Abstract
ABSTRACT Laboratory experiments were designed to mimic the frequency, duration, and magnitude of water velocity changes associated with towboat and barge passage at prominent mussel beds in the upper Mississippi River. Ambient water velocity was set to 11 cm·s−1, with regular 5-minute pulses of 45 cm·s−1 occurring every 0.5, 2.0, or 5.0 hours. After 49 days mussels showed no significant inter-treatment differences in respiration, nitrogen excretion, O:N ratio, or tissue condition index. Additional short-term (14 days) laboratory experiments indicated additive effects of increased total suspended solids (TSS) during brief, intermittent periods of high water velocity. The 5-minute episodes of high velocity (50 cm·s−1) occurred once every 0.5 and 3.0 hours against an ambient velocity of 7 cm·s−1. TSS rose from an ambient concentration of 20 mg·liter−1 to 120 mg·liter−1 during the episodes of high velocity. Significantly lower nitrogen excretion and higher O:N tended to occur in high versus ambient TSS treatments; however, these differences were small and inconsistent both among species within a treatment and among treatments for a particular species.
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