The effect of Hurricane Katrina on the mussel assemblage of the Pearl River, Louisiana

We sampled the mussels of the Pearl River along the eastern border of Louisiana, USA, in 1997 and 2007. Hurricane Katrina followed the pathway of the river in August 2005, and we were interested in detecting any resulting decreases in mussel abundance or diversity. The mussel assemblage was relatively stable, despite the hurricane and a drought in the Pearl River Basin in 2006 and 2007. We detected no change in average species richness per site, and the total number of mussels collected per site (e.g., collection per unit effort) actually increased in 2007. Species importance curves comparing the two studies did, however, suggest a reduction from 29 to 23 species. Monte Carlo simulations and comparisons of assemblage similarity also suggested that Glebula rotundata increased at the expense of Potamilus purpuratus and Villosa lienosa, possibly because it was more tolerant of salt water intrusion. Eight of 13 mussels with adequate sample sizes had different adult size structures in the two surveys, either increasing or decreasing in mean shell length during the 10 years. Large individuals were rare for the two species that decreased in relative abundance in 2007. Semi-quantitative sampling appeared adequate here to compare relatively long-term changes in mussel diversity and adult size structure. However, more research on natural variation in assemblage structure and mussel size distributions is needed to understand the importance of disturbances like hurricanes or droughts.