Award: OCE-1923965

Zones of low dissolved oxygen in aquatic ecosystems are on the increase, not only because of nutrient pollution but also because of warming. Warmer water holds less oxygen, and other physical characteristics exacerbate the re-aeration of low oxygen zones. Most aquatic life depends on oxygenated water, just as  humans depend on oxygen in the atmosphere to breathe.

We studied the complex interactions of fish communities with their environments under hypoxic (low oxygen) and normoxic (normal oxygen) conditions. We focused on the Baltic Sea, the world’s largest human caused “dead zone,” and on Lake Erie in the Laurentian Great Lakes. We were able to compare two fish species in Lake Erie and four species in the Baltic Sea. This was in collaboration with our partners at Texas A&M University at Corpus Christi, Ben Walther and his PhD student.

We used the techniques of analyzing trace elements in two structures that grow throughout the lifetimes of fishes, namely otoliths (ear-stones) and eye lenses. Otolith trace elemental chemistry provides information on lifetime chronologies of hypoxia exposure, metabolic status, and migrations; lens chemistry is particularly good for measuring mercury exposure.

Mercury in its toxic form (methylmercury) can increase in anoxia (zero oxygen) and persist in hypoxia, entering the food chain. We tested whether fishes exposed to hypoxia had enhanced mercury uptake. This was complicated by other factors, including the age and growth rates of fish; but we observed an increase in mercury in Lake Erie round goby exposed to hypoxia. Round goby live on the bottom and are the most in contact with hypoxia. When they accumulate mercury, it will be transferred up the food chain when predators eat them. Interestingly, we saw unexpected responses in yellow perch. Because it is more sensitive to hypoxia, low oxygen drove it up into the water column where it fed on plankton instead of on sediment-dwelling insect larvae. In contrast, yellow perch in the normoxic region of Lake Erie could forage on the bottom, and in fact their mercury levels were higher because they did not have to flee that habitat.

In addition we analyzed isotope ratios of carbon, nitrogen, and sulfur to understand how hypoxia impacts the food webs. We found an across-the-board reduction in dietary variety (“niche breadth”) in all the species studied. In addition, we found evidence of a biogeochemical shift in a Baltic Sea hypoxic area. Although the biogeochemistry occurs in microbes, it translated up the food web, such that the entire food web was impacted.

Finally, we developed and used sophisticated models of food webs in the Baltic Sea to understand the impacts of hypoxia on food web structure and also on fisheries yields. We found that as hypoxia intensifies, the main predator, cod, runs out of prey on the bottom and must feed on alternate prey higher up in the water. And when the water also warms, it holds less oxygen and the fish become smaller, partly because their metabolism increases in the warmer water, and partly because they can’t supplement the metabolic needs with additional food. We are also assessing the lost value of the fish biomass that never develops because of the low oxygen and warming.

We developed outreach with the help of our communications team. We used a novel technique called “data sonification” to convert numerical data into tones, and then we composed songs. We used the data from different fish in different phases of hypoxia exposure, and we tested how different audiences reacted and whether they were emotionally moved by listening to the songs. This was a way to connect listeners directly with fish from their environments; fishes coming from local (to listeners) environments resonated most strongly.

There were many people associated with our project, which we nicknamed “Project Breathless” and for which we developed a website (www.tinyurl.com/projectbreathless) to educate people of all ages about hypoxia. Our intent is to teach people about the connections of oceans to climate change, and what the implications are for fish communities, fisheries, and our well-being.

Last Modified: 04/07/2025
Modified by: Karin E Limburg