Temperature-induced changes in the relevance of viral lysis and microzooplankton grazing of Antarctic phytoplankton indicates future alterations in seasonal carbon flow.

Phytoplankton play a pivotal role as the primary producers in polar marine ecosystems. Despite evidence suggesting that production rates and loss factors vary from year to year, and thus drive dynamic ecosystem functioning, interannual comparisons remain sparse. In this study, we examined viral lysis and microzooplankton grazing rates on Antarctic phytoplankton during two productive seasons and compared them with published data from a previous year. Higher rates of phytoplankton gross growth and total mortality during the warmer productive season suggest global warming induced increases in the magnitude of ecosystem carbon flow. Viral lysis rates appear to be relatively independent of average seasonal temperatures, whereas grazing rates were lower during the colder productive seasons (average temperature <0°C). This resulted in a greater relative impact of viral lysis on phytoplankton mortality, particularly pronounced during periods of phytoplankton accumulation. The interannual variations in phytoplankton fate are likely due to a stronger coupling between rates of viral infection and phytoplankton growth compared with grazing. Our results emphasize the importance of monitoring rates of viral lysis, specifically in combination with the size and taxonomy of the phytoplankton community. Collectively these factors determine the relative significance of the different carbon fates, and hence the ocean's efficacy as a carbon sink.