DOI: 10.57778/qwzq-r614
title: Forest and salinity data across a tidal gradient in the lower Mobile Tensaw River Delta, USA 

description: 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.
description: 

A series of forest surveys (n = 47) were conducted from March to August 2023 using 400-m 2 circular plots established. Sites were established up and downstream of water stations (see below) and randomly along designated river reaches to maximize spatial range along the Mobile Tensaw River Delta. For each plot, a center point was established 60-m perpendicular from the river edge and the plot perimeter flagged. Inside each plot, canopy trees with ˃10 cm diameter at breast height (DBH, 1.3 m high) and a total height >15 m were identified, measured, and used to calculate species basal area (m 2 /ha). Using forest survey data, canopy tree species' relative density and basal area were calculated per station using the following equations:

    Relative density = (no. of individuals of species x)/(no. of individuals of all species) *100
    Relative dominance = (total basal area of species x)/(total basal area of all species) *100
    Importance value (IV200)= relative density + relative dominance

Certain species, such as those in the Fraxinus genus, proved difficult to identify in the field and were therefore classified strictly at the genus level. Quercus laurifolia and Quercus nigra also present identification challenges and thus were also grouped at the genus level.

Solinst Levelogger 5 data loggers (n = 9 water stations) were installed in November of 2021 at various tidal inlets to determine the approximate tidal reach of lower rivers in the study site. For each station, the instruments were in-stalled on fence posts near/at a tidal outlet draining the adjacent TFFWs. Each instrument was programmed to measure and record water level, salinity, and temperature for over two years at 15-minute intervals via the Solinst level logger program (Levelogger 4.6.2). Stations were purposely staggered throughout the tidal shrub and forested zones in the lower MT River Delta to gauge tidal influence on wetland forests.