Project Hypoxolith

Tracking effects of sub-lethal hypoxia exposure in fishes with otolith chemistry

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.

Otolith chemistry as a hypoxia indicator

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 water chemistry
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.  
Cod otolith chemistry

Gulf of Mexico "Dead Zone"

The northern Gulf of Mexico (nGoM) experiences seasonal summertime hypoxia, typically persisting between March and October after delivery of significant organic matter and nutrient loadings from the Mississippi-Atchafalaya River Basin followed by summertime stratification.  Variations in dissolved oxygen are mirrored by dissolved Mn, with higher ambient Mn in regions of extremely low oxygen (see below).

Gulf of Mexico hypoxia

For this system our study species is Atlantic croaker, an abundant bottom-water-associated species that can be exposed to hypoxia after leaving nearshore estuarine habitats in their first summer prior to spawning in the fall.  Samples will be obtained from the annual NOAA SEAMAP groundfish cruises.

More information about Gulf of Mexico hypoxia can be found at, an excellent website fun by the Louisiana Universities Marine Consortium (not affiliated with this project). 

Baltic Sea

The Baltic Sea, one of the largest semi-enclosed seas, is subject to natural hypoxia in its deepest basins due to circulation patterns.  However, due to excessive nutrient loading from the surrounding drainage basin, hypoxia has been increasing in severity in the 2000sMuch of the impact is seasonal, when organic matter decomposition consumes oxygen.  The figure below shows hypoxia (gray) and anoxia (black), from Hansson and Martinsson (2013).

Preliminary studies showed strong correlations between manganese in cod and flounder otoliths and the presence of hypoxia.  We will expand on these studies by examining archived otolith samples, following year classes of Baltic cod and flounder born during mild and severe periods of hypoxia.

Lake Erie

Lake Erie, the shallowest of the Laurentian Great Lakes, has suffered serious water quality problems since the 1960s.  Although water quality improved considerably with implementation of the Clean Water Act and reduction of point source pollution, non-point source pollution continues.  In particular, the central basin (CB) of Lake Erie experiences seasonal hypoxia events that are spatiotemporally variable.  

This portion of the project will focus on yellow perch, an important sport and commercial species in Lake Erie.   PI Limburg has begun collaboration with the Ohio Department of Natural Resources (DNR) to probe yellow perch otoliths from CB and WB collections.  We will continue to work with the Ohio DNR on new and archived samples of yellow perch.

The above figure shows XRF scans of CB yellow perch otoliths for strontium (left) and manganese (right) showing elevated Mn during summertime hypoxia.