Linking Hypoxia Exposure to Fish Trophic Dynamics in the Northern Gulf of Mexico Using Otolith Elements and Tissue Isotopes

Seasonal hypoxia affects benthic ecosystems by altering feeding behaviors and trophic relationships. The sublethal effects of hypoxia on mobile fishes are difficult to evaluate without reliable permanent biomarkers describing individual lifetime exposure histories. Hypoxia alters bottom water chemistry due to reductive release of dissolved manganese (Mn²⁺) from the sediments, which may be incorporated into the otoliths of fish inhabiting hypoxic waters and indicate individual exposure histories. To test these hypotheses, water samples and Atlantic croaker (Micropogonias undulatus) were collected from hypoxic and normoxic sites in the northern Gulf of Mexico in the fall of 2011 and summer of 2012. We examined trace elements in fish otoliths as chronological geochemical records of hypoxic redox conditions using manganese, and distinguished estuarine from coastal habitat use using barium. Tissue-specific stable isotopes of carbon and nitrogen were used to reveal feeding histories over long (muscle~months) and short (liver~weeks) time scales and compared to otolith chemistry results to quantify relationships between hypoxia exposure and shifts in trophic structure. A combination of otolith chemistry and tissue stable isotopes further enhances our understanding of responses to sublethal hypoxia and the potential consequences for ecosystem functioning.