LAUSR

Summary Seston availability and quality can affect the condition, nutrient stoichiometry and nutrient excretion of dreissenid mussels and other aquatic consumers. Nutrient excretion by dreissenid mussels may affect phytoplankton community composition by altering nitrogen:phosphorus (N:P) ratios of the water and may be an important accessory factor leading to increased Cladophora and toxic Microcystis blooms in mussel-invaded lakes. We manipulated phosphorus enrichment levels [no (L), moderate (M) and high (H)] and zebra mussel concentrations (1, 2 and 4 g dry mass m−2) to produce a total of nine treatment combinations, each one held in a 31 m3 enclosure in an oligotrophic lake. We measured zebra mussel condition, carbon:nitrogen:phosphorus (C:N:P) tissue stoichiometry, feeding rate and nutrient excretion and egestion as related to varying conditions of chlorophyll a (Chl), particulate phosphorus (PP), particulate organic nitrogen (PON) and seston C:N:P ratios at three time periods: 5–7, 18–20 and 32–34 days subsequent of adding mussels to the enclosures. Consistent with approximate homeostatic control of N and P, there were only modest differences in C:N:P ratios in mussel soft tissue despite greatly different seston C:N:P ratios among enrichment treatments. Mussel condition (mass per unit length) decreased with increased seston N:P, C:P and C:N ratios and percent composition of Cyanobacteria, and increased with percentage composition of cryptophytes and other flagellates. Assimilation rates of Chl and calculated potential assimilation rates of N and P linearly increased (P < 0.05) with increasing seston Chl, PON and PP concentrations. P excretion measured as soluble reactive phosphorus (SRP) significantly decreased in exponential fashion by two orders of magnitude as C:P (R2 = 0.71) and N:P ratios (R2 = 0.66) increased by a factor of 4. P excretion was significantly correlated with seston PP concentration, which varied over a 19-fold range; however, there was much scatter in the relationship (R2 = 0.29). In contrast, NH4-N excretion significantly decreased (R2 = 0.31) with N:P ratio by a factor of 2 over this same N:P range, and was not significantly correlated with PON concentration. Soluble P excretion was significantly correlated with potential P assimilation, whereas NH4 excretion was not significantly correlated with potential N assimilation. The ratio of N:P excreted showed a significant exponential increase with seston N:P ratio. P and N egestion rates were higher than corresponding P and N excretion rates from the same trials; however, the fate of this egested material – whether recycled by resuspension or remaining in the benthos – is not known. Mussel excretion and its impacts are highly context dependent, varying with algal composition, seston stoichiometry, and mussel abundance and feeding rate. The low P excretion but high N excretion observed when mussel feeding stops implies that under poor feeding conditions typical of summer seston, mussels excrete little P but continue excreting N, which would slow production rate of producers such as Cladophora and Microcystis in low-P systems. In contrast, NH4 excretion by mussels may prolong Microcystis blooms as nitrate is used up by the bloom in moderate-P systems.