Deep-Sea Research, 1969, Vol. 16, pp. 329 to 334. Pergamon Press. Printed in Great Britain

Nitrate distribution in the subarctic Northeast Pacific Ocean*

G. C. ANDERSON'S, T. R. PARSONS~ and K. STEPHENS~

(Received 6 January 1969)

Abstract--Data are presented on the distribution of nitrate in the subarctic Northeast Pacific Ocean. Over most of the area nitrate is present at the surface throughout the year at a concentration above that limiting phytoplankton growth. However, within coastal regions and south of about 45°N, nitrate may become depleted during the spring and summer. It is suggested that the maintenance of high nitrate concentrations over much of the area is due to relatively intensive entrainment of deep water into the upper zone coupled with a slow rate of removal of nitrate by the primary producers. Nitrate depletion in the southern area may be explained by less intensive entrainment of deep water and the utilization of nutrients by a subsurface phytoplankton maximum beneath the seasonal thermocline.

INTRODUCTION

THE DISTRIBUTION of phosphate in the subarctic Northeast Pacific Ocean was shown to be maximal at the centre of the Alaska Gyre and to decrease from the centre toward the coast (REID, 1962). Data on the distribution of nitrate in the same area has been lacking except in a region off the coast of Washington and Oregon, where nitrate, but not phosphate, became depleted during the spring and summer (STEFANS-

SON and RICHARDS, 1963). The question of whether nitrate depletion is a generalized event for the Gulf of Alaska can now be examined with the presentation of nitrate data for the area. These data were collected as parts of continuing studies on pro- duction processes in the subarctic Northeast Pacific Ocean by the University of Washington and the Fisheries Research Board of Canada. Data on other oceano- graphic parameters were collected simultaneously with the nitrate information reported here and have confirmed the general oceanographic features governing production as described in earlier publications and discussed later in this paper.

MATERIALS AND METHODS

Samples were collected during 1966 and 1967 at Ocean Weather Station P (50°N, 145°W) and as a result of cruises carried out in the North Pacific Ocean from 1964 to 1967 for the primary purpose of fish tagging. Data from these cruises have been reported by STEPHENS (1968). An extensive amount of nitrate data was collected during cruises of the R.V. Brown Bear in 1964 and 1965 from off the Washington and Oregon coasts seaward to 145°W (BECK, 1966, 1967). Nitrate analyses were carried out using amalgamated cadmium filings for the reduction of nitrate to nitrite, followed by diazotization and development of the azo dye with N-(1-naphthyl)-ethylenediamine (STRICKLAND and PARSONS, 1965).

*Contribution No. 484 from the Department of Oceanography, University of Washington. tDepartment of Oceanography, University of Washington, Seattle, Washington 98105. :~Fisheries Research Board of Canada, Biological Station, Pacific Oceanographic Group, Nanaimo,

B.C.

329

330 G.C. ANDERSON, T. R. PARSONS and K. STEPHENS

RESULTS

Surface nitrate data have been averaged by 5 ° squares of latitude and longitude, regardless of season or year (Fig. 1). Maximum and minimum nitrate values are given together with the number of samples analyzed. From this figure it may be seen that nitrate depletion does not occur over most of the Gulf of Alaska. However, in the southern region, near 45°N and east of 140°W, depletion of nitrate during summer is the rule. Observations are not available west of45°N, 140°W, to determine whether nitrate ever falls to zero, although the level of phosphate concentration is known to be low in this area (REIo, 1962).

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Distribution of surface nitrate (t~g atoms/L) in the subarctic Northeast Pacific Ocean. Station P

The time-series study of nitrate concentrations with depth at Ocean Weather Station P (Fig. 2) confirms the lack of any period of nitrate depletion and shows, in addition, the depth and extent to which nitrate is utilized during the summer in this area. In presenting these data, one anomaly was excluded from January 1967, in which nitrates of less than 15 t~g atoms/1, were encountered at 150 and 200 m. It is believed that this anomaly was due to a sampling error, but the intrusion of subtropical water with low nitrate concentrations cannot be entirely discounted.

Seasonal changes in nitrate concentration may also be demonstrated where

Nitrate distribution in the subarctic Northeast Pacific Ocean 331

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observations are numerous in the region between 45 ° and 50°N, from the coast to Sta. P (Fig. 3). During winter, surface nitrate values are lower near 45°N than 50°N but increase toward Sta. P. During spring, extensive utilization of nitrate occurs near the coast but the remaining area shows little change from winter conditions. By midsummer, however, nitrate concentration is reduced considerably over the entire area. Nitrate depletion is widespread near the coast and in the southern area, but again nitrate values increase toward Sta. P from a southeasterly direction. During autumn, concentrations remain low near 45°N and along the coast, but a small increase has occurred over much of the region presumably caused by increased mixing at the onset of autumn storms.

DISCUSSION

The physical oceanography of the subarctic Pacific Ocean has been described by a number of authors (e.g., UDA, 1963; DODIMEAD, FaVORITe and HIRANO, 1963; TULLY, 1964 and references cited therein). From the point of view of biological oceanography, the most important physical processes contained in these descriptions are those which affect the stability of the water mass and the input of solar radiation. The effect of these two variables in determining the extent and timing of the spring bloom in the oceanic region of the subarctic Northeast Pacific has been discussed by PARSONS and LEBRASSEUR (1968), while the influence of the Columbia River on the same water mass has been discussed by A~rDERSON (1964). From these references, it is possible to describe some processes that may account for the nitrate distributions shown in Figs. 1-3.

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Nitrate distribution in the subarctic Northeast Pacific Ocean 333

The subarctic Pacific is characterized by a distinctive salinity structure in which the excess of precipitation over evaporation is confined in a near isohaline zone from the surface to 100 m depth. The surface waters circulate around the Gulf of Alaska in a counterclockwise motion and water is lost by a northward flow through Bering Strait, but more so by the southerly flow of the West Wind Drift into the subtropic region. Sub-halocline water is entrained into the upper zone at an approxi- mate average upward velocity of 20 4- 10 m/year. According to TULLY and BARBER (1960), this entrainment is due in part to shear resulting from velocity differences between waters above and below the halocline, similar to the situation in coastal embayments. The shear apparently contributes energy for the preferential upward transfer of deep water into the upper zone. The most intense entrainment appears to occur in the dynamic centre of the gyre. Thus from the point of view of nutrients, the continual replenishment of surface nitrate, especially toward the centre of the Gulf of Alaska, is assured.

Seasonal changes in the upper zone (above 100 m) are marked by the establishment o fa thermocline during summer, when winds are light and there is increased insolation. The relationship between the depth of the thermocline and the amount of sunlight entering the water has been shown (PARSONS and LEBRASSEUR, 1969) to lead to the establishment of the spring bloom, starting in March around the edge of the Gulf of Alaska and terminating in May in a central portion of the gulf, including Ocean Weather Station P. Thus a second reason for the persistence of nitrate-rich water, especially toward the centre of the gulf, is that conditions for plant growth are not well established as nearer the coast (see ANDERSON, 1964) until nearly two months after the spring equinox. This shorter period of plant growth from the coast outward toward the centre of the gulf must lead to a reduced level of nutrient removal from offshore oceanic waters as compared with coastal waters, where it has been shown (STEFANSSON and RICHARDS, 1963) that nitrate becomes exhausted in the surface layers during the spring and summer. In addition, it was postulated earlier (McALLISTER, PARSONS and STRICKLAND, 1960) that the secondary production at Sta. P controlled the standing stock of primary producers so that it was impossible for the latter to increase to a point where it could rapidly exhaust the supply of nutrients.

Over the area where nitrate depletion occurs, there is present during summer a subsurface chlorophyll maximum situated between the permanent halocline and the seasonal thermocline (ANDERSON, in press). This chlorophyll maximum, which represents as much as a 20-fold greater concentration than that at the surface, develops apparently after nitrate is exhausted in the upper mixed layer during the spring bloom and when transparency of the water increases after the decline of the phytoplankton bloom. It has been suggested that the phytoplankton community making up the chlorophyll maximum acts as a nutrient trap. Nutrients which might normally be supplied to surface waters during summer by diffusion and mixing from below are utilized by the considerable photosynthesis which has been measured at the site of the chlorophyll maximum. As a result, after nitrate depletion at the surface, primary production declines and remains low through the summer since nutrient replenishment by physical processes does not occur until autumn and winter mixing, at which time the chlorophyll maximum disappears.

Another reason for nitrate depletion in the southern region is that the total quantity of nutrients supplied to the surface layers during winter mixing is less than in

334 G.C. ANDERSON, T. R. PARSONS and K. STEPHENS

areas to the north. This is in part a result of less intensive entrainment of sub-halocline water compared with Sta. P but is also due to high photosynthetic activity in waters between the seasonal thermocline and permanent halocline. Consequently, the nutrient pool between the two pycnoclines is small in regard to replenishment of nutrients to surface waters during the period of winter mixing.

In conclusion it is suggested that the maintenance of high nitrate concentrations in the oceanic waters over much of the Gulf of Alaska is due to the continual replen- ishment of surface water by deep, nutrient-rich water, and to the slow rate of removal of nitrate by primary producers. Factors contributing to nitrate depletion in the southern part are the less intensive entrainment of deeper waters and the utilization of nutrients below the seasonal thermocline by a phytoplankton community adapted to low light intensity.

Acknowledgements--The study by one of us (G.C.A.) was supported by the U.S. Atomic Energy Commission under Contract AT(45-1)- 1725 (ref. RLO-1725-131), and the Office of Naval Research under Contract Nonr-477 (37), Project NR 083 012.

R E F E R E N C E S

ANDERSON G. C. (1964) The seasonal and geographic distribution of primary productivity off the Washington and Oregon coasts. Limnol. Oceanogr., 9, 284-302.

ANDERSON G. C. (In press) Subsurface chlorophyll maximum in the Northeast Pacific Ocean. LimnoL Oceanogr.

BECK J. R. and the DATA ANALYSIS SECTION (1966) Physical, chemical and biological data from the Northeast Pacific Ocean: Columbia River effluent area, 1964. Univ. Washington, Depart. Oceanogr. Techn. Rept. 180; vol. II, 266 pp. (Unpublished manuscript).

BECK J. R. and the DATA ANALYSIS SECTION (1967) Physical, chemical and biological data from the Northeast Pacific Ocean: Columbia River effluent area, 1965. Univ. Washington, Depart. Oceanogr. Techn. Rept. 182; vol 1, 330 pp. (Unpublished manuscript).

DODIMEAD A. J., F. FAVORITE and T. HIRANO (1963) Salmon of the North Pacific Ocean. Part I[: Review of oceanography of the subarctic Pacific region. Bull. htt. North. Pac. Fish. Comm., No. 13, 195 pp.

McALLISTER C. D., T. R. PARSONS and J. D. H. STRICKLAND (1960) Primary productivity at Station P in the northeast Pacific Ocean. J. Cons. int. explor. Met., 25, 240-259.

PARSONS T. R. and R. J. LEBRASSEUR 0969) A discussion of some critical indices of primary and secondary production for large scale ocean surveys. Calif. mar. Res. Comm., Calcofi Rep., 12, 54-63.

REID J. L., Jr. (1962) On circulation, phosphate phosphorus content, and zooplankton volumes in the upper part of the Pacii~,: Ocean. Limnol. Oceanogr., 7, 287-306.

STEF~,NSSON U. and F. A. RICHARDS (1963) Processes contributing to the nutrient distribu- tions off the Columbia River and Strait of Juan de Fuca. Limnol. Oceanog., 8, 394-410.

STEPHENS K. (1968) Data Record. Primary production data from the Northeast Pacific Ocean, January 1966 to December 1967. MS Rept. Set'. Fish. Res. Bd. Can. No. 957, 58 pp. (unpublished manuscript).

TULLY J. P. (1964) Oceanographic regions and assessment of temperature structure in the seasonal zone of the north Pacific Ocean. J. Fish. Res. Bd. Can., 21, 941-970.

TULLY J. P. and F. G. BARBER (1960) An estuarine analogy in the subarctic Pacific Ocean. J. Fish. Res. Bd. Can., 17, 91-112.

UDA M. (1963) Oceanography of the subarctic Pacific Ocean. J. Fish. Res. Bd. Can., 20, 119-179.