Viruses are ubiquitous in aquatic ecosystems and contribute to nutrient cycling through lysis of phytoplankton and bacteria.  The role of viral lysis in carbon cycling is well established in marine microbial food webs, but less is known for other elements or about viruses in freshwater ecosystems. Phosphorus (P) is usually the limiting nutrient in freshwater ecosystems and viral lysis could be important in releasing P; this may be especially significant in oligotrophic ecosystems, but this has not been examined. We posed the question: Does viral abundance and type differ between freshwater ecosystems of differing trophic status? To address this question, we compared the viral communities in oligotrophic Lake Michigan and a eutrophic urban pond. Surface water samples from nearshore Lake Michigan (<100 m from shore and <10 m deep) offshore Lake Michigan (>2 km from shore and >10 m deep) and the urban pond (4047 m² and 1.8 m deep) were characterized using epifluorescense and transmission electron microscopy (TEM).

Results/Conclusions

The average viral abundance in samples was 1.26x10⁶ and 4.68x10⁵ viruses mL^-1 for nearshore and offshore Lake Michigan, respectively, while bacterial abundances per mL were 4.67x10⁴ (nearshore) and 2.23x10⁴ (offshore). In the pond, the average viral and bacterial abundance was higher than in Lake Michigan (5.31x10⁶ viruses mL^-1 and 7.82x10⁵ bacteria mL^-1) suggesting that bacterial and viral abundance are linked to trophic status. Under TEM, viruses included those with single icosahedral heads, single round heads, and round heads with tails, but the dominant viral forms (35 – 63%) had icosahedral head and a tail, suggesting that phages are the numerically dominant virus in all three ecosystems. The abundance and morphological diversity of virus types in these two ecosystems suggest that viral lysis of bacteria and phytoplankton could contribute to P-cycling. Cyanophages and/or bacteriophages may be especially important in P-cycling due to the prevalence of this viral form. Quantifying P release due to viral lysis is necessary to clarify the ecological role of viruses in phosphorus cycling in freshwater ecosystems of differing trophic status.