Model investigations of the North Atlantic spring bloom initiation

- Spring bloom onset cannot be explained as a purely bottom-up or top-down process.
- Conceptual bases of critical depth and dilution-recoupling hypotheses are true under realistic conditions.
- Mixed layer fluctuations can shift bloom timing by modifying nutrient and light availability.
- Changes in zooplankton concentrations produce only negligible shifts in bloom timing.

The spring bloom - a massive growth of phytoplankton that occurs annually during the spring season in mid and high latitudes - plays an important role in carbon export to the deep ocean. The onset of this event has been explained from bottom-up and top-down perspectives, exemplified by the “critical-depth” and the “dilution-recoupling” hypotheses, respectively. Both approaches differ in their key expectations about how seasonal fluctuations of the mixed layer affect the plankton community. Here we assess whether the assumptions inherent to these hypotheses are met inside a typical onedimensional Nutrient-Phytoplankton-Zooplankton-Detritus (NPZD) model, optimized to best represent climatological annual cycles of satellite-based phytoplankton biomass in the Subpolar North Atlantic. The optimized model is used in idealized experiments that isolate the effects of mixed layer fluctuations and zooplankton grazing, in order to elucidate their significance. We analyzed the model sensitivity qualitatively and using a second-order Taylor series decomposition of the model equations. Our results show that the conceptual bases of both bottom-up and top-down approaches are required to explain the process of blooming; however, neither of their bloom initiation mechanisms fully applies in the experiments. We find that a spring bloom can develop in the absence of mixed layer fluctuations, and both its magnitude and timing seem to strongly depend on nutrient and light availability. Furthermore, although zooplankton populations modulate the phytoplankton concentrations throughout the year, directly prescribed and physically driven changes in zooplankton grazing do not produce significant time shifts in bloom initiation, as hypothesized. While recognizing its limitations, our study emphasizes the processes that require further testing in order to discern among competing hypotheses.