Submitted to: Estuaries and Coasts - Journal of the Estuarine Research Federation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2008
Publication Date: 6/1/2009
Citation: Lambert, M.S., Ozbay, G., Richards, G.P. 2009. The Influence of Feral Horse Activity on Water and Shellfish (Gukensia demissa) Quality Along the Western Coast of Assateague Island National Seashore, Maryland. Estuaries and Coasts - Journal of the Estuarine Research Federation. 57:405-415. Interpretive Summary: Approximately 150 wild horses inhabit the Assateague Island National Seashore (ASIS), Maryland, which is a barrier island popular with tourists and recreational fishermen. This study evaluated physical and chemical qualities of seawater along ASIS as well as bacteriological qualities of ribbed mussels that grow in these waters. Horse density was highly variable, as were the level of dissolved oxygen, phosphorous, ammonia nitrogen, and nitrite in the seawater. Total bacterial counts and total vibrio bacterial levels were nearly 10 million per gram of mussel tissue during the summer months. Total phosphorous levels in the seawater generally exceeded acceptable thresholds, while the levels of dissolved oxygen were deficient. The total amount of nutrients derived from the horses is not expected to have as great an impact on the water quality along ASIS as the anticipated runoff from agricultural fields and animal farms.
Technical Abstract: Feral horses (Equus caballus) inhabit portions of the Western United States and some barrier islands along the East Coast. Approximately 150 feral horses are located on Assateague Island National Seashore (ASIS), Maryland, a barrier island popular with tourists and recreational fishermen. This study surveyed the temperature, pH, salinity, dissolved oxygen, total phosphorus, total ammonia nitrogen, and nitrite levels for seawater as well as total aerobic plate counts and total Vibrionaceae levels of ribbed mussels (Gukensia demissa) at six sites along the Maryland portion of Assateague Island during the summer of 2006 and showed, to the extent possible, the effects of horse activity on water and shellfish quality. Horse density data for the individual sites was highly variable due to the migratory nature of the horses, but ranged from 0 to 0.99 horses/ha. Areas without active horse activity still had a persistent presence of manure. The average monthly DO ranged from 2.65 mg/L in June to 8.70 mg/L in October. Total phosphorus generally exceeded the maximum level of 0.1 mg/L, as set forth in the management objective for the Maryland Coastal Bays Program. Spatial and temporal differences were observed in total bacteria and total Vibrionaceae counts with heterotrophic bacterial counts as high as 9.5 million cells/g mussel tissue of which an average of 76% were Vibrionaceae. The contribution of feral horse manure to nitrogen, phosphorous, and bacterial levels in seawater or mussels of ASIS appears minor compared to contributions expected from agricultural runoff into the bays.