Bruno Zakardjian

Seasonal versus synoptic variability in planktonic production in a high‐latitude marginal sea: The Gulf of St. Lawrence (Canada)

The Gulf of St. Lawrence (Canada) is a subarctic marginal sea characterized by highly variable hydrodynamic conditions that generate a spatial heterogeneity in the marine production. A better understanding of physical-biological linkages is needed to improve our ability to evaluate the effects of climate variability and change on the gulfs planktonic production. We develop a three-dimensional (3-D) eddy permitting resolution physical-biological coupled model of plankton dynamics in the Gulf of St. Lawrence. The planktonic ecosystem model accounts for the competition between simplified herbivorous and microbial food webs that characterize bloom and post-bloom conditions, respectively, as generally observed in temperate and subarctic coastal waters. It is driven by a fully prognostic 3-D sea ice-ocean model with realistic tidal, atmospheric, and hydrological forcing. The simulation shows a consistent seasonal primary production cycle, and highlights the importance of local sea ice dynamics for the timing of the vernal bloom and the strong influence of the mesoscale circulation on planktonic production patterns at subregional scales.

Biological and chemical signs of upward motions in permanent geostrophic fronts of the western Mediterranean

Upward motions are often invoked to explain the high productivity of permanent geostrophic fronts in the Western Mediterranean while physical evidence of such upward advections is seldom reported. The goal of this study is to define biological and chemical criteria, which can be used to localize such upward motions zones. We use a one-dimensional, time-dependent model of phytoplankton dynamics to test the effects of upward advection on the vertical distribution of phytoplankton biomass, nutrients, and dissolved oxygen. Simulations also include the effects of advective motions of the phytoplankton cells in the light field on phytoplankton growth. In conformance with the continuity equation, boundary conditions were defined to allow horizontal flow of the upwelled water within the upper mixed layer. Low upward advections (≤3 m d -1 ) led to a shallowing and sharpening of the nitracline, oxycline. and deep maxima of phytoplankton biomass and oxygen and to an increase in phytoplankton biomass. By confining the phytoplankton-nutrient system in the surface mixed layer, higher upward advections lead to homogeneous phytoplankton biomass and oxygen vertical distributions in the upper mixed layer, the nitracline and the oxycline being then at the top of the pycnocline. Data collected during the Prolig 2 cruise (May 1985) on the heavy side of the Liguro-Provencal front are interpreted as an illustration of these numerical results. Computed primary production rates are compared with measurements conducted in the Almeria-Oran front during the Almofront 1 cruise (April 1991) in a similar situation. In both fronts, upward advections of 1-2 m d -1 would be sufficient to account for the observed vertical distributions and the increased primary production. Ecological implications for the phytoplankton-nutrient system are discussed, particularly the spatial uncoupling of phytoplankton biomass and primary production in permanent geostrophic fronts.

A numerical study of primary production related to vertical turbulent diffusion with special reference to vertical motions of the phytoplankton cells in nutrient and light fields

Abstract Assuming stationary physical processes, in particular the light field and turbulent activity [K(z)], we described steady-state and convergent solutions obtained from a simple time-dependent vertical model of phytoplankton dynamics. Simulations included vertical turbulent motions experienced by the cells in the light and nutrient fields. Parallel simulations made with a classical formulation of phytoplankton growth, i.e., neglecting vertical turbulent motions, are discussed. From two typical situations of stratification in the Western Mediterranean, we identified two distinct systems of new production, as the consequence of Low (LTR) and High Turbulent Regime (HTR) in the photic zone respectively. Data from the Prolig-II (1985) and Almofront-I (1991) cruises supported the LTR system of new production. The results of the second part of the Mediprod-I (1969) cruise show several patterns that specifically appeared in the HTR simulation. Regenerated production was not influenced by the turbulent activity situation. In natural conditions, regenerated production depends on the specific phytoplankton-grazers system that develops according to the level of new production; such ecological dynamics were not considered in our model. Differences with the reference model changed the relationships between the vertical distributions of biomass and new production. Particularly, the HTR simulation led to distinct vertical distribution of biomass and new production. Such a pattern did not occur with the reference model. Although the vertical turbulent motions affected both the level and vertical distribution of new production, a significant effect on the depth-integrated production finally depends on how the phytoplankton biomass interacts with its environmental conditions. It is shown that the minimum of K(z) in the euphotic zone determined the system of new production, whereas its values below the euphotic zone scaled the production and biomass levels. The two distinct systems of new production, LTR and HTR, may be diagnosed from simple cast measurement by examining the relationships between the distributions of parameters implicated in new production (biomass maximum, nitracline and oxycline) and the density profile.

Spatial and temporal variability of ice algal production in a 3D ice–ocean model of the Hudson Bay, Hudson Strait and Foxe Basin system

Primary production, the basic component of the food web and a sink for dissolved inorganic carbon, is a major unknown in Arctic seas, particularly ice algal production, for which detailed and comprehensive studies are often limited in space and time. We present here a simple ice alga model and its coupling with a regional 3D ice–ocean model of the Hudson Bay system (HBS), including Hudson Strait and Foxe Basin, as a first attempt to estimate ice algal production and its potential contribution to the pelagic ecosystem on a regional scale. The ice algal growth rate is forced by sub-ice light and nutrient availability, whereas grazing and ice melt control biomass loss from the underside of the ice. The simulation shows the primary role of sea-ice dynamics on the distribution and production of ice algae with a high spatio-temporal variability in response to the great variability of ice conditions in different parts of the HBS. In addition to favourable light and nutrient conditions, there must be a sufficient time lag between the onset of sufficient light and ice melt to ensure significant ice algal production. This suggests that, in the context of enhanced warming in Arctic and sub-Arctic regions, earlier melt could be more damaging for ice algal production than later freezing. The model also includes a particulate organic matter (POM) variable, fed by ice melting losses to the water column, and shows a large redistribution of the POM produced by the ice ecosystem on a regional scale.

Exploiting coastal altimetry to improve the surface circulation scheme over the central Mediterranean Sea

This work is the first study exploiting along track altimetry data to observe and monitor coastal ocean features over the transition area between the western and eastern Mediterranean Basins. The relative performances of both the AVISO and the X-TRACK research regional altimetric data sets are compared using in situ observations. Both products are cross validated with tide gauge records. The altimeter-derived geostrophic velocities are also compared with observations from a moored Acoustic Doppler Current Profiler. Results indicate the good potential of satellite altimetry to retrieve dynamic features over the area. However, X-TRACK shows a more homogenous data coverage than AVISO, with longer time series in the 50 km coastal band. The seasonal evolution of the surface circulation is therefore analyzed by conjointly using X-TRACK data and remotely sensed sea surface temperature observations. This combined data set clearly depicts different current regimes and bifurcations, which allows us to propose a new seasonal circulation scheme for the central Mediterranean. The analysis shows variations of the path and temporal behavior of the main circulation features: the Atlantic Tunisian Current, the Atlantic Ionian Stream, the Atlantic Libyan Current, and the Sidra Gyre. The resulting bifurcating veins of these currents are also discussed, and a new current branch is observed for the first time.

Inter-annual variations of surface currents and transports in the Sicily Channel derived from coastal altimetry

Twenty-year coastal altimetry data set (X-TRACK) is used, for the first time, to gain insight into the long-term inter-annual variations of the surface circulation in the Sicily Channel. Firstly, a spectral along with a time/space diagram analysis are applied to the monthly means of the X-TRACK geostrophic currents over the period 1993-2013. They reveal a regionally coherent current patterns from track to track with a marked inter-annual variability that is unequally shared between the Atlantic Tunisian Current and Atlantic Ionian Stream inflows in the Sicily Channel and the Bifurcation Tyrrhenian Current outflow northeast of Sicily. Secondly, an empirical altimetry-based transport-like technique is proposed to quantify volume budgets inside the closed boxes formed by the crossing of the altimetry tracks and coastlines over the study area. A set of hydrographic measurements is used to validate the method. The inferred altimetry transports give a well-balanced mean eastward Atlantic Waters baroclinic flow of 0.4 Sv and standard deviations of 0.2 Sv on a yearly basis throughout the Sicily Channel and toward the Ionian Sea, which is fairly coherent with those found in the literature. Furthermore, the analysis allows to quantify the intrusions of Atlantic Waters over the Tunisian Shelf (0.12 +/- 0.1 Sv) and highlights two main modes of variability of the main surface waters path over the Sicily Channel through the Bifurcation Atlantic Tunisian Current and Atlantic Ionian Stream systems. Some physical mechanisms are finally discussed with regards to changes in the observed currents and transports.