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

Abstract

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.

Introduction

The vertical flux of particulate organic matter displays a high degree of variability over small time scales of a few days (Fowler et al., 1987; Taylor, 1989; Miquel et al., 1995); short pulses of enhanced sedimentation lasting only 48 hours were, for example, recorded by Miquel et al. (1995). In fact, the downward flux of matter depends on biological, chemical and physical processes (e.g. primary production, grazing of zooplankton, particle aggregation, internal waves, wind events) that vary on short time scales, of 1 h to a few days. Diurnal processes such as vertical migration of zooplankton could play a determining role in the downward flux of fecal pellets and marine snow or in the redistribution of nitrogen throughout the nutricline (Angel, 1989; Longhurst and Harrison, 1989; Lampitt et al., 1993). On the other hand, wind-induced vertical mixing is one example of episodic variability. Several studies have provided evidence of increased availability of nitrate and enhanced phytoplankton production and biomass following such transient events (Walsh et al., 1978; Hitchcock et al., 1987; Eppley and Renger, 1988; Marra et al., 1990; Nielsen and Kiørboe, 1991). Wind events have also been reported to give rise to short-lived blooms of diatoms (Hitchcock et al., 1987; Marra et al., 1990; Nielsen and Kiørboe, 1991), shifts in the zooplankton community structure and dynamics (Haury et al., 1990; Nielsen and Kiørboe, 1991; Kiørboe, 1993), coagulation of dissolved organic matter (Kepkay and Johnson, 1988; Kepkay, 1991), and enhanced vertical biogenic fluxes (Taylor, 1989). In oceanic regimes, transient increases in phytoplankton biomass account for an important fraction of the annual primary production (Platt et al., 1989) and therefore of the organic export to the deep sea.

There are few data sets that address short-term changes in biological variables and effects of wind events in the open ocean. Most of these observations are available for nearshore environments (e.g. Walsh et al., 1978; Eppley and Renger, 1988; Nielsen and Kiørboe, 1991) or in areas occasionally subjected to horizontal advection or mesoscale events (for example, Ortner et al., 1984; Dickey et al., 1991). In the framework of the JGOFS-France program, there is a time-series station located offshore that has been occupied monthly for several years (DYFAMED station). We organized the DYNAPROC cruise (May 1995) in the vicinity of this station. This cruise was devoted to the study of the DYNAmics of the rapid PROCesses that control the vertical flux of matter at time scales of a few hours to a few days. Often, part of the complexity in the interpretation of time series is due to the likely occurrence of horizontal advection. The DYNAPROC study was therefore conducted at a fixed station in the open central zone of the Ligurian Sea (Northwestern Mediterranean), where currents are weak. Throughout the cruise, this was verified by several types of observations indicating that horizontal advection was negligible in the vicinity of the time-series station.

The three specific objectives of the DYNAPROC cruise were (i) to estimate the variation of phytoplankton biomass and related physical and chemical parameters at hourly and daily time scales, (ii) to characterize the autotrophic and heterotrophic processes that can influence the vertical flux of matter on a day/night scale, (iii) to assess the influence of wind events on the dynamics of the ecosystem. The fulfilment of these objectives will provide a scheme of the pertinent relationships between the pelagic ecosystem dynamics and the flux of matter on short time scales, and a multidisciplinary data set to parameterize, constrain and validate process models and one-dimensional physical-biological models. The model of Klein and Coste (1984) simulates how wind-stress amplitude and short term variability affect the nutrient transport into the mixed layer. To accurately test such a model, data had to be acquired during a post bloom period and in conditions of weak stratification and a shallow nitracline; the month of May was therefore chosen as the observational period. On the other hand, nitrate injections to the euphotic zone and subsequent increases in phytoplankton biomass are transient events, lasting a few days, while longer time lags can occur in the response of the zooplankton community; measurements were therefore performed over a one-month period.

In this paper, after presenting the strategies and multidisciplinary observation methods of the DYNAPROC cruise, we describe the main physical, chemical and biological features encountered throughout the one-month observational period. These features provide the necessary background for the interpretation of the specific results, such as those detailed in the five following papers in this issue. We also give in the present paper an overview of the effects of wind events and of the trophic transition of the system.