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Influence of the thermocline on the vertical migration of medusae during a 48 h sampling period

1992; Taylor & Francis; Volume: 27; Issue: 2 Linguagem: Africâner

10.1080/02541858.1992.11448262

0254-1858

F. Pagès, Josep María Gili,

Oil Spill Detection and Mitigation

AbstractThe vertical distribution and migratory movements of epiplanktonlc medusae were studied at an oceanic station off northern Namibia (18°00’S /10°30’E). Samples were collected from five separate depth strata (200–100 m, 100–60 m, 60–40 m, 40–20 m, and 20–0 m) using a multiple opening and closing RMT 1 x 6 net over a 48–h period. The area was characterized by mixing of the Angola Current surface waters with the northernmost subsurface waters of the Benguela Current. During the sampling period there was a continuous flow of water from Angola, giving rise to a strong thermocline between 20 and 40 m depth. A total of 17 species of medusae were captured, of which Aglaura hemistoma, Liriope tetraphylla, and Solmundella bitentaculata were the most abundant. Both the number of species and the number of individuals were higher on the second than on the first day of sampling, chiefly in the 20–0 stratum. Differences in abundance appeared to be related to the patch size of each species. The depth diistributlon of the medusae population was characterized by the existence of two assemblages, one associated with each current, and by the presence of the thermocline, which acted as a boundary between the two water masses and did not facilitate migratory movements of the most abundant species. The non-migratory distribution pattern of most of the species was attributable to the high concentration of potential prey items and to the absence of predators in the layer above the thermocline throughout the sampling period.Die vertikale verspreiding en migratoriese bewegings van epipianktoniese meduse is bestudeer by ’n oseaanstasie in Noord-Namibië (18°00’S /10°30’0). Monsters is versamei vanaf vyf verskillende dieptestrata (200–100 m, 100–60 m, 60–40 m, 40–20 m, and 20–0 m) deur gebruik te maak van ’n veelvuldige oop- en toemaak RMT 1 x 6 net oor ’n tydperk van 48 h. Die gebied is gekenmerk deur die vermenging van oppervlakwater van die Angolastroom met die mees noordeiike onderoppervlakwater van die Benguela- stroom. Gedurende die bemonsteringstydperk was daar ’n aanhoudende vloei van water vanaf Angola, wat tot die vorming van ’n sterk termoklien tussen 20 en 40 m diepte aanleiding gegee het. ’n Totaal van 17 medusae-spesies is gevang, waarvan Aglaura hemistoma, Liriope tetraphylla, en Solmundella bitentaculata die talrykste was. Die aantal spesies sowel as die aantal individue was hoer op die tweede as die eerste dag van monsterneming, hoofsaaklik in die 20–0 stratum. Verskille in die talrykheid het geblyk ’n verwantskap te toon met die koigrootte van elke spesie. Die diepteverspreiding van die medusaebevolking is gekermerkdeur die bestaan vam twee groeperings, een geassosieer met elke seestroom, en deur die teenwoordigheid van die termoklien wat as grens tussen die twee wateriiggame gedien het en nie die migratoriese bewegings van die mees talryke spesies vergemaklik het nie. Die nie-migratoriese verspreidingspatroon van die meeste van die spesies kon toegeskryf word aan die hoë konsentrasie van potensiële prooi-items en die afwesigheid van predatore in die laag bokant die termoklien tydens die hele bemonsteringsperiode.