Freshwater Mussels as Biofilters

Freshwater mussels (Bivalvia: Unionidae) are filter feeders, removing phytoplankton and other suspended particulate matter from the water. The removal of suspended matter from water is often considered desirable, in order to reduce algal blooms and in the treatment of drinking water. This thesis investigates the potential role of freshwater mussels as living filters, or “biofilters”, in a variety of settings. Initial measurements of the filtration rates of British freshwater mussels showed that individual mussels can filter up to half a litre of water per hour. Calculations of the filtration rates of mussel populations in four British rivers indicate that mussel filtering removes between 7% and 30% of the particulate matter in a parcel of water travelling 10km downstream. This implies that mussels play an important role in the removal of suspended particulate matter in river ecosystems. In a large-scale experiment on the Ouse Washes RSPB reserve, mussels were placed in three eutrophic ditches to assess their potential use in the biomanipulation of these ditches. Although mussels suffered high mortality in two ditches, in the third ditch 70% of mussels survived, and the section of ditch containing mussels remained clear of floating macrophytes throughout the summer. However, mussels had little effect on the water quality in ditches, and further work is needed before they are used in future biomanipulations. The novel use of mussels in drinking water treatment was investigated by placing mussels in large flow-through tanks at Coppermills drinking water treatment plant (operated by Thames Water). Mussels reduced the concentration of chlorophyll a and suspended solids in the water flowing through tanks, and increased sedimentation through the production of faeces and pseudofaeces. Therefore mussels behaved as flocculators, and could be used in the early stages of drinking water treatment. In order to assess the feasibility of producing the large numbers of mussels needed for their use as biofilters, freshwater mussels were cultured in the laboratory. Juveniles of $\textit{Anodonta anatina}$ and $\textit{A. cygnea}$ were successfully reared for over a year, and reached 14mm in length (mean = 11.3mm, n = 17) with 20% survival. $\textit{Unio pictorum}$ and $\textit{Pseudanodonta complanata}$ were also reared for 274 and 100 days respectively, although they had lower survival and growth. The apparatus used in these rearing attempts was small and inexpensive, and could be scaled up to produce the required number of mussels for their use as biofilters. Additionally, the rearing of $\textit{P. complanata}$ is vital for the conservation of this rare mussel species, and offers the first opportunity to study its juvenile morphology and habitat requirements.