DIATOM AND SILICOFLAGELLATE FLUXES AT THE WALVIS RIDGE - AN ENVIRONMENT INFLUENCED BY COASTAL UPWELLING IN THE BENGUELA SYSTEM

Citation
Uf. Treppke et al., DIATOM AND SILICOFLAGELLATE FLUXES AT THE WALVIS RIDGE - AN ENVIRONMENT INFLUENCED BY COASTAL UPWELLING IN THE BENGUELA SYSTEM, Journal of marine research, 54(5), 1996, pp. 991-1016
Citations number
75
Categorie Soggetti
Oceanografhy
Journal title
ISSN journal
00222402
Volume
54
Issue
5
Year of publication
1996
Pages
991 - 1016
Database
ISI
SICI code
0022-2402(1996)54:5<991:DASFAT>2.0.ZU;2-C
Abstract
Seasonal fluctuations in the total particle, biogenic opal, diatom and silicoflagellate fluxes were observed in sediment traps deployed at 5 99 m and 1648 m in the Walvis Ridge area, within the Benguela upwellin g system, from March 1989 to March 1990 (station WR 2: 20 degrees 02.8 'S, 09 degrees 09.3'E). Fluxes were directly related to wind stress va riations (wind maxima preceding flux maxima by several weeks), and inv ersely related to SST changes (derived from measured concentrations of C-37 alkenones; range: 14.6 degrees-23.6 degrees C). The biogenic par ticle composition at different depths reflected the complicated hydrol ogy of the area with a combination of tropical, temperate and subantar ctic water masses. Biogenic opal content varied from about 2 to 12% of the total mass flux in the upper trap and from about 4 to 17% in the lower trap. Diatoms were the main contributor to the opal fraction (me an daily flux of ca. 5.5 a 10(6) valves m(-2) day(-1)), followed by si licoflagellates (ca. 2.6 10(5) skeletons m(-2) day(-1)). Two seasona l maxima, in May and June (austral autumn) and from October to Novembe r (austral spring), were observed; silicoflagellates yielded also a th ird moderate maximum in August/September (austral winter). At 1648 m f luxes peaked from May to July (data were available for the period 18 M ar to 27 Aug 1989 only). Few diatoms were abundant; 19 taxa accounted for 50% of all the diatoms identified, and about 32 for the 75% level. Specific diversity of diatoms at 599 m was highest during times of lo west fluxes, in the austral winter, late spring and summer. The diatom taxa occurring at 599 m and at 1648 m were the same, with some flux e nrichment with depth due to advection of particles into the lower trap by resuspension and downslope movement. The relatively high concentra tions of the Antarctic-Subantarctic species Fragilariopsis kerguelensi s in the upper trap solely, was probably linked to selective entrainme nt and transport within a ring of southerly origin (south of the Suban tarctic/Subtropical Convergence Zone). The enrichment of moderately re sistant and robust taxa in the sediments in conjunction with the rarit y or absence of delicate taxa points to preferential concentration in the sediments of some taxa and dissolution of others. The occurrence o f phytoliths in the traps and in the sediment sample can be linked to the ''berg'' winds, which are typical for the entire Benguela region d uring fall and winter.