A study of time-dependent primary productivity in a natural upper- ocean mixed layer using a biophysical model

Abstract

A one-dimensional biophysical model of time-dependent photosynthesis in the upper-ocean mixed layer was applied near 36°N, 74.5°W. The photosynthesis submodel was formulated and parameterized based on daylong, light-manipulation experiments performed on two phytoplankton communities collected prior to dawn on October 13, 1992: one from 5 m depth in the upper mixed layer and the other from 28 m depth below the pycnocline. Time course biological measurements included chlorophyll a concentration, primary productivity and DCMU fluorescence ratio. The biophysical model was then used to predict the physical and biological response of the water column on October 14, 1992. A pre-dawn conductivity-temperature-depth-fluorescence- transmission-rosette (CTDFT-rosette) cast provided data to initialize the water column stratification and the distribution of phytoplankton biomass. The biophysical model was forced using meteorological and oceanographic measurements collected continuously throughout the subsequent daylight period. CTDFT-rosette casts and measurements of chlorophyll a concentration, primary productivity and DCMU fluorescence ratio throughout that daylight period provided the data for comparison with the model predictions. In general, the biophysical model predicted the physical and biological evolution of the sampled water column. Although the inclusion of vertical mixing initially improved the accuracy of prediction, agreement decreased with time, especially in the lower part of the water column. The one- dimensional model suffered from the effects of excluded horizontal gradients.