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| Titel |
Plankton in the open Mediterranean Sea: a review |
| VerfasserIn |
I. Siokou-Frangou, U. Christaki, M. G. Mazzocchi, M. Montresor, M. Ribera d'Alcalá, D. Vaqué, A. Zingone |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 7, no. 5 ; Nr. 7, no. 5 (2010-05-18), S.1543-1586 |
| Datensatznummer |
250004770
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| Publikation (Nr.) |
 copernicus.org/bg-7-1543-2010.pdf |
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| Zusammenfassung |
| We present an overview of the plankton studies conducted during the last 25
years in the epipelagic offshore waters of the Mediterranean Sea. This
quasi-enclosed sea is characterized by a rich and complex physical dynamics
with distinctive traits, especially in regard to the thermohaline
circulation. Recent investigations have basically confirmed the
long-recognised oligotrophic nature of this sea, which increases along both
the west-east and the north-south directions. Nutrient availability is low,
especially for phosphorous (N:P up to 60), though this limitation may be
buffered by inputs from highly populated coasts and from the atmosphere.
Phytoplankton biomass, as chl a, generally displays low values (less than
0.2 μg chl a l−1) over large areas, with a modest late
winter increase. A large bloom (up to 3 μg l−1) is observed
throughout the late winter and spring exclusively in the NW area. Relatively
high biomass values are recorded in fronts and cyclonic gyres. A deep
chlorophyll maximum is a permanent feature for the whole basin, except during
the late winter mixing. It is found at increasingly greater depths ranging
from 30 m in the Alboran Sea to 120 m in the easternmost Levantine basin.
Primary production reveals a west-east decreasing trend and ranges
between 59 and 150 g C m−2 y−1 (in situ measurements).
Overall, the basin is largely dominated by small autotrophs,
microheterotrophs and egg-carrying copepod species. The microorganisms
(phytoplankton, viruses, bacteria, flagellates and ciliates) and zooplankton
components reveal a considerable diversity and variability over spatial and
temporal scales, although the latter is poorly studied. Examples are the wide
diversity of dinoflagellates and coccolithophores, the multifarious role of
diatoms or picoeukaryotes, and the distinct seasonal or spatial patterns of
the species-rich copepod genera or families which dominate the basin. Major
dissimilarities between western and eastern basins have been highlighted in
species composition of phytoplankton and mesozooplankton, but also in the
heterotrophic microbial components and in their relationships. Superimposed
to these longitudinal differences, a pronounced biological heterogeneity is
also observed in areas hosting deep convection, fronts, cyclonic and
anti-cyclonic gyres or eddies. In such areas, the intermittent nutrient
enrichment promotes a switching between a small-sized microbial community and
diatom-dominated populations. A classical food web readily substitutes the
microbial food web in these cases. These switches, likely occurring within
a continuum of trophic pathways, may greatly increase the flux towards higher
trophic levels, in spite of the apparent heterotrophy. Basically, the
microbial system seems to be both bottom-up and top-down controlled. A
"multivorous web" is shown by the great variety of feeding modes and
preferences and by the significant and simultaneous grazing impact on
phytoplankton and ciliates by mesozooplankton. |
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