Disturbance effects on nekton communities of seagrasses and bare substrates in Biloxi Marsh, Louisiana

With marshes deteriorating rapidly across the Louisiana coast, the role of seagrasses in sustaining fisheries has come into question. Seagrasses are known to shelter small fish from larger predators. Seagrasses also act as a foraging ground for commercially important species, including spotted seatrout. Thirty samples, fifteen from seagrass habitats and fifteen from bare substrate habitats, were collected inside Biloxi Marsh using a drop sampler in May 2005. After the passage of Hurricanes Cindy and Katrina on July 6, 2005 and August 29, 2005, respectively, the seagrasses in Biloxi Marsh were either uprooted, buried, or both. The May 2005 sites were re-sampled in August 2005 and May 2006 for comparison of pre- and post-hurricane communities. Multidimensional Scaling (MDS) analysis, followed by MANOVA was used to assess overall community structure and species composition, whereas canonical correlation was used to assess the influence of environmental variables on communities. Results show differences in seagrass and bare substrate communities prior to hurricane passage in May 2005. However, post-Cindy, the sites with remaining seagrass resembled the seagrass sites of May 2005, while those without seagrass resembled the bare sites of May 2005. After Cindy, canonical correlation analysis showed that the variables of bottom type, habitat type, month, salinity, turbidity, dry weight of seagrasses, Farfantepenaeus aztecus, Lucania parva, Palaemonetes pugio and Syngnathus scovelli loaded heavily on the first canonical variable, while habitat type, month, substrate, and Sphoeroides parvus loaded heavily on the second. Post-Katrina, all sites were bare, and thus resembled the bare substrate sites of May 2005. Post-Katrina canonical correlation revealed heavy loadings of bottom type, habitat type, year, salinity, dissolved oxygen, turbidity, dry weight, Anchoa mitchilli, Lucania parva, Mysidopsis spp., Palaemonetes pugio, and Syngnathus scovelli. Changes in community structure and species composition observed after the passage of Cindy and Katrina occurred only in seagrass stations that suffered either removal of seagrasses, influx of salt water from storm surges, or both. Additionally, dry weight of seagrasses and/or habitat type were the variables that loaded most heavily on the canonical variables, indicating that presence of seagrasses was the main factor leading to observed changes in community