Salinity Variability in Tidal Creeks and the Role of Watershed Development

Abstract Salinity variability across estuaries is attributed to patterns of freshwater inflow, and there is increasing awareness of the effect it can have on the ecological function of estuaries. In addition to natural causes related to climate and proximity to freshwater sources, there are anthropogenic factors that may increase salinity variability. One less considered urban impact on estuaries is the potential for increased stormwater runoff and water yield to tidal creeks which may artificially shift salinity regimes. To better understand natural and anthropogenic causes of salinity variation, we installed conductivity sensors and data loggers near the outlet of 12 s- and third-order, mesohaline tidal creeks across an urban gradient in Alabama and west Florida, USA. Like many coastal zones, this area is currently experiencing rapid urban development characterized by the spread of low- to moderate-density suburban and commercial land use. Conductivity and temperature data were logged in each creek every 15 min and used to generate a time series of daily mean and 1-h mean salinity (ppt) between June 2019 and May 2020. Using 1-h mean salinity data, variability was calculated from time series and compared using three different measures: mean salinity timestep Δ (MSTΔ), mean daily salinity range (MDSR), and salinity variance. We found that proximity to major river outlets strongly affected three of the monitored creeks and was an overriding factor in both mean daily salinity and salinity variability. The other creeks less affected by nearby rivers were more comparable, and data showed that all three measures of salinity variability had positive relationships with watershed urban land use. Salinity variation changed markedly with seasons, but the positive relationship between watershed urban land use and salinity variability was consistent. Our results suggest that urbanization, even at low- to moderate levels, can lead to detectable shifts in salinity patterns. Because climate change is leading to both more intense storms and rising sea levels, further research is needed to understand how these changes may interact with continued coastal development to affect the ecological functioning of estuaries.