GREAT OUTDOORS: Low dissolved oxygen levels detected in Lake Erie

Observations made decades ago showed the link between warm water temperature, low wind speed and depleted dissolved oxygen levels in the highly enriched Western Basin of Lake Erie. Normally, the shallow Western Basin has enough wave action to stir the water column and transport dissolved oxygen to the water/sediment interface where decomposer bacteria consume dead algae and other organic matter. However, on July 9, NOAA issued a bulletin citing that hypoxic bottom waters, meaning water with less than 2 parts per million of dissolved oxygen, have been identified in the Western Basin on both the Ohio and Canadian sides, and expected persistent light winds will maintain this condition. Most vulnerable are bottom-oriented aquatic life, especially mussels, snails, insect larvae, crayfish, scuds and other macroinvertebrates. Some fish, especially round gobies, sculpins and darters cannot readily move to the higher oxygenated upper portions of the water column. The well-documented severe oxygen depletion that occurred over an extensive area in Lake Erie's Western Basin during the hot, calm summer of 1953 was blamed for the catastrophic collapse of the mayfly population that took decades to recover. While some shoreline area residents would celebrate having fewer mayflies to clean up after they emerge during the summer, they support the high numbers of yellow perch, walleyes and other fish. Many species of birds, reptiles, amphibians and spiders also benefit from the abundance of this pollution-intolerant insect. When Ohio Sea Grant Director Chris Winslow was an instructor at Stone Laboratory on Gibraltar Island, he supervised student research projects that deployed water quality monitoring probes around the islands. In 2009, at the same time as low dissolved oxygen levels were present near the bottom, they discovered that round gobies were coincidentally disappearing from their fish trawl catches. Gobies do not have a swim bladder that allows them to suspend in the water column for extended periods of time, thus suffocate while forced to endure low dissolved oxygen on the bottom. Last week, gobies were seen washing up along the shoreline near the Marblehead Lighthouse. Meanwhile, dead freshwater drum and channel catfish continue to be seen in noticeable numbers in Sandusky Bay and drifting offshore in June and July. Because their deaths predate the current hypoxic event, spawning mortality (coupled with high water temperatures) is more likely the cause, according to Lake Erie administrator Travis Hartman. "We were cold for a long time and spawning was potentially delayed, but regardless of the timing of spawning, fish that died stayed on the bottom because the cold temperatures delayed their decomposition," Hartman said. "When we got really warm really fast, the fish started decaying faster and filled with gas, which caused them to float, but at the end of June and early July instead of late May." Also, during the first half of July, a hypoxic upwelling event that moved poorly oxygenated bottom water into the nearshore zone killed various species in the Lorain-to-Sheffield area of the Central Basin. According to Hartman, this situation appeared to have subsided by mid-month. However, it happens each time wind direction allows hypoxic water to push inland and engulf the entire water column near the shoreline. In fact, on July 16, NOAA issued an updated bulletin stating: "The Lake Erie Hypoxia Forecast predicts the potential for localized hypoxic upwelling along the Ohio coast near Sandusky from Thursday July 17 through Saturday July 19." Cooler water temperatures and enough wind to infuse the entire water column with dissolved oxygen is needed to restore the Western Basin to healthy conditions and prevent further bottom organism mortalities.