Feeding and nutrient excretion of the New Zealand freshwater mussel Echyridella menziesii (Hyriidae, Unionida): implications for nearshore nutrient budgets in lakes and reservoirs

Native freshwater mussels (Bivalvia: Unionoida) can represent a large portion of benthic biomass, but their functional role is still poorly understood. We sampled Echyridella menziesii (Hyriidae) in six lakes from two regions of the North Island of New Zealand to develop general allometric relationships for predicting filtration, excretion and biodeposition rates. Experimental containers were set up in the field (no-flow conditions) to measure filtration and biodeposition using natural phytoplankton communities, and excretion in filtered lake water. Filtration rates were 0.02–1.3 L mussel −1 h −1 and increased with increasing mussel size (R 2 = 0.13, P = 0.023). Stable isotope analysis suggests that mussels also assimilate food from non-planktonic origins. Nitrogen excretion rates were 4–50 μg N mussel −1 h −1 and increased with mussel size (R 2 = 0.70, P 0.3). In contrast, phosphorus excretion rates did not vary with mussel size, and were much lower in Rotorua than Waikato lakes (2 vs 5 μg P mussel −1 h −1 ). The reason for this regional difference is unclear, but suggests that mussels could contribute different N:P ratios to nearshore nutrient budgets in different types of lakes. Biodeposits represented 50–70% of the N, and 25–70% of the P recycled. Echyridella filters and excretes nutrients at rates similar to those measured in North American and European mussels (Unionidae, Margaritiferidae). Mussels could be important contributors of nutrients in areas where they are abundant, and their inclusion into nutrient budget models could improve the predictions of nearshore nutrient fluxes in lakes and reservoirs.