James T. Rogala

Filtration and Excretion by Zebra Mussels: Implications for Water Quality Impacts in Lake Pepin, Upper Mississippi River.

ABSTRACT Filtration and soluble nutrient excretion were examined over a range of zebra mussel (Dreissena polymorpha) shell lengths (range = 6 to 32 mm) in experimental laboratory systems and combined with in situ shell length frequency distribution and areal estimates of zebra mussel population density to make predictions of overall areal filtration and soluble nutrient excretion rates in Lake Pepin, upper Mississippi River, USA. Zebra mussels removed seston and excreted ammonia and soluble phosphorus in laboratory systems. When normalized with respect to ash-free dry mass (i.e., μg g−1 AFD mass d−1), smaller zebra mussels filtered seston and excreted soluble nutrients at a higher rate than larger zebra mussels. Although overall zebra mussel density is currently very low in Lake Pepin (~150 ind. M−2), lakewide chlorophyll filtration rates of ~ 7 mg m−2 d−1 were equivalent to chlorophyll loading into Lake Pepin via external sources and represented a turnover of 11 days for chlorophyll standing crop in the lake. Lakewide estimates of excretion of soluble phosphorus of ~ 3 mg m−2 d−1 by zebra mussels in Lake Pepin were comparable to internal phosphorus loading from anoxic sediments in other eutrophic freshwater aquatic systems. Our results suggest that zebra mussels are currently having a modest impact on phytoplankton dynamics and P and N recycling in this system.

Addressing among-group variation in covariate effects using multilevel models

Multilevel models are used to model processes associated with hierarchical data structures. Despite infrequent use in the biological and environmental sciences, the use of these models with hierarchically-structured data conveys multiple advantages. These include the assessment of whether covariate effects differ among groups or clusters, and separate estimation of covariate effects by hierarchical level (thereby addressing atomistic and aggregation fallacy concerns). We illustrate these advantages using larval mayfly count data derived from annual surveys on the Mississippi River and a continuous covariate (water depth).