The functional role of burrowing bivalves in freshwater ecosystems

1. Freshwater systems are losing biodiversity at a rapid rate, yet we know little about the functional role of most of this biodiversity. The ecosyste m roles of freshwater burrowing bivalves have been particularly understudie d. Here we summarize what is known about the functional role of burrowing b ivalves in the orders Unionoida and Veneroida in lakes and streams globally . 2. Bivalves filter phytoplankton, bacteria and particulate organic matter f rom the water column. Corbicula and sphaeriids also remove organic matter f rom the sediment by deposit feeding, as may some unionids. Filtration rate varies with bivalve species and size, temperature, particle size and concen tration, and flow regime. 3. Bivalves affect nutrient dynamics in freshwater systems, through excreti on as well as biodeposition of faeces and pseudofaeces. Excretion rates are both size and species dependent, are influenced by reproductive stage, and vary greatly with temperature and food availability. 4. Bioturbation of sediments through bivalve movements increases sediment w ater and oxygen content and releases nutrients from the sediment to the wat er column. The physical presence of bivalve shells creates habitat for epip hytic and epizoic organisms, and stabilizes sediment and provides refugia f or benthic fauna. Biodeposition of faeces and pseudofaeces can alter the co mposition of benthic communities. 5. There is conflicting evidence concerning the role of resource limitation in structuring bivalve communities. Control by bivalves of primary product ion is most likely when their biomass is large relative to the water volume and where hydrologic residence time is long. Future studies should conside r exactly what bivalves feed upon, whether feeding varies seasonally and wi th habitat, and whether significant overlap in diet occurs. In particular, we need a clearer picture of the importance of suspension versus deposit fe eding and the potential advantages and tradeoffs between these two feeding modes. 6. In North America, native burrowing bivalves (Unionidae) are declining at a catastrophic rate. This significant loss of benthic biomass, coupled wit h the invasion of an exotic burrowing bivalve (Corbicula), may result in la rge alterations of ecosystem processes and functions.