The seasonal dynamics of nutrient and chlorophyll a concentrations on the SE Mediterranean shelf-slope

Barak Herut, Ahuva Almogi-Labin, Natasja Jannink, Isaac Gertman

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Abstract

We report the results of a sequence of twelve research cruises over a two-year period, from June 1996 through May 1998 that examined the seasonal variations in the nutrient (NO3/-, PO4/3-, and Si(OH)4) and in the chlorophyll a (Chl a) concentrations at two permanent stations across the SE Mediterranean continental shelf and slope (120 and 400 m water depth off the Israeli coast). Seasonally-dependent Chl a concentrations ranged between 0.003 and 0.415 mg m-3. Following autum and winter a distinct biomass phytoplankton peak was produced (∼ 30 mg Chl a m-2 in the upper 120 m) and a subsequent moderate spring peak was observed. In the decomposition zone (120-400 m), an opposite pattern was observed with decreasing amounts of NO3/- and PO4/3- during winter. The integrated Si(OH)4 pattern in the euphotic zone indicates that diatoms grow during spring and summer and that their frustules are rapidly settled during the breakdown of stratification. At depths between 150 and 400 m, the geometric mean NO3/-/PO4/3- ratio (∼ 24) was significantly higher than in the upper 150 m (∼ 7), where it varied seasonally with low ratios (3, 5) during autumn and winter compared to summer and spring (9, 11). At the present time, it is not possible to reconcile the relatively low (< 16) NO3/-/PO4/3- ratios and apparent N dependence in the euphotic zone with the high (> 20) ratios of exogenous upwelling and atmospheric inputs. The determination of the possible presence of additional nutrient phases and preferential uptake of nutrients by the phytoplankton is an important next step toward the understanding of nutrient limitation in such ultra-oligotrophic systems.

Original languageEnglish
Pages (from-to)771-782
Number of pages12
JournalOceanologica Acta
Volume23
Issue number7 SUPPL.
DOIs
StatePublished - 2000
Externally publishedYes

Bibliographical note

Funding Information:
The authors would like to express their gratitude to L. Izraelov, Y. Gertner, E. Shoam-Frider and E. Shefer for their technical assistance in the sample collection and analyses, and to the crew of the R/V Shikmona . This project was supported by the Geo- and Life Sciences Foundation with financial aid from the Netherlands Organisation for Scientific Research, project-number 750-19-610A and by the Israeli Ministry of Infrastructure.

Funding

The authors would like to express their gratitude to L. Izraelov, Y. Gertner, E. Shoam-Frider and E. Shefer for their technical assistance in the sample collection and analyses, and to the crew of the R/V Shikmona . This project was supported by the Geo- and Life Sciences Foundation with financial aid from the Netherlands Organisation for Scientific Research, project-number 750-19-610A and by the Israeli Ministry of Infrastructure.

FundersFunder number
Geo- and Life Sciences Foundation
Israeli Ministry of Infrastructure
Nederlandse Organisatie voor Wetenschappelijk Onderzoek750-19-610A

    Keywords

    • Chlorophyll a
    • Mediterranean
    • Nutrients
    • Oligothropy

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