On Tuesday 8 December 2020, Tristan Biggs will defend his PhD thesis
|Date||8 December 2020|
The main aim of this thesis was to better understand the seasonal dynamics of polar phytoplankton in relation to viruses and zooplankton predators. Polar oceans are especially sensitive to global warming and play a pivotal role in global climate regulation as they are an important sink for atmospheric CO2.
Phytoplankton form the base of most marine food webs, however, in cold polar waters the influence of seasonal variations in phytoplankton growth-controlling factors still requires a better understanding. Moreover, the ecological importance of viral lysis is vastly understudied, despite it impacting the energy and carbon flow very differently than grazing. Viral lysis results in a more regenerative food web as it diverts matter and energy away from higher trophic levels.
To improve insight in the combined influence of bottom-up and top-down factors on polar phytoplankton populations, I investigated (A) how environmental factors structured polar phytoplankton communities and to what extent changing light and temperature affect phytoplankton net growth and polar host-virus interactions, (B) the importance of viral lysis compared to grazing over 2 seasonal cycles, and (C) how phytoplankton dynamics influences key copepod grazers.
The multi-trophic level approach of this thesis provided the following new insights: (i) viral lysis is a major loss factor for all taxonomic groups of Antarctic phytoplankton and is critically important for its seasonal mass balance, (ii) ice type can influence the size class of ice-associated diatoms, (iii) the efficiency of acclimation to low light intensities influences bloom phenology and magnitude, (iv) temperature is not only a regulator of phytoplankton and zooplankton growth rates but also of viral infectivity and production, and (v) the combined effects influence copepod lipid content, dormancy behaviour, lifecycle and annual reproductive success.
These findings not only highlight the importance of taking a multifaceted approach but indicate major implications for ecosystem productivity and the biological carbon pump under global climate change.