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Denitrification and hydrogen sulfide in the Peru upwelling region during 1976
Deep~Sea Research= 1977, Vol, 24, pp. 601 to 608, Pergamon Press. Printed in Great Britain
Preliminary Comunicatlon DENITRIFICATION AND HYDROGEN SULFIDE IN THE PERU UPWELLING REGION DURING 19761
R.C. DUGDALE 2, J.J. GOERING 3, R.T. BARBER 4, R.L. SMITH 5, and T.T. PACKARD 2
Denitrification,
one of the several processes determining the nitrogen
balance of the sea, is defined as the reduction of NO3 to a gaseous end product, N 2 or N20.
It is carried out by bacteria that use nitrate as a
hydrogen accepter when dissolved oxygen concentrations fall to very low or undetectable levels. When nitrate concentrations fall to low or undetectable levels as a result of denitrification,
SO42 is used as a hydrogen
accepter in anaerobic respiration with H2S as the end product. These processes are best known in the marine environment from studies of enclosed bays, fjords, or basins with shallow sills inhibiting communication with adjacent open seawater (Richards, 1965).
The occurrence of denitrification
in the open sea has been demonstrated in the north eastern tropical Pacific Ocean by direct measurements
(Goering, 1968) and from considerations of
the relationships between the concentrations of NO3 and ~t (Goering, Richards, Codispoti and Dugdale, 1973; Codispoti and Richards, 1976). Fiadeiro and Strickland (1968), also using a similar technique, provided convincing evidence that denitrification occurs off the coast of Peru. In each of these studies, the quantity of NO3 removed from the water column was not great, about i0 to 20% of that originally present. Evidence that massive denitrification occurs in the Peru upwelling system under certain unusual conditions was obtained in April, 1976 during the first phase of the JOINT 116 expedition of the Coastal Upwelling Ecosystems Analysis
(CUEA) project.
The region studied is at about 15 ° S
on the Peru coast where a persistent plume of cold upwelling water identified as an upwelling center by Gunther (1936) has been studied on two previous iContribution No. 76015 from Bigelow Laboratory for Ocean Sciences. 2Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Maine 04575. 31nstitute for Marine Sciences, University of Alaska, College, Alaska 99701. 4Duke University Marine Laboratory, Beaufort, North Carolina 28516. 5School of Oceanography, Oregon State University, Corvallis, Oregon 97331. 6jOINT II is the name of a multiship endeavor off the coast of Peru. 601
602
cruises,
Preliminary Communication
the R/V ANTON BRUUN cruise 15 in 1965 and R/V THOMAS G. THOMPSON
cruise 36 in 1969.
Both cruises were in late March or early April and the
JOINT II expedition was scheduled to begin in mid-March,
1976.
The R/V ALPHA
HELIX, the experimental biology ship for the expedition, began its work on March 22. An array of current meters was moored near 15 ° S: three along the CUEA "C" line and one on the "B" line (Figure i). The mooring at C-5 is I
JOINT ]~ AREA 14040~-
MET STATIONS ® CURRENT METERS
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Fig. i.
Tso,o ' I
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HYDRO S T A T I O N S
~_
,
NA~ICAL UILES
.............
~' I
CUEA study area and current meter locations near 15 ° S l a t . the coast of Peru.
referred to as Lobivia.
along
The work of the R/V ALPHA HELIX was concentrated
at C-3; daily stations were occupied and a wide variety of variables measured.
These included temperature, nutrients,
production,
chlorophyll,
14C primary
15N uptake of nitrate and an~nonia, 30Si measurements of Si(OH)4
uptake and regeneration, phytoplankton species composition,
zooplankton
biomass and species composition and regeneration of nutrients by zooplankton grazing.
The ship assigned to make hydrographic stations for the expedition,
the R/V THOMAS G. THOMPSON, was nearly a month late in arriving and the R/V ALPHA HELIX occupied additional stations along the C-line in an attempt to fill the void during this period.
Analytical methods used!were: NO3, NO;,
PO43, and Si(OH)4 by AutoAnalyzer according to Strickland and Parsons (1968) and NH~ by AutoAnalyzer according to Slawyk and Maclsaac (1972). RESULTS Following initial occupation of a series of stations along the C-line, it became apparent that the NO3 profiles had an unusual appearance, often showing a decrease in concentration with depth. On Sta. 16, taken April i,
Preliminary Communication
I0 O--
TEMPERATURE°C I~ 200
NO3-N,~g Atlliter I0 20
NO2 -N,~Atlliter 30012345678
NH4"N,/A) At I lltw 0123456789
603
f~4lpj~g At/liter I
2
Si(OH) 4 - Si,~l At/liter
4
I0
20
30
40
lOG
w
2OC
_z
N
~00."
CUEA
Fig. 2.
JOINT~ AREA LOCATION C - 5
Profiles of temperature and nutrients at location C-5.
1976, at location C-5 (Figure i) the nutrients were determined immediately following collection. Samples with undetectable NO3 concentrations were noted at depth and in samples freshly drawn from these Nansen bottles the smell of hydrogen sulfide was easily detected. The same location was reoccupied a few hours later at Sta. 17 using a sample spacing designed to obtain details of the region of anomolous NO3 distribution. The data from these two stations were combined to produce the profiles shown in Figure 2. Analyses of NO3 for the upper i00 m were lost from Sta. 16 so values of NO3 from Sta. 21, taken at the same location two days later, on April 3, 1976, are plotted also in Figure 2 to give some idea of the near-surface distribution. Nitrate concentrations were undetectable from 130 to 175 m, and NO2 was similarly undetectable, with high concentrations above and below. The NO2-free layer is somewhat more restricted than for NO3, extending from about 150 to 175 m. The NH~ profile shows a peak coinciding with the NO3-
free region, as do Si(OH) 4 and P043. The dashed lines bracket the depths where the odor of H2S was apparent. The temperature profile reflects the characteristically shallow mixed layer of the Peru upwelling region. DISCUSSION The profiles (Figure 2) agree with the classical picture of water column denitrification developed during studies of anoxic basins and fjords. The presence of H2S and absence of NO3 are highly diagnostic even in the absence of reliable dissolved 02 data. The NH~, P043, and Si(OH)4 accumulations result from the oxidation of organic matter accompanying the reduction of first NO3 and then S042 (Richards, 1965). Our observations are apparently the first obtained for an open sea station where complete denitrification had
50
604
Preliminary Co,,munieation
occurred.
Ivanenkov and Rozanov (1961) report the occurrence of H2S in the
Indian Ocean, a condition that signals complete denitrification;
however,
NO3 was reported to be present in the same samples, The denitrified layer observed in April 1976 was apparently contained within the Peru - Chile undercurrent, a warm saline current originating along the northern coast of Peru and fed from equatorial waters (Wooster and Gilmartin,
1961).
The undercurrent can be identified by low 02 concentrations,
high salinities, and the presence of large concentrations of NO2 (Wooster, Chow and Barrett, 1965).
These authors showed that a region of oxygen defi-
cient water occurs along the Peru coast and that a layer of high NO2 concentration occurs beginning somewhat north of Callao and extending southward to northern Chile.
Wooster and Gilmartin (1961), using drogues, observed flow
in the undercurrent to be generally poleward with weak northwest flow in the near surface regions.
Wooster, et al.,
(1965) showed that the thickness of
the high NO2 layer increases downstream.
They show profiles of N02 in the
north with smooth peaks and profiles to the south with double peaks that could be interpreted as the result of depletion within the NO2-rich layer. The NO2 profile in Figure 2 can be viewed as the result of the same processes that produced the double peak profiles described by Wooster, et al. (1965), Because the profiles of Figure 2 clearly are the result of denitrification, a coherent pattern of downstream processes emerges, with a sequence from north to south of low 02 with generation of high NO2 from reduction of NO3, later reduction of NO2 as well, and when both are completely exhausted as in April 1976, reduction of sulfate with the evolution of hydrogen sulfide. The current meter measurements from March and April 1976 were similar to observations in the same area in March and April 1969 (Smith, Enfield, Hopkins and Pillsbury, 1971).
The flow through the "C" line between 20 and
about 300 or 400 m was generally toward the southeast, parallel to the coast and opposite to the prevailing wind.
Presumably the Peru-Chile undercurrent
extended inshore over the inner continental slope and shelf, flowing parallel to the topography.
The data from the mooring at C-5 (Lobivia) is particu-
larly interesting:
the flow was variable but principally toward either 120
or 300 ° true, roughly "alongshore" toward the southeast or northwest.
The
mean "alongshore" flow was computed for the period 28 March to 28 April 1976 from the current meter records, which had been filtered to remove the tides. The mean flow at 106 m was 15.7 _+12.7 cm s -I toward the southeast, but at 206 m it was only 0.3 +16.4_ cm s -I toward the southeast, while at 406 m it was 4.2 !8.0 cm s -I toward the northwest.
The low mean flow at 206 m was the
result of several reversals, which led to intervals over which the apparent net displacement of a water parcel would be negligible. The progressive vector diagram (PVD), or pseudo-trajectory, at 206 m is shown in Figure 3. The reversals may have upset the normally precarious balance between oxygen supply and utilization in the undercurrent, with subsequent complete denitrification and sulfate reduction; i.e., oxygen-depleted water may have returned
Preliminary Communication
605
30
-30
I
~;;
I
I
i
150
--90
- -120
2 0 6 METERS AT LOBIVIA
Fig.
3.
4 3 . 9 DAYS STARTING 2 2 2 9 2 4 MAR 76
Progressive vector diagram for current meter at 206 m, near location C-5 in 650 m of water. Squares indicate 0000 GMT. Axes are N-S and E-W, distances in km.
under regions of high productivity to suffer even further reduction. such reversals occur in other years and other seasons is unknown,
Whether
nor do we
have data for earlier in March to know if such reversals may have set the stage for the denitrification observed in early April. The Peru - Chile undercurrent
is a major source of the water upwelling
onto the Peru coast (Wooster and Gilmartin, R/V THOMPSON 36 cruise in March - April,
1961).
Sections made during the
1969 in the region of 15 ° S l a t .
show clearly the upward slope of isotherms and of NO] isopleths in the surface waters characteristic
of coastal upwelling.
Downwarping of the iso-
therms is evident in these sections below i00 m and indicates of the Peru - Chile undercurrent.
Nitrate concentrations
the presence
at the surface
near the coast were between 15 and 20 ug at I -I, while concentrations
of NO]
were somewhat depressed at about 250 m near the shelf break and within the undercurrent. No synoptic sections are available for the first week of April, However,
1976.
stations occupied along the C-line during the period April I to 4
show low values of NO] throughout the upper 400 m of the water column.
Some
indication of the potential nutrient impoverishment arising from this condition can be obtained by comparing the depths of the 20 ~g at i "I NO3-N isopleth next to the coast, about 20 m in 1969 vs. 450 m in 1976. The physical and chemical conditions
described above occurred within a
period when a massive bloom of the dinoflagellate in progress.
Gymnodinium splendens was
The red tide resulting may be one of the most extensive ever
606
Preliminary Co~nunication
observed.
Packard (1976) reported that during transit of the ALPHA HELIX
from San Diego to the 15 ° S study area, the Gymnodinium bloom was first observed just south of the Galapagos Islands and appeared to be continuous southward from there.
Continuous recordings of chlorophyll fluorescence
made along the C-line at a depth of 3 m by ALPHA HELIX showed extremely high concentrations of chlorophyll seaward of the shelf break.
Microscopic obser-
vations confirmed that the phytoplankton population was strongly dominated by Gymnodinium splendens
(D. Blasco, personal communication).
face was visibly red during the daylight periods.
The sea sur-
Continuous fluorescence
recordings made on the same transect at night showed sharply reduced fluorescence, indicating a daily vertical migration pattern.
Dinoflagellate
populations occur in most coastal upwelling areas (Brongersma-Sanders, 1948) and their distribution in the 15 ° S area of Peru was described for the PISCO cruise by Blasco (1971).
An outbreak of red water occurs annually along the
Peru coast in the weak upwelling period of January or February, giving way to diatom dominated populations as upwelling strengthens (B. Rojas de Mendiola, personal communication).
In 1976, however, the red water was observed ear-
lier, in December (B. Rojas de Mendiola, personal communication) and persisted into mid-May, according to observations made on ALPHA HELIX (Barber, 1976).
Large numbers of medusae, locally termed "malagua," occurred through-
out this period, and there were reports in the newspapers of mortalities of a variety of organisms along the beaches. The phenomenon described above coincides with the normal concept of red tide and carries the designation aquaje along the west coast of South Ameri-
Aquaje does not necessarily coincide with
ca (Popovici and Popovici, 1966).
E1Ni~o
(Brongersma-Sanders, 1957), the strong incursion of warm water from
the north in some years that brings heavy rains to coastal Peru.
Gunther
(1936) suggested that areas of aquaje occur along the Peru coast where an onshore flow of warm water occurs.
Another feature of a major aquaje may be
the occurrence of hydrogen sulfide as described by Lavalle y Garcia (1917), as indicated both by smell and by the darkening of paint and brass where H2S is actively evolved to the atmosphere as observed by Burtt (1852) in the Bay of Callao. Unfortunately, no historical record of major occurrences of aquaje seems to exist for Peru waters.
However, it appears that these events occur
no more often than major E1Ni~o
, perhaps on the scale of about i0 to 20
years as scientists at the Instituto del Mar del Peru do not recall seeing such a strong aquaje as that of 1976 or that described by Lavalle y Garcia (1917) during their work at the Institute, about 20 years (B. Rojas de Mendiola, personal con~nunication).
Clearly, the CUEA program has been fortunate
in observing a major but rare perturbation of the Peru upwelling system. Already we have shown that denitrification can occur so strongly in the open sea that hydrogen sulfide is found in the water column and the nitrogen budget of an upwelling site strongly affected.
As collection of data will
Preliminary Communication
607
continue through June of 1976 and the analysis of these data proceeds,
impor-
tant additional effects on the Peru upwelling ecosystem are certain to become apparent. This research was supported by grants from the National Science Foundation, OCE 76 00136, OCE 75 23718, OCE 75 23722, OCE 76 00132, OCE 76 82835, and OCE 76 01309 to investigators as part of the International Decade of Ocean Exploration
(IDOE) Coastal Upwelling Ecosystems Analysis program.
We
wish to thank especially Dr. Blanca Rojas de Mendiola for helpful discussions about the 1976 Aquaje. The effort of Dr. J. Topinka in carrying out the nutrient analyses is greatly appreciated. REFERENCES BARBER R.T. (1976) Cruise report, ALPHA HELIX Leg 2. Ecosystems Analysis Newsletter, 5, 8.
Coastal Upwelling
BLASCO D. (1971) Composicidn y distribueidn del fitoplankton en la regidn del afloramento de las costas peruanas. Investigacion Pesquera, 35, 61-112. BRONGERSMA-SANDERS M. (1948) The importance of upwelling water to vertebrate paleontology and oil geology. Verhandelingen der Koninklyke Nederlandsche. Tweede Sectie, Deel XLV (4), 112 pp. BRONGERSMA-SANDERS M. (1957) Mass mortality in the sea. In: Treatise on Marine ecology and paleontology, J. Hedgepeth, editor, Geological Society of America Memoir 67, 941-1010. BURTT J.L. Callao.
(1852) On fish destroyed by sulphuretted hydrogen in the Bay of American Journal of Science, Series 2, 13, 433-434.
CODISPOTI L.A. and F.A. RICHARDS (1976) An analysis of the horizontal regime of denitrification in the eastern tropical North Pacific. Limnology and Oceanography, 21, 379-388. DUGDALE R.C. (1972) Chemical oceanography and primary productivity in upwelling regions. Geoforum, ii, 47-61. FIADEIRO M. and J.D.H. STRICKIJ~D (1968) Nitrate reduction and the occurrence of a deep nitrite maximum in the ocean off the west coast of South America. Journal of Marine Research, 26, 187-201. GOERING J.J. (1968) Denitrification in the oxygen minimum layer of the eastern tropical Pacific Ocean. Deep-Sea Research, 15, 157-164. GOERING J.J., F.A. RICHARDS, L.A. CODISPOTI and R.C. DUGDALE (1973) Nitrogen fixation and denitrification in the oceans. Biochemical budgets. Proceedings of the International Symposium on Hydrogeochemistry and B£~geochemistry, 2, 12-27. GUNTHER E.R. (1936) Variations in the behavior of the Peru coastal current with a historical introduction. Journal of the Royal Geographical Society, 88, 37-61. IVANENKOV V.N. and A.G. ROZANOV (1961) Hydrogen sulfide contamination of the intermediate waters of the Arabian Sea and the Bay of Bengal. Okeonologia, I, 443-449. ¢
.
LAVALLE Y GARCIA J.A. DE (1917) Informe prellmlnar sobre la causa de la mortalidad de las aves ocurrida en el mes de marze del presente a~o. Memoria Companla Administradova del Guano, Lima, 8a, 61-88.
608
PACKARD T.T. 5, 6.
Preliminary Communication
(.1976) Cruise report on ALPHA HELIX, Leg 0.
CUEA Newsletter,
POPOVICI F. and G. CHACON DE POPOVICI (1966) E1 "Aquaje" en el Pacifico Peruano. Memoria del I seminario Latinamericano sobre el oc~ano Paclfico Oriental, Universidad Nacional Mayor de San Marcos, Lima, 168-171. RICHARDS F.A. (1965) Anoxic basins and fjords. In: Chemical Oceanosraphy, Vol. I, J.P. Riley and G. Skirrow, editors, Academic Press, 611-645. SLAWYK G. and J.J. MACISAAC (1972) The comparison of two automated ammonium methods in a region of coastal upwelling. Deep-Sea Research, 19, 521-524. SMITH R.L., D.B. ENFIELD, T.S. HOPKINS and R.D. PILLSBURY (1971) The circulation in an upwelling ecosystem: The PISCO Cruise. Investigacion Pesquera, 35, 9-24. STRICKLAND J.D.H. and T.R. PARSONS (1968) A practical handbook of seawater analysis. Fisheries Research Board of Canada Bulletin, 167, 311 pp. WOOSTER W.S. and M. GILMARTIN (1961) of Marine Research, 19, 97-122.
The Peru - Chile undercurrent.
Journal
WOOSTER W.S., T.J. CHOW and I. BARRETT (1965) Nitrite distribution in Peru Current waters. Journal of Marine Research 23, 200-221.
Preliminary Comunicatlon DENITRIFICATION AND HYDROGEN SULFIDE IN THE PERU UPWELLING REGION DURING 19761
R.C. DUGDALE 2, J.J. GOERING 3, R.T. BARBER 4, R.L. SMITH 5, and T.T. PACKARD 2
Denitrification,
one of the several processes determining the nitrogen
balance of the sea, is defined as the reduction of NO3 to a gaseous end product, N 2 or N20.
It is carried out by bacteria that use nitrate as a
hydrogen accepter when dissolved oxygen concentrations fall to very low or undetectable levels. When nitrate concentrations fall to low or undetectable levels as a result of denitrification,
SO42 is used as a hydrogen
accepter in anaerobic respiration with H2S as the end product. These processes are best known in the marine environment from studies of enclosed bays, fjords, or basins with shallow sills inhibiting communication with adjacent open seawater (Richards, 1965).
The occurrence of denitrification
in the open sea has been demonstrated in the north eastern tropical Pacific Ocean by direct measurements
(Goering, 1968) and from considerations of
the relationships between the concentrations of NO3 and ~t (Goering, Richards, Codispoti and Dugdale, 1973; Codispoti and Richards, 1976). Fiadeiro and Strickland (1968), also using a similar technique, provided convincing evidence that denitrification occurs off the coast of Peru. In each of these studies, the quantity of NO3 removed from the water column was not great, about i0 to 20% of that originally present. Evidence that massive denitrification occurs in the Peru upwelling system under certain unusual conditions was obtained in April, 1976 during the first phase of the JOINT 116 expedition of the Coastal Upwelling Ecosystems Analysis
(CUEA) project.
The region studied is at about 15 ° S
on the Peru coast where a persistent plume of cold upwelling water identified as an upwelling center by Gunther (1936) has been studied on two previous iContribution No. 76015 from Bigelow Laboratory for Ocean Sciences. 2Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Maine 04575. 31nstitute for Marine Sciences, University of Alaska, College, Alaska 99701. 4Duke University Marine Laboratory, Beaufort, North Carolina 28516. 5School of Oceanography, Oregon State University, Corvallis, Oregon 97331. 6jOINT II is the name of a multiship endeavor off the coast of Peru. 601
602
cruises,
Preliminary Communication
the R/V ANTON BRUUN cruise 15 in 1965 and R/V THOMAS G. THOMPSON
cruise 36 in 1969.
Both cruises were in late March or early April and the
JOINT II expedition was scheduled to begin in mid-March,
1976.
The R/V ALPHA
HELIX, the experimental biology ship for the expedition, began its work on March 22. An array of current meters was moored near 15 ° S: three along the CUEA "C" line and one on the "B" line (Figure i). The mooring at C-5 is I
JOINT ]~ AREA 14040~-
MET STATIONS ® CURRENT METERS
\
o
~
i
T6°oo'
I
Fig. i.
Tso,o ' I
\ \
cA
HYDRO S T A T I O N S
~_
,
NA~ICAL UILES
.............
~' I
CUEA study area and current meter locations near 15 ° S l a t . the coast of Peru.
referred to as Lobivia.
along
The work of the R/V ALPHA HELIX was concentrated
at C-3; daily stations were occupied and a wide variety of variables measured.
These included temperature, nutrients,
production,
chlorophyll,
14C primary
15N uptake of nitrate and an~nonia, 30Si measurements of Si(OH)4
uptake and regeneration, phytoplankton species composition,
zooplankton
biomass and species composition and regeneration of nutrients by zooplankton grazing.
The ship assigned to make hydrographic stations for the expedition,
the R/V THOMAS G. THOMPSON, was nearly a month late in arriving and the R/V ALPHA HELIX occupied additional stations along the C-line in an attempt to fill the void during this period.
Analytical methods used!were: NO3, NO;,
PO43, and Si(OH)4 by AutoAnalyzer according to Strickland and Parsons (1968) and NH~ by AutoAnalyzer according to Slawyk and Maclsaac (1972). RESULTS Following initial occupation of a series of stations along the C-line, it became apparent that the NO3 profiles had an unusual appearance, often showing a decrease in concentration with depth. On Sta. 16, taken April i,
Preliminary Communication
I0 O--
TEMPERATURE°C I~ 200
NO3-N,~g Atlliter I0 20
NO2 -N,~Atlliter 30012345678
NH4"N,/A) At I lltw 0123456789
603
f~4lpj~g At/liter I
2
Si(OH) 4 - Si,~l At/liter
4
I0
20
30
40
lOG
w
2OC
_z
N
~00."
CUEA
Fig. 2.
JOINT~ AREA LOCATION C - 5
Profiles of temperature and nutrients at location C-5.
1976, at location C-5 (Figure i) the nutrients were determined immediately following collection. Samples with undetectable NO3 concentrations were noted at depth and in samples freshly drawn from these Nansen bottles the smell of hydrogen sulfide was easily detected. The same location was reoccupied a few hours later at Sta. 17 using a sample spacing designed to obtain details of the region of anomolous NO3 distribution. The data from these two stations were combined to produce the profiles shown in Figure 2. Analyses of NO3 for the upper i00 m were lost from Sta. 16 so values of NO3 from Sta. 21, taken at the same location two days later, on April 3, 1976, are plotted also in Figure 2 to give some idea of the near-surface distribution. Nitrate concentrations were undetectable from 130 to 175 m, and NO2 was similarly undetectable, with high concentrations above and below. The NO2-free layer is somewhat more restricted than for NO3, extending from about 150 to 175 m. The NH~ profile shows a peak coinciding with the NO3-
free region, as do Si(OH) 4 and P043. The dashed lines bracket the depths where the odor of H2S was apparent. The temperature profile reflects the characteristically shallow mixed layer of the Peru upwelling region. DISCUSSION The profiles (Figure 2) agree with the classical picture of water column denitrification developed during studies of anoxic basins and fjords. The presence of H2S and absence of NO3 are highly diagnostic even in the absence of reliable dissolved 02 data. The NH~, P043, and Si(OH)4 accumulations result from the oxidation of organic matter accompanying the reduction of first NO3 and then S042 (Richards, 1965). Our observations are apparently the first obtained for an open sea station where complete denitrification had
50
604
Preliminary Co,,munieation
occurred.
Ivanenkov and Rozanov (1961) report the occurrence of H2S in the
Indian Ocean, a condition that signals complete denitrification;
however,
NO3 was reported to be present in the same samples, The denitrified layer observed in April 1976 was apparently contained within the Peru - Chile undercurrent, a warm saline current originating along the northern coast of Peru and fed from equatorial waters (Wooster and Gilmartin,
1961).
The undercurrent can be identified by low 02 concentrations,
high salinities, and the presence of large concentrations of NO2 (Wooster, Chow and Barrett, 1965).
These authors showed that a region of oxygen defi-
cient water occurs along the Peru coast and that a layer of high NO2 concentration occurs beginning somewhat north of Callao and extending southward to northern Chile.
Wooster and Gilmartin (1961), using drogues, observed flow
in the undercurrent to be generally poleward with weak northwest flow in the near surface regions.
Wooster, et al.,
(1965) showed that the thickness of
the high NO2 layer increases downstream.
They show profiles of N02 in the
north with smooth peaks and profiles to the south with double peaks that could be interpreted as the result of depletion within the NO2-rich layer. The NO2 profile in Figure 2 can be viewed as the result of the same processes that produced the double peak profiles described by Wooster, et al. (1965), Because the profiles of Figure 2 clearly are the result of denitrification, a coherent pattern of downstream processes emerges, with a sequence from north to south of low 02 with generation of high NO2 from reduction of NO3, later reduction of NO2 as well, and when both are completely exhausted as in April 1976, reduction of sulfate with the evolution of hydrogen sulfide. The current meter measurements from March and April 1976 were similar to observations in the same area in March and April 1969 (Smith, Enfield, Hopkins and Pillsbury, 1971).
The flow through the "C" line between 20 and
about 300 or 400 m was generally toward the southeast, parallel to the coast and opposite to the prevailing wind.
Presumably the Peru-Chile undercurrent
extended inshore over the inner continental slope and shelf, flowing parallel to the topography.
The data from the mooring at C-5 (Lobivia) is particu-
larly interesting:
the flow was variable but principally toward either 120
or 300 ° true, roughly "alongshore" toward the southeast or northwest.
The
mean "alongshore" flow was computed for the period 28 March to 28 April 1976 from the current meter records, which had been filtered to remove the tides. The mean flow at 106 m was 15.7 _+12.7 cm s -I toward the southeast, but at 206 m it was only 0.3 +16.4_ cm s -I toward the southeast, while at 406 m it was 4.2 !8.0 cm s -I toward the northwest.
The low mean flow at 206 m was the
result of several reversals, which led to intervals over which the apparent net displacement of a water parcel would be negligible. The progressive vector diagram (PVD), or pseudo-trajectory, at 206 m is shown in Figure 3. The reversals may have upset the normally precarious balance between oxygen supply and utilization in the undercurrent, with subsequent complete denitrification and sulfate reduction; i.e., oxygen-depleted water may have returned
Preliminary Communication
605
30
-30
I
~;;
I
I
i
150
--90
- -120
2 0 6 METERS AT LOBIVIA
Fig.
3.
4 3 . 9 DAYS STARTING 2 2 2 9 2 4 MAR 76
Progressive vector diagram for current meter at 206 m, near location C-5 in 650 m of water. Squares indicate 0000 GMT. Axes are N-S and E-W, distances in km.
under regions of high productivity to suffer even further reduction. such reversals occur in other years and other seasons is unknown,
Whether
nor do we
have data for earlier in March to know if such reversals may have set the stage for the denitrification observed in early April. The Peru - Chile undercurrent
is a major source of the water upwelling
onto the Peru coast (Wooster and Gilmartin, R/V THOMPSON 36 cruise in March - April,
1961).
Sections made during the
1969 in the region of 15 ° S l a t .
show clearly the upward slope of isotherms and of NO] isopleths in the surface waters characteristic
of coastal upwelling.
Downwarping of the iso-
therms is evident in these sections below i00 m and indicates of the Peru - Chile undercurrent.
Nitrate concentrations
the presence
at the surface
near the coast were between 15 and 20 ug at I -I, while concentrations
of NO]
were somewhat depressed at about 250 m near the shelf break and within the undercurrent. No synoptic sections are available for the first week of April, However,
1976.
stations occupied along the C-line during the period April I to 4
show low values of NO] throughout the upper 400 m of the water column.
Some
indication of the potential nutrient impoverishment arising from this condition can be obtained by comparing the depths of the 20 ~g at i "I NO3-N isopleth next to the coast, about 20 m in 1969 vs. 450 m in 1976. The physical and chemical conditions
described above occurred within a
period when a massive bloom of the dinoflagellate in progress.
Gymnodinium splendens was
The red tide resulting may be one of the most extensive ever
606
Preliminary Co~nunication
observed.
Packard (1976) reported that during transit of the ALPHA HELIX
from San Diego to the 15 ° S study area, the Gymnodinium bloom was first observed just south of the Galapagos Islands and appeared to be continuous southward from there.
Continuous recordings of chlorophyll fluorescence
made along the C-line at a depth of 3 m by ALPHA HELIX showed extremely high concentrations of chlorophyll seaward of the shelf break.
Microscopic obser-
vations confirmed that the phytoplankton population was strongly dominated by Gymnodinium splendens
(D. Blasco, personal communication).
face was visibly red during the daylight periods.
The sea sur-
Continuous fluorescence
recordings made on the same transect at night showed sharply reduced fluorescence, indicating a daily vertical migration pattern.
Dinoflagellate
populations occur in most coastal upwelling areas (Brongersma-Sanders, 1948) and their distribution in the 15 ° S area of Peru was described for the PISCO cruise by Blasco (1971).
An outbreak of red water occurs annually along the
Peru coast in the weak upwelling period of January or February, giving way to diatom dominated populations as upwelling strengthens (B. Rojas de Mendiola, personal communication).
In 1976, however, the red water was observed ear-
lier, in December (B. Rojas de Mendiola, personal communication) and persisted into mid-May, according to observations made on ALPHA HELIX (Barber, 1976).
Large numbers of medusae, locally termed "malagua," occurred through-
out this period, and there were reports in the newspapers of mortalities of a variety of organisms along the beaches. The phenomenon described above coincides with the normal concept of red tide and carries the designation aquaje along the west coast of South Ameri-
Aquaje does not necessarily coincide with
ca (Popovici and Popovici, 1966).
E1Ni~o
(Brongersma-Sanders, 1957), the strong incursion of warm water from
the north in some years that brings heavy rains to coastal Peru.
Gunther
(1936) suggested that areas of aquaje occur along the Peru coast where an onshore flow of warm water occurs.
Another feature of a major aquaje may be
the occurrence of hydrogen sulfide as described by Lavalle y Garcia (1917), as indicated both by smell and by the darkening of paint and brass where H2S is actively evolved to the atmosphere as observed by Burtt (1852) in the Bay of Callao. Unfortunately, no historical record of major occurrences of aquaje seems to exist for Peru waters.
However, it appears that these events occur
no more often than major E1Ni~o
, perhaps on the scale of about i0 to 20
years as scientists at the Instituto del Mar del Peru do not recall seeing such a strong aquaje as that of 1976 or that described by Lavalle y Garcia (1917) during their work at the Institute, about 20 years (B. Rojas de Mendiola, personal con~nunication).
Clearly, the CUEA program has been fortunate
in observing a major but rare perturbation of the Peru upwelling system. Already we have shown that denitrification can occur so strongly in the open sea that hydrogen sulfide is found in the water column and the nitrogen budget of an upwelling site strongly affected.
As collection of data will
Preliminary Communication
607
continue through June of 1976 and the analysis of these data proceeds,
impor-
tant additional effects on the Peru upwelling ecosystem are certain to become apparent. This research was supported by grants from the National Science Foundation, OCE 76 00136, OCE 75 23718, OCE 75 23722, OCE 76 00132, OCE 76 82835, and OCE 76 01309 to investigators as part of the International Decade of Ocean Exploration
(IDOE) Coastal Upwelling Ecosystems Analysis program.
We
wish to thank especially Dr. Blanca Rojas de Mendiola for helpful discussions about the 1976 Aquaje. The effort of Dr. J. Topinka in carrying out the nutrient analyses is greatly appreciated. REFERENCES BARBER R.T. (1976) Cruise report, ALPHA HELIX Leg 2. Ecosystems Analysis Newsletter, 5, 8.
Coastal Upwelling
BLASCO D. (1971) Composicidn y distribueidn del fitoplankton en la regidn del afloramento de las costas peruanas. Investigacion Pesquera, 35, 61-112. BRONGERSMA-SANDERS M. (1948) The importance of upwelling water to vertebrate paleontology and oil geology. Verhandelingen der Koninklyke Nederlandsche. Tweede Sectie, Deel XLV (4), 112 pp. BRONGERSMA-SANDERS M. (1957) Mass mortality in the sea. In: Treatise on Marine ecology and paleontology, J. Hedgepeth, editor, Geological Society of America Memoir 67, 941-1010. BURTT J.L. Callao.
(1852) On fish destroyed by sulphuretted hydrogen in the Bay of American Journal of Science, Series 2, 13, 433-434.
CODISPOTI L.A. and F.A. RICHARDS (1976) An analysis of the horizontal regime of denitrification in the eastern tropical North Pacific. Limnology and Oceanography, 21, 379-388. DUGDALE R.C. (1972) Chemical oceanography and primary productivity in upwelling regions. Geoforum, ii, 47-61. FIADEIRO M. and J.D.H. STRICKIJ~D (1968) Nitrate reduction and the occurrence of a deep nitrite maximum in the ocean off the west coast of South America. Journal of Marine Research, 26, 187-201. GOERING J.J. (1968) Denitrification in the oxygen minimum layer of the eastern tropical Pacific Ocean. Deep-Sea Research, 15, 157-164. GOERING J.J., F.A. RICHARDS, L.A. CODISPOTI and R.C. DUGDALE (1973) Nitrogen fixation and denitrification in the oceans. Biochemical budgets. Proceedings of the International Symposium on Hydrogeochemistry and B£~geochemistry, 2, 12-27. GUNTHER E.R. (1936) Variations in the behavior of the Peru coastal current with a historical introduction. Journal of the Royal Geographical Society, 88, 37-61. IVANENKOV V.N. and A.G. ROZANOV (1961) Hydrogen sulfide contamination of the intermediate waters of the Arabian Sea and the Bay of Bengal. Okeonologia, I, 443-449. ¢
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LAVALLE Y GARCIA J.A. DE (1917) Informe prellmlnar sobre la causa de la mortalidad de las aves ocurrida en el mes de marze del presente a~o. Memoria Companla Administradova del Guano, Lima, 8a, 61-88.
608
PACKARD T.T. 5, 6.
Preliminary Communication
(.1976) Cruise report on ALPHA HELIX, Leg 0.
CUEA Newsletter,
POPOVICI F. and G. CHACON DE POPOVICI (1966) E1 "Aquaje" en el Pacifico Peruano. Memoria del I seminario Latinamericano sobre el oc~ano Paclfico Oriental, Universidad Nacional Mayor de San Marcos, Lima, 168-171. RICHARDS F.A. (1965) Anoxic basins and fjords. In: Chemical Oceanosraphy, Vol. I, J.P. Riley and G. Skirrow, editors, Academic Press, 611-645. SLAWYK G. and J.J. MACISAAC (1972) The comparison of two automated ammonium methods in a region of coastal upwelling. Deep-Sea Research, 19, 521-524. SMITH R.L., D.B. ENFIELD, T.S. HOPKINS and R.D. PILLSBURY (1971) The circulation in an upwelling ecosystem: The PISCO Cruise. Investigacion Pesquera, 35, 9-24. STRICKLAND J.D.H. and T.R. PARSONS (1968) A practical handbook of seawater analysis. Fisheries Research Board of Canada Bulletin, 167, 311 pp. WOOSTER W.S. and M. GILMARTIN (1961) of Marine Research, 19, 97-122.
The Peru - Chile undercurrent.
Journal
WOOSTER W.S., T.J. CHOW and I. BARRETT (1965) Nitrite distribution in Peru Current waters. Journal of Marine Research 23, 200-221.