Historical ecosystem disturbance and recovery of Sarasota Bay recorded in bottlenose dolphin stable isotopes
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, some 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 dredge and fill habitat alteration, 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.
Stable isotopes are unique forms of the same element which differ only in mass. For instance, in nature the most abundant form of nitrogen is 14N or “nitrogen 14” meaning that it has an atomic mass of 14 derived from 7 protons and 7 neutrons. Less abundant is 15N, which is like 14N but has one additional neutron.
The ratio of the abundance of 15N to the abundance of 14N is an indicator of an animal’s place in their food web also known as trophic level. This is because animals preferentially excrete more 14N than 15N and, thus, the ratio of 15N to 14N of an organism will be higher than that of their diet. Nitrogen isotope ratios are unit-less but are commonly expressed as isotope values which are denoted with a per mil (‰) sign. One trophic level results in a 3.2‰ difference between diet and consumer.
For example, if a dolphin fed exclusively on a single prey species which had a nitrogen isotope value of 4‰, the dolphin in question would likely possess an isotope value of 7.2‰. The tips of bottlenose dolphin teeth record an animal’s diet prior to 1 year of age and remain inert for the rest of the dolphin’s life. Thus, by sampling a tooth from a dolphin which died in 1994 and was 50 years old, we can assess the diet of the dolphin from 1944. We analyzed stable nitrogen isotopes from the tips of teeth from 69 dolphins from the Sarasota Bay population to assess how their trophic position may have changed in response to ecosystem disturbances occurring between 1944 and 2007.
The most striking feature of our data was a 3‰ increase in average nitrogen isotope value of dolphins from 1944 to 1989.
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 wide spread habitat destruction.
Nitrogen isotope values increased as human population grew in the Sarasota Bay area.
We determined that the increase in nitrogen isotope value was not caused by an increase in trophic level but rather probably the incorporation of human produced wastewater, high in 15N, into the Sarasota Bay food web.
Since 1989 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 1989, 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.
This work was conducted as part of Sam Rossman’s dissertation at Michigan State University and is being funded though a National Science Foundation Graduate Research Fellowship.