The influence of viruses on phytoplankton and bacterial productivity in Hamilton Harbour, an impaired embayment of Lake Ontario

This study investigated the ecological effects of viruses on the microbial community in Hamilton Harbour, a Lake Ontario embayment. A modified Landry-Hassett style dilution assay was used to assess the relative impact of microzooplankton grazers and viruses on phytoplankton community and population growth and mortality in August 2013. Although viral effects were not observed in this experiment, significant grazing mortality was detected in all instances. Similar dilution methodology was then applied in conjunction with standard bacterial and size-fractionated primary productivity assays (using 3H-leucine and Na14CO3, respectively). The first iteration of this experiment (August 2013) involved the use of a single dilution treatment with grazer-and-virus-free filtrate, and led to the observation that the combined activity of grazers and viruses appeared to stimulate productivity for phytoplankton < 2 μm and bacteria. The approach was then modified to discriminate between grazer and viral effects by incorporating an additional treatment involving grazer-free filtrate. Neither grazer nor viral regulation of algal or bacterial productivity was observed in this second iteration of the experiment (November 2013). However, in a third iteration of the experiment (November 2014), productivity was measured before, and at the end of a prolonged environmental incubation period. Viral infection/lysis appeared to limit productivity for phytoplankton 2-20 μm (and possibly < 2 μm) in size, while productivity for phytoplankton > 20 μm may have first been limited by viral infection/lysis and then stimulated by grazer dynamics. Our results provide the first evidence that viruses can actively affect microbial productivity in the Harbour.