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To examine the potential impact of invasive dreissenid mussels on in situ populations of phytoplankton and nutrients in western Lake Erie, we combined mussel population estimates from a 2018 survey, results from mussel excretion, grazing, and in situ growth experiments, along with nutrient measurements on collected lake water. We calculated the proportion of the water column filtered per day, based on both clearance rates from grazing experiments and mussel biomass. In most cases the water column was filtered less than once per day. Based on mussel densities from nearby survey sites, we found that mussels could be expected to clear less than 5% of phytoplankton from the water column each day. We combined measurements of nitrogen and phosphorus excretion by mussels with survey densities and found that concentrations of nitrogen and phosphorus from excretion were much less than the ambient inorganic nitrogen and phosphorus measured throughout the season. Despite the modest potential impact that we measured, spatial variability in mussel density and temporal variability in nutrients and seston suggest that more substantial impact likely occurs in some conditions. Lastly, we used a mass balance approach to compare flows of nitrogen and phosphorus attributable to mussel assimilation, growth, and excretion. The proportion of assimilated nitrogen (0.01-0.21) and phosphorus (0.007-0.08) due to growth changed markedly throughout the season, but the excretion rate sometimes exceeded the apparent assimilation rate. These differences in growth:assimilation suggest changes in food quantity or quality, fluctuations in growth rates over time, or other physiological effects can lead to short-term imbalance in nutrient cycling by mussels, which could lead to locally important impacts on phytoplankton and algal blooms. Moreover, this work underscores the importance of mapping mussel densities at fine spatial scales and across interannual variation. [ABSTRACT FROM AUTHOR]

We measured Quagga Mussel grazing-induced changes in seston concentration in 20-L laboratory mesocosms containing lake water, during May-October, as part of a study to investigate the present role of mussel feeding and nutrient excretion in affecting phytoplankton composition. A variety of measuring methodologies including size-fractionated chlorophyll (< 2, 2-20, and >20 µm), phytoplankton groups by FluoroProbe, cyanobacterial genera by 16S rRNA gene sequencing, and particulate C, N, and P gave us different insights into seasonal phytoplankton dynamics and mussel selective feeding and assimilation. High clearance rates and/or high assimilation rates were seen during late May-early July across all size categories, with high rates seen particularly in cryptophytes and diatoms as revealed by FluoroProbe. Starting in late July cyanophytes dominated the phytoplankton and feeding rate on them was generally zero or low. Sequencing analyses suggested that there was a diverse cyanobacterial community present, with Microcystis, contrary to expectation, dominating only in late June. Cyanobium was the dominant genera at most times during summer-fall, and as expected, Microcystis was rejected by mussels relative to other cyanobacteria. Planktothrix, dominant in spring, was readily ingested as well as Anabaena in late June. We show that a combination of methods is helpful to make progress in understanding plankton succession and grazing. These methods were important adjuncts to microscopic analysis (Carrick et al, this issue). Our results support the hypothesis that dreissenid mussels, when abundant, can affect seasonal succession of phytoplankton shifting composition to cyanobacteria and even changes within the cyanobacterial community; however, impacts are likely modest now due low mussel biomass (Carter et al., this issue). Assimilation of C, N, and P was generally high, which is important for mussel population maintenance. [ABSTRACT FROM AUTHOR]