Francesca Vidussi

Phytoplankton pigment distribution in relation to upper thermocline circulation in the eastern Mediterranean sea during winter

Using a sampling grid of 67 stations, the influence of basin-wide and subbasin-scale circulation features on phytoplankton community composition and primary and new productions was investigated in the eastern Mediterranean during winter. Taxonomic pigments were used as size class markers of phototroph groups (picophytoplankton, nanophytoplankton and microphytoplankton). Primary production rates were computed using a light photosynthesis model that makes use of the total chlorophyll a (Tchl a) concentration profile as an input variable. New production was estimated as the product of primary production by a pigment-based proxy of the ƒ ratio (new production/total production). For the whole eastern Mediterranean, Tchl a concentration was 20.4 mg m−2, and estimated primary and new production were 0.27 and 0.04 g C m−2 d−1, respectively, when integrated between the surface and the depth of the productive zone (1.5 times the euphotic layer). Nanophytoplankton and picophytoplankton (determined from the pigment-derived criteria) were the dominant size classes and contributed to 60 and 27%, respectively, of Tchl a, while microphytoplankton contributed only to 13%. Subbasin and, to a certain extent, mesoscale structures (cyclonic and anticyclonic gyres) were exceptions to this general trend. Anticyclonic gyres were characterized by low Tchl a concentrations (18.8±4.2 mg m−2, with the lowest value being 12.4 mg m−2) and the highest picophytoplankton contribution (40% of Tchl a). In contrast, cyclonic gyres contained the highest Tchl a concentration (40.3±15.3 mg m−2) with the highest microphytoplankton contribution (up to 26% of Tchl a). Observations conducted at a mesoscale in the Rhode gyre (cyclonic) region show that the core of the gyre is dominated by microphytoplankton (mainly diatoms), while adjacent areas are characterized by high chlorophyll concentration dominated by picophytoplankton and nanophytoplankton. We estimate that the Rhodes gyre is a zone of enhanced new production, which is 9 times higher than that in adjacent oligotrophic areas of the Levantine basin. Our results confirm the predominance of oligotrophic conditions in the eastern Mediterranean and emphasize the role of subbasin and mesoscale dynamics in driving phytoplankton biomass and composition and, finally, biogeochemical cycling in this area.

Phytoplankton pigment variations during the transition from spring bloom to oligotrophy in the northwestern Mediterranean sea

Phytoplankton taxonomic pigments and primary production were measured at the JGOFSFrance time-series station DYFAMED in the northwestern Mediterranean Sea during May 1995 to investigate changes in phytoplankton composition and the biogeochemical implications (DYNAPROC experiment). The study period covered the transitional situation from late spring bloom to pre-oligotrophic. The late spring bloom situation, occurring at the beginning of the study, revealed high chlorophyll a concentrations (maximum 3 mg m~3 at 30 m) and high primary production (maximum 497 mg C m~2 14 h~1). At the end of the experiment, the trophic regime shifted towards pre-oligotrophic and was characterized by lower chlorophyll a concentrations (( 1m g m~3), although primary production still remained high (659 mg C m~2 14 h~1). At termination of the spring bloom, the phytoplankton community was composed of chromophyte nano#agellates (38

Planktonic ciliates in the Mediterranean Sea: longitudinal trends

4%), diatoms (29

Diel vertical distribution of planktonic ciliates within the surface layer of the NW Mediterranean (May 1995)

2%), cryptophytes (12

Determination of chlorophylls and carotenoids of marine phytoplankton: separation of chlorophyll a from divinylchlorophyll a and zeaxanthin from lutein

1%) and cyanobacteria (8

Pigments at time series station DYNAPROC

1%). During the transition to the pre-oligotrophic period, the contribution of small cells increased (e.g. cyanobacteria 18

Particulate dimethylsulfoniopropionate at DYNAPROC station

2%, green #agellates 5

Pigments at station DYNAPROC_266

1%). Vertical proles of pigments revealed a partition of the phytoplankton groups: cyanobacteria were most abundant in the surface layer, nano#agellates containing 19@-HF#19@BF at the depth of chlorophyll maximum, whereas diatoms were located below the chlorophyll maximum. At termination of the spring bloom, a wind event induced vertical transport of nutrients into the euphotic layer. Phytoplankton groups responded di!erently to the event: initially, diatom concentrations increased (for 24 h) then rapidly decreased. In contrast, all others groups

Pigments at station DYNAPROC_125

Abstract We analysed samples taken through the euphotic zone from 18 stations between the Ligurian Sea (6°E) and the Levantin Basin (32°E) from 24 May to 25 June 1996. Both ciliate and chlorophyll concentrations ranged over a factor of about 7, but ciliate concentrations (0.4–2.8xa0mgxa0Cxa0m 3 ) varied irregularly compared to a longitudinal decline, west to east, in chlorophyll concentration (0.07–0.48xa0mgxa0m 3 ). The lower chlorophyll concentrations (0.1xa0mgxa0m 2 ) of the eastern basin stations corresponded with a relatively high stock of ciliates (0.5xa0mgxa0Cxa0m 2 ). Large mixotrophic ciliates were more abundant, in both absolute and relative terms, in the eastern Mediterranean stations with less chlorophyll. The species diversity of tintinnid ciliates appeared higher in the central and eastern basins compared to the west. Our results suggest a shift from the western to eastern Mediterranean in the planktonic food towards a microbially dominated system.

Pigments at station DYNAPROC_116

The composition and vertical distribution of planktonic ciliates within the surface layer was monitored over four diel cycles in May 95, during the JGOFS-France DYNAPROC cruise in the Ligurian Sea (NW Mediterranean). Ciliates were placed into size and trophic categories: micro- and nano-heterotrophic ciliates, mixotrophic ciliates, tintinnids and the autotrophic Mesodinium rubrum. Mixotrophic ciliates (micro and nano) represented an average of 46% of oligotrich abundance and 39% of oligotrich biomass; nano-ciliates (hetero and mixotrophic) were abundant, representing about 60 and 17% of oligotrich abundance and biomass, respectively. Tintinnid ciliates were a minor part of heterotrophic ciliates. The estimated contribution of mixotrophs to chlorophyll a concentration was modest, never exceeding 9% in discrete samples. Vertical proles of ciliates showed that chlorophyll-containing ciliates (mixotrophs and autotrophs) were mainly concentrated and remained at the chlorophyll a maximum depth. In contrast, among heterotrophic ciliates, a portion of the population appeared to migrate from 20}30 m depth during the day to the surface at night or in the early morning. Correlation analyses of ciliate groups and phytoplankton pigments showed a strong relationship between nano-ciliates and zeaxanthin, and between chlorophyll-containing ciliates and chlorophyll a ,a s well as other pigments that were maximal at the chlorophyll a maximum depth. Total surface layer concentrations showed minima of ciliates during nightime/early morning hours. ( 2000 Elsevier Science Ltd. All rights reserved.