Diurnal variation and primary production in the ocean preliminary results of a Lagrangian ensemble model

Abstract

The diurnal variation of insolation influences the rate of primary production in the upper ocean in two ways. Firstly, light energy is only available for photosynthesis during the day, and secondly solar heating of the upper ocean during the day changes the depth of the surface mixed layer. The consequences of these physical changes have been investigated using a one-dimensional model in which the vertical motion and energy uptake of each member of an ensemble of phytoplankters is calculated deterministically on 3 min time steps. The depth change and energy uptake of a single plankter, and the depth distribution and total energy uptake of the ensemble of multiplying cells are described. Plankton become rapidly concentrated into a band centred just below the maximum depth of mixing in the previous 24 h. This provides a physical explanation for the depth of the maximum in measured chlorophyll profiles. The seasonal variation of production rate has been calculated, assuming no grazing, nutrient limitation or self shading. Sverdrup's classic explanation for the onset of the spring plankton bloom is re-interpreted, taking account of the diurnal cycle of insolation. It is argued that the Lagrangian-ensemble model of the growth of an ensemble of phytoplankters described in this paper is inherently better than Eulerian-continuum models, because it averages after integrating nonlinear equations, rather than vice-versa . The model can easily by extended to investigate the consequences of physiological hypotheses not considered in this paper.