Habitat associations and drought responses of mussels in the lower Flint River Basin, southwest Georgia
Johnson, Paula Marie
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Freshwater mussels are among the least understood and most threatened of freshwater organisms in the United States. Approximately 70% of 300 North American unionids are at risk, with habitat associations, host fish requirements and factors determining mussel distribution patterns are not understood for most species. In 1999, we identified mussel community composition, mesohabitat conditions (reach-scale habitat factors) and macrohabitat conditions (watershed-scale factors) were identified at 46 locations in the lower Flint River Basin (FRB), southwest Georgia. Canonical correspondence analysis demonstrated clustering of mussels into four groups: riffle, pool, non-specialist rare, and non-specialist common. Mussel richness, diversity and abundance were positively associated with average stream depth, d-link magnitude (a metric indicating functional stream size) and percent riparian wetland forest cover. During record drought conditions from July through October 2001, weekly mussel mortality, water depth, velocity, dissolved oxygen concentrations and temperature were recorded in 3-5 4m2 quadrats at nine study sites in the lower FRB. Unionid mortality increased sharply when water velocities decreased below 0.01 m/s and dissolved oxygen conditions fell below 5 mg/L. Average weekly mortality for common species exposed to hypoxia (dissolved oxygen levels below 5 mg/l) was 9%, significantly lower than that for three endangered species (Lampsilis subangulata, Pleurobema pyriforme, Medionidus pencilatus) and one riffle species (Elliptio crassidens), whose weekly mortality averaged 44% under hypoxia. Infrequent, extreme drought events may strongly shape mussel community structure and drive observed distribution patterns.