Abnormal Alligators and Organochlorine Pesticides in Lake Apopka, Florida

Abstract

Lake Apopka is a 12,400-ha hypereutrophic lake in central Florida that was the recipient of nutrient and pesticide pollution from adjacent agricultural operations for 50 years. The abnormal American alligator (Alligator mississsippiensis) population in Lake Apopka has been the object of a number of studies including investigations of a population crash, the epidemiology of egg failure, and anomalous endocrine function. Several hypotheses of the causes of these abnormalities have been proposed and examined by multiple research organizations over the past three decades. Initially, organochlorine pesticide (OCP) contamination was considered the most likely factor causing poor reproductive success. DDE concentrations in alligator eggs sampled in 1984–1985 were approximately 4 mg/kg and toxaphene concentrations were approximately 2.5 mg/kg. These levels were known to cause reproductive failure in certain birds. However, transmissible diseases, population age and density, cyanotoxins, nutritional deficiencies, and combinations thereof, were also investigated for their contribution to poor alligator reproductive success. Investigations of an alligator mortality and reproductive failure event on Lake Griffin, a lake similar to Lake Apopka but with lower OCP levels, revealed analogous reproductive abnormalities that were associated with a dietary thiamine deficiency. Thiamine deficiency appeared to be associated with a diet of almost exclusively gizzard shad, which contain thiaminase, an enzyme that breaks down thiamine. OCP contaminants may contribute to these maladies, perhaps through endocrine disruption and increased stress. The findings of the past 30 years of work at Lake Apopka have affected local management decisions as well as policy at the national level.