Retention of N and P by zebra mussels (<Emphasis Type="Italic">Dreissena polymorpha</Emphasis> Pallas) and its quantitative role in the nutrient budget of eutrophic Lake Ekoln,Sweden

We quantified cover, population densities, size distribution and biomass of zebra mussels along 7 transects in eutrophic Lake Ekoln (Sweden). We also analyzed the elemental (C, N, P) composition of zebra mussel soft tissue and computed their retention rates of N and P their quantitative role in the lake’s nutrient budget. We hypothesized that zebra mussels play an important role in the nutrient budget of the lake and speculate that the successive harvesting of cultured mussels could contribute to the lake’s rate of recovery from cultural eutrophication. At depths exceeding 5 m, mussels covered consistently less than 5% or were absent. Similarly, mean densities were 3,158 ± 2,143 ind m⁻² between 2 and 4 m, but rapidly declined at larger depths. Calculated clearance rates averaged 19.4 ± 2.3 km³ y⁻¹, implying the entire lake is filtered every 8–10 days. Concentrations of N and P in mussel soft tissue averaged 100.9 ± 1.5 mg N g⁻¹ DW and 9.3 ± 0.2 mg P g⁻¹ DW. The lake population was estimated to 22.2 ± 2.6 × 10¹⁰ mussels, corresponding to a standing stock biomass of 362 ± 42 ton DW, or conservative estimates of 36.6 ± 4.3 ton N and 3.4 ± 0.4 ton P. Assuming a life span of 2–3 years gives a retention estimate of 1.2–1.8 ton P y⁻¹ by mussels, corresponding to 50–77% of the annual P influx from Uppsala sewage treatment plant to the lake. Similarly, annual N-retention by zebra mussels makes up 13–20 ton N y⁻¹, largely equaling the annual N-deposition from atmospheric sources on the lake’s surface. These retention rates correspond to only a few percent of the annual P-load from agricultural sources, but we argue that the quantitative role of zebra mussels in nutrient budgets is much larger if these budgets are adjusted for the bias introduced by coarse estimates of N and P pools that include a large share of refractory P.