Temporal and spatial variation in CO2 exchange in a salt marsh dominated estuary (PIE LTER)

Salt marshes are important carbon sinks, but large uncertainties about current rates of carbon exchange with the atmosphere and the ocean remain. These need to be constrained for a better assessment of changes in long-term drivers such as sea level and climate. At the Plum Island Ecosystems LTER, we are expecting a transition from the current Spartina patens dominated high marsh to a more frequently flooded Spartina alterniflora dominated low marsh with increasing sea level. We have set up two eddy covariance sites, one in a high marsh (starting in 2013) and one in a low marsh (starting in 2015) to study net ecosystem CO2 exchange and evapotranspiration (ET). We use a broad-band NDVI to monitor phenology at both sites, which is tightly coupled to the CO2 fluxes. While the temporal dynamics do not vary much between the years, the magnitude in NDVI and CO2 fluxes does: For the high marsh site, we observe lower NDVI (and smaller overall net CO2 uptake) in years with low rainfall during the growing season, e.g. in 2014 and likely in 2016. In 2014, a low rainfall period occurred at the beginning of the growing season, during which ET was slightly higher than in other years, which likely increased soil salinity. In 2016, the period of low rainfall has extended much longer into the growing season (on-going) which seems to have an overall stronger effect (i.e. decrease) on low marsh net CO2 uptake than on the high marsh. We will discuss our findings in the context of salt marsh hydrology and carbon cycling in high and low marsh.