Effects of inorganic sediment and food concentrations on energetic processes of the zebra mussel, Dreissena polymorpha: Implications for growth in turbid rivers

We examined the effects of inorganic suspended sediment (0, 1, 10, and 100 mg·L^-1) and food concentrations (0.1, 0.5, and 2.0 mg particulate organic matter (POM)·L^-1) on clearance, ingestion, respiration, feces, and pseudofeces production rates and assimilation efficiencies of adult zebra mussels (Dreissena polymorpha) under laboratory conditions at 20 ± 1°C. Food quality indices (the proportion of inorganic to organic material in feeding suspensions = IO ratios) ranged from 0.304 to 63.04. Suspended inorganic sediment above 1 mg·L^-1 greatly reduced clearance rates, ingestion rates, and assimilation efficiencies of zebra mussels, and ingestion of POM was maintained only at high food concentrations. Pseudofeces were produced copiously by zebra mussels in the 100 mg·L^-1 sediment treatment, and there appeared to be an increased metabolic cost associated with pseudofeces production. Declining food quality caused exponential declines in clearance, ingestion, and absorption rates, water processing potential, and assimilation efficiencies, with the sharpest declines between IO ratios of 0.304 and 5.0. IO ratios in turbid rivers often exceed 1.71, our experimental threshold where scope for growth for zebra mussels was zero. We contend that zebra mussels in turbid rivers may exhibit low growth potential and may not stabilize at the high population densities seen in their lake counterparts.