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Grazing dynamics of Euphausia spinifera in the region of the Subtropical Convergence and the Agulhas Front

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Abstract:

The feeding ecophysiology of the subtropical euphausiid Euphausia spinifera was investigated in the Indian sector of the Southern Ocean during January–February 1999. Gut pigment levels varied from 1.7 to 40.6 ng chlorophyll a (Chl a) equiv.·individual–1 in adults and from 0.3 to 9.3 ng Chl a equiv.·individual–1 in juveniles. Highest levels were observed at the Subtropical Convergence (Chl a concentrations 0.4–0.6 µg·L–1) and minima in the area north of the Agulhas Front (Chl a concentrations 0.2–0.3 µg·L–1). Gut evacuation rates ranged between 0.59 and 0.96·h–1. Gut pigment destruction levels were among the highest ever recorded in euphausiids, with 94.2–98.5% of total pigments ingested converted to nonfluorescing end products. Size-selectivity experiments with natural phytoplankton showed that E. spinifera ingests mainly cells in the 0.7- to 20-µm range. The grazing dynamics of this important species are compared with those of subantarctic (Euphausia vallentini) and Antarctic krill (Euphausia superba). Individual ingestion rates, estimated from the autotrophic component of its diet (2.78–4.46 µg Chl a equiv.·individual–1·day–1), were just sufficient to account for respiratory requirements. While E. spinifera is clearly able to ingest large amounts of heterotrophic prey, it is not known to what extent these contribute to its total energy budget.

Nous avons étudié l'écophysiologie de l'alimentation chez l'euphausiidé subtropical Euphausia spinifera dans le secteur indien de l'Océan Austral en janvier et février 1999. Les quantités de pigments dans le tube digestif variaient de 1,7 à 40,6 ng equiv.-chlorophylle a (Chl a)·individu–1 chez les adultes et de 0,3 à 9,3 ng equiv.-Chl a·individu–1 chez les jeunes. Les valeurs les plus élevées ont été observées à la Convergence subtropicale (concentrations de Chl a : 0,4–0,6 µg·L–1) et les plus basses dans la région au nord du Front des Agulhas (concentrations de Chl a : 0,2–0,3 µg·L–1). Les taux d'évacuation du tube digestif variaient de 0,59 à 0,96·h–1. Les taux de destruction des pigments dans le tube digestif étaient parmi les plus élevés jamais enregistrés chez les euphausiidés: 94,2–98,5% des pigments totaux ingérés se transformaient en produits terminaux non-fluorescents. Des études de sélectivité de la taille du phytoplancton naturel montrent qu'E. spinifera ingère surtout des cellules dans l'intervalle de tailles 0,7–20 µm. La dynamique de broutage de cette espèce importante a pu être comparée à celles du krill subantarctique (Euphausia vallentini) et du krill antarctique (Euphausia superba). Les taux d'ingestion d'un individu, estimés d'après la fraction autotrophe de son régime (2,78–4,46 µg equiv.-Chl a·individu–1·jour–1), suffisent tout juste à combler ses besoins respiratoires. Bien qu'E. spinifera soit capable d'ingérer de grandes quantités de proies hétérotrophes, on ne sait pas dans quelle mesure celles-ci contribuent à son bilan énergétique total.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: February 1, 2001

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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