Photoacclimation, production and critical depth: a comparison of phytoplankton dynamics in Lagrangian and Eulerian models

Abstract

Marine phytoplankton growth is controlled by non-linear processes, such as the photosynthetic and photoacclimative response to irradiance. Traditional Eulerian models calculate rates of primary production using the assumption that phytoplankton have identical properties, whereas Lagrangian models simulate phytoplankton as individual particles, tracking their trajectories through the light field. It might therefore be expected that photoacclimation in Lagrangian models would have an impact on seasonal cycles. In this thesis, I construct a Lagrangian ecosystem model, applying it to two questions: whether the individual responses of phytoplankton to their local irradiance affects the overall rates of primary production, and whether Lagrangian models are necessary for the study of the mechanisms surrounding the spring bloom, due to their representation of phytoplankton growth in response to mixing.
The study begins by addressing some of the fundamental assumptions underpinning Lagrangian models, and provides novel solutions for some of the difficulties. The model was set up for Ocean Weather Station India (OWSI) in the North Atlantic, and the predicted seasonal cycles of primary production were shown to not differ from those predicted by an Eulerian equivalent, due to the phytoplankton being mixed too fast to have time to acclimate to local irradiances. Additionally, the results suggested a closer relationship between the timescales of growth and mixing, demonstrating that vertical profiles of phytoplankton could form within a well-mixed layer, resulting in changes to the overall rates of primary production. The model was next used to investigate the controls of the spring bloom at OWSI, by investigating the critical depth, critical turbulence and disturbance-recovery hypotheses. Although the use of Lagrangian model did highlight a possible source of inaccuracy when calculating critical depth with an Eulerian model, overall an Eulerian model could have performed the experiments with the same results, given information about the vertical profile of phytoplankton in the mixed layer. However, the study successfully reconciled the three hypotheses, showing how each describes a mechanism that can affect the critical depth.

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