluxes were measured at 1000 μS cm^-1; however, fluxes rebounded with largest rates occurring at 2000 μS cm^-1. By 96 hours the trends reversed, where 1000 μS cm^-1 had the largest CO₂ fluxes. The same pattern occurred for both vegetation types; however, rates of CO₂ from green litter were approximately twice those of the senesced leaves. These results suggest that freshwater salinization could increase the decomposition of leaf litter and CO₂ released from streams; however, length of exposure and vegetation quality likely affect those rates.