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Temporal and spatial patterns in the surface-water biomass of phytoplankton in the North Water
Deep-Sea Research II 49 (2002) 4947–4958
Temporal and spatial patterns in the surface-water biomass of phytoplankton in the North Water Tsuneo Odatea,*, Toru Hirawakea, Sakae Kudoha, Bert Kleinb, Bernard LeBlancb, Mitsuo Fukuchia a
National Institute of Polar Research, 1-9-10 Kaga, Itabashi-ku, Tokyo 173-8515, Japan b D!epartment de biologie, GIROQ, Universit!e Laval, Qu!ebec, Canada G1K 7P4
Received 1 November 2000; received in revised form 8 August 2001; accepted 20 February 2002
Abstract Temperature, salinity, and in vivo fluorescence of surface seawater in the North Water were recorded continuously, using a CTD+fluorometer, in August 1997, April–July 1998 and August–October 1999. The phytoplankton bloom started in the polynya on the Greenland side in April. In April and May, high phytoplankton biomass coincided with saline water on the Greenland side, while biomass was low on the Ellesmere Island (Canada) side where a deep mixed layer prevailed. High phytoplankton biomass extended over the whole polynya in June, when surface temperature increased due to solar heating and salinity decreased due to freshwater input. The initiation of the bloom was about 2 months earlier on the Greenland than the Canadian side. In July and August, phytoplankton biomass became low in the southern survey area, indicating that the phytoplankton bloom had ended. In September, relatively saline and warm water occurred in the southeastern part of the study area where, consistent with the change in water properties, high concentrations of chlorophyll a were observed again. These results imply that both the earlier start of the algal bloom in spring and the eventual increase in phytoplankton biomass in summer contribute to the high annual primary production along the Greenland side, thus influencing the structure and biological productivity of the entire North Water ecosystem. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction The North Water, located between Greenland and Ellesmere Island (Canada), is one of the largest polynyas and the most productive ecosystems within the Arctic Circle (Stirling, 1997). The ice-free and highly productive waters are considered to serve as feeding, mating, spawning, and *Corresponding author. Tel.: +81-3-3962-4363; fax: +81-33962-5743. E-mail address: [email protected] (T. Odate).
overwintering grounds for huge populations of key species of birds and mammals, although few scientific studies have been conducted in the area until recently. Lewis et al. (1996) observed an east–west gradient in phytoplankton biomass in the polynya in May 1991, i.e. chlorophyll a biomass integrated to 30 m depth decreased from 506 mg m2 in the east to 50 mg m2 in the west. They suggested that a diatom bloom may have been initiated by the melting of sea ice, due to the upward input of sensible heat, and that the bloom moved
0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 2 ) 0 0 1 7 2 - 8
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T. Odate et al. / Deep-Sea Research II 49 (2002) 4947–4958
westwards, progressively exhausting the nutrients (Lewis et al., 1996). That study was limited, however, to a period of 48 h, so that comprehensive studies of the physical, chemical, and biological aspects of the bloom were still needed. The fieldwork of the International North Water Polynya Study (NOW) was conducted from 1997 to 1999, with the general hypothesis that the physical forces that open the North Water are also responsible for its unusually high biological productivity (Fortier et al., 2001). Polynyas usually belong to two main types, i.e. latent heat or sensible heat polynyas, based on their dominant mechanism of formation (Smith et al., 1990). In the first case, the algal bloom has been hypothesized to be delayed until solar heating stratifies the surface layer. In a sensible heat polynya, however, stratification of the surface layer resulting from the melting of the ice cover has been suggested to trigger an algal bloom immediately after the opening of the water. The season of planktonic production (the period between the algal bloom and the freeze-up in autumn) could last up to 6 months in the sensible heat case, 4 months in the latent heat case, and 2 months at similar latitudes outside of the North Water. The latent and sensible heat mechanisms could act together in the case of the North Water (Mysak and Huang, 1991). The present study was aimed at determining the spatial and temporal variability in phytoplankton biomass, based on in vivo fluorescence, in the North Water from spring to autumn, and at comparing the variability in phytoplankton biomass with water properties. The latter goal was to test the idea that the factors responsible for generating the polynya strongly influence its biological productivity.
2. Materials and methods Temperature, salinity, and in vivo fluorescence of seawater, which was pumped from the bottom of the C.C.G.S. Louis S. St. Laurent (draft, 9.91 m) in 1997 and the C.C.G.S. Pierre Radisson (draft, 7.16 m) in 1998 and 1999, were recorded during three summer seasons using a CTD+fluorometer (AquaPack, Chelsea Instruments, Ltd.). Exactly
speaking, the pumped seawater was not the surface water. However, we considered it as the surface water, since our purpose was to show temporal and spatial patterns in water temperature, salinity, and phytoplankton biomass based on seawater collected from similar depths, which were near the sea surface throughout the cruises. Moreover, our data on temperature may be slightly higher than ambient water temperature, since the temperature of the pumped water may have increased slightly due to room temperature during the seawater supply to the water tank in which the CTD+fluorometer was immersed. Flow rate was about 1 l mm1 and the volume of the seawater in the tank was about 10 l. Consequently, turnover rate of seawater in the tank was too short (10 min) to allow increases in phytoplankton biomass to occur; none were ever detected. Observations were made from 18 to 26 August in 1997, from 12 April to 18 May and 3 to 19 July in 1998, and from 28 August to 1 October in 1999. In April and May 1998, the survey area was restricted due to thick ice (see Mei et al., 2002; Mundy and Barber, 2001). The time intervals between records were 2 min in 1997 and 1 min in 1998 and 1999. During the 1999 fieldwork, 24-h observations were conducted at southern stations S2 (761170 N, 711500 W) and S4(761170 N, 741090 W), when the vessel occupied these stations for recovery of mooring arrays (see Fig. 1 in Ingram et al., 2002, for a map of these station locations). During the periods of observation, 34 (1997), 39 (1998), and 123 samples (1999) of surface seawater were collected from a drain of the water tank in which the CTD+fluorometer was immersed. The seawater was filtered onto Whatman GF/F filters, which were immediately placed in tubes containing N;N-dimethylformamide (Suzuki and Ishimaru, 1990), except in 1997 and 1998 when the filters were first frozen (201C and dark). Concentrations of chlorophyll a were determined fluorometrically using a Turner Design Model 10R Fluorometer (Parsons et al., 1984) in the laboratory. These data were used for transforming the in vivo fluorescence data into chlorophyll a concentrations. Details on water-column sampling and determination of chlorophyll a concentrations are de-
T. Odate et al. / Deep-Sea Research II 49 (2002) 4947–4958
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Fig. 1. Variations in temperature, salinity, in vivo fluorescence intensity, and extracted chlorophyll a concentration over a 24-h period at: (a) station S4 (761170 N, 741090 W) and (b) station S2 (761170 N, 711500 W).
scribed by Klein et al. (2002). Results of selected CTD casts (provided by G. Ingram; see also B#acle et al., 2002) were used for illustrating vertical profiles of temperature and salinity.
3. Results 3.1. In vivo fluorescence intensity During the first 24-h observation, from 12:00 on 28 August to 12:00 on 29 August 1999 (local time) at south-central station S4, in vivo fluorescence
intensity varied between 0.08 and 1.86, with a mean value7standard deviation (SD) of 0.6870.39 (Fig. 1a). The coefficient of variation (CV) was 58%. Similarly relatively large variation was observed in the concentration of extracted chlorophyll a, i.e. mean7SD=2.2471.01 and CV=45%. At this station, there was a relatively large CV for water temperature (18%), suggesting that the water mass changed during the period since the vessel stayed at almost the same position. In contrast, fairly constant water temperature and salinity were noted during the second 24-h observation period, from 12:00 on 6 September to
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12:00 on 7 September 1999 (local time) at southeastern station S2 (Fig. 1b). The CVs of water temperature and salinity were 3% and 0.4%, respectively. At that station, temporal variations of in vivo fluorescence and extracted chlorophyll a concentration were also small (CV=8% and 7%, respectively). No trend of higher in vivo fluorescence intensity during the night was detected. Relationships between in vivo fluorescence intensity and concentration of extracted chlorophyll a are illustrated in Fig. 2. For all 3 years, significant regressions were obtained, although the values of the slope and intercept varied. Using these equations, in vivo fluorescence intensity was converted to chlorophyll a concentration. 3.2. Distributions of temperature, salinity, and chlorophyll a concentration Surface distribution of water temperature is shown in Fig. 3 for a composite seasonal sequence from April to September. The surface-water temperature was below zero in almost the whole survey area in April and May. In June, the surfacewater temperature increased from 11C to 201C,
although cold water was observed in the northern part of the survey area. Warm water (>31C) sometimes occurred south of ca. 771N in July and August. In the second half of September, the warm water disappeared and the surface-water temperature was o31C throughout the study area. The temporal pattern of salinity distribution coincided with that of temperature (Fig. 4). Surface salinity was 32.5 to 33.5 over the survey area in April and May, when cold water covered the whole area. During these months relatively high salinity was observed in the eastern part of the area. In June the surface salinity decreased to o32.5 in the whole area. Less saline water (o30.0) occurred east of ca. 721300 W in July (1998) and west of ca. 751W and north of ca. 761300 N in August and the early half of September (1999). During the latter half of September, saline water (32.5–33.0) was observed in the southeastern part of the survey area. Chlorophyll a concentrations were low in April, although slightly higher concentrations (1– 2 mg l1) were noted locally in the eastern part of the survey area (e.g., around 771200 N and 741W and 771500 N and 731W; Fig. 5). The
Fig. 2. Relationships between in vivo fluorescence intensity and extracted chlorophyll a concentration during the cruises of the International North Water Polynya Study (NOW): (a) NOW97; (b) NOW98; and (c) NOW99.
May 6-17 1998
June 4-28 1998
August 18-26 1997
August 28September 14 1999
September 17October 1 1999
July 1-19 1998
-2 -1 0 1 2 3 Temperature (ºC)
4
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April 12-29 1998
Fig. 3. Distribution of surface-water temperature, where red lines indicate cruise tracks and the white hole indicates temperature >41C.
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4952
May 6-17 1998
June 4-28 1998
August 18-26 1997
August 28September 14 1999
September 17October 1 1999
July 1-19 1998
29
30
31 32 Salinity
Fig. 4. Distribution of surface salinity, where red lines indicate cruise tracks and the black hole in the last panel indicates salinity o29.
33
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April 12-29 1998
T. Odate et al. / Deep-Sea Research II 49 (2002) 4947–4958
concentrations were similar in the southern part (below 771N and 74–761W). A strong increase in chlorophyll a concentration occurred from April to May–June. Concentrations >10 mg l1 were observed around 771300 N and 751W in June. In July, chlorophyll concentrations had decreased everywhere except in the north; extremely low chlorophyll water (o0.1 mg l1) was frequently present in the southern part of the area in July– August. From late August to early September, however, chlorophyll a concentrations increased in the southeastern area around 761200 N and 721W. Some examples of east–west distributions of surface-water temperature, salinity, and chlorophyll a concentration along 771500 N are shown in Fig. 6. This northerly study transect was bounded on the west by station 14 (771490 N, 751290 W) and on the east by station 18 (771500 N, 731W) (see Fig. 2 in B#acle et al., 2002, for a map of the station locations defining this and other transects). In April, the eastward increase of salinity occurred east of ca. 741W, while chlorophyll concentrations slightly increased east of ca. 731250 W (Fig. 6a). Warm and saline water occupied the shallower layer on the Greenland side, at stations 18 and 27 (771220 N, 731500 W, on another northerly transect), than on the Canadian side, at stations 14 and 22 (771200 N, 761290 W, on another northerly transect) (Fig. 7a and b). As shown by surface observations, salinity was higher on the Greenland than the Canadian side, but the difference in surface temperature between the two sides was small. In the former area, chlorophyll concentrations were high above the thermocline (Fig. 7c), which was deeper than 60 m. In the latter area, temperature was almost constant over the upper 150 m, where chlorophyll concentrations were low. In May, the increase of chlorophyll corresponded well to salinity (Fig. 6b). Eastward increases of salinity and chlorophyll were observed east of 741W. Vertical profiles of water temperature and salinity on the Greenland side at station 18 in May were similar to those in April, although a slight increase in surface water temperature was observed (Fig. 7d and e). At that station, high chlorophyll concentration occurred above the thermocline (Fig. 7f) as observed in April. In contrast, the water-column properties changed at
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station 27, a slightly more southerly location on the Greenland side. In particular, water temperature and salinity decreased in the top 40 m and became similar to values at stations 14 and 18 along the northerly transect at 771500 N. Similar vertical distributions of chlorophyll were noted at stations 14, 18, and 27, where values were low. As described above, surface salinity largely decreased and chlorophyll concentration increased from May to June. High chlorophyll concentrations occurred east of 741W and relatively saline water occurred east of 731200 W in June (Fig. 6c). Vertical profiles show that a sharp increase of temperature and decrease of salinity occurred in the surface layer on both the Greenland and Canadian sides (Fig. 7g and h). Chlorophyll concentrations increased in the upper layers on both sides (Fig. 7i). Eastward increase of salinity also was observed during the late summer season (Fig. 6d). High chlorophyll concentrations occurred east of 741W, although high concentrations were also detected around ca. 751W where relatively saline water occurred. As mentioned above, the similar trend of high chlorophyll concentration with saline water became clear in the southern part of the polynya (Fig. 8). Along the southerly transect at 761200 N, salinity sharply increased from west to east around 751W, while high chlorophyll concentrations were observed ease of 751W.
4. Discussion The present study aims at determining the spatial and temporal variability in phytoplankton biomass in the surface waters of the North Water. For this purpose, continuous observations of in vivo fluorescence of surface water in the polynya were conducted during several cruises over 3 years. The interpretation of in vivo fluorescence in terms of phytoplankton biomass, however, is not simple, because the in vivo fluorescence from natural phytoplankton communities is well known to show diurnal variations (Falkowski and Kiefer, 1985). Changes in the quantum yield of fluorescence are considered to be a major source of variation of fluorescence in the field (Pre! zelin and
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May 6-17 1998
June 4-28 1998
August 18-26 1997
August 28September 14 1999
September 17October 1 1999
July 1-19 1998
0.1
1 10 -1 Chlorophyll a (µgl )
Fig. 5. Distribution of surface chlorophyll a concentration, estimated from in vivo fluorescence intensity, where red lines indicate cruise tracks and black indicates chlorophyll concentration o0.1 mg l1.
T. Odate et al. / Deep-Sea Research II 49 (2002) 4947–4958
April 12-29 1998
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Fig. 6. East–west distribution of temperature, salinity, and chlorophyll a concentration along the northerly transect of 771500 N in: (a) April; (b) May; (c) June; and (d) September. Values within 10 longitude are shown.
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Fig. 7. Vertical profiles of temperature (a, d, g), salinity (b, e, h), and chlorophyll a concentration (c, f, i) on the Greenland side, at stations 18 (771500 N, 731W) and 27 (771220 N, 731500 W), and on the Canadian side, at stations 14 (771490 N, 751290 W) and 22 (771200 N, 761290 W) in: (a–c) April; (d–f) May; and (g–i) June.
Fig. 8. East–west distributions of temperature, salinity, and chlorophyll a concentration along the southerly transect of 761200 N in September. Values within 10 longitude are shown.
Ley, 1980; Falkowski and Kiefer, 1985). Our 24-h series at station S2 showed that the diurnal variation of fluorescence was small in spite of the reappearance of a day–night cycle and that
chlorophyll a concentration, determined from extracted pigments, did not change within the same water mass (Fig. 1b). Hence, we considered that the variations of in vivo fluorescence intensity mostly represented spatial variations of phytoplankton biomass. The relatively large changes observed at the central station S4 reflected advection, i.e. the Carey Island water was replaced by Central Northern Baffin Bay water as shown by B#acle et al. (2002). The development of the phytoplankton bloom in the North Water also is described from discrete station data by Klein et al. (2002) and Mei et al. (2002). The phytoplankton biomass started to increase on the Greenland side (between 761 and 771300 N and o751W) in April, when the polynya
T. Odate et al. / Deep-Sea Research II 49 (2002) 4947–4958
opened (Mundy and Barber, 2001), and spread north in May–June, when the maximum biomass occurred (Klein et al., 2002; Mei et al., 2002). The present study shows the fine-scale distribution of phytoplankton biomass based on the continuous record of surface-water parameters along the cruise track, indicating that relatively high phytoplankton biomass was first observed in the eastern part of the survey area in April and May, as shown by Klein et al. (2002) and Mei et al. (2002). In that season, low water temperature reached the depth of about 150 m on the Canadian side, meaning that latent heat prevailed; the deep surface mixed layer prevented phytoplankton from developing. Mei et al. (2002) observed chlorophyll a concentrations >10 mg l1 in the southeasterly area around 761200 N and 72–741W in May. Unfortunately, we did not have observations in that area. Consequently, the highest concentration in our data set was lower than that in Klein et al. (2002) and Mei et al. (2002) for May. Since the saline water originates from depth, high phytoplankton biomass in the eastern part of the polynya seems to be associated with deep west Greenland current water entrained into the surface along the Greenland coast (Melling et al., 2001), i.e. sensible heat. In other words, the melting or slower formation of sea ice due to the upwelling of warm water could result in haline stratification of the surface layer, which triggers the phytoplankton bloom (Fortier et al., 2001). However, we did not observe decreased salinity due to ice melt. Melling et al. (2001) showed that upwelled water does not reach the surface and that heat is sufficient only to reduce ice formation, not cause ice melt. At station 27 on the Greenland side, surface salinity decreased from April to May, but the decrease did not result from the melting of sea ice but from the movement of water masses, since surface-water temperature and salinity were similar to those at stations 14 and 22 on the Canadian side. Water mass observed at stations 14 and 22 prevailed eastward at least at station 27 in May. High phytoplankton biomass in the saline water means that the sensible heat scenario was not responsible for the earlier development of the algal bloom in the polynya, but the physical structure nevertheless determined the timing of the phytoplank-
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ton bloom on the Greenland side of the North Water. The bloom did not begin on the Canadian side until June, when surface temperature increased and salinity decreased due to solar heating and melting of ice, respectively. The timing of the bloom was thus about 2 months earlier on the Greenland than the Canadian side. Mei et al. (2002) show that the bloom development is related to the depth of the mixed layer. Phytoplankton production decreased in June– July in the southern part of the survey area, due to exhaustion of nutrients in surface waters (Tremblay et al., 2002). Our results also show low phytoplankton biomass in the south in summer. In late summer, however, high local phytoplankton biomass occurred in the southeastern part of the survey area. One of the reasons for the high biomass could be horizontal advection from Kane Basin in the north, where relatively high phytoplankton biomass also was observed in July 1998 and August 1997 (Fig. 5). To explain the increase of phytoplankton biomass in the southeastern part by advection from the north is difficult, however, since the water that passes through Kane Basin moves from north to southwest along the Ellesmere Island coastline (B#acle et al., 2002; Melling et al., 2001). Furthermore, water from the West Greenland Current, moving from southeast to north, penetrates farther north along the Greenland coast in winter than in summer (Melling et al., 2001), so that upwelling intensity is low in summer in the northern part of the survey area. A certain physical event must nevertheless occur in the southern part along the Greenland Coast in summer to sustain phytoplankton growth and result in increased phytoplankton biomass. The co-occurrence of high phytoplankton biomass with saline water observed in the present study supports this interpretation, although further studies are required to understand the local increase of phytoplankton biomass in the Greenland Coast in summer. The results of this study suggest that the occurrence of saline water determines the timing of the spring bloom in the northern part of the North Water. As the polynya opens more in summer, the occasional increases in phytoplankton biomass in the southeasterly part seem to
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result from a physical event that involves an increase in salinity along the Greenland Coast. The early start of the algal bloom in spring and the increases in phytoplankton biomass in summer contribute to the high annual primary production along the Greenland Coast, thus influencing the structure and biological productivity of the entire polynya ecosystem.
Acknowledgements Some data used in this study were provided by the physics group of the NOW project; we thank G. Ingram, physics group leader, for permission to use CTD data at several stations. We gratefully acknowledge the captains, officers, and crews of the C.C.G.S. Louis S. St. Laurent during the NOW97 cruise and of the C.C.G.S. Pierre Radisson during the NOW98 and 99 cruises and all colleagues involved for their help in the field. We also thank L. Legendre, Z.-P. Mei and two reviewers for their comments on the manuscript. This study was supported by a grant-in-aid for Scientific Research from the Japan Society for the Promotion of Science (No. 11208203) and is a contribution to the International North Water Polynya Study (NOW).
References B#acle, J., Carmack, E.C., Ingram, R.G., 2002. Water column structure and circulation under the North Water during spring transition: April–July, 1998. Deep-Sea Research II, this issue (PII: S0967-0645(02)00185-6). Falkowski, P., Kiefer, D.A., 1985. Chlorophyll a fluorescence in phytoplankton: relationship to photosynthesis and biomass. Journal of Plankton Research 7, 715–731. Fortier, L., Fortier, M., Fukuchi, M., Barber, D., Gratton, Y., Legendre, L., Odate, T., Hargrave, B., 2001. The international north water polynya study (now): a progress report. Memoirs of National Institute of Polar Research 54 (Special issue), 343–348.
Ingram, R.G., B#acle, J., Barber, D.G., Gratton, Y., Melling, H., 2002. Physical processes in the North Water. Deep-Sea Research II, this issue (PII: S0967-0645(02)00169-8). Klein, B., LeBlanc, B., Mei, Z.-P., Beret, R., Michaud, J., " Roy, Mundy, C.-J., von Quillfeldt, C.H., Garneau, M.-E., S., Gratton, Y., Cochran, J.K., B!elanger, S., Larouche, P., Pakulski, J.D., Rivkin, R.B., Legendre, L., 2002. Phytoplankton biomass, production and potential export in the North Water. Deep-See Research II, this issue (PII: S09670645(02)00174-1). Lewis, E.L., Ponton, D., Legendre, L., LeBlanc, B., 1996. Springtime sensible heat, nutrients and phytoplankton in the North Water polynya, Canadian Arctic. Continental Shelf Research 16, 1775–1792. Mei, Z.-P., Legendre, L., Gratton, Y., Tremblay, J.-E., LeBlanc, B., Mundy, C.J., Klein, B., Gosselin, M., Larouche, P., Papakyriakou, T.N., Lovejoy, C., von Quillfeldt, C.-H. 2002. Physical control of spring–summer phytoplankton dynamics in the North Water, April–July 1998. Deep-Sea Research II, this issue (PII: S09670645(02)00173-X). Melling, H., Gratton, Y., Ingram, G., 2001. Ocean circulation within the North Water polynya of Baffin Bay. Atmosphere-Ocean 39, 301–325. Mundy, C.J., Barber, D.G., 2001. On the relationship between spatial patterns of sea–ice type and the mechanisms which create and maintain the North Water (NOW) polynya. Atmosphere-Ocean 39, 327–341. Mysak, L.A., Huang, F., 1991. A latent- and sensible-heat polynya model for the North Water, Northern Baffin Bay. Journal of Physical Oceanography 22, 596–608. Parsons, T.R., Maita, Y., Lalli, C., 1984. A Manual of Chemical and Biological Methods for Seawater Analysis. Pergamon Press, Oxford, 173pp. Pr!ezelin, B.B., Ley, A.C., 1980. Photosynthesis and chlorophyll a fluorescence rhythms of marine phytoplankton. Marine Biology 56, 295–307. Smith, S.D., Muench, R.D., Pease, C.H., 1990. Polynyas and seals: an overviews of physical processes and environment. Journal of Geophysical Research 95, 9461–9479. Stirling, I., 1997. The importance of polynyas, ice edges and leads to marine mammals and birds. Journal of Marine Systems 10, 9–21. Suzuki, R., Ishimaru, T., 1990. An improved method for the determination of phytoplankton chlorophyll using N,Ndimethylformamide. Journal of the Oceanography Society of Japan 46, 190–194. Tremblay, J.-E., Gratton, Y., Fauchot, J., Price, N.M., 2002. Climatic and oceanic forcing of new, net and diatom production in the North Water. Deep-Sea Research II, this issue (PII: S0967-0645(02)00171-6).
Temporal and spatial patterns in the surface-water biomass of phytoplankton in the North Water Tsuneo Odatea,*, Toru Hirawakea, Sakae Kudoha, Bert Kleinb, Bernard LeBlancb, Mitsuo Fukuchia a
National Institute of Polar Research, 1-9-10 Kaga, Itabashi-ku, Tokyo 173-8515, Japan b D!epartment de biologie, GIROQ, Universit!e Laval, Qu!ebec, Canada G1K 7P4
Received 1 November 2000; received in revised form 8 August 2001; accepted 20 February 2002
Abstract Temperature, salinity, and in vivo fluorescence of surface seawater in the North Water were recorded continuously, using a CTD+fluorometer, in August 1997, April–July 1998 and August–October 1999. The phytoplankton bloom started in the polynya on the Greenland side in April. In April and May, high phytoplankton biomass coincided with saline water on the Greenland side, while biomass was low on the Ellesmere Island (Canada) side where a deep mixed layer prevailed. High phytoplankton biomass extended over the whole polynya in June, when surface temperature increased due to solar heating and salinity decreased due to freshwater input. The initiation of the bloom was about 2 months earlier on the Greenland than the Canadian side. In July and August, phytoplankton biomass became low in the southern survey area, indicating that the phytoplankton bloom had ended. In September, relatively saline and warm water occurred in the southeastern part of the study area where, consistent with the change in water properties, high concentrations of chlorophyll a were observed again. These results imply that both the earlier start of the algal bloom in spring and the eventual increase in phytoplankton biomass in summer contribute to the high annual primary production along the Greenland side, thus influencing the structure and biological productivity of the entire North Water ecosystem. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction The North Water, located between Greenland and Ellesmere Island (Canada), is one of the largest polynyas and the most productive ecosystems within the Arctic Circle (Stirling, 1997). The ice-free and highly productive waters are considered to serve as feeding, mating, spawning, and *Corresponding author. Tel.: +81-3-3962-4363; fax: +81-33962-5743. E-mail address: [email protected] (T. Odate).
overwintering grounds for huge populations of key species of birds and mammals, although few scientific studies have been conducted in the area until recently. Lewis et al. (1996) observed an east–west gradient in phytoplankton biomass in the polynya in May 1991, i.e. chlorophyll a biomass integrated to 30 m depth decreased from 506 mg m2 in the east to 50 mg m2 in the west. They suggested that a diatom bloom may have been initiated by the melting of sea ice, due to the upward input of sensible heat, and that the bloom moved
0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 2 ) 0 0 1 7 2 - 8
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T. Odate et al. / Deep-Sea Research II 49 (2002) 4947–4958
westwards, progressively exhausting the nutrients (Lewis et al., 1996). That study was limited, however, to a period of 48 h, so that comprehensive studies of the physical, chemical, and biological aspects of the bloom were still needed. The fieldwork of the International North Water Polynya Study (NOW) was conducted from 1997 to 1999, with the general hypothesis that the physical forces that open the North Water are also responsible for its unusually high biological productivity (Fortier et al., 2001). Polynyas usually belong to two main types, i.e. latent heat or sensible heat polynyas, based on their dominant mechanism of formation (Smith et al., 1990). In the first case, the algal bloom has been hypothesized to be delayed until solar heating stratifies the surface layer. In a sensible heat polynya, however, stratification of the surface layer resulting from the melting of the ice cover has been suggested to trigger an algal bloom immediately after the opening of the water. The season of planktonic production (the period between the algal bloom and the freeze-up in autumn) could last up to 6 months in the sensible heat case, 4 months in the latent heat case, and 2 months at similar latitudes outside of the North Water. The latent and sensible heat mechanisms could act together in the case of the North Water (Mysak and Huang, 1991). The present study was aimed at determining the spatial and temporal variability in phytoplankton biomass, based on in vivo fluorescence, in the North Water from spring to autumn, and at comparing the variability in phytoplankton biomass with water properties. The latter goal was to test the idea that the factors responsible for generating the polynya strongly influence its biological productivity.
2. Materials and methods Temperature, salinity, and in vivo fluorescence of seawater, which was pumped from the bottom of the C.C.G.S. Louis S. St. Laurent (draft, 9.91 m) in 1997 and the C.C.G.S. Pierre Radisson (draft, 7.16 m) in 1998 and 1999, were recorded during three summer seasons using a CTD+fluorometer (AquaPack, Chelsea Instruments, Ltd.). Exactly
speaking, the pumped seawater was not the surface water. However, we considered it as the surface water, since our purpose was to show temporal and spatial patterns in water temperature, salinity, and phytoplankton biomass based on seawater collected from similar depths, which were near the sea surface throughout the cruises. Moreover, our data on temperature may be slightly higher than ambient water temperature, since the temperature of the pumped water may have increased slightly due to room temperature during the seawater supply to the water tank in which the CTD+fluorometer was immersed. Flow rate was about 1 l mm1 and the volume of the seawater in the tank was about 10 l. Consequently, turnover rate of seawater in the tank was too short (10 min) to allow increases in phytoplankton biomass to occur; none were ever detected. Observations were made from 18 to 26 August in 1997, from 12 April to 18 May and 3 to 19 July in 1998, and from 28 August to 1 October in 1999. In April and May 1998, the survey area was restricted due to thick ice (see Mei et al., 2002; Mundy and Barber, 2001). The time intervals between records were 2 min in 1997 and 1 min in 1998 and 1999. During the 1999 fieldwork, 24-h observations were conducted at southern stations S2 (761170 N, 711500 W) and S4(761170 N, 741090 W), when the vessel occupied these stations for recovery of mooring arrays (see Fig. 1 in Ingram et al., 2002, for a map of these station locations). During the periods of observation, 34 (1997), 39 (1998), and 123 samples (1999) of surface seawater were collected from a drain of the water tank in which the CTD+fluorometer was immersed. The seawater was filtered onto Whatman GF/F filters, which were immediately placed in tubes containing N;N-dimethylformamide (Suzuki and Ishimaru, 1990), except in 1997 and 1998 when the filters were first frozen (201C and dark). Concentrations of chlorophyll a were determined fluorometrically using a Turner Design Model 10R Fluorometer (Parsons et al., 1984) in the laboratory. These data were used for transforming the in vivo fluorescence data into chlorophyll a concentrations. Details on water-column sampling and determination of chlorophyll a concentrations are de-
T. Odate et al. / Deep-Sea Research II 49 (2002) 4947–4958
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Fig. 1. Variations in temperature, salinity, in vivo fluorescence intensity, and extracted chlorophyll a concentration over a 24-h period at: (a) station S4 (761170 N, 741090 W) and (b) station S2 (761170 N, 711500 W).
scribed by Klein et al. (2002). Results of selected CTD casts (provided by G. Ingram; see also B#acle et al., 2002) were used for illustrating vertical profiles of temperature and salinity.
3. Results 3.1. In vivo fluorescence intensity During the first 24-h observation, from 12:00 on 28 August to 12:00 on 29 August 1999 (local time) at south-central station S4, in vivo fluorescence
intensity varied between 0.08 and 1.86, with a mean value7standard deviation (SD) of 0.6870.39 (Fig. 1a). The coefficient of variation (CV) was 58%. Similarly relatively large variation was observed in the concentration of extracted chlorophyll a, i.e. mean7SD=2.2471.01 and CV=45%. At this station, there was a relatively large CV for water temperature (18%), suggesting that the water mass changed during the period since the vessel stayed at almost the same position. In contrast, fairly constant water temperature and salinity were noted during the second 24-h observation period, from 12:00 on 6 September to
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12:00 on 7 September 1999 (local time) at southeastern station S2 (Fig. 1b). The CVs of water temperature and salinity were 3% and 0.4%, respectively. At that station, temporal variations of in vivo fluorescence and extracted chlorophyll a concentration were also small (CV=8% and 7%, respectively). No trend of higher in vivo fluorescence intensity during the night was detected. Relationships between in vivo fluorescence intensity and concentration of extracted chlorophyll a are illustrated in Fig. 2. For all 3 years, significant regressions were obtained, although the values of the slope and intercept varied. Using these equations, in vivo fluorescence intensity was converted to chlorophyll a concentration. 3.2. Distributions of temperature, salinity, and chlorophyll a concentration Surface distribution of water temperature is shown in Fig. 3 for a composite seasonal sequence from April to September. The surface-water temperature was below zero in almost the whole survey area in April and May. In June, the surfacewater temperature increased from 11C to 201C,
although cold water was observed in the northern part of the survey area. Warm water (>31C) sometimes occurred south of ca. 771N in July and August. In the second half of September, the warm water disappeared and the surface-water temperature was o31C throughout the study area. The temporal pattern of salinity distribution coincided with that of temperature (Fig. 4). Surface salinity was 32.5 to 33.5 over the survey area in April and May, when cold water covered the whole area. During these months relatively high salinity was observed in the eastern part of the area. In June the surface salinity decreased to o32.5 in the whole area. Less saline water (o30.0) occurred east of ca. 721300 W in July (1998) and west of ca. 751W and north of ca. 761300 N in August and the early half of September (1999). During the latter half of September, saline water (32.5–33.0) was observed in the southeastern part of the survey area. Chlorophyll a concentrations were low in April, although slightly higher concentrations (1– 2 mg l1) were noted locally in the eastern part of the survey area (e.g., around 771200 N and 741W and 771500 N and 731W; Fig. 5). The
Fig. 2. Relationships between in vivo fluorescence intensity and extracted chlorophyll a concentration during the cruises of the International North Water Polynya Study (NOW): (a) NOW97; (b) NOW98; and (c) NOW99.
May 6-17 1998
June 4-28 1998
August 18-26 1997
August 28September 14 1999
September 17October 1 1999
July 1-19 1998
-2 -1 0 1 2 3 Temperature (ºC)
4
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April 12-29 1998
Fig. 3. Distribution of surface-water temperature, where red lines indicate cruise tracks and the white hole indicates temperature >41C.
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May 6-17 1998
June 4-28 1998
August 18-26 1997
August 28September 14 1999
September 17October 1 1999
July 1-19 1998
29
30
31 32 Salinity
Fig. 4. Distribution of surface salinity, where red lines indicate cruise tracks and the black hole in the last panel indicates salinity o29.
33
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April 12-29 1998
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concentrations were similar in the southern part (below 771N and 74–761W). A strong increase in chlorophyll a concentration occurred from April to May–June. Concentrations >10 mg l1 were observed around 771300 N and 751W in June. In July, chlorophyll concentrations had decreased everywhere except in the north; extremely low chlorophyll water (o0.1 mg l1) was frequently present in the southern part of the area in July– August. From late August to early September, however, chlorophyll a concentrations increased in the southeastern area around 761200 N and 721W. Some examples of east–west distributions of surface-water temperature, salinity, and chlorophyll a concentration along 771500 N are shown in Fig. 6. This northerly study transect was bounded on the west by station 14 (771490 N, 751290 W) and on the east by station 18 (771500 N, 731W) (see Fig. 2 in B#acle et al., 2002, for a map of the station locations defining this and other transects). In April, the eastward increase of salinity occurred east of ca. 741W, while chlorophyll concentrations slightly increased east of ca. 731250 W (Fig. 6a). Warm and saline water occupied the shallower layer on the Greenland side, at stations 18 and 27 (771220 N, 731500 W, on another northerly transect), than on the Canadian side, at stations 14 and 22 (771200 N, 761290 W, on another northerly transect) (Fig. 7a and b). As shown by surface observations, salinity was higher on the Greenland than the Canadian side, but the difference in surface temperature between the two sides was small. In the former area, chlorophyll concentrations were high above the thermocline (Fig. 7c), which was deeper than 60 m. In the latter area, temperature was almost constant over the upper 150 m, where chlorophyll concentrations were low. In May, the increase of chlorophyll corresponded well to salinity (Fig. 6b). Eastward increases of salinity and chlorophyll were observed east of 741W. Vertical profiles of water temperature and salinity on the Greenland side at station 18 in May were similar to those in April, although a slight increase in surface water temperature was observed (Fig. 7d and e). At that station, high chlorophyll concentration occurred above the thermocline (Fig. 7f) as observed in April. In contrast, the water-column properties changed at
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station 27, a slightly more southerly location on the Greenland side. In particular, water temperature and salinity decreased in the top 40 m and became similar to values at stations 14 and 18 along the northerly transect at 771500 N. Similar vertical distributions of chlorophyll were noted at stations 14, 18, and 27, where values were low. As described above, surface salinity largely decreased and chlorophyll concentration increased from May to June. High chlorophyll concentrations occurred east of 741W and relatively saline water occurred east of 731200 W in June (Fig. 6c). Vertical profiles show that a sharp increase of temperature and decrease of salinity occurred in the surface layer on both the Greenland and Canadian sides (Fig. 7g and h). Chlorophyll concentrations increased in the upper layers on both sides (Fig. 7i). Eastward increase of salinity also was observed during the late summer season (Fig. 6d). High chlorophyll concentrations occurred east of 741W, although high concentrations were also detected around ca. 751W where relatively saline water occurred. As mentioned above, the similar trend of high chlorophyll concentration with saline water became clear in the southern part of the polynya (Fig. 8). Along the southerly transect at 761200 N, salinity sharply increased from west to east around 751W, while high chlorophyll concentrations were observed ease of 751W.
4. Discussion The present study aims at determining the spatial and temporal variability in phytoplankton biomass in the surface waters of the North Water. For this purpose, continuous observations of in vivo fluorescence of surface water in the polynya were conducted during several cruises over 3 years. The interpretation of in vivo fluorescence in terms of phytoplankton biomass, however, is not simple, because the in vivo fluorescence from natural phytoplankton communities is well known to show diurnal variations (Falkowski and Kiefer, 1985). Changes in the quantum yield of fluorescence are considered to be a major source of variation of fluorescence in the field (Pre! zelin and
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May 6-17 1998
June 4-28 1998
August 18-26 1997
August 28September 14 1999
September 17October 1 1999
July 1-19 1998
0.1
1 10 -1 Chlorophyll a (µgl )
Fig. 5. Distribution of surface chlorophyll a concentration, estimated from in vivo fluorescence intensity, where red lines indicate cruise tracks and black indicates chlorophyll concentration o0.1 mg l1.
T. Odate et al. / Deep-Sea Research II 49 (2002) 4947–4958
April 12-29 1998
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Fig. 6. East–west distribution of temperature, salinity, and chlorophyll a concentration along the northerly transect of 771500 N in: (a) April; (b) May; (c) June; and (d) September. Values within 10 longitude are shown.
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Fig. 7. Vertical profiles of temperature (a, d, g), salinity (b, e, h), and chlorophyll a concentration (c, f, i) on the Greenland side, at stations 18 (771500 N, 731W) and 27 (771220 N, 731500 W), and on the Canadian side, at stations 14 (771490 N, 751290 W) and 22 (771200 N, 761290 W) in: (a–c) April; (d–f) May; and (g–i) June.
Fig. 8. East–west distributions of temperature, salinity, and chlorophyll a concentration along the southerly transect of 761200 N in September. Values within 10 longitude are shown.
Ley, 1980; Falkowski and Kiefer, 1985). Our 24-h series at station S2 showed that the diurnal variation of fluorescence was small in spite of the reappearance of a day–night cycle and that
chlorophyll a concentration, determined from extracted pigments, did not change within the same water mass (Fig. 1b). Hence, we considered that the variations of in vivo fluorescence intensity mostly represented spatial variations of phytoplankton biomass. The relatively large changes observed at the central station S4 reflected advection, i.e. the Carey Island water was replaced by Central Northern Baffin Bay water as shown by B#acle et al. (2002). The development of the phytoplankton bloom in the North Water also is described from discrete station data by Klein et al. (2002) and Mei et al. (2002). The phytoplankton biomass started to increase on the Greenland side (between 761 and 771300 N and o751W) in April, when the polynya
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opened (Mundy and Barber, 2001), and spread north in May–June, when the maximum biomass occurred (Klein et al., 2002; Mei et al., 2002). The present study shows the fine-scale distribution of phytoplankton biomass based on the continuous record of surface-water parameters along the cruise track, indicating that relatively high phytoplankton biomass was first observed in the eastern part of the survey area in April and May, as shown by Klein et al. (2002) and Mei et al. (2002). In that season, low water temperature reached the depth of about 150 m on the Canadian side, meaning that latent heat prevailed; the deep surface mixed layer prevented phytoplankton from developing. Mei et al. (2002) observed chlorophyll a concentrations >10 mg l1 in the southeasterly area around 761200 N and 72–741W in May. Unfortunately, we did not have observations in that area. Consequently, the highest concentration in our data set was lower than that in Klein et al. (2002) and Mei et al. (2002) for May. Since the saline water originates from depth, high phytoplankton biomass in the eastern part of the polynya seems to be associated with deep west Greenland current water entrained into the surface along the Greenland coast (Melling et al., 2001), i.e. sensible heat. In other words, the melting or slower formation of sea ice due to the upwelling of warm water could result in haline stratification of the surface layer, which triggers the phytoplankton bloom (Fortier et al., 2001). However, we did not observe decreased salinity due to ice melt. Melling et al. (2001) showed that upwelled water does not reach the surface and that heat is sufficient only to reduce ice formation, not cause ice melt. At station 27 on the Greenland side, surface salinity decreased from April to May, but the decrease did not result from the melting of sea ice but from the movement of water masses, since surface-water temperature and salinity were similar to those at stations 14 and 22 on the Canadian side. Water mass observed at stations 14 and 22 prevailed eastward at least at station 27 in May. High phytoplankton biomass in the saline water means that the sensible heat scenario was not responsible for the earlier development of the algal bloom in the polynya, but the physical structure nevertheless determined the timing of the phytoplank-
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ton bloom on the Greenland side of the North Water. The bloom did not begin on the Canadian side until June, when surface temperature increased and salinity decreased due to solar heating and melting of ice, respectively. The timing of the bloom was thus about 2 months earlier on the Greenland than the Canadian side. Mei et al. (2002) show that the bloom development is related to the depth of the mixed layer. Phytoplankton production decreased in June– July in the southern part of the survey area, due to exhaustion of nutrients in surface waters (Tremblay et al., 2002). Our results also show low phytoplankton biomass in the south in summer. In late summer, however, high local phytoplankton biomass occurred in the southeastern part of the survey area. One of the reasons for the high biomass could be horizontal advection from Kane Basin in the north, where relatively high phytoplankton biomass also was observed in July 1998 and August 1997 (Fig. 5). To explain the increase of phytoplankton biomass in the southeastern part by advection from the north is difficult, however, since the water that passes through Kane Basin moves from north to southwest along the Ellesmere Island coastline (B#acle et al., 2002; Melling et al., 2001). Furthermore, water from the West Greenland Current, moving from southeast to north, penetrates farther north along the Greenland coast in winter than in summer (Melling et al., 2001), so that upwelling intensity is low in summer in the northern part of the survey area. A certain physical event must nevertheless occur in the southern part along the Greenland Coast in summer to sustain phytoplankton growth and result in increased phytoplankton biomass. The co-occurrence of high phytoplankton biomass with saline water observed in the present study supports this interpretation, although further studies are required to understand the local increase of phytoplankton biomass in the Greenland Coast in summer. The results of this study suggest that the occurrence of saline water determines the timing of the spring bloom in the northern part of the North Water. As the polynya opens more in summer, the occasional increases in phytoplankton biomass in the southeasterly part seem to
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result from a physical event that involves an increase in salinity along the Greenland Coast. The early start of the algal bloom in spring and the increases in phytoplankton biomass in summer contribute to the high annual primary production along the Greenland Coast, thus influencing the structure and biological productivity of the entire polynya ecosystem.
Acknowledgements Some data used in this study were provided by the physics group of the NOW project; we thank G. Ingram, physics group leader, for permission to use CTD data at several stations. We gratefully acknowledge the captains, officers, and crews of the C.C.G.S. Louis S. St. Laurent during the NOW97 cruise and of the C.C.G.S. Pierre Radisson during the NOW98 and 99 cruises and all colleagues involved for their help in the field. We also thank L. Legendre, Z.-P. Mei and two reviewers for their comments on the manuscript. This study was supported by a grant-in-aid for Scientific Research from the Japan Society for the Promotion of Science (No. 11208203) and is a contribution to the International North Water Polynya Study (NOW).
References B#acle, J., Carmack, E.C., Ingram, R.G., 2002. Water column structure and circulation under the North Water during spring transition: April–July, 1998. Deep-Sea Research II, this issue (PII: S0967-0645(02)00185-6). Falkowski, P., Kiefer, D.A., 1985. Chlorophyll a fluorescence in phytoplankton: relationship to photosynthesis and biomass. Journal of Plankton Research 7, 715–731. Fortier, L., Fortier, M., Fukuchi, M., Barber, D., Gratton, Y., Legendre, L., Odate, T., Hargrave, B., 2001. The international north water polynya study (now): a progress report. Memoirs of National Institute of Polar Research 54 (Special issue), 343–348.
Ingram, R.G., B#acle, J., Barber, D.G., Gratton, Y., Melling, H., 2002. Physical processes in the North Water. Deep-Sea Research II, this issue (PII: S0967-0645(02)00169-8). Klein, B., LeBlanc, B., Mei, Z.-P., Beret, R., Michaud, J., " Roy, Mundy, C.-J., von Quillfeldt, C.H., Garneau, M.-E., S., Gratton, Y., Cochran, J.K., B!elanger, S., Larouche, P., Pakulski, J.D., Rivkin, R.B., Legendre, L., 2002. Phytoplankton biomass, production and potential export in the North Water. Deep-See Research II, this issue (PII: S09670645(02)00174-1). Lewis, E.L., Ponton, D., Legendre, L., LeBlanc, B., 1996. Springtime sensible heat, nutrients and phytoplankton in the North Water polynya, Canadian Arctic. Continental Shelf Research 16, 1775–1792. Mei, Z.-P., Legendre, L., Gratton, Y., Tremblay, J.-E., LeBlanc, B., Mundy, C.J., Klein, B., Gosselin, M., Larouche, P., Papakyriakou, T.N., Lovejoy, C., von Quillfeldt, C.-H. 2002. Physical control of spring–summer phytoplankton dynamics in the North Water, April–July 1998. Deep-Sea Research II, this issue (PII: S09670645(02)00173-X). Melling, H., Gratton, Y., Ingram, G., 2001. Ocean circulation within the North Water polynya of Baffin Bay. Atmosphere-Ocean 39, 301–325. Mundy, C.J., Barber, D.G., 2001. On the relationship between spatial patterns of sea–ice type and the mechanisms which create and maintain the North Water (NOW) polynya. Atmosphere-Ocean 39, 327–341. Mysak, L.A., Huang, F., 1991. A latent- and sensible-heat polynya model for the North Water, Northern Baffin Bay. Journal of Physical Oceanography 22, 596–608. Parsons, T.R., Maita, Y., Lalli, C., 1984. A Manual of Chemical and Biological Methods for Seawater Analysis. Pergamon Press, Oxford, 173pp. Pr!ezelin, B.B., Ley, A.C., 1980. Photosynthesis and chlorophyll a fluorescence rhythms of marine phytoplankton. Marine Biology 56, 295–307. Smith, S.D., Muench, R.D., Pease, C.H., 1990. Polynyas and seals: an overviews of physical processes and environment. Journal of Geophysical Research 95, 9461–9479. Stirling, I., 1997. The importance of polynyas, ice edges and leads to marine mammals and birds. Journal of Marine Systems 10, 9–21. Suzuki, R., Ishimaru, T., 1990. An improved method for the determination of phytoplankton chlorophyll using N,Ndimethylformamide. Journal of the Oceanography Society of Japan 46, 190–194. Tremblay, J.-E., Gratton, Y., Fauchot, J., Price, N.M., 2002. Climatic and oceanic forcing of new, net and diatom production in the North Water. Deep-Sea Research II, this issue (PII: S0967-0645(02)00171-6).