Stable isotope analyses of manatee bones measure historical nitrogen pollution in Florida waters, 1975–2010

Abstract

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.