Historical ecosystem disturbance and recovery of Sarasota Bay recorded in bottlenose dolphin stable isotopes

Jan 17, 2012 2 comments By , , , and


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.

Nitrogen isotope values from teeth of bottlenose dolphins

Nitrogen isotope values from teeth of bottlenose dolphins resident to Sarasota Bay highlighting the incorporation of nitrogen pollution into the Sarasota Bay food web prior to 1989.

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.


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About the author

I am a Ph.D student duel enrolled in Zoology and Ecology, Evolutionary Biology and Behavior at Michigan State University. I first became interested the foraging habits of large carnivores as an undergraduate at Michigan State University and actively sought out a graduate student opening in the stable isotope laboratory of Dr. Peggy Ostrom. I am fascinated by our ability to derive complex foraging behavior from the tissue of bottlenose dolphins. During my career I hope to use current forensic techniques to learn more about how wild animals forage. I then want to apply my findings to conservation and management of wild populations and resources.

About the author

Michigan State University

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US Geological Survey

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No extra information is available at this time

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Randall Wells, PhD, is the Director of the Chicago Zoological Society’s Sarasota Dolphin Research Program (SDRP). He began studying bottlenose dolphins in Sarasota as a high school volunteer at Mote Marine Laboratory in 1970. He received his BA in Zoology from the University of South Florida in 1975, a Master’s in Zoology from the University of Florida in 1978, a PhD in Biology from the University of California, Santa Cruz in 1986, and a Post-doctoral Fellowship in Biology from the Woods Hole Oceanographic Institution in 1987. Employed by the Chicago Zoological Society since 1989, he is a Senior Conservation Scientist, and in this capacity he also manages Mote Marine Laboratory’s Dolphin Research Program. As a Professor of Ocean Sciences (adjunct) at the University of California, Santa Cruz, he serves as major advisor for MS and PhD students, and he is an adjunct Professor with the University of North Carolina, Wilmington, Duke University, and the University of Florida’s College of Veterinary Medicine. Wells was President of the Society for Marine Mammalogy (2010-2012).

2 Responses to “Historical ecosystem disturbance and recovery of Sarasota Bay recorded in bottlenose dolphin stable isotopes”

  1. Blair Irvine says:

    Feel free to post our link. Different researchers are using isotopes in multiple studies.

  2. Camille Franicevich says:

    Thanks for this informative article! We would like to post a link to it on our isotope-specific blog, http://www.isotope.info. You can also find us on Facebook at our IsotopeDigest page.

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