Phytoplankton Community Response to Multiple Stressors: a Trait-based Modeling Approach

This thesis based on trait-based modeling approach and long term mesocosms data is the first modeling study, which mechanistically and systematically investigated the impact of interplay of multiple stressors and phytoplankton physiological properties on shaping the community size structure. Using a trait-based modeling approach with assuming cell size as the master trait, this study illustrates (i) how uncertainties in allometric relationship of major growth-nutrient uptake traits influence the size structure of the community, (ii) how community composition responds to bottom up control via nutrient enrichment from deep nutrient rich water, (iii) how nutrient enrichment and removal of grazers as two concurrently occurring multiple stressors reshape the community structure and why/when additive/non-additive interaction types are observed, (iv) how elevated CO2 and zooplankton (by selective grazing) (in)directly influence the community. A central question then will be how phytoplankton community response to a combination of stressors, and how this response is governed by trade-offs between eco-physiological traits. The first chapter addresses importance of allometric relationship of major growth-nutrient uptake traits, which are introduced into a size-based multi-species nutrient-phytoplankton-detritus model to obtain a more realistic phytoplankton dynamic under periodic nutrient enrichment. Secondly, the previously developed model is extended to resolve multi-species phytoplankton-nutrients-detritus-zooplankton dynamics within the upper mixed layer. A novel approach is proposed here to correlated community net growth rate to sudden shifts in the environmental conditions and linking it with trait-mediated adaptive response helped to assess why and when non-additive interactions are observed. Finally, the extended model is adopted to asses the net impact of elevated CO2 and selective grazing on the phytoplankton community