subsurface chlorophyll maximum in the northeast pacific ocean

Subsurface Chlorophyll Maximum in the Northeast Pacific OceanAuthor(s): G. C. AndersonSource: Limnology and Oceanography, Vol. 14, No. 3 (May, 1969), pp. 386-391Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2833802 .

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SUBSURFACE CHLOROPHYLL MAXIMUM IN THE NORTHEAST PACIFIC OCEAN'

G. C. Anderson Department of Oceanography, University of Washington, Seattle 98105

ABSTRACT A well-developed subsurface chlorophyll maximum is present during summer in oceanic

waters off the Oregon coast. It appears to be formed at depth by a photosynthetically active phytoplankton community that is adapted to low light intensity. It may be present and may affect the distributions and concentrations of primary production, oxygen, and nutri- ents over large areas of the North Pacific Ocean.

INTRODUCTION

It has been known for several years that the maximum concentration of chlorophyll may be found at depth between a seasonal pycnocline and the permanent halocline in oceanic waters off the Washington and Oregon coasts (Anderson 1964). Until re- cently, little was known of its distribution and concentration. This subsurface chlo- rophyll maximum begins beyond the con- tinental shelf and extends seaward. It should not be confused with a coastal chlorophyll maximum which appears over the continental shelf and is associated with upwelling.

The subsurface maximum chlorophyll layer has been charted in detail from two cruises of the RV Thomas G. Thompson. These cruises, during August 1966 and July 1968, covered large areas off the Oregon coast extending seaward to a distance of more than 250 nautical miles (470 kmi). In 1966, standard hydrographic casts with 6- liter plastic water samplers were made to collect seawater samples at closely spaced depths within the interval where the chlo- rophyll maximum occurs. Acetone extracts of chlorophyll were made according to the UNESCO procedure (UNESCO 1966). At a very few stations, these data revealed large concentrations of chlorophyll a (up to 20 times greater concentration than in surface waters) between 50 and 75 m. At

1 Supported by U.S. Atomic Energy Commis- sion Contract AT(45-1)-1725 (Ref: RLO-1725- 130). I thank Dr. C. A. Barnes and Mr. E. E. Collias for their helpful discussions and assistance. Contribution No. 468 from the Department of Oceanography, University of Washington, Seattle.

most other stations, much lower values were recorded in this depth zone indi- cating that the maximum was patchy and that it was probably confined to such a thin layer that water samplers were missing the peak concentration. In 1968, the sub- surface chlorophyll maximum was again investigated in the same area using a re- cently developed method for in vivo mea- surement of chlorophyll a by fluorescence (Lorenzen 1966). Vertical profiles of chlorophyll concentration were made at 78 stations by pumping seawater, with sub- mersible pump and hose, to a shipboard fluorometer (Strickland 1968). Photosyn- thesis was measured in situ by 14C uptake at one station (Steemann Nielsen 1952). At each station measurements were made of nitrate, phosphate, silicate, and dis- solved oxygen concentrations (Strickland and Parsons 1965). Vertical profiles of salinity and temperature were made with a salinity-temperature-depth recorder (Bis- sett-Berman Corp.). Particle counts were made with a Coulter counter (Model B) (Sheldon and Parsons 1967).

RESULTS AND DISCUSSION

The subsurface chlorophyll maximum is a well-developed feature during summer in all oceanic waters investigated off the Oregon coast. Its peak concentration is confined to a relatively thin layer, typically found between 55 and 65 m (Fig. 1) depths not ordinarily sampled by routine hydrographic casts. It is located in a layer of water bounded by two discontinuity layers, a seasonal halocline and pycnocline above and the permanent halocline and

386

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SUBSURFACE CHLOROPHYLL MAXIMUM IN PACIFIC OCEAN 387

PHOTOSYNTHESIS (mg C mr3 day-1) CHLOROPHYLL (mg mr3 x 101)

O 2 4 6 8 IS 12 14 O j X m I , ' 7100

25 / S , ' DISSOLVED PHOTOSYNTHESIS I V OXYGEN

- 50 1-CHLOROPHYLL I

w

7 5 6 7 8

DISSOLVED OXYGEN (ml liter I)

FiG. 1. Vertical distributions of chlorophyll a, photosynthesis, and dissolved oxygen at 450 10' N lat, 1260 56' W long, 26 July 1968. The right ordinate represents depths to which specified amounts of subsurface light penetrate, expressed as percentage of light at the sea surface.

pyonocline below, corresponding usually to depths of about 40 m and 100 m respec- tively (Budinger, Coachman, and Barnes 1964) (Fig. 2). These conditions suggest that the maximum would be subject to little loss by mixing from above. The maximum layer, although continuous over the area investigated, is somewhat patchy. Maximum chlorophyll concentrations were generally 3 to 10 times those in surface waters. Inspection of chlorophyll measure- ments made from hydrographic casts on summer cruises in 1964 and 1965 (Depart- ment of Oceanography 1966, 1967) extend- ing out to more than 700 nautical miles (1,300 km) off Oregon shows that the maxi- mum is continuous seaward, although peak concentrations were probably missed at most stations.

The chlorophyll maximum observed at this depth is most likely a result of phyto- plankton growth there. Processes that ex- plain chlorophyll maxima in other areas have involved consideration of cells sinking from above and subsequent concentration at depth (Riley, Stommel, and Bumpus 1949; Steele and Yentsch 1960), although in situ formation was recently used to ex- plain the presence of a chlorophyll maxi- mum in the metalimnion of a lake in Japan (Ichimura, Nagasawa, and Tanaka 1968).

SALINITY M%o) 32.0 32 5 330 33 5

SIGMA - t 23 24 25 26

25

S0 -

75 I SIGMA- t

-S / I

E 100 I

Lo S

125 _ / SALINITY-is

150 - k- TEMPERATURE

75 1 -

8 10 12 14 16 18

TEMPERATURE (?C)

FIG. 2. Vertical distribution of temperature, salinity, and density (sigma-t) at 450 10' N lat, 126? 56' W long, 26 July 1968.

Off the Washington and Oregon coasts, after the formation of the seasonal pycno- cline during spring, high phytoplankton photosynthesis in upper waters reduces nitrate to undetectable concentrations; as a result, phytoplankton concentration de- clines and remains low throughout summer (Anderson 1964; StefYansson and Richards 1963). Beneath the upper pycnocline, nu- trient concentrations increase with depth and remain relatively high. Light trans- mission, although low, is enhanced by the paucity of phytoplankton above. Photosyn- thesis exhibited a peak in the maximum layer at the same depth as that of chloro- phyll and was detectable throughout the

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388 G. C. ANDERSON

region even though light was exceedingly low at the lower depth (less than 0.1% of surface light at 90 m) (see Fig. 1). The more than threefold increase in chlorophyll at the maximum over that found in upper waters probably does not represent a cor- responding increase in phytoplankton vol- ume. Particle counts using the Coulter counter showed little more than a twofold increase in particle volume at the maxi- mum. Assuming that the difference is due to phytoplankton, these results suggest adaptation of the cells to low light by an increase in their chlorophyll content; the increase is well within the range of values reported from physiological studies of pig- ment concentration adaptation by algae (Steemann Nielsen and Jorgensen 1968). Therefore, although the maximum appears to be formed by growth of the phytoplank- ton within its recorded depth zone, a large part of the chlorophyll increase may re- sult from adaptation to low light. Steele (1964) suggested that midwater chlorophyll maxima in the Gulf of Mexico may be due to an increase of chlorophyll rather than to an accumulation of plants due to sinking.

The presence of this well-developed maximum over an extensive region of the North Pacific Ocean can be expected to have a considerable effect on related oceanographic features and processes; for example, it should be significant to studies of primary production in general and to the distributions of oxygen and nutrients in the North Pacific Ocean.

Ordinarily, when primary production measurements are made on oceanographic cruises, the selection of sampling depths is based on a rather arbitrary definition of a euphotic zone; that is, the region extending from the surface to a depth where 1% surface light occurs, the latter depth com- monly thought of as being near to compen- sation depth. Because net photosynthesis by definition is zero at compensation depth, measurements are usually made only at depths shallower than the 1% level. Con- sequently, a large part of the primary pro- duction may not be measured in regions where phytoplankton communities adapted

to low light exist at considerable depth. In this instance, compensation depth, assum- ing 14C uptake measures close to net pho- tosynthesis (Strickland 1960), extends even below the 0.1% light level and 15% of the total primary production lies below the 1% depth. Based on a value of 0.3 cal cm-2 min-' (24-hr mean) for incident radiation measured during 26 July 1968 and assum- ing that less than half would penetrate the surface of the ocean, light energy at most would be 1.50 x 10-3 cal cm-2 min-1 at the 1% light level. This compares favorably with a commonly used value of 1.45 x 10-3 cal cm-2 min-' for compensation intensity for Coscinodiscus (Jenkin 1937), but both values are too high by an order of magni- tude for the present case.

Over large areas of the North Pacific Ocean during summer, a subsurface oxygen maximum is found beneath the seasonal pycnocline at a depth corresponding to the lower part of the winter mixed layer, simi- lar to the depth of peak concentrations of chlorophyll in the area of this investigation. It is best developed as a broad band (350 to 450 N lat) extending across the entire North Pacific Ocean (Kitamura 1958; Reid 1962). Its occurrence has been explained by summer loss of oxygen above the max- imum layer owing to warming of the upper waters (Reid 1962; Pytkowicz 1964); it has also been suggested that offshore movement and subsequent sinking of coastal upwelled water enriched by oxygen from photosynthesis at the surface contrib- utes significantly to the observed maximum off the Washington and Oregon coasts (Stef'ansson and Richards 1964).

From these results, it is suggested that the oxygen maximum is formed largely by photosynthesis, at least in areas where the chlorophyll maximum is present. That summer loss of oxygen in upper waters does occur is not disputed, but it appears invalid to attribute the total oxygen incre- ment to this process, for the following reasons. Surface dissolved oxygen for the area reaches a maximum value of about 6.8 ml liter-' during early spring (Pytko- wicz 1964). In summer 1968, the oxygen

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SUBSURFACE CHLOROPHYLL MAXIMUM IN PACIFIC OCEAN 389

increment was 1.6 ml liter-' (surface 6.0, maximum 7.6) but only half of this amount represents a loss and is confined to waters above the seasonal pycnocline (Fig. 1). An equal amount of oxygen is gained by the maximum layer and can be explained by photosynthesis. From the data presented in Fig. 1, the contribution of oxygen by pho- tosynthesis at the chlorophyll maximum is approximately 0.2 ml liter-' month-'. If the chlorophyll maximum begins to form shortly after the start of thermal stratifica- tion in April, the total amount of oxygen contributed by photosynthesis by the end of July would be 0.6 ml liter-'. This value is in reasonable agreement with the ob- served gain of 0.8 ml liter-', especially when one considers that only a single series of in situ photosynthesis measurements is available, that photosynthesis was not mea- sured at precisely the depth of maximum chlorophyll (62 mi), and that dissolved oxy- gen was not measured during spring 1968.2 It is conceivable that the oxygen increment between 350 and 450 N lat is greater than the generally reported values of 1-2 ml liter-' since routine hydrographic sampling would likely miss peak concentrations in the relatively thin oxygen maximum layer. Also, it is known from this study that the chlorophyll concentration may be at least three times greater than the value used in the above computation. Thus, if one as- sumes a corresponding increase in photo- synthesis, the oxygen increment could be much larger.

Other features of the chlorophyll maxi- mum can be elucidated if it is reasonable to assume that the distribution and con- centration of the oxygen maximum reflects the distribution of chlorophyll. The weak- ening of the oxygen maximum south of

NO3, SIO4 (/,9g-at liter-)

00 1 2 3 4 5 6 7

I ) NO3 ,_

25 -

3. , 50 4 50 | *,.~~S; 04 s

75 - S

Dol 0 1 0.2 0 3 0 4 0 5 0 6 0.7

PO3 (,49 - atI t-I) 044 0.7e

FIG. 3. Vertical distribution of phosphate, ni- trate, and silicate at 450 10' N lat, 1260 56' W long, 26 July 1968.

350 N lat coincident with the disappear- ance of the permanent halocline suggests the importance of stability to formation of the chlorophyll maximum. However, a similar weakening of the oxygen maximum occurs north of 450 N lat where the perma- nent halocline is well developed. This may be a result of other conditions in northern waters such as diminution in incident radi- ation, increased turbidity of surface waters, and greater turbulence-none of which fa- vors phytoplankton development at depth.

The persistent nutrient deficiency in summer, especially nitrate depletion, in these surface waters has been mentioned. At the same time, below the seasonal pyc- nocline, nitrate is detectable only at depths beneath the level of the chlorophyll maxi- mum (Fig. 3). Hence, waters that are mixed into the surface layer as the pycno- cline deepens during summer are nitrate deficient. As the mixed layer depth in- creases, the chlorophyll maximum may gradually deepen leaving a zone of nitrate deficient water between it and the upper pycnocline. Therefore, nutrients that might normally be supplied to surface waters by diffusion and mixing from below are mostly used by the photosynthetically ac-

2 Computation of oxygen production based on phosphate utilization lends further support to the argument that photosynthesis accounts for the sub- surface dissolved oxygen gain during summer. The difference between phosphate concentration oc- curring in the surface mixed layer before spring stratification and the concentration observed at the maximum in July is approximately 0.3 Ag-at./ liter. This represents a gain of 0.9 ml liter-' dis- solved oxygen, assuming a AO/AP ratio of 276:1 by atoms (Redfield, Ketchum, and Richards 1963).

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390 G. C. ANDERSON

tive phytoplankton community making up the maximum zone. Intensive studies for many years by Canadian oceanographers at Weather Station "P" in the Gulf of Alaska (500 N lat, 1450 W long) show a high concentration of nitrate in upper wa- ters during summer but no marked chlo- rophyll concentration at depth (McAllister, Parsons, and Strickland 1960). Recently, it has been shown that nitrate is present at the surface throughout the year over much of the subarctic northeast Pacific Ocean but that it becomes depleted during spring and summer south of about 450 N lat and toward the coast (Anderson, Parsons, and Stephens, in press). The maintenance of high nitrate concentrations in waters north of 450 N lat was attributed to relatively intensive entrainment of deep water into the upper zone coupled with a slow rate of removal of nitrate by the primary pro- ducers. These observations agree with the proposed relationship of the chlorophyll maximum to the distribution of nutrients. Thus, the phytoplankton maximum may act as an efficient subsurface nutrient trap over a large oceanic area. The result is markedly low productivity of surface wa- ters during summer.

NOTE ADDED IN PROOF

Dr. P. H. Wiebe recently called to my attention the chlorophyll data collected during the Ursa Major Expedition by Scripps Institution of Oceanography in summer 1964 (Scripps Institution of Ocean- ography 1967). Chlorophyll measurements were made at standard depths (0, 5, 10, 15, 20, 35, 50, 75, 100, 150, 300 m) on a section at 155000'W long from Kodiak, Alaska, to near 26000'N lat. Although peak concentrations of chlorophyll were prob- ably missed, the general distribution of the subsurface maximum can be followed. The layer was well developed from about 480 N lat to 320 N lat with a gradual deepen- ing from less than 50 m in the northern tip to about 100 m in the southern section. These observations agree well with the boundaries suggested in this paper for the north-south extension of the chlorophyll maximum and lend further support to the

suggestion that its distribution may be transpacific.

REFERENCES

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