Survival Rates of Unionid Species Following a Low Oxygen Event

In July of 2000 a spill from an agribusiness resulted in at least 20,000 gallons of runoff mixed with fermented grain, molasses, and other organic substances finding its way into the upper Big Darby Creek, a state and national Scenic River in central Ohio. The spill resulted in what was primarily a low dissolved oxygen (DO) event extending several miles downstream, the worst of which lasted for roughly a week. At the peak of the event readings approaching zero DO were recorded, especially at night. Thousands of fish and mussels were killed.

The Ohio Department of Natural Resources permitted staff from the Ohio State Museum of Biological Diversity to salvage dead mussel shells from the spill zone. Over the course of several visits -- both during and after the period of low DO -- surviving mussels were noted and rough estimates were made of the percentage of the fauna that had made it through the event (Table 1).

Although most individual mussels showed some reaction to the event (i.e. most either partially or completely emerged from the substrate), differences in survival rates among species were remarkable. For instance, almost all Lampsilis fasciola and Ptychobranchus fasciolaris succumbed very quickly, while virtually every Fusconaia flava and Amblema plicata made it through with no apparent lasting effect. Another discrepancy was noted between the survival rate for individuals of a species living in riffles vs. individuals of the same species living in pools, with the latter having greater success. It is possible that pool individuals were better adapted to lower DO levels. It is also possible that age was a factor, as pool individuals were generally older than riffle individuals. Juveniles appeared to fare poorly.

The pattern of species-selective kills may explain why this section of Big Darby does not support some of the rarer, presumably more sensitive unionids living elsewhere in the Darby watershed -- e.g., Epioblasma rangiana, E. triquetra, Pleurobema clava, and Quadrula cylindrica -- despite large populations of many other species. Perhaps similar low oxygen events have quietly eliminated these species over the years.

Despite the tragedy of the Darby spill, perhaps some good will come of it. By recording species-specific responses to known organic spills we might begin to lay the groundwork for identifying the causes and origins of other more mysterious mussel kills in the future.

Table 1. Estimated survival rates for various unionid species following a low oxygen event in Big Darby Creek, Union County, Ohio, 2000.

Common Name	Scientific Name	Abundance at spill site	Estimated survival after 3 weeks
paper pondshell	Utterbackia imbecillis	uncommon	15 percent
giant floater	Pyganodon grandis	fairly common	70-80 percent
cylindrical papershell	Anodontoides ferussacianus	rare*	50 percent
squawfoot	Strophitus undulatus	common	50 percent
slippershell	Alasmidonta viridis	rare*	none observed
elktoe	Alasmidonta marginata	rare*	50 percent
fluted shell	Lasmigona costata	common	90 percent
creek heelsplitter	Lasmigona compressa	uncommon	80 percent
threeridge	Amblema plicata	very common	near 100 percent
Wabash pigtoe	Fusconaia flava	fairly common	100 percent
spike	Elliptio dilatata	abundant	50 percent
kidneyshell	Ptychobranchus fasciolaris	abundant	less than 5 percent
fragile papershell	Leptodea fragilis	rare*	12.5 percent
rainbow	Villosa iris	common	50 percent
fat mucket	Lampsilis radiata luteola	abundant	33 percent
plain pocketbook	Lampsilis ventricosa	common	30 percent
wavy-rayed lampmussel	Lampsilis fasciola	very common	5 percent