Rapp. P.-v. Réun. Cons. int. Explor. Mer, 180: 184-201. 1982.
A comparison of the nutrient regimes off Northwest Africa, Peru, and Baja California1
L. A. Codispoti2, R. C. Dugdale3, and H. J. Minas4
Although all are eastern boundary regions in which upwelling produces high surface nutrient concentrations, a comparison of Northwest Africa with Peru and Baja California reveals considerable differences in their nutrient regimes. These differen ces may be summarized as follows: 1. The upwelling source waters off Baja California and Peru are generally higher in nutrients and poorer in oxygen than those off Northwest Africa, largely as a result of the global scale circulation. 2. The relationship between the local winds and the nutrient fields appears to be weaker off Peru than off Northwest Africa. 3. The year-to-year variability is much greater off Peru than off Northwest Africa. 4. Dinoflagellate blooms that can cause significant decreases in the specific uptake rates of nutrients may be more important off Peru and Baja California than off Northwest Africa. 5. Denitrification and (sometimes) hydrogen sulfide production occur between about 75 and 400 m off Peru. 6. The highest rates of nitrate uptake have been measured off Peru, but ammonia uptake in the three regions seems to be about the same. 7. The nutrient regeneration regime off Peru (at least near 15°S) may differ conside rably from that off Northwest Africa (at least near 21°40'N), because of factors such as a tendency towards an offshore bottom flow over the Peruvian shelf and high turbidities in the inshore zone off Northwest Africa. Les régions côtières du nord-ouest de l'Afrique, du Pérou et de la Basse Californie, dans lesquelles les remontées d’eau provoquent de fortes concentrations superficiel les de sels nutritifs, sont toutes les trois situées sur la bordure orientale des grands systèmes océaniques; toutefois, l’étude comparative de ces régions révèle des diffé rences considérables dans la circulation des sels nutritifs, leur assimilation et leur régénération. 1. Les eaux alimentant les résurgences côtières de la Basse Californie et du Pérou sont en général plus riches en sels nutritifs et plus pauvres en oxygène que celles des côtes NW-africaines; ceci est essentiellement dû à la circulation océanique générale. 2. Les relations entre les vents locaux et les remontées d’eau apparaissent moins évidentes au Pérou que sur les côtes NW-africaines. 3. La variabilité inter-annuelle devant les côtes du Pérou est plus grande que devant le littoral NW-africain. 4. Les poussées de Dinoflagellés qui peuvent être à l’origine d’une baisse du taux d’assimilation spécifique des éléments nutritifs, sont probablement plus importan tes au large du Pérou et de la Basse Californie que devant les côtes NW-africaines. 5. Une dénitrification et (parfois) une production d'hydrogène sulfuré se produi sent à des profondeurs entre 75 et 400 mètres au Pérou. 6. Les taux d'assimilation de nitrates les plus élevés ont été mesurés dans les eaux péruviennes, alors que l’assimilation de l’azote ammoniacal est à peu près la même dans les trois régions. 7. Le régime de régénération des sels nutritifs au Pérou (tout au moins vers 15°S) semble se différencier nettement de celui des côtes NW-africaines (tout au moins vers 21°40‘N) en raison de facteurs tels que l’existence, au Pérou, d’un courant de fond au-dessus du plateau continental et dirigé vers le large et la forte turbidité dans certaines eaux littorales NW-africaines.
1 Contribution 81015, Bigelow Laboratory for Ocean Sci ences. 2 Bigelow Laboratory for Ocean Sciences, McKown Point, West Boothbay Harbor, Maine 04575, USA. 3 Department of Biological Sciences, Allen Hancock Found ation, University of Southern California, Los Angeles, California 90007, USA. 4 Station Marine d’Endoume, Laboratoire d’Océanographie, Centre Universitaire de Luminy, 13288 Marseille Cedex 9, France. 184 30' 3 0 ' 30' 3 0 ’ lfm
30‘ 20 ' 182 9m 54 9m
40* • CABO BARBAS
30*
AFRICA - 20°
30' 20 '
CABO COR BE IRO
LAT 2 1®42.2'N ▲ LONG 18° 09.2'W ATLANTIS-H UW JOINT-I
Y X ¥• FF LEG I LEG n 30' LEG HT — 30' LEG 12 OSU MOORINGS
f
KILOMETERS 20
NAUTICAL MILES
LAT 21° 02' N .«-▼LONG 18° I9.5W
00' 0 0 ’
NOU ADHIBOU
ICAP BLANC
30'
Figure 161. Station locations for RV “Atlantis-11" cruise 82 (JOINT-I), March-May 1974, and the Oregon State University (OSU) current meter array locations.
185 3 0 ' 3 0 '
30' 3 0 ’
lOOfm
50 fm
C, NAZCA
C3, i BAH IA 'C4 >AN JUAN C5 cr.
'CIO
3 0 ‘ •C 12 3 0 '
3 0 ' 3 0 '
Figure 162. Average station locations for the main hydrographic line (the “C”-line) during the March-June 1976 portion of the JOINT-II experiment off Peru.
Introduction comparison may also provide insights that are not eas The nutrient regimes off Northwest Africa, Peru, and ily achieved by studying the regions separately. Baja California share some basic similarities. For The differences arise from a number of causes, vary example, upwelling produces high surface nutrient con ing in scale from the nature of the exchange between centrations in these locales (e.g., Strickland et al., the oceans to local differences in turbidity. We shall 1969; Dugdale and Goering, 1970; Jones, 1972; Walsh emphasize the contrasts between some results taken et al., 1974; Fraga and Manriquez, 1975), thereby satis near Cape Corveiro during the JOINT-I experiment fying one of the requirements for high primary produc (Fig. 161; Barton et al., 1975; Friebertshauser et al., tion rates. These areas have relatively well-developed 1975; Codispoti et al., 1976a) and some data collected oxygen minimum zones (e.g., Richards, 1957; Reid, off Peru at ~ 15°S during the first phase of the JOINT- 1965), indicating that their subsurface waters have II experiment (Fig. 162; Codispoti et al., 1976b; Bar been generously supplied with nutrients regenerated at ton, 1977; Friebertshauser et al., 1977; Friederich et depth; and they are significantly affected by subsurface al., 1977; Robles and Barton, 1977). These areas were countercurrents that contain water with distinct chemi chosen because the JOINT-I and JOINT-II experi cal characteristics (e.g., Wooster et al., 1965; Codispoti ments provided very comprehensive suites of meso- and Richards, 1976; Codispoti and Friederich, 1978). scale and local data. However, this choice will con Despite the general conformity outlined above, even found our inter-ocean comparison somewhat because a cursory examination of conditions off Northwest the regions have different morphologies. For example, Africa, Peru, and Baja California reveals considerable the JOINT-II region has a narrow shelf (Fig. 162) and differences, and these differences prompt us to make a frequently has a three-layered cross-shelf flow (Brink comparative study of the three nutrient regimes. The et al., 1978; J.C. Van Leer, personal communication) necessity for keeping the differences in mind when while the JOINT-I region has a broader shelf and a using the knowledge gained in one region as a guide for two-layered cross-shelf flow (Fig. 161; Barton et al., studying another provides a sufficient rationale, but a 1975; Mittelstaedt et al., 1975; Halpern et al., 1977). It
186 45' 30' 45' 30' 30' 30'
SAN PABLO PT.
o MESCAL- I • MESCAL- n
•SAN HI POL I TO PT. 27° 00'
ABREOJOS'/ 45' Ï.-..PT r
5 0 fm
30' 30' 1 0 0 fm
45' 30' I 14° 00' 45' 30'
Figure 163. Location chart for the MESCAL experiments off Baja California. MESCAL-I took place during March 1972 and MESCAL.-II during April 1973.
Methods is our belief that we can separate some of the differ ences that are due to the local morphology from the Much of the data discussed here was collected by inter-ocean effects and thereby make a reasonable routine methods that require little or no explanation. comparison. More importantly, the morphological dif However, a number of ammonia and nitrate uptake ferences may shed some light on differences due to measurements have also been included, and the intra-ocean variations. We shall also consider some methodology and rationale behind these techniques are data taken off Baja California (Fig. 163) during the worthy of some discussion. MESCAL-I and MESCAL-II experiments (Walsh et Primary production associated with nitrate uptake al., 1974; Walsh et al., 1977; Whitledge and Bishop, may be termed new production; ammonium is pro 1972 and 1973), and when nitrogen uptake is consi duced by regenerating processes; and the associated dered we shall supplement our JOINT-I and JOINT-II primary production is sometimes called regenerated data with additional results from the same locales. production (Dugdale and Goering, 1967). Productivity measurements made with 14C include both regenerated and new production and therefore give no information
187 about the proportion of the primary production that Berger, 1970) that this inter-ocean fractionation arises may be available as yield from the system under steady- largely because the Atlantic is “lagoonal” (deep-water state conditions. Measurements of new and regener outflow) and the Indo-Pacific is “estuarine” (deep- ated production using the 15N method with labeled water inflow). In order for Indo-Pacific surface waters nitrate and ammonium have been made in the upwell to replace the Atlantic’s loss of deep-water, the ing areas of Peru, Northwest Africa, and Baja Califor enriched Indo-Pacific must have a net upwards motion. nia, allowing a comparative assessment of the inorganic Because of this and other mixing and advective proces nitrogen economies of these areas. ses, the upwelling source waters off Peru can benefit Corrections for enhancement of uptake values by the from the inter-ocean fractionation even though they nitrogen added as isotope have been made according to are often found at depths of ~ 100 m or less (see Figs. the model of Dugdale and Maclsaac (1971) using 166-168). Data from Baja California are also in gen Michaelis-Menten expressions for the effect of irra eral agreement with the above theory. For example, diance and substrate concentration on uptake rates data from MESCAL-I and MESCAL-II indicate that (Maclsaac and Dugdale, 1969 and 1972). The results dissolved silicon concentrations at ~ 100 m were fre are expressed in two ways: 1) as transport rates (q) with quently greater than 25 (Agat/l and sometimes greater units of mgat N/m2/d, or 2) as specific uptake rates (V) than 30 |xgat/l (Whitledge and Bishop, 1972 and 1973). obtained by normalizing the transport rates for the par In addition, nitrate concentrations at ~ 100 m averaged ticulate nitrogen (PN) concentration of the samples, about 10 |xgat/l higher than those observed off North with units/h. When the terms Qmax and Vmax are used, west Africa during JOINT-I. Maximum surface values we are referring to transport rates and specific uptake for nitrates were about the same during the MESCAL rates that were not limited by the amount of nitrate or and JOINT-I experiments, but this situation appears to ammonium that was present. In practice, these are the be the result of relatively weak upwelling during the values obtained before correcting for enhancement, MESCAL experiments and is not really in conflict with and they will only be a good approximation of the in the other evidence. situ rates for samples taken from nutrient-rich waters. Although high nutrient concentrations are not the Specific rates may be related to phytoplankton growth only requirement for high primary production rates, rates under some circumstances, but are subject to the nutrient enrichment of the upwelling source waters errors arising from dilution of the living phytoplankton off Peru probably contributes to the exceptional rates nitrogen by detrital particulate nitrogen; the transport that are often observed. For example, relatively weak rates are not subject to this error (Dugdale and Goer- upwelling off Peru could supply the surface waters with ing, 1967). When specific uptake rates are compared, significant amounts of nutrients, and during weak we assume that the contribution of detrital particulate upwelling, surface heating is more likely to produce the nitrogen is approximately the same in the different stratification necessary for good light conditions for areas studied. We also assume that the average PN phytoplankton growth. during the course of an incubation is equal to the final Since a portion of the organic matter produced by PN. This could result in an underestimate of V, espe the exceptional surface productivity off Peru is lost to cially when large increases in PN occur during the depth, the subsurface respiration rates are relatively course of an experiment. At specific uptake rates of high (Codispoti and Packard, 1980). In addition, the 0-05/h, this error may amount to about 25 % and it is inter-ocean fractionation process ensures that the usually lower at lower V values. waters arriving off Peru will have relatively low oxygen concentrations (e.g., Richards, 1965a). In these ways, the quantitative differences induced by the inter-ocean fractionation mechanism contribute to an important Differences arising from global scale qualitative difference between the dissolved oxygen processes: the > 100 year time scale and nutrient regimes found off Northwest Africa and Peru; off Northwest Africa oxygen concentrations are The Pacific and Indian Oceans are considerably richer seldom (if ever) low enough to interfere with “normal” in nutrients and poorer in oxygen than the Atlantic biological processes (Figs. 164,165), but off Peru there (particularly the North Atlantic). Broecker (1974) esti is a large volume of oxygen-deficient ( 0 2 < ~ 0-1 ml/1) mates that the deep waters of the Pacific have twice as water with relatively high nitrite concentrations (Figs. much nitrate and reactive phosphorus and five times as 166-168) that restricts the vertical motion of higher much dissolved silicon as the deep waters of the Atlan organisms (Smith et al., 1981; Judkins, 1980) and per tic. Although these inter-ocean differences can some mits nitrate reduction and denitrification to become times be obscured by intra-ocean effects (e.g., eastern prominent respiratory processes (e.g. Fiadeiro and vs western boundary regions), they are quite evident in Strickland, 1968; Codispoti, 1973). The existence of a our mesoscale sections (Figs. 164-168) from Northwest prominent nitrite maximum in the oxygen minimum Africa and Peru. zone off Peru from about 10° to 23°S (e.g., Wooster et There is general agreement (e.g., Richards, 1965a; al., 1965) is the result of nitrate reduction in the oxy-
188 STATION POSITION
50
100
26.7f 150
LU 200 (A ) SIGMA - t 8 MARCH 250
300
6 7 12 10
50 10 * 50
100 6 — - 100 >15 30' 30: ,22“ 22' iCABO 150 150 :ORBEIRO • • (B) 30' 30‘- 200 200 NITRATE DISSOLVED SILICON 8 MARCH 8 MARCH 250 250
30' 30' 300 300 42 11 10 6 7 12 11 10 6 7
50 50
100 100
150 150
200 200 DEPTH (m) DEPTH (m )
AMMONIA DISSOLVED 0 8 MARCH 250 8 MARCH 250
300 300 60 50 40 30 2060 50 40 30 20 DISTANCE OFFSHORE (km) DISTANCE OFFSHORE (km)
Figure 164. Sigma-/ (A), nitrate (B), dissolved silicon (C), ammonia (D), and dissolved oxygen (E) sections taken off Northwest Africa on 8 March 1974. Nitrate, dissolved silicon, and ammonia concentrations are in [i.moles/1. Oxygen concentrations are in ml/1. Nitrite concentrations were < 1-3 jimoles/l everywhere.
189 STATION POSITION 35 3334
26.67, 26.67 50 26.6 100
26.7. 150 26.8
(A) 200 26.9 SIGMA -t 15 MARCH 250
300 35 34 33 31 32 35 34 33 32 < 5 5
50 50
100 100 30' 17°W 30! 22' S.CABO )0RBEIR0 150 150 35, 33 30' 32/ 30' . (B) Å 200 (C) 200 NITRATE DISSOLVED SILICON 15 MARCH 25015 MARCH 250
30' 30' 300 300 35 3433 31 32 35 34 33 32 > 5 > 5 •
50 50
100 100
150 150
200 (E) 200 DEPTH (m) DEPTH (m ) AMMONIA DISSOLVED 0; 15 MARCH 250 15 MARCH 250
300 300 60 50 40 30 20 60 50 40 30 20 DISTANCE OFFSHORE (km) DISTANCE OFFSHORE (km) Figure 165. Sigma-f (A), nitrate (B), dissolved silicon (C), ammonia (D), and dissolved oxygen (E) sections taken off Northwest Africa on 15 March 1974. Nitrate, dissolved silicon, and ammonia concentrations are in ^moles/1. Oxygen concentrations are in ml/1. Nitrite concentrations were < 0-5 ^moles/1 everywhere.
190 STATION POSITION
46ASC 45 44 43 42 41 40 39B 38 46C 45 44 43 42 4I 40 398 38 25.2:
26.0: 20 14 26.2 25' 100 - 25 26.3
30 26.4 30 200
26.5 f 3 0 0 26.6
26.7. 4 0 0 26.8
TEMPERATURE SIGMA-t 5 0 0 -26.9 DISSOLVED Si 30 APRIL- 1 MAY 30 APRIL- 1 MAY 40' 27.0" 30 APR-1 MAY 45' 6 0 0
45 44 43 42 41 40 396 38 44 43 42 41 40 398 38 46C 45 44 43 424I 4039B38 ,0.5- •<1
100
20 200
X 3 0 0
30 4 0 0
<1 5 0 0 NH 40 30 APR - 1 MAY30 APR-1 MAY 30 APR - 1 MAY30 30 APRIL-1 MAY
6 0 0 Figure 166. The “C’Mine (~ 15°S off Peru) section of 30 April - 1 May 1976. Nitrite, nitrate, dissolved silicon, and ammonia are in ^moles/1. The “ticks” at the top of the figures denote the average “C"-line positions shown in Figure 162.
gen-deficient waters, and this “secondary maximum” tions of Northwest Africa, nitrate minima are also constitutes an obvious difference between the nutrient possible (see Fig. 164), but these seem to be more regimes off Northwest Africa and Peru. As one would limited in time and space and they probably arise from expect, the respiratory reduction of nitrate can some the interleaving of nutrient-rich South Atlantic Central times override the normal nitrate increase with depth Water (SACW) (e.g., Codispoti and Friederich, 1978) in the upper ~ 1000 m and produce a nitrate minimum with ambient North Atlantic Central Water (NACW). zone in the waters off Peru (Figs. 166-168). Off por Studies off Peru and in the analogous oxygen-defi
191 STATION POSITION
8 8 8 7 8 6 85 84 83 82 8 8 87 8 6 85 84 83 82 8 8 8 6 8 4 8 3 82
26.1' ' 20 - 26.2 —25 100 30 26.3.
26.4 200
26.5,
<30 26.6. 30
4 0 0
26.8' TEMPERATURE Ä SIGMA-t 5 0 0 -26.9 \Y 3 MAY DISSOLVED Si ~ 3 MAY 3 MAY
6 0 0
6 6 6 6 6 4 6 3 82 8 6 87 8 6 8 4 8Î 82 88 86 8 4 83 82 8 8 8 6 8 4 82
20 - ■25. 0.5 10 100
>4 200 <0.5
20 3 0 0
-3 4 0 0 - 2 -3 0 -1 -35 < 1 5 0 0 NO 2 NOi DISSOLVED 02 "
3 MAY 3 MAY 3 MAY
6 0 0
Figure 167. The “C ’-line section (~ 15°S off Peru) of 3 May 1976. Dissolved oxygen concentrations are in ml/1. Nitrite, nitrate, dissolved silicon, and ammonia concentrations are in nmoles/1. The “ticks” at the top of the figures denote the average “C”-line positions shown in Figure 162.
cient zone in the eastern tropical North Pacific suggest toplankton, the denitrification process may be one of that a portion of the nitrate originally present in the the feed-back mechanisms that control the productivity oxygen-deficient zone is reduced beyond nitrite by of the ocean (Piper and Codispoti, 1975). However, denitrifying bacteria and that the probable end-product the upwelling source waters off Peru are normally only is free nitrogen (e.g., Fiadeiro and Strickland, 1968; modestly affected by the denitrification process. They Codispoti, 1973; Codispoti and Richards, 1976). lie above the nitrite maximum and, even within the Because free nitrogen cannot be utilized by most phy maximum, the amount of nitrate that is reduced be-
192 DEPTH (m) ert-er aitos se eo) w wl peet a effect. larger a present had will have may we denitrification where case below), (see discuss rates variations we When year-to-year Peru. production off primary periods normal during on even mod effect a has suppressive probably estly layers shallower the with waters origi amount the half than less usually is nitrite yond al peet Nvrhls, iig f h denitrified the of mixing Nevertheless, present. nally nitrate, dissolved silicon, and ammonia concentrations are in [xmoles/1. The “ticks” at the top of the figures denote the average average the denote figures the of top the at “ticks” [xmoles/1. 162. in The are Figure in shown concentrations positions ammonia “C"-line and silicon, dissolved nitrate, 13 Rapports et Procès-Verbaux et Rapports 13 Nitrite, ml/1. in are concentrations oxygen Dissolved 30—31 1976. of May Peru) 15°S off (~ section ’-line “C The 168. Figure 5 10 418416 145144 149148147146 150 151 o UJ 1- 0 0 0 3 x E . 1 31 MAY -3 0 3 0 0 6 0 0 4 0 0 5 200 100 , , ■ , , - TEMPERATURE' 03 MAY 30-31 151 150 1 4 9148147146 145144 145144 9148147146 4 1 150 151 ^25^\ 0 2 > 20 30- 2 ( 20 31 MAY -3 0 3 NOi TTO POSITION STATION 27.0 ■ 25.4 26.9 26.8 26.7 26.6 25 8; 8; 25 26.4 , 0 26 25.9' 26 2 26 26.5 25.5: 03 MAY 30-31 ain i te daet cas Hwvr tee s little is there However, oceans. adjacent the in var iations inter-annual produce also border Sahara, lands southern the inthe ing drought severe of years cause that scale time year > the It is likely that meteorological variations, such as those as such variations, meteorological that likely is It Differences between years: between Differences 5 10 149148147 150 151 146 146144 SIGMA-t I 25 NH ■30 20 5 149) 4 4 1 5 4 1 6 4 1 7 4 1 6 )4 9 4 1 150 ISLE Si‘ DISSOLVED 03 MAY 30-31 151 >4 >4 / 0 5 1 ' <0.5 4 4 1 5 4 1 6 4 1 7 4 1 8 4 1 9 4 1 ISLE 02 DISSOLVED 0 frN' 03 MAY 30-31 193 or no comment on inter-annual variability in the litera ship between the warm anomalies and the subsurface ture dealing with upwelling off Northwest Africa. chemical perturbations noted during 1976 is that While this situation could arise from insufficient data, another, and even more anomalous, feature was pre it seems unlikely that the variations are as dramatic as sent: a red tide (caused by Gymnodinium splendens) of those off Peru, where the upwelling system was famous immense proportions. This feature extended from just for its inter-annual variations even before modern south of the Galapagos to at least 15°S (Dugdale et al., oceanographic studies of the region began. 1977). The large-scale red tide of 1976 was referred to The best known of the phenomena that produce as an “Aguaje”, a name that is also used for smaller anomalous years off Peru is El Nino. Numerous inves scale biological discolorations of the sea observed off tigators have commented on this condition (e.g., Zuta the west coast of South America (see Popovici and and Guillen, 1970; Guillen, 1971; Quinn, 1974; Steven Popovici, 1966). The smaller scale red tides are rela son and Wicks, 1975; Wyrtki, 1975; Harvey and Pat- tively common and red tides may be even more com zert, 1976; Wyrtki et al., 1976; Cowles et al., 1977). El mon during El Ninos (e.g., Wooster, 1960). However, Nino’s known effects on the oceanographic regime off Aguajes of the magnitude noted in 1976 may be more Peru may be summarized as follows: rare than major El Ninos (Dugdale et al., 1977). An outbreak of red water occurs annually along the 1. There is a general warming of the surface layers. Peruvian coast during austral summer (B. Rojas de 2. Surface layer ( ~ upper 50 m) nutrient concentra Mendiola, personal communication), and Gunther tions are lowered. (1936) suggests that Aguajes are associated with 3. Primary productivity is reduced. onshore flows of warm water. Consequently, it is poss 4. The anchoveta fishery is adversely affected. ible that there is a relation between the warm anomaly 5. The guano bird populations are reduced. of 1976, the occurrence of the immense Aguaje, and the significant changes that were observed in the sub It is known that El Ninos occur as a response to large- surface nutrient regime. We hope eventually to be able scale meteorological perturbations (e.g., Quinn, 1974), to unravel the possible relationships between the warm and it is also becoming clear that the mechanisms for anomalies, the Aguaje, and the unusual chemical con warming the coastal waters off Peru are more complex ditions observed near 15°S and reported by Dugdale et than the standard textbook descriptions (e.g., Wooster al. (1977). For now all we can do is summarize the and Guillen, 1974). Major El Ninos last for 10-16 unusual chemical conditions as follows. months (Wooster and Guillen, 1974; Wyrtki, 1975 and 1. Although denitrification normally occurs in the 1978) and in recent years (post-1950) they have begun oxygen-deficient strata off Peru, only a fraction of the in 1957, 1965, 1972, and 1976. There is no general nitrate present is removed (see above). In April 1976, agreement on how other recent years with significant however, denitrification had gone to completion in warm anomalies should be classified, but anomalously portions of the water column near 15°S. warm intrusions were noted in 1951, 1953, 1963, 1969, 2. Hydrogen sulfide was detected in many of the and 1975 (Wooster and Guillen, 1974; Clark, 1975; samples in which denitrification had gone to comple Wyrtki et al., 1976; D. Enfield, personal communica tion (N 03_ + N 0 2_ = 0), in agreement with the theory tion). that hydrogen sulfide production only becomes signifi While it is clear that considerable decreases in sur cant when nitrate and nitrite are absent (e.g., face nutrient concentrations are associated with El Richards, 1965b). Ninos and with some of the less dramatic warm 3. A subsurface ammonia maximum coincided with anomalies (e.g., Guillen, 1971; Cowles et al., 1977), the hydrogen sulfide zone, a situation that is also in little is known about any associated perturbations in agreement with results from anoxic basins and fjords the subsurface nutrient and dissolved oxygen regimes. (e.g. Richards, 1965b). We should expect some changes since the transport of 4. Reactive phosphorus and dissolved silicon maxima organic material to depth may be affected and because also coincided with the hydrogen sulfide zone. El Ninos can significantly affect the subsurface T-S and 5. Either the enhanced removal of nitrate in the oxy current structure (e.g., Wyrtki, 1975; Enfield and gen-deficient zone pr the presence of a large dinoflagel- Urquizo, 1976). Subsurface nutrient data were col late population that did not require silicon, or both, lected near 15°S during the anomalously warm years of had an apparent effect on the composition of the sur 1969 (University of Washington, 1970) and 1976 face waters, because dissolved silicon/nitrate ratios (Codispoti et al., 1976b; Dugdale et al., 1977), but they were unusually high. do not permit a resolution of this matter. The 1969 data 6. Hydrogen sulfide could not be detected after suggest no striking differences, but they were relatively April, but abnormally low nitrate concentrations and limited in space and time. The 1976 data do indicate high dissolved silicon/nitrate ratios in the water upwell major changes, but it is not clear how strongly these ing over the inner shelf persisted until about 9 May. changes are related to the warm anomalies. During the latter half of 1976 and during the 1977 One reason why we cannot be sure of the relation JOINT-II observations (March-May 1977) another 194 chemical anomaly appeared. When compared with past strong as or stronger than normal. Consequently, it data (e.g., Zuta and Guillen, 1970), the secondary ni seems reasonable to conclude that days with winds 2: trite maximum often seemed particularly well de 10 m/s are more common in the JOINT-I region than veloped, with nitrite concentrations frequently being ~ off Peru. 10 ugat/1 vs the ~ 4 [xgat/1 that seems more typical for 2. Off Africa (near 21°N), periods with wind direc the coastal waters near 15°S. tions that are unfavorable for upwelling occur with Although we are not always sure of the causes, it is some regularity, although the average monthly winds clear from the above that both the subsurface and sur are always from upwelling-favorable directions. Off face nutrient regimes off Peru can vary markedly from Peru (at least near 15°S), winds with directions un year to year. We can only speculate on the biological favorable for upwelling appear to be extremely rare significance of hydrogen sulfide production in the open (Stuart, 1975; Stuart et al., 1976; D.W . Stuart, per ocean off Peru, but it could be profound since hydro sonal communication). gen sulfide is a poison for most higher organisms. 3. As a result of 1 and 2, temporal variations in the wind stress can be more dramatic off Northwest Africa. A striking difference between conditions observed off Northwest Africa during JOINT-I and conditions observed off Peru during JOINT-II was the degree of Mesoscale and local differences: coupling between the longshore flow over the shelf and the > day < month time scale the wind field. Although flows counter to the winds may occur seaward of the shelf at 21°40'N off North A. General west Africa (e.g., Mittelstaedt, 1982), the coupling be In this section we shall compare data from mesoscale tween winds and the currents over the shelf was good regions off Northwest Africa (Fig. 161), Peru (Fig. (e.g., Mittelstaedt et al., 1975). In contrast, at ~ 15°S 162), and Baja California (Fig. 163). As stated at the off Peru the mean longshore transport measured over outset, many of the variations to be considered arise the shelf appears to be poleward, whereas the mean because of differences in general conditions, but others wind stress is equatorward (Smith et al., 1971; Brink et may be due to the particular natures of the three al., 1978). locales. Insofar as possible, we shall distinguish the Apparently, only a relatively thin surface layer flows inter-regional differences from those due to more local with the wind. These direct current meter observations effects. off Peru were made during the anomalously warm years of 1969 and 1976, and during 1977, a year that may also prove to be anomalous (see above). Since B. Differences in local winds and their correlation southward flow may be enhanced during warm periods, it is not clear how typical these results are. Neverthe with the nutrient fields less, much of the northward flow along the South Data taken during the JOINT-I (Fig. 161) and JOINT- American coast turns seaward near 25°S (e.g., Wyrtki, II (Fig. 162) experiments are used in this analysis. Both 1978) even during normal years, so the weak coupling of these regions are sites of enhanced and persistent between winds and currents in the nearshore zone near upwelling imbedded in larger scale upwelling zones 15°S may be a normal feature. (e.g., Zuta and Urquizo, 1972; Stuart, 1975; Wooster The above comments suggest that one should expect et al. 1976; Schulz et al., 1978). The average monthly a stronger correlation between the local winds and nu winds in both locales are similar insofar as they are trient fields off Northwest Africa than off Peru. Our always from upwelling-favorable directions. There are, initial results indicate that this notion may be correct. It however, considerable differences in the exact nature is clear, for example, that there was a strong correla of the upwelling winds and in the correlations between tion between the local winds and nutrient fields during the local winds and nutrient fields. The investigations the portion of the JOINT-I experiment (February- of our colleagues permit us to outline some of the dif April 1974) that included meteorological, physical, and ferences in the JOINT-I and JOINT-II wind regimes as chemical observations. Barton et al. (1977) demon follows: strated the correlation between the local winds and the 1. Even when some daily wind data from a portion of temperature and density fields. They identified five the JOINT-II experiment with relatively strong winds periods of enhanced upwelling (“events”), and were (August-September 1976) are compared with similar able to show that the two largest events had a similar data from the JOINT-I experiment, it appears that life cycle: the density structure appeared to respond to wind speeds > 10 m/s are less common off Peru (Bar the local winds with a time lag of about one day. With ton et al., 1977; Brink et al., 1978). While the data the onset of favorable upwelling winds, relatively cold from Peru were taken during the 1976 El Nino, the and dense water would first appear over the inner available data (e.g., Stuart, 1975; Medina, 1978; Brink shelf. Then, with continuing favorable winds the center et al., 1978) suggest that the winds near 15°S were as of most intense upwelling would migrate to the vicinity 13* 195 of the shelf break and remain there until the system significant chemical variability. The effects of such relaxed. Codispoti and Friederich (1978) were able to waves seem to be obscured by wind effects off North show that much of the nutrient variability could be west Africa. related to these “event” cycles, and Figures 164 and The effects of biological processes off Peru also seem 165 give some idea of conditions prior to and at the to confuse the relationships between winds and nu height of the first of the two largest events. The trient fields more than they do off Africa. The high JOINT-I data also suggest a brief period of downwell- dissolved silicon/nitrate ratios observed during the ing during a phase with weak winds from directions Aguaje (see above) is an obvious complication. The unfavorable for upwelling (e.g., Codispoti and complicated nature of the temporal variations in sub Friederich, 1978). Another aspect of the nutrient surface nutrient distributions that is evident in our regime off Northwest Africa may also be correlated Peruvian sections probably also arises partially from with the local and mesoscale wind regime. A narrow mesoscale variations in biological processes. poleward flowing undercurrent carries nutrient-rich It is possible that the relatively weak relationship South Atlantic Central Water into the region, and between the winds and nutrient fields observed near there is some evidence to suggest that this countercur 15°S off Peru (when compared with conditions at ~ rent is relatively weak (at least above 300 m) when the 21°40'N off Northwest Africa) is characteristic of the upwelling-favorable winds are strong (Mittelstaedt et entire coast. For example, winds measured at 12°S al., 1975). Codispoti and Friederich (1978) have shown (Enfield et al., 1978) seem more constant than those that nutrient concentrations in the upwelling source observed during the JOINT-I experiment off North water are lowered by a weakening of the undercurrent, west Africa, and winds measured from a nearshore so it is possible that the local and mesoscale wind mooring near 5°S are also remarkably constant (Brock- regime may exert this additional influence on the nu mann et al., 1978). In addition, some of the other trient concentrations observed during JOINT-I. sources of variance in the nutrient fields off Peru, such While it is probably weaker than in the JOINT-I as coastal trapped waves and high biological activity, region there is some correlation between the local should operate along all or most of the coast. Conse winds and nutrient fields over the Peruvian shelf near quently, the relatively weak co-variance between winds 15°S. For example, Brink et al. (1978) present evidence and nutrient fields that we observed in our mesoscale suggesting that the cross-shelf flow does respond to the data from Peru may reflect a true inter-regional differ local wind, and that the subsurface longshore flow may ence between Peru and Northwest Africa. sometimes be modulated by the mesoscale wind field even though the average longshore transport over the shelf is opposite to the wind stress. Figures 166 and 167 C. Differences in nutrient regeneration are in agreement with their results, suggesting a re sponse of the nearshore temperature, density, and nu Since regions of high ammonia concentration are main trient fields to an increase in the average daily long tained by local and mesoscale regenerative processes shore wind from about 2 m/s on 30 April to about 7 m/s (e.g., Codispoti and Friederich, 1978), an examination on 3 May. of their distribution is helpful in any attempt to com While the above data suggest some similarities in the pare the nutrient regeneration regimes off Northwest co-variance of the winds and nutrient fields off North Africa and Peru. Relatively high ammonia concentra west Africa and Peru, considerable differences remain. tions can be found in both the JOINT-I and JOINT-II For example, temperature, density, and nutrient dis regions, but the distributions of these high values are tributions observed off Peru during 30-31 May 1976 different (Figs. 164-168). Off Africa, the highest val (Fig. 168) are not so easily related to the local wind ues were always associated with the inner shelf. Fre field. Data presented by Brink et al. (1978) indicate quently there was little difference between the surface that the wind regime during this period was not vastly and near-bottom values, but sometimes a high different from that of the 3 May period, but the cross ammonia tongue extended offshore at mid-depths (see shelf hydrographic and nutrient distributions are (com Codispoti et al., 1976). Off Peru, high values were fre pare Figs. 166-168). quently found in the nearshore zone, but they were About one day after this period, a current reversal often close to the bottom and near the surface with an that may have been due to a coastal trapped wave ammonia minimum in between. (Brink et al., 1978; K.H. Brink, personal communica One reason why the high ammonia concentrations tion) occurred, but it is difficult to attribute the abnor off Northwest Africa often did not show much varia mal 30-31 May hydrographic distributions to a subse tion with depth is that the nearshore water column was quent event. While the effect of such waves is not clear frequently well mixed (see Fig. 165). As a result of in this instance, Smith (1978) has shown that such these conditions and probably other factors as well, waves can account for a significant portion of the physi such as the nature of the sediments and the effects of cal variability off Peru, and we expect that our ongoing waves, the nearshore surface waters off Northwest studies will eventually show that these waves do cause Africa were often quite turbid. Codispoti and 196 Friederich (1978) have emphasized the association of there is off Northwest Africa. For example, current the high ammonia concentrations off Northwest Africa measurements made during 1977 indicate a well- with this zone of high turbidity. The turbidity appeared developed poleward undercurrent over mid-shelf at to produce a poor light regime for phytoplankton 10°S where the Peruvian shelf is fairly wide (Enfield et (Huntsman and Barber, 1977), thereby inhibiting al., 1978). In addition, many authors have noted that ammonia uptake. In addition, the nutrient regenera the upwelling waters off Peru come from relatively tion rates/unit volume were relatively high in the near shallow depths (e.g., Wyrtki, 1963) and this situation shore zone (e.g., Rowe et al., 1977; Smith and Whit- should favor the development of a three-layered flow. ledge, 1977). Codispoti and Friederich (1978) show that the high regeneration rates may be a partial result of a net transport of organic matter into the nearshore environment. They invoke the interaction of four fac D. Differences in ammonia and nitrate uptake tors to account for the net input of organic matter: the Tables 24 and 25 present data that deal with nitrate and net onshore motion of the bottom layer, the high prim ammonia uptake in our mesoscale study regions off ary production rate on the mid and outer shelf (Hunts Northwest Africa (Fig. 161), Peru (Fig. 162), and Baja man and Barber, 1977), the sinking of a fraction of this California (Fig. 163). The JOINT-I and JOINT-II data surface productivity, and the relatively wide and shal have been supplemented with additional data taken low shelf (when compared with Peru at ~ 15°S) which from the same locales and seasons. Some of the prob prevents the organic matter from sinking below the lems with these data are pointed out in the Methods onshore flow. section, and we must also note that the number of Friederich and Codispoti (1979) have also suggested observations is small in relation to the temporal and how the above factors, when abetted by a higher sink spatial variations that one should expect. Nevertheless, ing rate for biogenic silica, may combine to produce the data are sufficient to permit some insight into the relatively high dissolved silicon regeneration rates over variability in nitrate and ammonia uptake. the Northwest African shelf near Cape Corveiro. When we exclude the data from periods when dino- Contributory factors that may cause the ammonia flagellates dominated, some aspects of the nitrogen distributions off Peru to differ from those off North productivity are remarkably similar. The range of the west Africa include the following: mean value of Vmax-N H 4 is from 0-0139 to 0-0183/h; 1. The nearshore waters are more stratified; their the ammonium productivity (q) range is from 4 to 10 average depths are greater; and maximum winds are mgat/m2/d; and the mean proportion of nitrate produc weaker. These factors probably help to keep ammonia tivity, expressed as a percentage of combined nitrate concentrations in the nearshore surface waters rela and ammonium productivity, shows a range of 52 to 78 tively low by providing a more favorable light regime % (Table 24). The differences occur in the values of for phytoplankton and by preventing the products of nitrate productivity and Vmax- N 0 3, both being nearly bottom and near-bottom regeneration from reaching twice as high in Peru as in the other two upwelling the surface. regions. From these observations we may conclude that 2. There is a tendency for the cross-shelf flow in the a major difference in productivity between Peru and JOINT-II region to be three layered (Brink et al., the other regions lies in the ability of the phytoplank 1978; J. C. Van Leer, personal communication), with ton to take up nitrate at a high specific rate, i.e., to offshore surface and bottom flows, and maximum grow rapidly on nitrate. Quantitatively, the amount of shoreward flow at mid-depths. Certainly, the ammonia nitrate production available to make up losses, e.g., by distribution in Figure 166 is suggestive of offshore flow conversion into zooplankton or fish biomass, by sink along the bottom of the shelf. It should be noted that ing or by cross-shelf advection losses to seaward, is an offshore bottom flow would reduce the supply of twice as high off Peru as off Northwest Africa and Baja regenerated nutrients to the photic zone, particularly California at the times these studies were made. Conse for those nutrients (such as dissolved silicon) that are quently, attention is focused upon the possible factors incorporated into particles with relatively rapid sinking that may result in the high specific maximal uptake rates. rates for nitrate, V ^ - N O s . Poleward undercurrents tend to induce an offshore Evidence was obtained on the JOINT-I cruise to component in the near-bottom layer (e.g., Mittel Northwest Africa that strong winds and a relative ab staedt, 1982), so areas off Northwest Africa with nar sence of stability in the water column produced a deep row and steep shelves may also have a three-layered mixed layer, with a mean depth of about 38 m com flow, if they permit the undercurrent to impinge upon pared with 12 and 13 m in two cruises off Peru and 14 m the shelf. We should also point out, once again, that obtained from the MESCAL-II data (Table 24). Low our observations may be for a period when the Peru values for V ^ - N O a were noted during JOINT-I, and vian Undercurrent was particularly well developed (see experiments were made to examine the possible role of above). Nevertheless, we think that there is generally a deep mixing and low average light levels on the maxi greater tendency for a three-layered flow off Peru than mal nitrate uptake rate. During periods of strong mix- 197 Table 24. Mean values of nitrate and ammonium uptake parameters for a series of cruises to three upwelling regions. Region Cruise e e e Nitrate Knax-N03-/h Vmax-N H 4-/h Zm Nitrate Ammonium Total N productivity mixed- productivity productivity productivity (%) layer (mgat/m2/d) (mgat/m2/d) (mgat/m2/d) depth3 (m) Peru Anton Bruun-15 -- - 0-0367 0-0167 12 Pisco 30 10 40 75 0-0358 0-0172 13 JOINT-II Alpha Helixb -- - - 0-0096 0-0112 Baja Cali MESCAL-II 14 4 18 78 0-0233 0-0183 14 fornia MESCAL-Ib 1.2C 1-5C 0-0086 0-0089 Northwest JOINT-I 15-2 6-6 21-8 70 0-0192 0-0139 38 Africa CINECA- Charcot II 10-6 9-7 20-3 52 0-0221 0-0139 a Examples of the mixed-layer depths encountered when the nitrogen uptake data were taken. They may differ from the depths that would be obtained from an examination of all the stations taken during each cruise. b Dinoflagellate-dominated (red tide). 0 These are pmax values from experiments where corrections for enhancement could not be made. ing, Vmax_ N 0 3 was found to be low at all depths sam ing regions studied appear to provide an important clue pled within the euphotic zone. However, if these sam to understanding the underlying basis for the differ ples were held for 24 hours in 50 % light-screened bot ences in productivity of these regions. tles in deck incubators, the values increased to those The above comments give the impression that the ex observed in a shallow mixed layer when winds de ceptionally high nitrate uptake rates off Peru represent creased. A rough inverse correlation could also be a true inter-regional difference between this zone and shown between wind velocity and Vmax—N 0 3 for a Northwest Africa and Baja California, and this is prob series of 50 % level samples. The high winds and deep ably true under average conditions in the three areas. mixing apparently reduced the potential productivity of However, it must be recognized that nutrient concen- the Northwest African upwelling region near 21°40'N, a conclusion reached by Huntsman and Barber (1977) on the basis of 14C measurements. Table 25. Maximum and minimum values of for nutrient- The range of Vmax values for nitrate and ammonium saturated uptake of nitrate and ammonium in the upper eu uptake are presented for each area by cruises in Table photic zone3 from a series of cruises to three upwelling regions. 25. Minimum values may not be particularly useful since very low values would probably appear in any Region Cruise Vr max -]N 0 3“/h Vr max - NH3/h sufficiently long time series. However, the maximum min max min max values reinforce the conclusions reached above: 1) the Peru Anton Bruun-15 0-0170 0-0547 0-0129 0-0222 ammonium Vraa!1-N H 4 does not show the same mag nitude of increases as does Vmax—N 0 3, and 2) the Peru Pisco 0-0089 0-0595 0-0268 0-0358 vian upwelling region exhibits the highest Vmax- N 0 3 0-0076 0-0151 values. Alpha Helixb 0-0038 0-0181 The conclusion to be drawn from this preliminary Baja Cali MESCAL-IF 0-0144 0-0450 0-0121 0-0216 analysis is that the high biological productivity of the fornia Peruvian upwelling system may be linked to the ability MESCAL-Ib 0-0046 0-0134 0-0059 0-0115 of the phytoplankton to take up and utilize nitrate at an Northwest JOINT-I 0-0028 0-0329 0-0046 0-0261 extraordinary rate. The factors leading to these espe Africa cially high rates off Peru appear to be the co-occur rence of high nutrient concentrations in the euphotic a Defined here as the 100 to 30 % light-penetration zone. zone and relatively shallow mixed layers. Other factors Most values are from the 50 % light depth. b Dinoflagellate-dominated (red tide). may also be important in determining the size of the c Stations 9-39, mean of 100, 50, and 30 % light levels when standing stock of phytoplankton. However, the differ nitrate (for Vmax- N 0 3_) or ammonia (for Vmax-N H 3) were ences in specific uptake rates for nitrate in the upwell present in saturating concentrations. 198 trations in the upwelling source waters off Northwest G. Friederich, D. Harmon, and G. Gruenseich for Africa increase considerably to the south of the their help in producing the nitrogen uptake data. We JOINT-I region, that at times the upwelling off Baja are particularly grateful to W. S. Wooster for his help California may supply nutrients to the photic zone at a ful comments. Financial support was provided by the much higher rate than was observed during the MES National Science Foundation’s International Decade of CAL experiments (e.g., Walsh et al., 1977), and that Ocean Exploration Program under grants ID 072- mixed-layer depths near 15°S off Peru are sometimes 06422, OCE76-00136, OCE76-04825, and OCE77- deeper than those observed during the cruises cited in 27128; by the National Science Foundation’s Interna Table 24. Consequently, some of the differences may tional Biological Program under grants GB-8648, GB- arise from more local effects and from the timing of the 18568, and GB-35880X; by the Office of Naval experiments. Research under contract N-00014-76-C-0271; and by So far, these comments on nitrate and ammonia up the Government of France under grant number RCP- take have neglected those cases when dinoflagellates 247-CNRS. dominated the phytoplankton populations. In consid ering these data (Tables 24 and 25) we see that uptake during periods of dinoflagellate dominance tends to be relatively low. This may be partially the result of experimental artifacts. For example, the ability of References dinoflagellates to congregate often produces popula tions that are large enough to exhaust the nutrient sup Barton, E. D. 1977. RV Thomas G. Thompson cruise 108, leg 1, CTD measurements off the coast of Peru near Cabo ply quickly in the incubation flasks. Nevertheless, even Nazca, April-May 1976. Coastal Upwelling Ecosystems if we restrict our comparison to the maximal specific Analysis Data Rep., 39: 140 pp. rates (Table 25), it is obvious that dinoflagellates tend Barton, E. D., Huyer, A., and Smith, R. L. 1977. Temporal to have lower uptake rates. variation observed in the hydrographic regime near Cabo Corveiro in the Northwest African upwelling region, Feb This discussion and considerations advanced by ruary to April 1974. Deep-Sea Res., 24: 7-23. others (e.g., Blasco, 1977; Maclsaac, 1978) suggest Barton, E. D., Pillsbury, R. D., and Smith, R. L. 1975. A that dinoflagellate blooms may be more common off compendium of physical observations from JOINT-I, verti Baja California and Peru than off Northwest Africa (at cal sections of temperature, salinity, and sigma-/ from RV Gilliss data and low-pass filtered measurements of wind and least near 21°40'N), so it may be reasonable to infer currents. Oregon State University, School of Oceanogra that the range of nitrate uptake values off Northwest phy. Ref. 75-17: 60 pp. Africa is less than in the other locales. Berger, W. H. 1970. Biogenous deep-sea sediments: fraction ation by deep-sea circulation. Bull. geol. Soc. Am., 81: 1385-1402. Blasco, D. 1977. Red tide in the upwelling region of Baja Conclusion California. Limnol. Oceanogr., 22: 255-263. Brink, K. H., Allen, J. S., and Smith, R. L. 1978. A study of The differences between the nutrient regimes off low-frequency fluctuations near the Peru coast. J. phys. Oceanogr., 8: 1025-1041. Northwest Africa, Peru, and Baja California are con Brockmann, C., Fahrbach, E., and Urquizo, W. 1978. ESA- siderable, and they arise from a melange of processes CAN - data report. Berichte aus dem Institut für Meeres and features ranging in scale from local topography to kunde an der Christian-Albrechts-Universität Kiel. No. 51: the global circulation. There are additional contrasts 54 pp. Broecker, W. 1974. Chemical oceanography. Harcourt Brace that we are only now beginning to investigate. It is Jovanovich Inc., New York, 214 pp. likely, for example, that the seasonal cycles in the nu Clark, W. G. 1975. A study of the virtual population of the trient regimes off Peru and Northwest Africa are dis Peruvian anchoveta in the years 1962-1972. Ph.D. Thesis, similar. While further comparative studies will un University of Washington, Seattle, Washington, 300 pp. Codispoti, L. A. 1973. Denitrification in the eastern tropical doubtedly reveal more differences, it should already be North Pacific. Ph.D. thesis, University of Washington, clear that we must exercise caution when transferring Seattle, Washington, 118 pp. knowledge from one upwelling region to another. Codispoti, L. A., Bishop, D. D., and Friebertshauser, M. A. 1976a. JOINT-I, the Atlantis-II sections: RV Atlantis-II cruise 82. Coastal Upwelling Ecosystems Analysis Tech. Rep., 20: 76 pp. Codispoti, L. A., Bishop, D. D., Friebertshauser, M. A., and Friederich, G. E. 1976b. JOINT-II, RV Thomas G. Acknowledgements Thompson cruise 108, bottle data, April-June 1976. Coas tal Upwelling Ecosystems Analysis Data Rep., 35: 370 pp. We should like to express our profound gratitude to the Codispoti, L. A., and Friederich, G. E. 1978. Local and following individuals who assisted us in the preparation mesoscale influences on nutrient variability in the North of this paper: M. Minas, J. J. Maclsaac, M. C. Bonin, west African upwelling region near Cabo Corbeiro. Deep- D. Lowman, L. Lewis, S. Patterson, D. Doyle, D. Sea Res., 25: 751-770. Codispoti, L. A., and Packard, T. T. 1980. Denitrification Cromoga, A. Hafferty, and G. Friederich. Special rates in the eastern tropical South Pacific. J. mar. Res., 38: thanks are also due to J. J. Maclsaac, H. L. Conway, 453-477. 199 Codispoti, L. A., and Richards, F. A. 1976. An analysis of the Maclsaac, J. J. 1978. Diel cycles of inorganic nitrogen uptake horizontal regime of denitrification in the eastern tropical in a natural phytoplankton population dominated by Gony- North Pacific. Limnol. 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