Jean-Henri Hecq

Neustonic microplastic and zooplankton in the North Western Mediterranean Sea

Neustonic microplastic and zooplankton abundance was determined in the North Western Mediterranean Sea during a summer cruise between July 9th and August 6th 2010, with a break between July 22 th and 25th due to a strong wind event. Ninety percent of the 40 stations contained microplastic particles (size 0.3-5mm) of various compositions: e.g., filaments, polystyrene, thin plastic films. An average concentration of 0.116 particles/m(2) was observed. The highest abundances (>0.36 particles/m(2)) were observed in shelf stations. The neustonic plastic particles concentrations were 5 times higher before than after the strong wind event which increased the mixing and the vertical repartition of plastic particles in the upper layers of the water column. The values rise in the same order of magnitude than in the North Pacific Gyre. The average ratio between microplastics and mesozooplankton weights was 0.5 for the whole survey and might induce a potential confusion for zooplankton feeders.

Factors controlling the distribution of diatoms and Phaeocystis in the Ross Sea

Abstract The spatial and temporal distributions of phytoplankton pigments were investigated in the western and south central Ross Sea during austral spring 1994 and summer 1990. Large gradients in biomass and phytoplankton community composition were observed both in the east–west and south–north directions, in relation to differences in water column structure and stability, which themselves depend on the processes of ice retreat within the different areas. Important are melting in the western Ross Sea, which induces strong stratification, and ice breakup and wind stress in the south central Ross Sea, which result in deep mixing and weakly stratified waters. In the western Ross Sea, the highest chlorophyll a (chl. a) concentrations observed in this study (129–358 mg m−2 in the upper 100 m) were tightly coupled to the stratified region of meltwater influence and were dominated by diatoms, as indicated by elevated fucoxanthin concentrations (89–239 mg m−2 in the upper 100 m). In the diatom bloom area, high levels of phaeophorbides a (maximum value of 192 mg m−2 in the upper 100 m) indicated that the dominant grazers, identified as Limacina helecina and copepods [Hecq, J.H., Magazzu, G., Goffart, A., Catalano, G., Vanucci, S., Guglielmo, L., 1992. Distribution of planktonic components related to vertical structure of water masses in the Ross Sea and the Pacific sector of the Southern Ocean. In: Anonymous, Atti del 9° congresso A.I.O.L., Santa Margherita Ligure, 20–23 Novembre 1990, 665–678], transferred a sustained part of the diatom production to the herbivore trophic level. Synthesis of our data with published information suggests that the diatom bloom we observed in the western Ross Sea was dominated by the species Fragilariopsis curta, and occurs annually off the coast of Victoria Land from 72°30′S to 77°S within 100–250 km from the coastal ice edge. It is assumed to persist on the order of 2.5–3 months during summer. In the south central Ross Sea, which was characterized by a poorly or unstratified water column, moderate chl. a concentrations (55–186 mg m−2 in the upper 100 m) were found in the polynya and in the ice-edge area in early spring. The 19′-hexanoyloxyfucoxanthin (19′-HF), a biomarker for Phaeocystis, was the major contributor to the total carotenoid abundance (29–132 mg m−2 in the upper 100 m), corroborating the hypothesis that Phaeocystis are well adapted to develop in a relatively mixed water column. At the northern limit of the polynya, a quite narrow (ca. 30 km) diatom bloom (31–67 mg fucoxanthin m−2 in the upper 100 m) overlapped with the Phaeocystis bloom in the slightly stratified marginal ice zone. This was an area of increased grazing pressure by various types of herbivorous zooplankton, among which very large amounts of krill were observed.

Annual variation in neustonic micro- and meso-plastic particles and zooplankton in the Bay of Calvi (Mediterranean-Corsica)

The annual variation in neustonic plastic particles and zooplankton was studied in the Bay of Calvi (Corsica) between 30 August 2011 and 7 August 2012. Plastic particles were classified into three size classes, small microplastics (0.2-2mm), large microplastics (2-5mm) and mesoplastics (5-10mm). 74% of the 38 samples contained plastic particles of varying composition: e.g. filaments, polystyrene, thin plastic films. An average concentration of 6.2 particles/100 m(2) was observed. The highest abundance values (69 particles/100 m(2)) observed occurred during periods of low offshore wind conditions. These values rose in the same order of magnitude as in previous studies in the North Western Mediterranean. The relationships between the abundance values of the size classes between zooplankton and plastic particles were then examined. The ratio for the intermediate size class (2-5mm) reached 2.73. This would suggest a potential confusion for predators regarding planktonic prey of this size class.

A model of the seasonal dynamics of biomass and production of the seagrass Posidonia oceanica in the Bay of Calvi (Northwestern Mediterranean)

Modelling of seagrasses can be an effective tool to assess factors regulating their growth. Growth and production model of Posidonia oceanica, the dominant submerged aquatic macrophyte occurring in the Bay of Calvi (Corsica, Ligurian Sea, Northwestern (NW) Mediterranean) was developed. The state variables are the above- and below-ground biomass of P. oceanica, the epiphyte biomass, and the internal nitrogen concentration of the whole plant. Light intensity and water temperature are the forcing variables. The model reproduces successfully seasonal growth and production for each variable at various depths (10, 20 and 30 m). The model can simulate also a number of consecutive years. Sensitivity analysis of model’s parameters showed that the maximum nitrogen quota nmax rate is the most sensitive parameter in this model. The results simulations imply that light intensity is one of the most important abiotic factors, the diminution of which can cause an important reduction in seagrass density.

Molecular and pigment studies of the picophytoplankton in a region of the Southern Ocean (42–54°S, 141–144°E) in March 1998

Seven filtered seawater samples (depths between 30 and 55 m) collected during the SAZ project of the Austral summer of 1997–1998 were used for a simultaneous study of the picophytoplankton pigments based on high-performance liquid chromatography (HPLC) analyses and flow cytometry, and of the molecular diversity of the picophytoplankton based on their rDNA sequences. The sampling sites could be divided into three temperature zones, distinguished by their proximity to the Sub-Antarctic and Polar Fronts. HPLC analysis of total chlorophylls and carotenoids showed fairly low phytoplankton concentrations (77–262 ng chl a l � 1 ), with minimal values of the pigments in the two samples of the Polar Front Zone around 541S (water temperature of 41C at time of collection). In this zone, a similar decrease of particles, identified as cyanobacteria on the basis of their fluorescence, was observed by flow cytometry. Sequences very similar to the 16S rDNA sequence of Synechococcus WH8103 were present in all samples. This Synechococcus genotype is thus found in the Southern Ocean in addition to the Atlantic and Pacific locations where it has been previously observed. The yield of PCR products was lower in the two samples taken in the Polar Front Zone, showing a good agreement between molecular and pigment data. 16S rDNA sequences of plastids of eukaryotic algae also were retrieved, mostly related to those of an environmental clone called OM164, which has not been cultivated but has phylogenetic affinities to the Raphidophyceae. r 2002 Elsevier Science Ltd. All rights reserved.

Spatial patterns of phytoplanktonic pigments and primary production in a semi-enclosed periantarctic ecosystem: the Strait of Magellan

Abstract The spatial patterns of nitrate, phytoplankton size-fractionated chlorophyll a , photosynthetic pigments detected by HPLC and primary production were studied at the end of the austral summer (February–March 1991), in the Strait of Magellan to provide insight into the mechanisms governing this seemi-enclosed periantartic ecosystem. Climatological peculiarities, hydrodynamic constraints, runoff and land forcing were considered. The most important features identified for this area were the confinement of the microphytoplankton fraction to the external parts of the Strait and the rather uniform dimensional structure of the phytoplankton communities ( μ m) within the internal sectors. In particular, the nanoplanktonic fraction (10-2 μm) comprised 33%, while the picoplanktonic one (2-0.5 μm) represented 62% of the total. The concentration of active Chl. a and plant pigments detected by HPLC, nitrate and primary production showed an uneven but similar distribution, allowing for the characterization of different sectors within the Strait, according to different types and intensities of forcing factors. The different ecological sectors identified along the Strait were associated with distinct hydrographic typologies: an oligotrophic, wind-mixed coastal area, adjacent to the Pacific opening; a high runoff fjord and a divergence zone in the Andean sectors; a stratified inland sea in the basin-like part of the channel (Paso Ancho) and a shallow, tidally mixed system in the Patagonian sectors. Areas of relatively low and high phytoplankton biomass and primary production alternate along the Strait, according to the hydrographic structures of the different sectors. Phytoplankton biomass, in terms of active Chl. a (spectrofluorometric determination) ranged between 10 and 51 mg m −2 and primary production between 275 and 1170 mg C m −2 d −1 . The assemblage of plant pigments detected by HPLC indicated that the high levels of phytoplankton pigments and production were most likely attributable to Chlorophytes, Prasinophytes, Prymnesiophytes and/or Gyrodinium and Gymnodinium -like Dinoflagellates that were submitted to natural senescence and to superfluous feeding activity; while, when Diatoms, Cryptomonads and Prymnesiophytes were dominant, efficient zooplankton grazing occurred and characterized a balanced food chain. The spatial distribution of the parameters considered was related to hydrologic conditions indicating a Pacific water influence in the entire channel.

Shelf‐slope exchanges associated with a steep submarine canyon off Calvi (Corsica, NW Mediterranean Sea): A modeling approach

A three-dimensional, unsteady, nonlinear, high-resolution model is used to investigate the impact of the Calvi Canyon (NW Corsica) steep topography on the shelf-slope exchanges as well as on the circulation in the Calvi Bay in homogeneous winter and early spring conditions. A double σ coordinate system is considered in order to represent adequately the high depth gradients within the canyon. The studied region is under the influence of the West Corsica Current flowing northeastward along the NW Corsican coast (right-bounded flow). Model results show that the circulation in the Calvi Bay is determined by flow modifications in the canyon area. The mean horizontal flow is deviated southwestward upstream of the canyon to form an anticyclonic gyre in the western part of the Calvi Bay. Within the canyon the circulation is cyclonic leading to an offshore flow downstream of the canyon. Around the canyon rim, the cross-shelf currents become important, indicating that this region acts as a transition zone of high exchange between nearshore and offshore areas. Furthermore, the canyon topography generates high downwelling (upwelling) and downsloping (upsloping) velocities responsible for an intense vertical transport of material in the area. Numerical runs are performed for typical prevailing wind conditions. The wind is responsible for a drastic increase of cross-shore transports between the bay and the canyon area (3–4 times larger than in the no-wind case). SW winds induce a further enhancement of cross-shelf exchanges, whereas the effect of N-NE winds is to reduce exchange at the shelf break apart from the canyon head where an intense offshore flow occurs. Within the canyon, high vertical velocities are shown to be associated with high cyclonic vorticity which is enhanced (reduced) by the N-NE (SW) wind event. A comparison between model results and measured distributions of nitrate and chlorophyll a concentrations in the area shows the role played by this specific hydrodynamics as a strong constraint on the coastal pelagic ecosystem.

Influence of the residual circulation on the physico-chemical characteristics of water masses and the dynamics of ecosystems in the Belgian coastal zone

Abstract Classical models of the residual circulation in the North Sea predict a north-bound residual flow in the Southern Bight. A more refined model, taking into account the mesoscale Reynolds stress exerted in the mean on the residual flow by the non-linear interactions of mesoscale processes (tides, storm surges, etc.), shows on the contrary off the Belgian coast a south-bound coastal current in relation with a residual coastal gyre. Observations of the physico-chemical characteristics of coastal waters confirm the existence of the gyre. The dynamics of coastal ecosystems here are found to be determined by the gyre; successive stages of the pelagic food chain displaying a typical spatial distribution along the deflected plume of the Scheldt estuary around the gyre ‘outerlagoon’.

Water fluxes at an ocean margin in the presence of a submarine canyon

A 3-D, unsteady, nonlinear, high-resolution model is used to estimate shelf/slope exchanges through Calvi Canyon (NW Corsica, Mediterranean Sea) in various regimes of stratification and wind patterns. To evaluate the alongshore and cross-shore fluxes within the canyon area as well as the water exchanges between the canyon and Calvi Bay, volume transports are computed at the sides of two closed, interconnected boxes encompassing the canyon on the shelf and slope domains. Model results show that water transports between Calvi Bay and the open sea are determined by flow modifications in the canyon area. The mean horizontal flow deviates southwestward upstream of the canyon, generating an onshore transport in the western part of Calvi Bay. Within the canyon, the circulation is cyclonic and is responsible for an offshore transport downstream of the canyon and in the eastern part of the bay. The effect of stratification is shown to limit the vertical extent of the influence of canyon topography so that the alongshore flow above the canyon is quasi-undisturbed in strong stratified conditions, resulting in weak cross-shore exchange. Wind events are shown to be responsible for a strong increase of cross-shore transports between the bay and the canyon area.

One-dimensional modelling of the plankton ecosystem of the north-western Corsican coastal area in relation to meteorological constraints

In order to study the influence of wind mixing on the spring variability of the plankton production of the north western Corsican coastal area, a one-dimensional (1D), vertical, coupled hydrodynamic/biological model (ECOHYDROMV) is used. A hydrodynamic 1D model of the water column with a k–l turbulent closure is applied. The biological model comprises six state variables, representing the plankton ecosystem in the spring period: phytoplankton, copepods, nitrate, ammonium, particulate organic matter of phytoplanktonic origin and particulate organic matter of zooplanktonic origin. The system is influenced by turbulence (expressed by the vertical eddy diffusivity), temperature and irradiance. The model takes into account momentum and heat surface fluxes computed from meteorological data in order to simulate a typical spring atmospheric forcing for the considered area. Results show that primary production vertical structure is characterised by a subsurface maximum which deepens with time and is regulated by the opposite gradients of nitrate concentration and irradiance. Surface plankton productivity is mainly controlled by turbulent vertical transport of nutrients into the mixed layer. The short time scale variability of turbulent mixing generated by the wind appears to be responsible for the plurimodal shape of plankton blooms, observed in the considered area. Furthermore, the model is applied to the study of the spring evolution of the plankton communities off the bay of Calvi (Corsica) for the years 1986 and 1988. In order to initiate and validate the model, time series of hydrological, chemical and biological data have been used. The model reproduces accurately the spring evolution of the phytoplankton biomass measured in situ and illustrates that its strong variability in those years was in close relation to the variability of the wind intensity.