Recovery of Benthic Microalgal Biomass and Community Structure Following Beach Renourishment at Folly Beach, South Carolina

One method of preserving beaches against the effects of erosion and sea level rise is beach renourishment. While there have been many studies assessing the impact of renourishment on macrofauna, few studies have looked at its effects on microbes. Benthic microalgae (BMA) are important primary producers, representing the basis of nearshore food webs. BMA also secrete extracellular polymeric substances (EPS), which bind sediment and thus help prevent erosion. The objective of this study was to monitor recovery of BMA in terms of relative biomass (estimated as sediment chlorophyll a) and community structure (characterized using high-throughput DNA sequencing) following renourishment of Folly Beach, SC in 2014. We also examined the relationships among biomass, EPS, and erosion. Sediment samples were collected intermittently (n = 9) from two renourished and two control sites within three intertidal zones (high, mid, low) from June 2014 to January 2015. Biomass recovered in sequence from low to high intertidal, corresponding to when the artificially-raised beach once again experienced regular tidal inundation (between 93 and 169 days post-renourishment). Alpha diversity metrics misleadingly indicated recovery around this same time within the high intertidal, but compositional changes through time were unlike those seen in control samples, and these communities had yet to recover at ~ 7 months post-renourishment. Renourishment therefore appears to impact BMA communities via artificial elevation of the beach face. While there were relationships between chl a, EPS, and erosion, BMA most likely play a minimal role in sediment stabilization in high-energy environments like Folly Beach.