One-month study in the open NW Mediterranean Sea (DYNAPROC experiment,May 1995): overview of the hydrobiogeochemical structures and effects of wind events

Time-series observations were made over a one-month period (May 1995, DYNAPROC cruise) in the open northwestern Mediterranean at a fixed station where horizontal advection remained weak throughout the observational period. Changes in the dynamics of the pelagic ecosystem and influence of wind events were examined at time scales of a few hours to a few days. This paper gives a summary of the strategies and multidisciplinary observation methods of the cruise program. It describes in detail the physical and meteorological background and provides an overview of the chemical and biological features encountered over the course of the study. Effects of two different wind events were observed. The first wind event was associated with a low pressure system and an important wind stress curl. During this event, an upward advective shift of deeper layers, related to Ekman pumping, was superimposed upon the wind-induced mixing of the superficial layers. Despite the resulting enhanced availability of nitrate in the euphotic layer, phytoplankton biomass decreased drastically; the processes (vertical advection, primary production, grazing pressure, etc.) controlling this decrease are examined. In contrast, the second wind event, slightly lower and of longer duration, resulted in increased downward mixing and a higher phytoplankton biomass. However, it is shown that this transient increase would not have occurred without the previous influence of the first wind event. This study also allowed documentation of the transition from a mesotrophic to an oligotrophic system over a short time scale.