Imagine going about your day, walking the neighborhood with not a care in the world, when suddenly it becomes hard to breathe. Confused and a little scared, you turn around, wondering if maybe something is wrong with the local gas lines. You note with relief that breathing is getting easier again as you move away from that particular road. When city workers check out the area a few hours later, the air is clear and nothing out of the ordinary is found.
Sounds strange, right? For Lake Erie fish, a scenario like this isn’t as far-fetched as it may seem.
Hypoxia, an area of low-oxygen water, develops in the central basin of Lake Erie during the summer and early fall. It’s caused when bacteria at the lake bottom decompose dead algae and use up oxygen in the process faster than it can be replenished from the surface or from photosynthesis. When combined with stratification – the formation of a sharp border between an upper warm layer and a cold bottom layer of water – that region of the lake becomes hypoxic (low in oxygen) or even anoxic (no oxygen). This can lead to fish kills and other negative impacts on the ecosystem.
The central basin is one of three distinct parts of Lake Erie’s depth profile, which also includes the shallow western basin and the much deeper eastern basin. Because the central basin’s average depth is only about 18 meters, and the thermocline tends to form at around 15 meters of depth, it’s the most prone to developing hypoxia that impacts life at the bottom of the lake.
Monitoring data have shown that hypoxic waters don’t always remain in one place – water currents and waves can shift the edges of hypoxia into a shallow area and back out again in a matter of hours, potentially killing fish that were trapped in that area when hypoxia developed and that now can’t escape that intrusion of low-oxygen water.
Researchers from the Environmental Protection Agency (EPA) monitored water movement and oxygen concentrations at the 14-16 meter depth, where it’s most likely for hypoxic water to intrude into shallower areas with normal oxygen concentrations. Organisms that live there aren’t adapted for those lower oxygen levels, so it’s also where those intrusions can have serious negative impacts on aquatic life that can’t survive these conditions for more than a few minutes.
Continue reading this article on the Ohio Sea Grant website.