Balance of power: Dissolved nitrogen-to-phosphorus ratios and phytoplankton growth rate determine the balance between bottom-up and top-down processes in planktonic food webs

Abstract

Phytoplankton are globally responsible for ~50% of the global oxygen production via primary production, fuelling food webs, and can alter biogeochemical cycles. Grazing forms a massive loss factor of phytoplankton standing stocks. Since it can be challenging to measure variation in these relatively tiny organisms, most studies on planktonic predator-prey interactions overlook variation within groups and populations. It has recently become evident that neither prey populations nor predator populations can be viewed as homogeneous entities. Numerous physiological and behavioural differences can influence how predator-prey interactions act out in both phytoplankton and herbivorous zooplankton. Community organization may be influenced by variation in nutrient stoichiometry, cell quotas, and nutritional requirements and, particularly intraspecific (within-population) variation. I therefore concentrated on nutritional stoichiometry as the changeable trait among populations since the processes driving variation in this trait within populations are different in primary and secondary producers, which can result in mismatch phenomena. A series of grazing dilution experiments using field samples, a growth medium literature review, chemostat experiments, grazing experiments and trait-based approaches were used within this dissertation to investigate on different organisational levels the role of dissolved nitrogen-to-phosphorus ratios and phytoplanktonic growth on the balance between bottom-up and top-down processes in planktonic food webs. The dilution experiments indicated that zooplankton protists (i) actively select between and within phytoplankton and bacterioplankton prey populations, (ii) shift their grazing pressure depending on their nutritional requirements, as both prey items are plastic in their composition, and (iii) play roles in termination of spring phytoplankton and bacterioplankton blooms. The chemostat experiment showed changing stoichiometry as well as lower intercellular trait variability in the faster-growing isogenic phytoplankton populations. As there are multiple ways to grow slowly for phytoplankton, but only one way to grow fast, the recommendation for medium optimisation is a Redfield ratio-correction of the nitrogen-to-phosphorus ratio in future use growth media. The grazing experiments demonstrated that the secondary consumers get impacted by fluctuations in dissolved nitrogen-to-phosphorus ratios (resource quality) and growth rates of their prey. This dissertation provides ultimately more insights in biogeochemical cycling and planktonic trophodynamics with fine-tuning implications for food web models.