Stable isotope analyses of manatee bones measure historical nitrogen pollution in Florida waters, 1975–2010
Nutrient pollution is a key driver of ecological change in coastal marine environments. Over the last 150 years, the dominant source of nitrogen in coastal waters has shifted from sewage to synthetic fertilizer in many regions including the Caribbean. Here, we use stable nitrogen isotope analyses of bones from manatees (Trichechus manatus latirostris) from along the coast of Florida, USA, to determine whether they record changes in coastal nitrogen pools over a 35-year period. Nitrogen isotopic composition (δ15N) of 172 manatee bones declined statewide at a rate of 0.07‰ year−1, from 9.8‰ in 1975 to 7.3‰ in 2010, suggesting a greater relative abundance of fertilizer, which has a lower δ15N value than that of sewage. Of the four coastal regions of Florida we examined, δ15N values declined significantly in two: the northeast and northwest coasts. The southeast coast, which includes some of the most densely populated counties and cities, had the highest mean δ15N values. In contrast, δ13C values did not vary over time or among coastal regions within the state, indicating that the source of the plant material in the manatee diets did not change substantively over the course of the study period. The general decline in δ15N values may be due to an increase in synthetic fertilizer nitrogen use by the agriculture industry in Florida, but that trend has now abated as overall fertilizer use has stabilized. It is also likely that the increased use of deep injection wells for treated wastewater disposal, which diverts sewage-derived nitrogen away from coastal waters, contributed to the long-term decline in δ15N values in spite of the state’s rising human population.
Permit for accessing and sampling collection of manatees was granted by the US Fish and Wildlife Service (FWS) (MA14932A–0), Florida Fish and Wildlife Conservation Commission (FWC), and University of Florida’s Natural History Museum (UFNHM). We thank M. Snow for help with the census map, and N. Geeraert and E. Kim for their comments on the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
- Clark CT, Horstmann L, Misarti N (2017) Quantifying variability in stable carbon and nitrogen isotope ratios within the skeletons of marine mammals of the suborder Caniformia. J Archaeol Sci Rep 15:393–400Google Scholar
- FFWCC (2007) Florida manatee management plan, Trichechus manatus latirostris, Florida Fish and Wildlife Conservation Commission, Tallahassee, FL. http://myfwc.com/wildlifehabitats/managed/manatee/
- FLDEP (2014) general facts and statistics about wastewater in Florida, http://www.dep.state.fl.us/water/wastewater/facts.htm
- Halpern BS, Walbridge S, Selkoe KA, Kappel CV, Micheli F, D’Agrosa C, Bruno JF, Casey KS, Ebert C, Fox HE, Fujita R, Heinemann D, Lenihan HS, Madin EMP, Perry MT, Selig ER, Spalding M, Steneck R, Watson R (2008) A global map of human impact on marine ecosystems. Science 319:948–952. https://doi.org/10.1126/Science.1149345 CrossRefPubMedGoogle Scholar
- Newsome SD, Clementz MT, Koch PL (2010) Using stable isotope biogeochemistry to study marine mammal ecology. Mar Mammal Sci 26:509–572Google Scholar
- Paul JH, Rose JB, Brown J, Shinn EA, Miller S, Farrah SR (1995) Viral tracer studies indicate contamination of marine waters by sewage disposal practices in Key Largo, Florida. Appl Environ Microb 61:2230–2234Google Scholar
- Piñón-Gimate A, Soto-Jiménez MF, Ochoa-Izaguirre MJ, García-Pagés E, Páez-Osuna F (2009) Macroalgae blooms and δ15N in subtropical coastal lagoons from the Southeastern Gulf of California: discrimination among agricultural, shrimp farm and sewage effluents. Mar Pollut Bull 58:1144–1151. https://doi.org/10.1016/j.marpolbul.2009.04.004 CrossRefPubMedGoogle Scholar
- Reep RL, Bonde RK (2006) The Florida Manatee: biology and conservation. University of Florida, GainesvilleGoogle Scholar
- USDA (2016) Fertilizer use and price: https://www.ers.usda.gov/data-products/fertilizer-use-and-price/
- USEPA (2018) Class I industrial and municipal waste disposal wells. In: agency USEP (ed), https://www.epa.gov/uic/class-i-industrial-and-municipal-waste-disposal-wells
- York D, Potts E (1995) Domestic wastewater treatment plants in Florida. Fla Water Resour J 96:31–33Google Scholar