Increased salinity decreases annual gross primary productivity of a brackish wetland

Abstract:

Brackish wetland plant and microbial communities are a diverse mix of freshwater- and saltwater-adapted species in competition with each other. This has led researchers to predict that carbon cycling in brackish wetlands may be more resilient to changes in salinity than in fresh- or saltwater systems.¹ Rush Ranch, a brackish tidal wetland near Suisun Bay, California, experienced drought-induced salinization in the 2015 and 2016 growing seasons followed by a flushing event in 2017. Salinity rose from the baseline of 4.5 ppt to an average of 10.3 ppt, peaking at 12.5 ppt. During these summers, gross primary productivity decreased by 30%, similar to the response exhibited by a nearby freshwater tidal wetland.² In this study, we determined whether increased salinity was responsible for this decrease in GPP. Stepwise regression followed by multiple linear regression revealed that salinity was a major driver of GPP at this brackish wetland. We then trained a random forest model with site environmental data to predict GPP. When this random forest model was trained without salinity as an input, it consistently over-predicted GPP during years with drought-induced salinization and under-predicted GPP during non-drought years. These results provide ecosystem-scale evidence that increased salinity can decrease GPP at a brackish tidal wetland. These findings were supported by a negative relationship between daily light use efficiency and daily average salinity. This relationship is a starting point for incorporating the effect of changes in salinity on GPP in wetland carbon models, which could improve wetland carbon forecasting and management for climate resilience.

1. Chambers, L. G., Osborne, T. Z. & Reddy, K. R. Effect of salinity-altering pulsing events on soil organic carbon loss along an intertidal wetland gradient: a laboratory experiment. Biogeochemistry 115, 363–383 (2013).
2. Chamberlain, S. D. et al. Effect of Drought-Induced Salinization on Wetland Methane Emissions, Gross Ecosystem Productivity, and Their Interactions. Ecosystems (2019) doi:10.1007/s10021-019-00430-5.