Coastal Estuaries such as Sarasota Bay are reservoirs of biological diversity; however, out of the thousands of species that inhabit Sarasota Bay bottlenose dolphins are of special ecological importance for at least two reasons. They are large predators near the top of their food web and they are long lived. Top predators are useful indicators of marine ecosystem health because they depend on linkages throughout the food web. In addition, bottlenose dolphins in Sarasota Bay can live well over 50 years. During its lifetime a 50 year old Sarasota Bay dolphin would have experienced numerous alterations to their habitat including dredging, human population growth, the 1995 net fishing ban, and severe red tides. This ecological history of Sarasota Bay, is recorded in the tissues of bottlenose dolphins. Using stable isotope analysis, we can learn how dolphins responded to disturbances that changed their ecosystem for over six decades.
The most striking feature of our data was a 3‰ increase in average nitrogen isotope value of dolphins from 1944 to 1990. Traditionally, this would indicate the increase of one whole trophic level. However such an increase is highly unlikely for this time period in which fishing pressure increased, the human population in the area quintupled and there was widespread habitat destruction. Nitrogen isotope values increased as human population grew in Sarasota and Manatee counties. We determined that the increase in nitrogen isotope value was not caused by an increase in trophic level but rather the incorporation of human produced wastewater, high in 15N, into the Sarasota Bay food web. Since 1990 the amount of human-produced nitrogen entering Sarasota Bay has been greatly reduced predominately through advances in wastewater treatment. Nitrogen isotope values show no trend after 1990, indicating improved wastewater treatment was successful in reducing the amount of human produced nitrogen entering not only Sarasota Bay but its food web as well. In this capacity the dolphins of Sarasota Bay serve as historians of ecological change, allowing us to reconstruct disturbances that occurred more than 60 years ago.
To learn more please read the full manuscript: Rossman et al. 2013