Low Impact of Hurricane Katrina on Seagrass Community Structure and Functioning in the Northern Gulf of Mexico

Hurricanes are large-scale disturbances with the potential to exert extensive damage in coastal ecosystems. On August 29, 2005 Hurricane Katrina catastrophically impacted a large area of the Gulf of Mexico spreading from coastal Alabama to Louisiana. For five months before hurricane landfall we were assessing the structure and functioning of a mixed seagrass bed located in an area greatly affected by the hurricane. The storm provided an opportunity to assess the effect of a large-scale disturbance on the structure and functioning of that seagrass bed. A comparison of surveys before and after the hurricane showed no decreases in seagrass density, associated fauna, or the microalgal abundance in the water column and sediment. We found no major impact on gross primary productivity, respiration, or net productivity of the water column or the sediment, suggesting that the hurricane had little impact on the metabolism of the seagrass bed studied. Overall, natural temporal changes recorded before the storm were larger than any post-hurricane changes. These findings indicate that this seagrass meadow was naturally highly dynamic and very resistant to Hurricane Katrina. Seagrass meadows provide key ecological services to the marine environment (Hemminga and Duarte, 2000; Orth et al., 2006). They achieve high structural complexity, which provides habitat and refuge for a great diversity of adult and juvenile organisms (Orth et al., 1984; Heck and Crowder, 1991; Heck et al., 2003). Seagrass meadows are also highly productive systems (Duarte and Chiscano, 1999) that support large food webs and accumulate extensive pools of aboveand below-ground biomass (Romero et al., 1994; Duarte and Cebrian, 1996), but inhabit an environment that often experiences disturbances that may disrupt these functions (Preen et al., 1995). The damage of hurricanes to seagrass meadows as well as other coastal habitats is quite variable and depends on the hurricane itself and the location (Smith et al., 1994; Courtemanche et al., 1999; Paerl et al., 2001; Coles and Brown, 2007; Cebrian et al., 2008). For instance, a cyclone and associated rainfall caused the loss of approximately 1000 km2 of seagrass in Hervey Bay, Australia (Preen et al., 1995). On the other hand, other reports have shown overall small impacts of hurricanes on seagrass beds and associated macrophytes (e.g., Fourqurean and Rutten, 2004; Tilmant et al., 1994) and major Hurricanes Ivan in 2004 and Katrina in 2005 did not have a large impact on the distribution of seagrasses along the Alabama coast (Byron and Heck, 2006). Moreover, the impact of hurricanes can be very patchy, causing substantial damage in a number of scattered locations but leaving others unaltered. For instance, Steward et al. (2006) reported that roughly 4% of their study sites in Florida Bay suffered significant physical damage (e.g., complete seagrass eradication) after four major storms in 2004. BULLETIN OF MARINE SCIENCE, VOL. 85, NO. 1, 2009 46 The majority of studies assessing hurricane impacts on seagrass beds have focused on structural (distribution of abundance and biomass at different trophic levels) and dynamic properties of the dominant seagrass species such as plant abundance, density, biomass, and growth (Marba et al., 1994; Fourqurean and Rutten, 2004; Cruz-Palacios and van Tussenbroek, 2005; Byron and Heck, 2006; Ridler et al., 2006). However, to our knowledge, no studies have yet assessed the impacts of hurricanes on the abundance of the diverse producers and consumers that inhabit seagrass beds (e.g., community structure) and the consequences for the overall metabolic functioning (e.g., community respiration and net and gross production) of the seagrass ecosystem. In April 2005, we initiated a survey of community structure and function in a seagrass bed in the northern Gulf of Mexico. The survey assessed abundance of producers (seagrass, epiphytes, benthic microalgae, and phytoplankton) and fauna (epifauna, infauna, and fish), as well as benthic and water-column productivity and respiration in the bed. Hurricane Katrina, which was one of the most destructive storms in the history of the United States with winds over 264 km hr–1 and a maximum surge of 7.8–8.5 m in the western coast of Mississippi (Hsu et al., 2005), made landfall approximately 100 km west of our study site on August 29, 2005. The bed studied for 5 mo before landfall was well in range of the hurricane force conditions (Knabb et al., 2005). Indeed, the hurricane produced catastrophic property damage and destroyed most of the houses shoreward of the study site. Our goal was to assess the impact of Hurricane Katrina on seagrass ecosystem structure and functioning. We did so by continuing our measurements for a year after the passage of the storm and by comparing the preand post-storm dynamics of the bed.