Seasonal Development of Subsurface Chlorophyll Maxima in Slope Water and Northern Sargasso Sea of the Northwestern Atlantic Ocean

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1 Biological Oceanography ISSN: (Print) (Online) Journal homepage: Seasonal Development of Subsurface Chlorophyll Maxima in Slope Water and Northern Sargasso Sea of the Northwestern Atlantic Ocean J. L. Cox, P. H. Wiebe, P. Ortner & S. Boyd To cite this article: J. L. Cox, P. H. Wiebe, P. Ortner & S. Boyd (1982) Seasonal Development of Subsurface Chlorophyll Maxima in Slope Water and Northern Sargasso Sea of the Northwestern Atlantic Ocean, Biological Oceanography, 1:3, To link to this article: Published online: 1 Oct 213. Submit your article to this journal Article views: 76 View related articles Full Terms & Conditions of access and use can be found at

2 Seasonal Development of Subsurface Chlorophyll Maxima in Slope Water and Northern Sargasso Sea of the Northwestern Atlantic Ocean J. L. Cox Oceanic Biology Group, Marine Science Institute University of California, Santa Barbara Santa Barbara, CA P. H. Wiebe Woods Hole Oceanographic Institution Woods Hole, MA P. Ortner Ocean Chemistry Laboratory, NOAA/ AOML 15 Rickenbacker Causeway, Miami, FL S. Boyd Woods Hole Oceanographic Institution Woods Hole, MA Contribution No from the Woods Hole Oceanographic Institution (WHOI), Woods Hole, MA Biological Oceanography, Volume I, Number /82/1271-$2./ Copyright 1982 Crane Russak & Company, Inc. 271

3 272 J. L. Cox, P. H. Wiebe, P. Ortner, and S. Boyd Abstract Seasonal trends in the vertical distribution of chlorophyll a in slope water and the northwestern Sargasso Sea are analyzed, using data from a variety of sources, published and unpublished. Both slope water and Sargasso data indicate a seasonal submergence of the chlorophyll maximum and the formation of a summer to early fall deep chlorophyll maximum (DCM). Mechanisms of formation of the DCM appear to differin the two regions, as evidenced by differences in the degree of light penetration at the maximum, timing of DCM formation, abundance of chlorophyll, temperature at the maximum, and relationships between DCM parameters. Correlations between these parameters (DCM depth, DCM chlorophyll concentration, integral chlorophyll, nitracline depth, nitrite maximum depth, pycnocline depth) are generally high for data from both regions combined and in slope water alone. Correlations for Sargasso data alone are all nonsignificant, except for DCM depth vs. nitracline depth. The subsurface maximum of chlorophyll in the oceans is a widespread and consistent feature of many hydrographic regions. It is increasingly clear (see discussion of Cullen and Eppley, 1981) that factors that influence its formation can be quite different according to region and seasonal occurrence. Considering its potential importance as a food resource (Fairbanks and Wiebe, 198 Ortner et al., 198), documentation of aspects of its seasonal and hydrographic variability are of importance to understanding its role in pelagic communities. We present here a synthesis of data on the occurrence of chlorophyll maxima in the slope water off the Middle Atlantic Bight and in the northern Sargasso Sea. These data are used to reveal seasonal changes in the depth and chlorophyll densities at chlorophyll maxima and to document aspects of its occurrence relative to water column properties such as density, temperature, nutrients, and integral values for chlorophyll in the water column. Data from both hydrographic regions are compared and reveal that fundamental differences in descriptive and correlative aspects of chlorophyll maxima exist. Methods Data from oceanic water masses, the Slope Water off the Middle Atlantic Bight and the northern Sargasso Sea, were derived from published and

4 Subsurface Chlorophyll Maxima 273 unpublished data sources, including our own station data. Definitions of these water masses depended upon a combination of geographic and hydrographic limits. The northern Sargasso Sea stations were all taken north of 35 N latitude, west of 6 W longitude, and to the south and east of the Gulf Stream. Gulf Stream stations were rejected on the basis of vertical temperature profiles showing the 15 C isotherm lying between 2 and 45 m, combined with proximity to the axis of the Gulf Stream. Data sources for the Sargasso were: Wiebe et al. ( 1976), Ortner ( 1977), Anonymous (1961), Ketchum and Ryther (1965), and unpublished results ofrv/knorr cruises 62, 65, 71, and RV/Endeavor cruise 11 in the region. Stations that were taken in cold core rings were not used in the analysis. Slope water stations were geographically limited to the region west of 6 W longitude and seaward of the 2-m isobath off the shelf of the Middle Atlantic Bight. Slope water is a hydrographically complex area (see Cox and Wiebe, 1979, for discussion). Two features that substantially alter water column properties in the region are warm core rings and low salinity surface intrusions in the ''shelf water bulge'' extending over the continental slope. Stations in warm core rings were rejected on the basis of anomalously high temperature and salinity (Saunders, 1971). Our slope water stations strongly influenced by shelf water were arbitrarily judged to be those with lower than 34% salinity and were not used in the analysis. Gulf Stream stations occurring in the region normally occupied by slope water were identified by the temperature criterion described above and were not used in the analysis. Data sources for the slope water were Wiebe et al. (1976), Ortner (1978), Anonymous (1961), Ketchum and Ryther (1965), and unpublished results of RV/ Knorr cruise 65 and RV/Endeavor cruise 11 in the region. Techniques used in the analyses for chlorophyll, nutrients, temperature, and salinity are published in the sources for the data and are identical to those described by Wiebe et al. (1976) for the unpublished data. For the chlorophyll data, a total of 67 stations and 484 points were gathered for slope water, four of which were eliminated from the analysis, since they appeared to be erroneous. The Sargasso data consisted of a total of 43 stations and 379 points, with no eliminations. Not all of these stations had companion nutrient, temperature, and salinity data, so that some comparisons reflect a somewhat smaller data set. A complete listing of station data is available from the first author.

5 274 J. L. Cox, P. H. Wiebe, P. Ortner, and S. Boyd Results Figures 1 and 2 show composite vertical profiles of chlorophyll a from different months in the northern Sargasso Sea and slope water. Two features are particularly evident in these sets of profiles. First, while both water masses show the seasonal development of a deep chlorophyll maximum (DCM), the phenomenon is more intense and short-lived in slope water compared to the Sargasso, where the chlorophyll maximum is always subsurface. During the stratified period in slope water (May to September), DCM depths are significantly shallower (median depths at ca. 4 m) than in the northern Sargasso Sea (median depths at ca. 75 m). Both DCMs show submergence from shallower positions in winter months to greatest depths during summer and fall months, but the slope water DCM does not maintain the constant ''plateau'' of median depth of occurrence that is evident in the Sargasso. Second, the Sargasso profiles seem to show the same degree of variability as the slope profiles. As a simple index of profile variability, we have used the index V, which is the difference in the integrated chlorophyll a value for the minimum side and maximum side of the envelope of values, divided by the mean of both values. The null hypothesis of V sarg = V slope for all of the seasonal data is not rejected ((t-test).1 > p >.5); the range of variation between water masses with regard to chlorophyll profile envelopes is not significantly different. Figure 3 summarizes information on the median depth of the subsur- NORTHERN SARGASSO SEA CHLOROPHYLL (mg/m3) drftp rrtn ' J {... f "'" r:: r: r:. f,". [[ FIGURE 1. Composite vertical profiles of chlorophyll a from the northern Sargasso Sea. Solid lines represent arbitrary limits for high and low values from all the station data plotted and are not reflective of the values from any individual profile. Solid dots represent the maximum value for individual profiles, so the number of such dots in each composite plot is equal to the number of profiles used in its construction. In some cases, lower depth points are only represented by a single station, in which case the points are connected by a single line. Sources of data are listed in the text.

6 Subsurface Chlorophyll Maxima 275 SLOPE WATER CHLOROPHYLL (mgfm3) : I 5: UJ MAY-JUNE 1.5 NOVEMBER DECEMBER FIGURE 2. Composite vertical profiles of chlorophyll a from the slope water. Details of the profj.jes are outlined in the caption for Figure 1. Sources of data are listed in the text. face chlorophyll maximum, as derived from the values plotted in Figures 1 and 2. An important difference between the two environments is that slope water standing crop, while variable, remains relatively constant after the bloom in late April and May, despite deepening of the DCM during that period. Sargasso standing crops, in contrast, decline steadily after the DCM depth is established in May. Light values at the median DCM were computed according to Riley's

7 276 J. L. Cox, P. H. Wiebe, P. Ortner, and S. Boyd a. SLOPE WATER 12 N' E >.s...j...j > I a. a:...j I OJFMAMJJASOND 6 b. SARGASSO 4 2 _s ::J x <t: :J12 c. SLOPE WATER LJ,-l-:F:-1-:-MA...I..:-:M:L:-J.LJ-;-LA;-'-;S:;-'-;::;-'-;:N;-'-;::;-'D a. Or ,...J 6 4 LL. I 8... a. 12 d. SARGASSO JFMAMJJASOND MONTH OJFMAMJJASOND MONTH FIGURE 3. Chlorophyll a standing crop seasonal variation and depth of the chlorophyll a maximum. Graphs a and b were derived from the composite plots in Figures I and 2. The solid heavy line connecting the circles connects the median values for integral water column chlorophyll a for the periods plotted in Figures I and 2. The lighter lines represent integral values for the maximum and minimum lines plotted in those figures. Graphs c and dare piots of the ranges and median values for depth of the chlorophyll a values from Figures I and 2. The numbers that appear next to the circles (which represent the median subsurface chlorophyll maximum depths) are the percent of light values for that depth, computed on the basis of the average value of mg chlorophyll a/of for all the overlying data points. This average value, C, was used in the equation k = C +.54C 13 to solve for the extinction coefficient that was used to compute the percent of light at the plotted DCM depths. The equation is given in Riley (I956) and discussed in Riley (I975). (1956, 1975) equation for absorptive effects of chlorophyll in the overlying water column. These computed values, expressed as percent of surface intensity, appear in Figures 3c and 3d. Sargasso DCM light values during the ''plateau'' period from May to November range from 2.1 to.8%, a range that is only reached during September in slope

8 Subsurface Chlorophyll Maxima 277 water, where values are much higher owing to generally shallower DCM depths. Correlations between several variables derived from the data set, principally involving maxima, were computed from station data where such maxima were observable. Table 1lists correlation coefficients for a number of parameters and compares these to available literature values. Correlations were generally poor for the Sargasso data, except for the Znitracline vs. Zcmax correlation. High correlations were found for all comparisons using slope water data alone, except for Znitracune vs. nteg All correlations were highly significant except where noted. Temperatures at the depth of the chlorophyll maximum in the Sargasso were higher and less seasonally variable than in the slope water (Figure 4). The seasonal range of temperatures at DCM depths in the Sargasso is within 2 to 3 C of the mean temperature, and during the time when the DCM is at a relatively stable depth position (past May) the 21 oc isotherm will always fall within the envelope of DCM depth values shown in Figure 3d. Figure 5 shows the relationship between the slope of the pycnocline (ao-tl az) at the point of maximum nitracline slope ([a NO,j az] max). There is almost no overlap between Sargasso and slope water values, since both gradient features are much less extreme in the Sargasso than in slope water. In all but four cases the slope of the pycnocline at the nitracline depth was also the maximum slope for the water column, indicating that the nitracline depth is also highly correlated with the maximum pycnocline gradient. Discussion The procedure used for presenting chlorophyll data in Figures 1 and 2 represents a conservative approach for estimating the range of values to be expected in any particular month or period in either slope water or Sargasso. Although much attention has been paid to the fact that discrete sampling may tend to miss smaller peak features with a vertical extent less than that of the sampling intervals commonly employed in bottle casts (Strickland, 1968), the magnitude of error in estimates of per square meter chlorophyll introduced by discrete depth sampling is of the same order as error from small-scale horizontal differences (Derenbach et al., 1979) or small-scale temporal differences (Denman et al., 1977).

9 H erbland and Sarg. + Slope Slope Sarg. Voiturez (1979) Cullen +Eppley (1981) Zcmax VS. Cmax ') Zcmax VS. Cmax (In) a -.74 ZN2 max vs. z<1n3 max a.95 z<1n3 max vs. Clntel a -.8 Zcmax vs. z<1n3 max In our data and in Cullen and Eppley ( 1981) the Z <1.o, _, was computed as the depth where NO 3 concentrations reached the 1. p,gcat/1 concentration. The Herbland and Voiturez (1979) method for determining nitracline depth was to use the depth of the first non-zero concentration :We could not use this method, since detectable NO 3 often was observed in the mixed layer. N :--.. t--o "'1::l?:: "'1::l., c s C'-1 b:l Table 1 Correlations between parameters of deep chlorophyll maxima Correlation coeffjcieots not significantly different from zero.

10 Subsurface Chlorophyll Maxima I I- Q_ w I- <( - - w a: ::l I- <( a: w Q_ w I- 22 J FMAMJ J ASOND MONTH FIGURE 4. Temperatures at the depth of the chlorophyll a maximum. For all the stations that have data in the composite plots of Figures I and 2, and that also have corresponding temperature data, the mean temperatures at the depth of the chlorophyll maximum were averaged for the periods indicated. The vertical bars represent the standard error of the mean. Errors in a single, discrete, vertical profile, in fact, create only a slight negative bias in estimates of integral water column chlorophyll (Venrick et al., 1973), and such errors would tend to be eliminated in composite plots such as those employed in Figures 1 and 2. Since the observations were drawn from different cruises, in different years, from different station locations within the two hydrographic regions, and were made at different dates within each monthly period, we believe that the extent of the envelope is a good estimator of the variability of chlorophyll dis-

11 >< co.4 -E N.35 <J... C").3 z <J... LL.25 w :::J.2 _J <:3:: >.15 :::J X.1 <(.5 1. y = x r =.7 n =51 ; PYCNOCLINE SLOPE (.!lo-t/.!lz) at (N3/Z)max FIGURE S. Relationship of the magnitude of the density gradient at [N3 (Z] max and its depth of occurrence. The regression coefficient for the Sargasso data was significant ({3 +, p <.5), and the slope water data were not significant. 28

12 Subsurface Chlorophyll Maxima 281 tributions during the indicated times. Station-to-station variability may be due in part to sampling variation due to interval waves. The use of correlations to characterize relationships of the DCM is potentially useful for predictions based on limited data sets, as Herb land and Voiturez (1979) have shown. In their data taken from at least three different hydrographic regions, correlations indicate that vertical profiles of nitrate, nitrite, oxygen, and temperature can be used as estimators of integrated biomass and production in the water column. Our data indicate similar correlations can be obtained from the slope water data. In contrast, Sargasso Sea data, taken alone, show generally low and nonsignificant correlations for all but Znuracline vs. Zcmax Pooled data from slope water and the northern Sargasso do not differ significantly with regard to the correlation coefficients from slope data alone. Working from data that incorporate stations from fundamentally different water masses such as the northern Sargasso and the slope water can erroneously give the impression of continuity. In our data, using overall regression relationships for pooled data, estimates for the range encompassed by the Sargasso data alone are probably not valid. An illustration of this concept is shown by the Znitracline vs. ZN2max correlations in Table 1. The addition of a data set with a nonsignificant correlation coefficient (Sargasso) actually improves the value of the correlation coefficients for the pooled data relative to the slope data alone. An interesting aspect of the relationship between Zmax and Cmax (mg chlp a/m 3 at the maximum) is the fact that we obtained a higher correlation coefficient by using log transformed values of Cmax (Table 1). Thus, the log transformed data appear to fitthe regression line better. Transformation of the corresponding data of Herbland and Voiturez (1979) did not improve the correlation coefficient (Table 1). Using natural log transformation, the slope for our slope water data yielded a value of -.26m- 1 and for Herbland and Voiturez data (1979) a value of -.18m- 1 If chlorophyll a concentrations at the DCM (Cmax) decrease with depth primarily because of decreasing light availability, then these slopes should approximate a reasonable light extinction function, expressed as the light extinction coefficient, k (m- 1 ). Estimated k values for the stations all exceeded.5, which is approximately twice the slope estimated for the Zcmax vs. Cmax function just given. Thus, it can be concluded that as the DCM deepens, chlorophyll a at the DCM decreases much less rapidly than the decrease in incident

13 282 J. L. Cox, P. H. Wiebe, P. Ortner, and S. Boyd light intensity. Whether this effect is due to increased chlorophyll a/cell (Steele, 1964), increased quantum efficiency of production at the DCM (Taguchi, 198), or accumulation of cells sinking from above (Steele and Yentsch, 196) cannot be determined from our data. According to the model of Jamart et al. (1977), a major factor that affects the value of Cmax is grazing pressure. Recent observations of microzooplankton and macrozooplankton biomass in slope water and in the Sargasso, at DCM depths, (Ortner et al., 198) suggest that substantial seasonal and regional differences in zooplankton biomass density and composition may occur, although zooplankton aggregations at the DCM are a common feature, especially in the Sargasso Sea. Data-based oxygen isotope ratios indicate that foraminifera achieve most of their feeding in the DCM (Fairbanks and Wiebe, 198). Since the intensity of zooplankton grazing at DCM depths is of importance, the degree to which zooplankton can actively seek or aggregate in that zone will have an impact on Cmax. A possible environmental cue that has been suggested for such aggregative behavior by motile grazers is temperature (Harder, 1968; Boyd, 1973). The data in Figure 4 show quite dramatically the extreme unpredictability of temperature at the depth of the chlorophyll maximum in slope water relative to the Sargasso, both in terms of seasonal variability and variability within a given period. These data imply that while temperature cues for grazers seeking chlorophylla maxima could be quite effective in the northern Sargasso, they would be much less effective in slope water, although this conclusion may be mitigated by the extreme vertical compression of temperature gradients in slope water, which would lessen separation of food and grazer. These differences may be at least partially responsible for the noncorrelation of Zc.ru.x and Cmax in the Sargasso data, if grazers exert a greater influence on Cmax in the Sargasso Sea. As indicated in Table 1, both Sargasso and slope water data show significant correlations between Znitracline and Zcmax a result that has been observed consistently in reports of chlorophyll maxima (Anderson, 1969; Eppley, et al., 1973; Hobson and Lorenzen, 1972; Saigo, 1973; Venrick et al., 1973; Herbland and Voiturez, 1979; Cullen and Eppley, 1981). In our data, however, the depth separation between Znitracline and Zcmax was quite different in the Sargasso data when compared to the slope waterdata. Sargassodatashowedameandifferenceof45 ± 27m(S.D., n = 17), while the slope water mean difference was much smaller, with

14 Subsurface Chlorophyll Maxima 283 a mean of 12 ± 19m (S.D., n = 3). The t-test shows that these means are significantly different (p <. 1). Two explanations may account, at least in part, for this difference in slope water. First, the pycnocline occurs closer to the surface, as does the subsurface chlorophyll maximum; hence the amount of light available at Zcmax (see Figures 3c and 3d) is considerably greater, so presumably uptake rates of nitrate are higher, creating a sharper concentration gradient close to the chlorophyll maximum. In addition, density gradients at Znitracline in the slope water (Figure 5) can be at least an order of magnitude higher than in the Sargasso. This steep density gradient is an effective retardant of diffusive supply of nutrients to the DCM; hence for supply to be maintained at higher standing crop densities and uptake rates, the spatial separation of the uptake site (the DCM) and the zone of maximum diffusion potential (the depth of [dn 3 / dz] max) must be small. The pattern in slope water of a seasonally increasing depth of the chlorophyll maximum suggests that wind stress, internal waves, or other processes supplying mixing energy at the pycnocline, which work to increase the depth of the mixed layer, are the predominant factors in the continuing supply of ''new'' nitrogenous nutrients to the DCM. In contrast, the relatively stable position of the DCM in the Sargasso Sea during summer and fall suggests that such mixing is of relatively less importance in DCM nutrient supply there. The relatively greater spatial separation between the DCM and the nutricline suggests that other factors, such as light availability and regenerated nutrients, may exert a more important influence upon DCM depth and Cmax than cross-pycnal supply ofn 3. Acknowledgments This research was supported in part by NSF grant OCE A1 and ONR grant NOOOI4-74-C262NR83-4 to P.H.Wiebe. We gratefully acknowledge the assistance of WHOI ship operations personnel and others who participated as volunteers in the many cruises on which this work is based. References Anderson, G. C Subsurface chlorophyll maximum in the northeast Pacific Ocean. Limnol. Oceanogr. 14: Anonymous Biological, chemical, and radiochemical studies of the marine plankton for the period January 1-December 31, 196. Appendix C to WHOI Technical Reference 61-6 (unpublished report).

15 284 J. L. Cox, P. H. Wiebe, P. Ortner, and S. Boyd Boyd, C. M Small-scale spatial patterns of marine zooplankton examined by an electronic zooplankton detecting device. Neth. J. Sea Res. 7: Cox, J. L., and P. H. Wiebe Origins of oceanic plankton in the middle Atlantic bight. Estuarine Coast. Mar. Sci. 9: Cullen, J. J., and R. W. Eppley Chlorophyll maximum layers of the Southern California Bight and possible mechanisms of their formation and maintenance. Oceanologica Acta 4: Denman, K., A. Okubo, and T. Platt The chlorophyll fluctuation spectrum in the sea. Limnol. Oceanogr. 22: Derenbach, J. B., H. Astheimer, H. P. Hansen, and H. Leach Vertical microscale distribution of phytoplankton in relation to the thermocline. Mar. Ecol. Prog. Ser. 1: Eppley, R. W., E. H. Renger, E. L. Venrick, and M. M. Mullin A study of plankton dynamics and nutrient cycling in the central gyre of the North Pacific Ocean. Limnol. Oceanogr. 18: Fairbanks, R. G., and P. H. Wiebe Foraminifera and chlorophyll maximum: vertical distribution, seasonal succession, and paleooceanographic significance. Science 29: Harder, W Reactions of plankton organisms to water stratification. Limnol. Oceanogr. 13: Herbland, A., and B. Voiturez Hydrological structure analysis for estimating the primary production in the tropical Atlantic Ocean. J. Mar. Res. 37: Hobson, L. A., and C. J. Lorenzen Relationships of chlorophyll maxima to density structure in the Atlantic Ocean and Gulf of Mexico. Deep-Sea Res. 19: Jamart, B. M., D. F. Winter, K. Banse, G. C. Anderson, and R. K. Lane A theoretical study of phytoplankton growth and nutrient distribution in the Pacific Ocean off the northwestern U.S. Coast. Deep-Sea Res. 24: Ketchum, B., and J. H. Ryther Biological, chemical, and radiochemical studies, WHOI Technical Report (unpublished report). Ortner, P. B Investigations into the seasonal deep chlorophyll maximum in the Western North Atlantic, and its possible significance to regional food chain relationships. Ph.D. Thesis, Woods Hole Oceanographic Institution, 33 pp. Ortner, P. B., P. H. Wiebe, and J. L. Cox Relationships between oceanic epizooplankton distributions and the seasonal deep chlorophyll maximum in the Northwestern Atlantic Ocean. J. Mar. Res. 38: Riley, G. A Oceanography of Long Island Sound, II. Physical oceanography. Bull. Bingham Oceanogr. Colt. 15: Riley, G. A Transparency-chlorophyll relationships. Limnol. Oceanogr. 2: Saigo, Y The formation of the chlorophyll maximum in the Indian Ocean, pp /n: The Biology of the Indian Ocean, B. Zeitzschel and S. A. Gerlach, ed. Springer. Saunders, P. M Anticyclonic eddies formed from the Gulf Stream. Deep-Sea Res. 18:

16 Subsurface Chlorophyll Maxima 285 Steele, J. H A study of production in the Gulf of Mexico. J. Mar. Res. 22: Steele, J. H., and C. S. Yentsch The vertical distribution of chlorophyll. J. Mar. Bioi. Assn., U.K. 39: Strickland, J. D. H The comparison of profiles of nutrients and chlorophyll concentrations taken from discrete depths and by continuous recording. Limnoi. Oceanogr. 13: Taguchi, S Phytoplankton photosynthesis in the subsurface chlorophyll maximum layer of the tropical North Pacific Ocean. J. Exp. Mar. Bioi. Ecoi. 43: Venrick, E. L Systematic sampling in a planktonic ecosystem. Fish. Bull. 76: Venrick, E. L., J. A. McGowan, and A. W. Mantyla Deep maxima of photosynthetic chlorophyll in the Pacific Ocean. Fish Bull. 71: Wiebe, P. H., E. M. Hulbert, E. J. Carpenter, A. E. Jahn, G. P. Knapp, S. H. Boyd, P. B. Ortner, and J. Cox Gulf Stream cold core rings: large-scale interaction sites for open ocean planktonic communities. Deep-Sea Res. 23: