Evidence of upwelling on Rockall Bank

Deep-Sea Research, 1971, Vol. 18, pp. 261 to 268. Pergamon Press. Printed in Great Britain.

SHORTER CONTRIBUTION Evidence of upwelling on Rockall Bank J. H. STEELE,* I. E. BAIRD* and R. JOHNSTON*

(Received 26 March 1970; in revised form 25 June 1970; accepted 17 July 1970) Abstract--Nitrate and chlorophyll data are used to provide evidence of upwelling on Rockall Bank. THE EXISTENCEof a distinct mass of water on Rockall Bank has been apparent from hydrographic sections across this area which show water of lower salinity, and usually lower temperature, on the Bank compared with the near-surface waters on the eastern side (for example, N~mEN, 1913; TULLOCH and TArt, 1959; Fig. 5 of this paper). Zooplankton distributions can also show great differences in species composition on and off the Bank (S~TON, 1968) and the existence of an eddy on the Bank has been proposed to explain this. The water on the Bank, however, is very similar in character to that at depths of 300-600 m in the channel between Rockall and Scotland. Thus two mechanisms have been proposed to account for the water on the Bank. NANSEN(1913) suggested that this water is formed in winter by increased cooling and increased dilution by rain water in the relatively shallow mixed layer. This water then flows off the shelf in spring and summer. The alternative explanation (KNuvS~N, 1911) is that the divergence produced by the Bank in the general northerly flow in this area may induce a vertical component on the edges of the bank, causing upwelling. The purpose of this note is to indicate that the latter process can occur, although it is not necessarily the sole explanation. If upwelling takes place then the water brought to the surface should have, in addition to lower salinity, certain chemical and biological properties typical of deeper water; in particular high nutrient and low phytoplankton concentrations. Such upwelling is likely to be patchy ( ~ O N O et al., 1967; STr.ICgI~ND et al., 1969) so that continuous sampling of near-surface conditions from a ship underway is the most suitable method of investigation. In May, 1968 two surveys of the Bank were made by the F.R.S. Explorer (Fig. 1). Water pumped from 3 m in depth was used to measure nitrate and in rive pigment fluorescence continuously by the methods described by AltMSTgONOet al. (1967). The calibration of the latter measurements against discrete samples for chlorophyll a showed considerable variability, comparable to that described by STRICIO.~ND 0968). These fluctuations in pigment readings would obscure small changes in chlorophyll a but in this case the range of values was sufficiently large to indicate the areas of very low chlorophyll a. Surface temperature was recorded on a thermograph and salinity was sampled at frequent intervals from the pumped supply. The results from the two surveys are shown in Figs. 2 and 3. Both surveys took about a week and so cannot be considered as synoptic. The effect of the time lapse is seen in the chlorophyll data from the second survey (Fig. 3) where there were significant differences in chlorophyll along a line at the northeast corner of the Bank which was worked at the start and end of this survey. The main feature of the first survey is the area around 57° 30'N 13° 30'W where nitrate is high and chlorophyll is extremely low. This combination suggests water which has recently upwelled. Neither temperature nor salinity are able to indicate this feature precisely, although the temperature is generally lower over the Bank. Three short sections (Fig. 4), were worked from the centre of this area eastwards off the shelf and indicate not only the occurrence but also the complexity of the penetration of lower salinity to the surface layers. The second survey also showed an area of relatively high nitrate although this was less in absolute value than the maximum on the first survey. This area is definitely to the southwest of the previous *Marine Laboratory, Aberdeen, Scotland. 261

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maximum and again corresponds to an area of low chlorophyll although the minimum values, 0"2/~g/l. were not as low as in the first survey. These values could be regarded as the consequence of upwelling some days before the survey. Three sections were worked during this survey (Fig. 5), and all show the general upward displacement compatible with upwelling. The combination of high nitrate and low chlorophyll could be attributed also to a low rate of primary production but there is no evidence to support this: rather the reverse, since the thermocline is shallower in this region than over deep water and the zooplankton biomass is generally higher (ADAMS; personal communication). It appears probable that any northerly flow past Rockall Bank may have associated with it a vertical motion which will draw deep water to the surface layers on the Bank, particularly on the steeper eastern side. Water of o~ = 27-40-27.50, which generally occurs below 300 m off the Bank, is found just below the upper mixed layer (i.e. from 50 m downwards) on the Bank. Surface salinity is not a good index of this possible upwelling since it tends to decrease irregularly from east to west. For this reason vertical sections extending westwards from the Bank do not show so clearly those features that could indicate upwelling, as sections east of the Bank. Thus low salinity water on the Bank could be due to lateral rather than vertical intrusions. The lower temperatures on the Bank might also be due to lateral exchange with waters to the north. The occurrence of high nitrate coupled with low chlorophyll would seem the best index of upwelling. The two surveys suggest that this can occur at different positions on the Bank, at different times, being most intense in relatively small patches. The classic pattern associated with upwelling is that resulting phytoplankton blooms will occur some time later and some distance away from the original site of upwelling. There is some suggestion that the highest chlorophylls are found near the edges of the Bank. It is possible that this production, either because it is generally greater than that in deep water, or at least greater in occasional patches, may favour the production of populations of crustaceans which are one of the defining characters of the Bank zooplankton. REFERENCES ARMSTRONG F. A. J., C. R. STEARNSand J. D. H. STRICKLAND 0967) The measurement of upwelling and subsequent biological processes by means of the Technicon Autoanalyser and associated equipment. Deep-Sea Res., 14, 381-389. KNUDSEN M. (1911) Danish hydrographic investigations of the Faroe Islands in the spring of 1910. Meddr Kommn Havundors., Hydrogr., 2 (1), 17 pp. NANSEN F. (1913) The waters of the north-eastern North Atlantic. Int. Rev. ges. Hydrobiol. Hydrogr., 4 (Suppl.), 139 pp. SEATON D. D. (1968) Rockall Bank; May, June and September 1967. Annls biol., Copenh., 24 (1967), 86-90. STRICKLAND J. O. H. 0968) Continuous measurement of in vivo chlorophyll; a precautionary note. Deep-Sea Res., 15, 225-227. STRICKLAND J. D. H., R. W. EPPLEY and BLANCA ROJAS DE MENDIOLA (1969) Phytoplankton populations, nutrients and photosynthesis in Peruvian coastal waters. Boln Inst. Mar Pdru-Callao, 2 (l), 37-45. TULLOCH D. S. and J. B. TAIT (1959) Hydrography of the north-western approaches to the British Isles. Mar. Res. Scot. 1959, No. l, 32 pp.