Harmful algal bloom impacts on juvenile green turtle foraging ecology: insights from stable isotope analysis

Harmful algal blooms (HABs) are becoming increasingly frequent due to the effects of nutrient pollution. Some HABs affect the environment by decreasing light penetration, resulting in die-offs of aquatic primary producers. Particularly for species of conservation concern, there is a need to understand how HAB impacts on seagrasses and macroalgae relate to the foraging ecology of marine consumers. Stable isotope analysis (SIA) is a potentially useful tool to study these processes, but typical SIA study designs may not be appropriate due to the unpredictable nature and fast onset of HABs. Our goal was to gain insights into impacts of HABs on juvenile green turtles Chelonia mydas in the Indian River Lagoon, Florida, USA, by comparing long-term patterns in turtle carbon and nitrogen isotopic values to those of pinfish Lagodon rhomboides, a species with high site fidelity that serves as a reference for the lagoon. Mean isotopic values of carbon and nitrogen showed relatively small changes over time in both species. Green turtle stable carbon and nitrogen isotopic variance decreased during and after the 2011-2013 HABs in the Indian River Lagoon, while pinfish isotopic variance largely remained at pre-HAB levels except for an increase in nitrogen variance in 2013. The decrease in green turtle isotopic variance extended through 2018. We discuss potential causes for this decrease, including changes in diet diversity and habitat use. Our study highlights the need to further understand the impacts of nutrient pollution and HABs on protected marine species and the importance of long-term monitoring programs in achieving this understanding.