Effect of suspended sediment concentration on the clearance and biodeposition rates of an Australian freshwater mussel (Hyriidae: Alathyria jacksoni)

Abstract

Abstract Freshwater mussels are an ancient group of molluscs that are considered ecosystem engineers because of the many important functions they perform, including biofiltration and biodeposition. Associations between mussel declines and increased sedimentation are well documented, but the direct effects of suspended sediment (SS) concentrations on mussel functioning are poorly understood. This study investigated the effects of SS concentrations on the clearance and biodeposition rates of the Australian hyriid river mussel Alathyria jacksoni. Clearance rates of algae and sediment and biodeposition rates were measured in aquaria containing 3 L of aerated water for 1 hr, with six different concentrations of SS (bentonite clay: 0, 1, 10, 40, 100 and 300 mg/L). Clearance rates were estimated using flow cytometry and chlorophyll‐a with both methods giving similar results. Biodeposition rates were estimated by weighing the mussel biodeposits at the end of each trial. We found that SS concentrations did not have a significant effect on the clearance rates of algae, which ranged from 0.033 to 0.438 L mussel−1 hr−1, with an overall median of 0.118 L mussel−1 hr−1. By contrast, increasing SS concentrations had a significant effect on the clearance of sediment from the water column, with clearance rates peaking at 10 mg/L and declining thereafter. Clearance rates of suspended sediment ranged from 0.046 to 0.512 L mussel−1 hr−1, with an overall median of 0.182 L mussel−1 hr−1. We further detected a significant effect of SS concentration on the production of biodeposits, with inorganic and organic content in the biodeposits increasing as sediment concentrations increased. These results indicated that as SS concentrations increase, A. jacksoni rejects more inorganic and organic material in the form of pseudofaeces. This added maintenance work may result in an energetic cost to the mussel, and have potential consequences for growth, reproduction, and survival. Finally, population estimates of A. jacksoni in the lower Ovens River, Australia were combined with the clearance rates observed in the experiment, to estimate the potential impact mussel populations have in the clearance of total suspended matter from the open water of an unregulated south‐eastern Australian lowland river. A. jacksoni occurred patchily throughout the 35‐km section of the river surveyed, with an average overall mean and maximum density of 0.68, and 35 individuals/m, respectively. We estimate that under scenarios of 1 and 300 mg/L SS, the lower Ovens River mussel population can potentially clear up to 2–4 ML water/day, which equates to c. 1–2% of the average daily discharge in late summer, while transferring around 5.4–15.6 kg/day dry weight of organic material to the river bed. Increased SS concentrations negatively affect freshwater mussels around the world and many respond by reducing their clearance rates, but this was not the case for A. jacksoni. Instead, A. jacksoni increased production of inorganic and organic biodeposits rejecting more food particles in the form of pseudofaeces. For A. jacksoni and other species showing this response, the added energetic cost has the potential to impact growth rates, reproduction, and population dynamics.