Optimality-based approach for computationally efficient modeling of phytoplankton growth, chlorophyll-to-carbon, and nitrogen-to-carbon ratios

摘要

To increase the efficiency of computing phytoplankton growth rate (μ), chlorophyll-to-carbon (θ) and nitrogen-to-carbon ratios (QN) in three-dimensional ocean circulation models, it is preferable to directly calculateθandQNfrom ambient environmental factors instead of treating them as independent tracers. Optimality-based modeling has emerged as a novel and efficient approach to fulfill this task. However, it is still unclear precisely how the response of optimality-based models differs from conventional models. We compare a recent optimality-based phytoplankton model (PAHLOW model), based on which the familiar Droop function can be derived, to a commonly used Monod-type (MONOD) model. The two models generate similar patterns ofμwith some important differences. Compared to the MONOD model, the PAHLOW model predicts higherμunder light limitation. The PAHLOW model also predicts thatθdecreases with decreasing light under dim light and predicts decreasingQNwith increasing light even at constant nutrient levels. Compared to the MONOD model, these features of the PAHLOW model qualitatively agree better with laboratory data. The PAHLOW model also suffers from a few shortcomings including the underestimation ofθunder very low light and two times of computation time compared to the MONOD model. The two models generate striking differences ofQNandθin a one-dimensional implementation. Validation of such patterns will require more direct in situ measurements ofμ,θandQN.