Disturbance is recognized as a major organizing force in nearshore marine communities. Severe weather events such as tropical cyclones can defoliate and uproot emergent and submerged vegetated communities; these physical changes in ecosystem structure may cascade to habitat function. Hurricanes can impart extensive damage to coastal ecosystems depending on the intensity and proximity to the storm. Hurricane Harvey battered the Texas coast on August 25, 2017 with maximum winds of 130 knots and produced prolific rainfall as the storm stalled over the state. To assess the immediate impacts of this major hurricane on coastal marine ecosystems, we examined the response of two seagrass species and sediment dynamics to storm intensity. Following the landfall of Hurricane Harvey, we obtained measurements of percent cover and blade length for the dominant seagrass species, Halodule wrightii and Thalassia testudinum, at 525 permanent stations along the Texas coast. We collected sediment cores (60 ml) for the analyses of total organic carbon (TOC), grain size, and ammonium from a bay exposed to the storm’s eyewall. We compared post-storm measurements to pre-hurricane observations from the most previous sampling year for all analyses.
Results/Conclusions
For both species, greater wind intensity corresponded with a significant decrease in mean blade lengths relative to pre-storm measurements (R2 T. testudinum=0.42, R2 H. wrightii=0.54). Rapid growth rates of T. testudinum (up to 8 mm day-1) observed post-storm suggest that regrowth may have occurred in areas where aboveground tissues were severed but belowground biomass remained intact. Only T. testudinum displayed significant reductions in percent cover (R2 T. testudinum=0.13, R2 H. wrightii=0.01) with some stations exhibiting complete loss of aboveground and belowground biomass. We did not observe a difference in sediment ammonium concentrations. Mean TOC from post-storm measurements were higher than previous observations and displayed greater variance. We found significant differences among groups for rubble, sand, silt, and clay percentages (ANOVA, α=0.05) with a shift to sand-dominated sediments. The variance in TOC and grain size in the post-hurricane measurements compared to pre-storm compositions suggest the redistribution of sediments in remnant seagrass beds. Our observations document the acute effects of wind intensity on habitat structure in response to a category 4 hurricane. Severe wind damage, coupled with the poor colonization abilities of T. testudinum and the changes in sediment composition, may prolong the recovery of seagrass meadows directly impacted by Hurricane Harvey.