Is Salinity Variability a Benthic Disturbance?

Estuaries are subjected to variable salinity regimes governed by variable freshwater inflow and tidal regimes. Estuaries are less saline near the river (source of fresh water); salinities increase towards the inlet of the adjacent sea or ocean. Freshwater inflow is a driver to the functioning of estuaries, and average salinity is usually measured to identify the effects of inflow. However, salinity variability could act as a disturbance by producing unstable habitats. The purpose of this research was to determine if salinity variance is an indicator of benthic disturbance, and therefore a driver of community stability. The macrofauna communities of the five most southern estuaries on the Texas coastline were analyzed using a long-term data set. The estuaries lie in a climatic gradient and have different long-term salinity dynamics, thus salinity variance within and between estuaries can be compared. Benthic diversity, evenness, and richness (i.e., total number of species) were calculated and compared to salinity average and salinity variance to determine the efficacy of each diversity measure for determining community changes within and between estuarine systems. Salinity variance, rather than salinity average, was found to be more correlated to benthic diversity for each estuarine system. Freshwater inflow acts as a benthic disturbance both within and between estuaries. As salinity variance decreased (i.e. reduced freshwater inflow) diversity levels of benthic communities increased, while areas with more freshwater inflow displayed lower levels of benthic diversity. These findings advance a general theory of diversity maintenance. When communities are not influenced by persistent stressors, such as salinity variance, multiple stages of succession may occur with more species available to occupy the resulting open niches, thereby increasing diversity.