The Influence of Physical Factors on a Subsurface Chlorophyll Maximum in an Upwelling Area

Several fine-scale vertical profiles of current, conductivity, temperature and chlorophyll a fluorescence were taken during a 24-h period from a ship following a freely floating drogue at the depth of a pronounced subsurface chlorophyll maximum (SCM) in the Gulf of Arauco, a highly productive coastal embayment on the Chilean coast. A depth varying turbulent mixing coefficient Kz was calculated as a function of the gradient Richardson number for each pair of measurements, i.e. representative for each ʹtime stepʹ of several hours. This permitted calculation of the mass divergence d as a function of depth from the equation of conservation of mass. For each time step a hindcast of the phytoplankton biomass Q was made (as a function of depth) using a conservation equation for Q with the same Kz and d as if Q were a passive tracer. Comparing the hindcasted and measured Q at the end of each time step showed that the temporal changes due to the physical processes mixing and divergence are often at least of the same order of magnitude as the changes brought about by biology. The effect of the differential sinking of individual plankters was tested with a density-dependent sinking speed included in the conservation equation for Q, but was found to have negligible influence for such short time-scales. If the difference between measured and hindcasted Q at the end of each time step is interpreted as changes due to phytoplankton production-destruction the result from all the time steps taken together implies that there is a maximum of net production of phytoplankton biomass at the depth of the SCM. It is suggested that the time variation of the vertical distribution of high-frequency variance (spikiness) of fluorescence is due to vertically migrating zooplankton feeding on the phytoplankton in and around the SCM.