Past, Present, and Future: A Decadal-scale Case Study of Coastal Primary Producer Responses to Tidal Alterations

Manipulation of tidal patterns through dam or channel construction is often intended to improve ecosystem service provision. Tidal diversion projects along coastlines are common globally and include projects across the Gulf of Mexico coast. However, opportunities to study the long-term ecological impacts of these projects is much less common due to the long-time scales needed to understand the responses of primary producer communities with low turnover rates. One such case involves Rollover Pass, a constructed tidal channel through a coastal peninsula on the upper Texas coast that was built in 1954 and closed in 2019. It was opened in an effort to improve water quality and fishing conditions in eastern Galveston Bay, but was ultimately closed due to ongoing erosion issues. Over the 65 years that the Pass was open, eastern Bay marshes gradually transitioned from freshwater to dominance by halophytic species, with a decreasing salinity gradient moving inland along tidal creeks. When the Pass was closed in 2019, wetland sites closest to the pass were dominated by a few halophytes like Avicennia germinans and Spartina alterniflora, while assemblages further from the pass were more diverse and included more brackish species. While the pass was open, the surrounding wetlands were exposed to daily tidal cycles, resulting in variable salinities throughout the day and over longer timescales due the influence of rainfall and prevailing winds. Two years after the closure of Rollover Pass, overall salinity levels lowered and stabilized, with reduced variability due to isolation from tidal influx. Despite these rapid changes in the salinity regime, the emergent marsh community has not yet changed in diversity, cover, or composition. In addition, there has been no recovery of submerged aquatic vegetation in the area. Changes in the emergent and submerged vegetation assemblages will likely occur slowly, over much longer time scales than the few years that have thus passed. With both alterations and restorations to habitats increasing in frequency, studies such as this one are critical to understand the impacts of such events on ecologically relevant timescales and this study represents a unique opportunity to study the area prior to, during, and post alteration.