South African Journal of Marine Science

ISSN: 0257-7615 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/tams19

Sea-bottom temperature, salinity and dissolved oxygen on the continental margin off southwestern Africa R. V. Dingle & G. Nelson To cite this article: R. V. Dingle & G. Nelson (1993) Sea-bottom temperature, salinity and dissolved oxygen on the continental margin off south-western Africa, South African Journal of Marine Science, 13:1, 33-49, DOI: 10.2989/025776193784287220 To link to this article: http://dx.doi.org/10.2989/025776193784287220

Published online: 08 Apr 2010.

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Date: 26 January 2017, At: 07:32

S. Afr. J. mar. Sci. 13: 33-49 1993

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SEA-BOTIOM TEMPERATURE, SALINITY AND DISSOLVED OXYGEN ON THE CONTINENTAL MARGIN OFF SOUTH-WESTERN AFRICA R. V. DINGLE"" and G. NELSONt From a historical data base of temperature, salinity and dissolved oxygen for the Benguela shelf between Cape Agulhas and Cape Frio, average bottom patterns for each of these parameters are presented along with statistical justifications. Some physical interpretation is given for various features and variances, involving the poleward undercurrent, wind and upwelling, continental shelf waves and internal tides at the shelf edge and, in the case of oxygen, sediment-exchange chemistry. Uit 'n historiese databasis van temperatuur, soutgehalte en opgeloste suurstof vir die vastelandsplat van die Benguela tussen Kaap Agulhas en Kaap Frio word gemiddelde bodempatrone vir elk van hierdie parameters aangebied tesame met statistiese regverdiging. Daar word gepoog om 'n fisiese vertolking te gee vir verskeie kenmerke en variansies aan die hand van die poolwaartse onderstroom, wind en opwelling, vastelandsplatgolwe en interne getye aan die rand van die plat en, in die geval van suurstof, die skeikunde van sedimentuitwisseling.

Numerous surveys have been undertaken on the oceanic and abnospheric circulation of the South-East Atlantic, and especially on upwelling and the associated phenomena of the surface Benguela system off southwestern Africa (Hart and Currie 1960, Stander 1964, Gordon and Bosley 1991, and the summaries of Chapman and Shannon 1985, Shannon 1985a, b, Payne et al. 1987, Shannon et al. 1990). In contrast, relatively few systematic studies have been made on the bottom waters in the area (De Decker 1970, Bailey et al. 1985). Finally, to the knowledge of the present authors, there have been no accounts published of the regional distribution of sea-floor parameters over the West Coast continental margin. During a study of microfossils in sea-floor sediment samples from the south-eastern Cape Basin (Dingle 1992, 1993, Fig. I), information on various bottomwater parameters was required for an assessment of depositional environments. Data were acquired from the South African Data Centre for Oceanography (SADCO) and the Sea Fisheries Research Institute (SFRI), and in the present paper an account is given of regional sea-bottom temperature, salinity and dissolved oxygen (Figs 2, 3 and 4) as well as some oceanographic implications for water-column structure and circulation. The data were assembled for a specific purpose, namely as an adjunct to a study of modem and subfossil microfaunas (benthic Ostracoda), and this placed some constraints on the presentations. Because the microfossils were collected by means of a Van Veen grab that penetrated a few centimetres below the sedimentwater interface, the fossil assemblages consisted of

mixtures of specimens that probably ranged in age from modem to late Holocene (c. 5 000 years BP). Consequently, there was no requirement to recognize seasonality (if it exists) in the oceanographic data; on the contrary, they should represent long-term means. The data available covered only the period from the late 1920s to the 1980s, and all relevant readings at any site were included in the arithmetic averages of observations within rectangles of one quarter of a degree of latitude and longitude. The data set was too sparse in many areas for seasonal trends to be identified in a given rectangle (see Fig. 9). Theoretically, the maps under discussion should show mean values representative of a period of over 60 years. The temperature and salinity maps were constructed from the data provided by SADCO. Initially, c. 30000 readings were supplied, but most of these were taken some distance above the sea floor and, for the production of the maps, only those meeting the criteria shown in Table I were retained. These criteria were determined by the structure of the shelf stratification. A bottom mixed layer is encountered everywhere on the shelf, and it seldom has a thickness less than 3% of the water column. Examples of the near-bottom temperature gradient at representative locations on the shelf are shown in Figure 5 (see section on Data Quality and Variance later). On the uppermost slope, however, a ubiquitous bell-shaped distortion of the isopycnals occurs, where the bottom mixed layer is appreciably thicker, amounting to 10% of the total water depth (e.g. Fig. 6). Over the rest of the continental slope and rise, the mixed layer was relatively thin, but the in situ temperature did not

•. South African Museum, P.O. Box 61, Cape Town 8000, South Africa Research Institute, Private Bag X2, Rogge Bay 8012 Manuscript received: January 1992

t Sea Fisheries

South African Journal of Marine Science 13

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Fig. 1:

Bathymetry of the south-eastern Cape Basin and the continental shelf off south-western Africa (after Dingle at aI. 1987). Traverse off Cape Peninsula shows the location of Figure 6

1993

Dingle & Nelson: Sea-Bottom Environmental Parameters off S.W Africa

1993 s

25°

Fig. 2:

Average sea-bottom temperatures over the continental margin off south-western Africa. The dotted line is the shelf edge, which varies in depth from 300 m in the south to 500 m south-west of LOderitz, to 400 m in the north (see Fig. 9a for data sites and Table II for assessment of variance)

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South African Journal of Marine Science 13

36 s

Fig. 3:

Average sea-bottom salinities over the continental margin off south-western Africa.lhe dotted line is the shelf edge (see Fi!;l. 9b for data sites, and Table II for assessment of vanance)

1993

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Dingle & Nelson: Sea-Bottom Environmental Parameters off S.W Africa

37

s

'25°

2d'E Fig. 4:

Average sea-bottom dissolved oxygen (m!' ,-1) over the continental margin off south-westem Africa. The dotted line is the shelf edge (see Fig. 9c for data sites and Table II for assessment of variance)

South African Journal of Marine Science 13

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8

12

16

100

100

50

200

(d)

0.006

8

12

100

('cm-1

_(19")

4

48121620

20

100

-

1993

Zero

100 (e)

(40")

It

Fig. 5:

E

Temperature profiles at representative locations on the oontinental shelf, showing the temperature gradient near the bottom and the percentage of water oolumn over which it applies (all sections are from SFRI demersal cruise 088 in January 1991)

decrease rapidly within a 3% bottom boundary layer. Therefore, in I 000 m of water, temperatures 30 m off the sea floor were adequately representative of bottom temperatures. The map of dissolved oxygen values is a hybrid, combining the data in De Decker's (1970) study (undertaken over two years) for the area north of 28°S and 2 000 SFRI records (a data set for the years 19711991) for the area south of 28°S. The latter data were Table I: Criteria for data selection at various depths Water depth (m)

Location above bottom

0-599,9 600-999,9 I 000-1 999.9 >2000

3% or less of depth