Forest and salinity data across a tidal gradient in the lower Mobile Tensaw River Delta, USA

This research determined the current condition and function of the Mobile-Tensaw River Delta. A combination of field data collection and modeling were used to characterize tidal freshwater forested wetland (TFFW) hydrology, salinity, and vegetation along the tidal extent of three distributary river/creeks in the Mobile-Tensaw River Delta. A total of 47 forest survey plots (500-m 2 ) were stratified across different prominent forested wetland types and river reaches. Data were used to calculate species importance values of canopy-sized tree species and assess community assemblages related to tidal reach. Nine river-gaging water stations were established along a forested tidal gradient and within the Mobile-Tensaw River Delta project area. At each station, data loggers were used to continuously measure hourly water levels and salinity (converted from conductivity) at the surface water inlets of TFFWs along the river-wetland interface. Data were collected for approximately two years. Salinity data were used with other existing long-term data (Mobile River flow, Mobile Bay tide and salinity, and other data from the USGS, NOAA and other sources) to develop a 15-year hindcast salinity model for each gaging station. River salinity models were developed using a hybrid deep neural network model with a residual network technique to predict salinity dynamics at forest plots across the Mobile-Tensaw River Delta.

Finally, there remains significant uncertainty about the role that TFFW contribute in terms of export of organic matter (OM) and nutrients to the larger estuary. We utilized an isotopic approach to assess spatial and temporal trends in OM composition related to changes in river flow (and Mobile-Tensaw River Delta floodplain connectivity) and the OM sources to Mobile Bay. Changes in OM composition in river floodplains and tidal systems typically accompany changes in hydrology that affect salinity and nutrient regimes. These changes can be traced by measuring organic carbon (C) and nitrogen (N) stable isotope ratios that distinguish freshwater from marine influences and relative nutrient sources. We applied this approach to determine if changes in Mobile-Tensaw River Delta wetland structure or function may change OM sources due to tidal connectivity and river flows.