Hypoxia is a growing global threat that impairs the health and functioning of marine ecosystems. Although the potential impacts of hypoxic exposure are severe, there is little known about the consequences of systemic, sub-lethal exposure to hypoxic events for individuals, populations and communities of fishes. The objective of this project is to determine whether sub-lethal exposure to hypoxia during early life stages leads to sub-optimal growth and differential mortality. This project will use biogeochemical proxies in fish ear stones (otoliths) retrospectively to identify periods of hypoxia exposure. This project will capitalize on patterns of geochemical proxies such as Mn/Ca and I/Ca incorporated into otoliths to identify patterns of sub-lethal hypoxia exposure and ask whether exposure results in differential growth and survival patterns compared to non-exposed fish. The project will compare consequences of hypoxia exposure in several species from the Gulf of Mexico, the Baltic Sea, and Lake Erie, thus examining the largest anthropogenic hypoxic regions in the world spanning freshwater, estuarine, and marine ecosystems.
|Fish ear stones, or otoliths, have a number of properties that make them ideal life-long recorders of habitat
residency and environmental exposure histories.
Otoliths are paired structures in the inner ear of fishes that grow continuously through the life of a fish by the addition of successive
layers of calcium carbonate. In
recent decades, micro-scale variation in chemical constituents has been a key
focus of study. Although much otolith chemistry work has focused on chemical identifiers of migratory movements and stock discrimination, there is significant potential for geochemical indicators of hypoxia exposure to be recorded in otoliths. |
Hypoxia alters redox conditions such that Mn oxides are reduced; the reduced forms are soluble, and under suboxic/hypoxic conditions dissolved Mn can be released into the water column where it is available for uptake by fishes (see above figure).
In addition to Mn, hypoxia-induced changes in biogeochemical cycles of other redox-sensitive elements may be reflected in otolith profiles, including iodine. This means that otolith profiles of Mn/Ca ratio will increase and I/Ca ratios will decrease during periods of hypoxia exposure.
The figure to the right shows elemental maps of a Baltic cod otolith (microscope image: A) for strontium (B) and manganese (C), with the bottom panel (D) showing the transect from core to edge indicated by the red line.
Images taken from Limburg et al. 2014 Journal of Marine Systems.