Assateague Island National Seashore supports approximately 150 non-native horses (Equus caballus) and it is important to ensure they do not adversely affect native species. The objective of this study is to determine the relationship between feral horse activity and bacteria levels in ribbed mussels (Geukensia demissa). Understanding this relationship is important because shellfish host bacterial pathogens, including those within the Vibrionaceae and Aeromonaceae families, can negatively affect the health of aquatic organisms and human health. We test two hypotheses: 1.) whether there is a difference in Vibrionaceae and Aeromonaceae levels in ribbed mussels along the Island and 2.) if there is a difference in Vibrionaceae and Aeromonaceae levels in ribbed mussels in regards to levels of horse activity, using horse distribution data. Bimonthly from May to November 2006, three replicates of 50 g of ribbed mussels were collected at each site. The mussels were homogenized with 450 mL of buffer, diluted 1:10 with 0.1% peptone, and spread onto tryptic soy agar plates. After incubating at 37 C overnight, one countable plate from each replicate was used for total bacteria counts and then overlaid with a cellulose acetate membrane containing a substrate, which fluoresces when cleaved by enzymes present in Vibrionaceae and Aeromonaceae. ANOVA and post-hoc Tukey tests were used to determine if there were statistical differences between study locations. Lastly, correlation statistics were used to determine the relationship between feral horse activity and water quality. We were expecting to see variation in bacteria levels along the Island, such as elevated levels of Vibrionaceae and Aeromonaceae in areas of high horse activity due to increased nutrients in the water from their manure.
