Virginie Pons

Autotrophic carbon assimilation and biomass from size-fractionated phytoplankton in the surface waters across the subtropical frontal zone (Indian Ocean)

Abstract The composition of the phytoplanktonic communities in the surface waters of the La Reunion-Kerguelen transect (from 38°36S to 46°33S) has been investigated under spring conditions (Antarès 3 cruise, France-JGOFS, 28 September–8 November, 1995). The study, conducted at six stations in the subtropical frontal zone, involved size fractionations (threshold: 2 μm). The large variations in the overall biomass and autotrophic carbon fixation, calculated via Rubisco activity measurements and expressed respectively in terms of μg chlorophyll (a + b + c) per liter and nmol fixed carbon dioxide per liter and per hour, were attributable only to phytoplanktonic cells of >2 μm, with a peak observed in the frontal zone. The picophytoplankton (<2 μm) biomass remained constant throughout the transect, but the evolution of the species composition of the picophytoplanktonic population, as calculated from flow cytometry measurements through this frontal zone, changed. This study provides evidence, for the first time in this area, of the disappearance of prochlorophytes from the south of the frontal zone (42–47°S). Picoeukaryotes (<2 μm) and cyanobacteria populations, resolved by flow cytometry, were present all along the transect. However, their abundance decreased southward up to the quasi-disappearance of cyanobacteria at the southernmost station (52°S) that is characteristic of antarctic waters. The presence of prochlorophytes that is exclusive to the subtropical surface waters, and the low carbon fixation activity associated with these waters, may be linked to the specific hydrological features encountered. In contrast, the marked reduction in the cyanobacteria and the abundance of picoeukaryotes along the north-south transect is more likely to be a result of the reduction in temperature through the frontal zone.

Influence of nitrogen enrichment on size-fractionated in vitro carboxylase activities of phytoplankton from Thau Lagoon (Coastal Mediterranean Lagoon, France)

The influence of dissolved inorganic and organic nitrogen (DIN and DON) enrichments on pools of enzymes responsible for CO2 fixation by the Calvin-Benson (Rubisco) and beta-carboxylation pathways (beta-carboxylases) were studied in a natural plankton assemblage. The plankton community from a coastal Mediterranean lagoon were incubated in situ for 24 h with initially ammonium, nitrate and DON (taurine) enrichments and compared to a control without any enrichment. An increase of small picophytoplankton and diatom biomass was observed in the enriched inorganic nitrogen treatments 7 - 10 h after the initial enrichment. Phytoplankton biomass decreased in the control and under taurine enrichment suggesting an inorganic nitrogen limitation of phytoplankton growth.

Range-Wide Variation in the Ecological Niche and Floral Polymorphism of the Western Mediterranean Geophyte Narcissus dubius Gouan

Premise of research. Comparative studies of variation in the ecology and genetics of natural plant populations located at the limits and in the center of a species range provide fundamental insights into the historical formation of species distribution patterns. Methodology. In this study, we quantify variation in the ecological niche and the expression of a floral polymorphism across the range of the Mediterranean geophyte Narcissus dubius Gouan. An exhaustive data set of known locations was compiled to distinguish groups of geographically central and peripheral populations in both Spain and France. These occur across a double climatic gradient. First, there is a gradient from a hot and dry Mediterranean climatic regime in eastern Spain to a milder Mediterranean climatic regime in southern France (lower maximum temperature and shorter summer drought). Second, there is a shift to a more continental climate in peripheral populations in inland Spain. We also modeled the current climatic niche and produced historical projections of potential glacial refugia for this species. Pivotal results. Peripheral populations showed consistent patterns of ecological niche differences with a less rupicolous ecology, more bare soil, and a higher cover of annual species. The ecological niche was more variable among peripheral populations than among central populations. Peripheral populations showed a repeated pattern of loss of stigma-height polymorphism and floral traits indicative of a reproductive strategy based on within-morph mating (probably selfing to assure seed set). Climate models indicate that contemporary peripheral populations (particularly in Spain) occur in areas that were least likely to have served as glacial refugia. Conclusions. Historical isolation and recolonization may have shaped contemporary patterns of ecological niche and reproductive trait variation among central and peripheral populations.

Ecological niche differentiation in peripheral populations: a comparative analysis of eleven Mediterranean plant species

The ‘central-peripheral’ hypothesis has provided a baseline for many studies of population dynamics and genetic variability at species distribution limits. Although peripheral populations are often assumed to occur in ecologically marginal conditions, little is known about whether they effectively occur in a distinct ecological niche. A cross-taxa analysis of 11 Mediterranean vascular plants were studied. We quantified variation in the ecological niche between populations at the northern range limits of species in Mediterranean France and those in the central part of the distribution in continental Spain or Italy in 2013–2014. We analyzed both the macroecological niche where populations occur in terms of broad habitat and altitudinal range and the micro-ecological niche where individual plants grow in terms of soil and structural biotic and abiotic characteristics. Most species occur in a single broad habitat type common to central and peripheral populations and have a narrower altitudinal range in the latter. In contrast, for the micro-ecological niche we detected marked variation in several niche parameters among central and peripheral populations. Although many differences are species-specific some are common to several species. We found a trend towards narrower microniche breadth in peripheral populations. Our results illustrate the importance of studying the precise ecological characteristics where plants grow and the pertinence of a multi-species approach to correctly assess niche variation. The ecological originality of peripheral populations underlines their evolutionary potential and conservation significance.