Storm-induced convective export of organic matter during spring in the northeast Atlantic Ocean

Deep-Sea Research I 49 (2002) 1431–1444

Storm-induced convective export of organic matter during spring in the northeast Atlantic Ocean W. Koevea,b,*, F. Pollehneb, A. Oschliesa, B. Zeitzschela b

a Institut fur Weg 20, D-24105 Kiel, Germany . Meereskunde, Dusternbrooker . Institut fur Seestrae 15, 18119 Rostock, Germany . Ostseeforschung Warnemunde, .

Received 24 November 2001; received in revised form 15 March 2002; accepted 15 March 2002

Abstract Observations during a spring phytoplankton bloom in the northeast Atlantic between March and May 1992 in the Biotrans region at 471N, 201W, are presented. During most of the observation period there was a positive heat flux into the ocean, winds were weak, and the mixed layer depth was shallow (o40 m). Phytoplankton growth conditions were favourable during this time. Phytoplankton biomass roughly doubled within the euphotic zone over the course of about 7 days during mid-April, and rapidly increased towards the end of the study until silicate was depleted. However, the stratification of the water column was transient, and the spring bloom development was repeatedly interrupted by gales. During two storms, in late March and late April, the mixed-layer depth increased to 250 and 175 m, respectively. After the storm events significant amounts of chlorophyll-a, particulate organic carbon and biogenic silica were found well below the euphotic zone. It is estimated that between 56% and 65% of the seasonal new production between winter and early May was exported from the euphotic zone by convective mixing, in particular, during the two storm events. Data from the NABE 471N study during spring 1989 are re-evaluated. It is found that convective particle export was of importance during the early part of that bloom too, but negligible during the height of the bloom in May 1989. The overall impact of convective particle export during spring 1989 was equivalent to about 36% of new production. In view of these and previously published findings it is concluded that convective transport during spring is a significant process for the export of particulate matter from the euphotic zone in the temperate North Atlantic. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Carbon; Suspended particulate matter; Atmospheric forcing; Oceanic boundary layer; Algal blooms; Particulate flux; AN North Atlantic

1. Introduction The spring phytoplankton bloom is a characteristic feature of the temperate North Atlantic *Corresponding author. Marum, Fachbereich Geowissenschaften (FB5), Universit.at Bremen, Postfach 330440, D28334 Bremen, Germany. Fax: +49-431-565876. E-mail address: [email protected] (W. Koeve).

(Heinrich, 1962) that has attracted interest for more than a century (Mills, 1989). In view of its basin-wide extent (Esaias et al., 1986) and its importance for organic-matter flux to the deep sea (Deuser et al., 1981; Billet et al., 1983; Rice et al., 1986; Pfannkuche, 1993) and for element cycling in general (Ducklow, 1989), it has been the focus of several process studies within the Joint Global Ocean Flux Study (JGOFS), including the North

0967-0637/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 0 2 ) 0 0 0 2 2 - 5

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Atlantic Bloom Experiment (NABE; Ducklow and Harris, 1993), the Biogeochemical Ocean Flux Study (BOFS; e.g. Savidge et al., 1992), and other studies (Harrison et al., 1993). Nutrients supplied to the surface by deep mixing in winter, increasing solar irradiation, weakened mixing and moderate zooplankton grazing during spring allow for an increasing production and accumulation of phytoplankton. A pulse of sinking particles has been repeatedly observed after the culmination of the spring bloom (Honjo and Manganini, 1993; Kuss and Kremling, 1998; Lundgreen and Duinker, 1998), and it has been suggested that sinking of large particles such as aggregates or zooplankton faecal pellets dominate this export (Pilskaln and Honjo, 1987; Alldredge and Silver, 1988; Alldredge and Gotschalk, 1989). Such bloom sedimentation has been reported from a diverse suite of coastal, shelf and open ocean regions of the temperate North Atlantic (Wassmann, 1990; Lampitt and Antia, 1997; Antia et al., 1999). During spring blooms chlorophyll-a (chl-a) concentrations are typically about an order of magnitude larger in the euphotic zone than below (Lochte et al., 1993; Harrison et al., 1993; Dickey et al., 1994). Since, net primary production is restricted to the euphotic zone, such accumulation of biomass exclusively in the near-surface layer is an expected consequence of an unperturbed typical spring phytoplankton bloom under wellstratified conditions when mixing is shallower than, or equals, the depth of the euphotic zone. In contrast to this expected vertical biomass distribution, significant phytoplankton stocks well below the euphotic zone are indicative of a different type of spring bloom development. Evidence for such a vertical distribution of biomass during spring has existed for decades (Williams and Robinson, 1973; Williams, 1974; Williams and Hopkins, 1975, 1976), but it has not been recognised in relation to particle flux. More recently, observations from the Gulf of Maine (Townsend et al., 1992, 1994), the Marine Light Mixed Layer (MLML) Experiment near Ocean Weather Station India from 1991 (Stramska et al., 1995), and the subtropical North Atlantic (Altabet, 1989; Doney et al., 1996) have highlighted the occurrence and consequences of spring phyto-

plankton blooms in the absence of well-developed water column stratification. Diurnal variation of the mixing depth (Woods and Onken, 1982; Woods and Barkmann, 1986, 1993), the mixedlayer pump (Gardner et al., 1995), has been suggested to explain such patterns of vertical biomass distribution. This paper is based on observations from a spring bloom study in the northeast Atlantic carried out at the Biotrans site (471N, 201W) between March and May 1992 with the German research vessel FS Meteor (cruise 21). We present an example of a spring bloom that was characterised by a sequence of several short blooms interrupted by storm events. We use these observations together with data from remote sensing and weather hindcasting to explore the importance of convective mixing for particle export from the euphotic zone during spring in the northeast Atlantic.

2. Material and methods Field data presented in this paper were obtained during FS Meteor cruise 21/2 between mid-April and early May 1992. The sampling strategy during M21/2 included mesoscale surveys of the area from 171W to 211W and 461N to 481N and a drift experiment (Pfannkuche et al., 1993). For the latter, the water column was sampled daily at least along the track of a drogue system consisting of a sediment trap (120 m), floats and a surface buoy. A station plot is given in Fig. 1. Between April 23 and 25 sampling ceased because of increasingly harsh weather conditions (storm event 4 in Fig. 2). Sampling was with a 24-Niskin bottle rosette interfaced with a Neil Brown CTD system. Subsamples for nutrient and oxygen analyses were drawn off the bottles immediately after recovery and processed according to standard procedures (Grasshoff et al., 1983, 1999). Samples for the determination of chl-a, particulate organic carbon and nitrogen (POC, PON) and biogenic silica (BSi) were concentrated on glass fibre filters (Whatman GF/F) or nuclepore membrane filters (BSi only). Sampling depth was guided by standard light depths, as estimated from Secchi depth, and the

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Fig. 1. Stations sampled during FS Meteor cruise 21/2. The mesoscale grid survey, carried out between 16.4. and 19.4.1992, consisted of an east–west transect (transect I; sta. 21–30) and four smaller transects forming together a ‘rhomb’ like structure (transects II–V). Arrows indicate ship’s heading during the survey. The survey was completed with three additional stations (42, 43 and 44) related partly to handling of an early drifter. Three stations (25/36; 35/43 and 39/44) were sampled twice during the survey. The drift study (meandering solid line) was started on 20.4. (sta 46) and interupted after station 61 (23.4.) by a storm event. Water sampling started again on the morning of 26.4. (sta. 76) and lasted until 2.5. (sta. 106). The dashed line indicates the subjective best guess of the position of a frontal structure (for details see text) relative to the positions of stations from the survey and the drift study.

(1992)

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Fig. 2. Daily mean wind stress from ECMWF-re-analysis data (solid line) and SST (dashed line) at the Biotrans site during winter and spring 1992. SST data were interpolated from weekly mean data (Reynolds and Smith, 1995) available from the National Centers for Environmental Prediction (NCEP; http://www.ncep.noaa.gov) via the Ferret Live Access Server (http://www.ferret.noaa.gov/fbin/climate server). Wind stress data were taken from the re-analysis dataset (Gibson et al., 1997) of the European Center for Medium Weather Forecast (ECMWF; http://www.ecmwf.int/data/era.html).

vertical structure of the chl-a profile, as obtained from a preliminary analysis of on-line data from an in situ fluorometer mounted to the CTD-

system. Additional samples were taken below the euphotic zone (1% light depth). Analyses for chl-a, POC, PON and BSi were performed according to Herbland et al. (1985), Ehrhardt and Koeve (1999) and von Bodungen et al. (1991), respectively. To consider our field observations in a more general context, we analysed wind stress and heat flux data from the European Centre for Medium Weather Forecast (ECMWF) re-analysis data set (Gibson et al., 1997). Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) data were taken from the NASA Pathfinder Project (9 km data; Brown et al., 1993). Furthermore, we used selected data from the JGOFS North Atlantic Bloom Experiment, which were made available through the US JGOFS project data management (http://usjgofs. whoi.edu/). A detailed discussion of the hydrographic observations carried out during Meteor cruise 21/2 is presented elsewhere (Podewski and Deckers, 1999). Concomitant observations from

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benthic studies in March–August 1992 have been published by Pfannkuche et al. (1999).

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3.1. Mesoscale variability of temperature, nutrients and chl-a During spring basic features of mesoscale hydrography may be inferred from the distribution of temperature at the ocean surface. Because of persistent cloud cover during the grid survey valuable AVHRR SST data for the study region were available only for the week preceding our observations. Significant mesoscale variation of SST is apparent (Fig. 3); temperature range in the 2
41 area was about 2.51C. The most prominent feature was a temperature front oriented approximately along a NW–SE line and crossing the whole research area. Both the mesoscale survey and the stations from the drift study traversed this front (Figs. 3 and 4c). Surface nitrate concentrations at 10 m depth range between values as large as 6.6 mmol m3 in the eastern part of the research area and less than 3 mmol m3 in the northwestern part (Fig. 4a). Surface concentrations of chl-a varied by more than a factor of three (Fig. 4b). Highest values (mean7std.=0.970.1 mg m3) were observed

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The chief task of the pelagic work during M21/2 was to study the temporal development of plankton dynamics during early spring 1992 by means of a drift study. During this experiment a distinct body of water was marked with a drogue, and the sampling followed the path of this drifter (Fig. 1). Because of the complex mesoscale situation, which is typical for the region (McGillicuddy et al., 1995a, b), and the non-ideal behaviour of the drifting system (see below), however, a straightforward interpretation of our data as a time series of a distinct water mass is not possible. Therefore, we will first describe observations from a quasisynoptic survey of the region surrounding the drift track and from remote sensing of SST in order to describe the frame from within which the drift experiment is interpreted.

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3. Results

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Fig. 3. Sea surface temperature (SST) during April 1992 in the Biotrans region (26–481N, 17–211W). A composite was constructed from data obtained from the NOAA/NASA AVHRR Oceans Pathfinder Best SST archive (Brown et al., 1993; http://podaac-www.jpl.nasa.gov/sst/) which were available for the period 9–16 April. Data from 4 days (observations from daytime where used) were combined, providing a total spatial coverage of 83%. The grid of CTD stations is overlayed.

east of stations 27 and 34, and lowest concentrations south of the main east–west transect (sta. 31–34; 0.570.2 mg m3). chl-a concentrations below the euphotic zone, e.g. at 100 m depth, were surprisingly high (maximum 0.73 mg m3). Eastern stations showed about double the concentrations of western stations. Overall, higher chl-a concentrations were found in the colder (Figs. 3 and 4c) and nitrate-rich waters separated from the western area by the front. There were also differences in phytoplankton composition. Larger microphytoplankton, in particular diatoms, dominated in the east, and smaller mainly nanoplanktonic forms, in particular prymnesiophytes, dominated in the west (Deckers, 1997; Podewski and Deckers, 1999). Note that the geographical position of the front apparently changed during April. For this reason and because of the time delay between the AVHRR sea surface temperature observations (Fig. 3), the survey (Figs. 4, 5b and e) and the drift study itself (Fig. 5a, c, d and Fig. 6a, b), the dashed line shown in Fig. 1 does not show the actual geographic position of the front at any particular time but is a best guess of the relative

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Fig. 4. Mesoscale variation of nitrate concentration (a), chlorophyll-a (b) and temperature (c) near the surface (10 m) during April 1992.

position of the front relative to the geographic positions of the stations during the survey and the grid. 3.2. Observations during the drift study Starting at the northern corner of the surveyed grid the drift track during the first 3.5 days (sta. 46–61) was east of, and parallel to, grid-transect IV (sta. 36–39, see Fig. 1). SST, nitrate and chl-a

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concentrations (Fig. 5a) were similar to those at nearby stations from transect IV (in particular, sta. 37 and 38, Fig. 5b). SST varied between 13.31C and 13.51C, comparable to the 13.31C to 13.71C at sta. 37–39. Surface nitrate was between 3.3 and 3.8 mmol m3 (sta. 37–39: 3.4–3.6 mmol m3) and chl-a was 0.5–0.85 mg m3 (sta. 37 and 38: 0.61– 0.83 mg m3). Vertical distribution was homogeneous in the upper 50 m (Fig. 5a). Below this depth, temperature, chl-a concentration and oxygen saturation decreased, and nitrate concentration increased. At about 100–150 m depth, values were significantly different from surface values. The similarity of the vertical structures of temperature and nitrate at these stations and those at sta. 37 and 38 (Fig. 5b) implies that the drifter stayed in waters west of the frontal structure during the first 3.5 days. On the evening of April 23 (after CTD station 62), sampling was stopped because of harsh weather conditions (storm event 4 in Fig. 2). A detailed analysis of high-resolution temperature data (Podewski et al., 1993 in Pfannkuche et al., 1993) reveals that after the storm the mixed layer depth was about 175 m. Stations 64–76, which were sampled during the 4 days following the gale, were characterised by surface temperatures of 12.2–12.31C and nitrate concentrations of 5.1–5.6 mmol m3 (Fig. 5c). The most remarkable feature of these stations was the homogeneous distribution and the high concentrations of biogenic particles (chl-a, BSi and POC) between the surface and about 150 m (Figs. 5c and 6a). We suggest that convective mixing during storm event 4 had brought about this homogeneous distribution of biomass. During the final 4 days of the drift study a significant increase in surface chl-a and BSi, a slight increase in temperature and a decrease in nitrate and silicate gave evidence for a growing phytoplankton population (Figs. 5d and 6b). These changes were restricted to the upper 50 m or less of the water column. During this period surface nitrate and silicate concentrations decreased by about 1 mmol m3 each, down to minimum values of about 4.3 and 0.2 mmol m3, respectively. Most of this effect can be interpreted as a development within a water mass by

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oxygen saturation (%)

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Fig. 5. Vertical patterns of temperature (circles, left panel), nitrate (diamonds, left panel) and chlorophyll-a concentration (circles, right panel) and oxygen saturation (diamonds, right panel) during spring 1992 (a–e) and 1989 (f) in the Biotrans region. (a) the early part of the drift study (stations 46, 56 and 60, 20.–23.4.1992), (b) transect IV (stations 37 and 38, 18.4.1992), (c) stations sampled just after storm event 4 (stations 64, 65, 71 and 76; 26.–29.4.1992), (d) stations sampled during late April to early May (stations 82, 89, 102 and 103; 30.4.–2.5.1992), (e) sta. 24 (1.4.1992) on transect I, (f) during late April 1989 (RV Atlantis II, cruise 119/4, stations 10, 11 and 12; 25–27.4.1989)

considering the nutrient concentrations between 50 and 100 m, which stayed similar to those before the storm and the unchanged surface salinity values (the only conservative tracer) of about 35.5. During this period chl-a concentrations increased to up to 1.5 mg m3 in the upper 50 m, but decreased below 100 m. We expect the diatom spring bloom to have declined shortly after our study was terminated since surface silicate concentrations were already extremely low during the last stations.

4. Discussion 4.1. Interpretation of post-storm observations In this section we discuss the extent to which the observations after the storm event can be directly compared to those just prior to it during the drift experiment. We do this by means of a heat flux/ heat storage comparison for the storm period and by comparisons of the integrated stocks of salinity and biogenic particles from the pre- and

W. Koeve et al. / Deep-Sea Research I 49 (2002) 1431–1444 BSi (mmol m –3 ) 1

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Fig. 6. Vertical patterns of dissolved silicate (diamonds, dashed lines, left panel), particulate biogenic silicate (circles, solid lines, left panel) and particulate organic carbon (circles, right panel) at (a) stations sampled just after storm event 4 (same stations as in Fig. 5c) and (b) at stations sampled during late April–early May (same stations as in Fig. 5d).

post-storm observations. Immediately after the storm, surface temperatures and nitrate concentrations were about 11C lower and almost 2 mmol m3 higher compared to pre-storm stations. At first sight, these changes could have been a consequence of cooling at the surface and entrainment of nitrate-rich water during the storm. However, the depth integrated difference (0–150 m) of the heat content between stations sampled prior to and after the storm is about 600 MJ m2, which is more than ten times the time integrated heat loss during storm event 4 (50 MJ m2). Similarly, depth-averaged salinities over the upper 150 m between pre-storm (35.67170.043) and post-storm (35.51170.017) stations differ by 0.16 units. The mean integrated stocks of chl-a, biogenic silica and POC are significantly larger at post-storm stations (131718 mg chl-a m2, 258730 mmol Si m2 and 1.570.3 mol C m2) compared with pre-storm stations (51710 mg chl-a m2, 7476 mmol Si m2 and 1.170.1 mol C m2). Since deep mixing during the storm event will prevent net growth, we expect that within a given water body the integrated stocks of biogenic particles should stay constant or even decrease (by respiration, grazing, or other losses) during the storm event. Heat flux considerations, salinity and biogenic particle stocks suggest that post-storm sampling took place in waters significantly different from pre-storm stations. By comparing post-storm stations with data from the mesoscale survey, we find that surface values and depth weighted

averages of temperature, salinity and nitrate concentration indicate that post-storm stations were sampled in the cold, less saline and nitraterich water mass that we had observed east of the frontal system during the grid survey. Vertical averages from post-storm stations are 12.2670.061C, 35.51170.02 and 5.570.2 mmol NO3 dm3, which is similar to values observed at station 24 (12.331C, 35.541 and 6.16 mmol m3, Fig. 5e), a nearby station that was sampled during transect I of the grid survey east of the frontal structure. We conclude that our drogue system traversed the frontal region at the time the gale passed the area and that the interpretation of the drift-study data as a one-dimensional time series is not possible. Clearly, at pre- and post-storm stations of the drift study two different water masses were sampled, which were already obvious from the grid survey. Apart from vertical turbulent mixing as a result of the deepening surface mixed layer, there may be other physical processes contributing to the vertical export of organic matter. These include vertical circulation associated with mesoscale features like fronts and eddies (Woods, 1988), and Ekman pumping and suction due to the curl of the wind stress at the storm. Even for a stationary strong storm the latter process will yield velocities of not more than a few meters per day (upward below an atmospheric cyclone) and can therefore be neglected in our study. Vertical velocities associated with ocean fronts have, on the other hand, been found to reach up to

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40 m day1 (Pollard and Regier, 1992). This is significant with respect to the changes in the chlorophyll profiles described above. However, since many properties, like for example, salinity or nitrate, were found to be clearly distinct in the surface waters to both sides of the front, any export of organic matter by subduction at the front could be identified by the associated water mass properties. The profiles taken after the storm do not show any significant signals of water originating from southwest of the front. Surface salinity there was about 35.77 (sta. 37), whereas the salinity at the post-storm stations was close to 35.5 throughout the upper 300 m. Similarly, surface nitrate concentrations southwest of the front (Fig. 5a and b) were lower by several micromoles than those in subsurface waters sampled after the storm (Fig. 5c). We thus conclude that the POM found below the euphotic zone must have been exported by turbulent vertical mixing. 4.2. The importance of storm-induced convective mixing for particle export during spring 1992 To estimate the magnitude of convective particle export from the euphotic zone during storm event 4 we compare post-storm stations with station 24, which was sampled during the grid survey east of the front (Fig. 1), 10 days before post-storm sampling started. At that time surface chl-a was already elevated (Fig. 5e); the integrated stock (0–150 m) was about 100 mg m2 (Table. 1). About 42% of the standing stock was found within the upper 50 m (i.e. the euphotic zone). For an

estimate of convective export during storm event 4, we assume no net growth within the whole water column during the storm period and no concentration change for the depth range 50–150 m depth between the time when profile 24 was sampled and the storm. Given these assumptions, the increase of the chl-a standing stock for the depth layer 50–150 m observed just after the storm compared with the integrated stock of station 24 has to be due to convective export. The integral change over this depth range, 28 mg chl-a m2, is an estimate of the total convective chl-a export during storm event 4. Likewise, the 50–150 m integral from station 24, 58 mg chl-a m2, may be taken as a (minimum) estimate of convective export earlier in the year, during storm event 3, which deepened the mixed layer down to about 250 m (Podewski, IfMKiel, pers. comm.) and also by diurnal mixed-layer pumping. From the post-storm chl-a stock of the upper 150 m (133715 mg chl-a m2) and the increase of chl-a within the upper 50 m until the end of the study (20 mg chl-a m2), we estimate a cumulative chl-a stock of about 152717 mg m2. The maximum integrated chl-a stock between 50 and 150 m, 86 mg chl-a m2, taken as a (minimum) measure of integrated convective export makes up 56% of the cumulative standing stock of chl-a. For the time period between sampling of station 24 and the post-storm stations the integral change of chl-a below 50 m is equivalent to 85% of the stock increase in the upper 150 m, accentuating the importance of this storm for particle export. Our first order estimates from the vertical chl-a

Table 1 Integrated stocks of chlorophyll-a and the nitrate deficit at the Biotrans site during spring 1992 Depth range (m)

Grid survey 16.4.92 (station 24) (mg chl-a m2)

Post storm stations 26–29.4.92 (stations 64, 65, 71, and 76) (mg chl-a m2)

Late stations 30.4.–2.5.92 (stations 82, 89, 102, and 103) (mg chl-a m2)

Post storm stations 26–29.4.92 (stations 64, 65, 71, and 76) (mol N m2)

0–50 50–150 0–150

42 58 100

4774 86712 133715

6775 75713 146722

0.13070.012 0.34070.080a 0.47070.090a

a The total nitrate deficit is calculated between the surface and the interpolated depth where the observed nitrate concentration equals the winter time nitrate concentration (8 mmol m3). For the stations from the last column of the table this depths is 240750 m.

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distribution assume that grazing and sinking of particles from the upper 100 m do not affect our estimate of the convective export fraction and that background chl-a concentrations in this depth horizon are negligible. The latter is justified by data from summer and autumn in the region (e.g. Veldhuis et al., 1993). We estimate new production by calculating the nitrate deficit as the depth integrated difference between a given nitrate profile during early spring and the maximum winter nitrate concentration (Koeve, 2001). Here, we calculate the difference between the local winter nitrate concentration and the nitrate profiles from stations sampled just after the storm event. Our best estimate of the local winter nitrate concentration in the region east of the temperature front is about 8 mmol m3, which is at the upper end of the range estimated for 1992 based on data from M21/1 (Koeve, 2001). The seasonal, pre-storm new production for this region then amounts to 0.47070.090 mol N m2 (Table 1). The vertical distribution of the nitrate deficit during early spring can be used to estimate the share of convective particle export in seasonal new production. If we assume that pre-bloom net primary production was restricted largely to the upper 50 m—as it was during our study (Jochem, 1993)—any nitrate deficit below 50 m must be due to convective mixing after the onset of spring growth. After storm event 4, about 70–75% of the observed deficit was found below 50 m depth (Table 1). Until the end of our study, about a week later, an additional 0.04070.010 mol m2 of nitrate was taken up within the euphotic zone. Still about 65% of the total depth integrated new production is found below the euphotic zone. Depletion of silicate (0.2–0.3 mmol m3) indicates that the diatom bloom was over at that time. We conclude that a share of 56–65% of the seasonal new production during the diatom bloom in 1992 in the Biotrans region was exported from the euphotic zone by convective mixing. . Observations from 22 May (M21/3, T. Korner, IfM-Kiel, pers. comm., 1993) show little nitrate (1–1.5 mmol m3) at the surface. Assuming that nitrate was exhausted in the upper 25 m through the end of May and that nitrate concentrations

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between 25 and 50 m increased linearly from 0 to about 4 mmol m3 (like during NABE), we compute a seasonal new production of 0.2 mol N m2 for the period of the unperturbed bloom between late April and the end of May. As convective mixing is negligible during that time (see below) we finally estimate the fraction of total spring bloom new production that was exported by convective mixing to be about 49–53% of the total spring time new production of 0.67 mol N m2 during spring 1992. Both the chl-a and the nitrate data yield similar estimates of the fraction of new production that is exported by convective mixing (56% and 65%, respectively, for the time period until early May). Being influenced by zooplankton grazing, sinking and the light regime, the chl-a stock can be regarded as a weak proxy of new production. During our study the PON:chl-a ratio in the upper 50 m varied between 1.5 and 2.5 mol:g. Using a mean PON:chl-a ratio of 270.5 mol:g, the cumulative chl-a stock of 0.152 g m2 converts to a cumulative PON-stock of 0.370.1 mol m2, which is about 30% less than the nitrate-based estimate of new production (0.47 mol N m2). This difference is partly due to the depth reference used for the two estimates. The vertical resolution of chl-a data permitted an integration only down to 150 m, but the nitrate deficit was calculated down to the interpolated depth at which the observed nitrate concentration was equal to the winter concentration (see Table 1). The nitrate deficit between 0 and 150 m was 0.3770.04 mol NO3 m2, which agrees favourably with the chl-a-based estimate of 0.370.1 mol PON m2 for the same reference depth. 4.3. Magnitude of convective transport of biogenic particles during spring 1989 In the following we compare our observations with data from the North Atlantic Bloom Experiment in 1989. During the beginning of that study, at the end of April 1989, profiles of chl-a, nitrate and temperature provide evidence for convective particle export. For example, chl-a (Fig. 5f) was elevated down to 100 m. Later, during the diatom bloom in May 1989, surface chl-a increased to as

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4.4. Early spring bloom dynamics In this section we discuss possible causes for the differences in development of the spring blooms in 1989 and 1992. We evaluate the temporal development of the daily mean net heat flux (Fig. 7) during both years and discuss mixed-layer dynamics, phytoplankton growth conditions, and the importance of particle loss by convective mixing. During 1989, daily mean net heat flux as diagnosed from the ECMWF reanalysis was generally negative until mid-April (Fig. 7b), interrupted only by a number of short periods, typically of 1–3 days duration, during which slight warming

400 net heat flux (W m–2 )

much as 3 mg chl-a dm3 (Lochte et al., 1993), but this increase was restricted to the upper 20–30 m. Below, chl-a concentrations decreased rapidly to about 0.1 mg m3 at 100 m. From this and other observations (Gardner et al., 1993; Garside and Garside, 1993; Koeve, 2001), we infer an initial bloom development under transient stratification and convective particle export during the early part of the diatom bloom in 1989. Koeve (2001) estimates a seasonal new production during the time of transient stratification (pre-bloom new production) of about 0.275 mol N m2. We interpret the vertical distribution of the nitrate deficit during early spring as we did for the 1992 data and estimate the share of seasonal new production exported by convective mixing. About 75% of the pre-bloom nitrate deficit (new production) is found below 50 m depth (0.21 mol N m2). Again we assume that prebloom net primary production was restricted largely to the upper 50 m—as it was during NABE (Marra and Ho, 1993)—and any nitrate deficit below 50 m to be due to convective mixing after the onset of spring growth. There is no comparable evidence of convective particle export being important during the main spring phytoplankton bloom at Biotrans during NABE. Given that pre-bloom new production was slightly less than bloom new production in 1989 (0.3 mol N m2; Koeve, 2001), still up to 36% of the total (pre-bloom+bloom=0.575 mol N m2) new production was exported from the euphotic zone by convective transport.

(1992)

200 0 –200

(a) –400

net heat flux (W m –2 )

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(1989)

200 0 –200

(b) –400

FEB

MAR

APR

MAY

Fig. 7. Net heat flux during winter and spring of 1992 (a) and 1989 (b) taken from the ECMWF re-analysis data. Storm events 3 and 4 during spring 1992 are marked.

was observed. Despite the perpetual heat loss, which superficially indicates deep mixing and unfavourable growth conditions for the phytoplankton, estimates of pre-bloom new production (Koeve, 2001) show a significant share of total spring new production to have taken place prior to the permanent stratification of the water column. Furthermore, the vertical distribution of the nitrate deficit, of chl-a and of particulate matter (Fig. 5f; Gardner et al., 1993) during late April 1989 suggest that convective mixing had been important in early spring. Since an extended period of heat gain interrupted by storm events during this time cannot be found, diurnal variations of the mixing depth are a possible explanation for both observations. Such mixed-layer pumping (Gardner et al., 1995) is typical during the transition from winter cooling to summer warming (Woods and Barkmann, 1986), when daily mean heat flux is near zero or only moderately negative. Under such a scenario, phytoplankton is kept within a shallow mixed layer during daytime and can have positive net growth rates (Woods and Barkmann, 1993). Every night, however, the plankton is mixed down to deeper layers (Stramska et al., 1995; Gardner,

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1997), and a fraction of it will be lost in the dark after re-shallowing of the mixed layer, the following day (Woods and Barkmann, 1986). Plankton stocks in the euphotic zone increase under such conditions, though slowly, long before a permanent shallow stratification has developed. After April 15, 1989 the sign of the daily net heat flux changes, and the ocean gains heat continuously afterwards. Consequently, a rapid shallowing of the mixed layer was observed (Lochte et al., 1993), and the spring bloom proceeded quickly and uninterrupted until nutrients were exhausted about a month later. Heat flux patterns during March and April 1992 were different (Fig. 7a). Two clear periods with positive heat flux of 2–3 weeks duration are recognised, one in March and one in April. Both were terminated by strong storm events (storm events 3 and 4 in Figs. 2 and 7a). During periods of moderate positive (net) heat flux a significant diurnal variation of mixed-layer depth is unlikely and the depth of the mixing layer is controlled mainly by wind mixing. We approximate the depth of the wind mixed layer (h) according to Haney and Davies (1976; Eq. 1) as the minimum of the Monin-Obukhov length scale (L; Eq. (1a)) and the neutral planetary boundary layer length scale (P; Eq. (1b)) which we adopt from Cushmon-Roisin (1994). h ¼ minðL; PÞ;

ð1Þ

L ¼ w3 ðkagQnet =ðrcp ÞÞ1 ;

ð1aÞ

P ¼ 0:3w =f

ð1bÞ

with w * the friction velocity in the water, k the Ka! rma! n constant, a the thermal expansion coefficient with respect to temperature, g the gravity constant, Qnet the net heat flux, r the density of seawater, cp the specific heat capacity and f the coriolis parameter. From this analysis we find that during much of March and April 1992 moderate depths of the wind mixed layer of between 30 and 40 m prevailed. This is in agreement with actual mixed layer depths (Dt ¼ 0:021C) that we observed during April 1992 along the east–west transect of 1978 m (mean7std, calculated from CTD temperature profiles (S. Podewski, IfM-Kiel, pers.

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comm.; Podewski et al., 1993). This is shallow compared with an estimate of the depth of the euphotic zone of about 50 m, inferred from the vertical distribution of primary production (Jochem, 1993). We conclude that the mean availability of light and subsequently the mean growth conditions of phytoplankton during early spring were more favourable in 1992 than 1989. After the passage of the last spring storm in the area in late April 1992, daily mean heat flux was positive during most of May (except for a 4-day period with small negative heat flux). Heat flux patterns between late April and late May are similar in 1989 and 1992 (Fig. 7). Consequently, convective mixing was not a significant process for particle export from the euphotic zone during May either in 1989 or 1992. The ratio of early bloom or transient-bloom new production to (unperturbed) bloom new production was 0.9 and 2.35 for 1989 and 1992, respectively. That is, production under conditions of transient stratification was more important during 1992. We estimate that the fraction of total spring-bloom new production exported by convective mixing was 49–53% in 1992 compared to about 36% during 1989. Different short-term weather patterns during spring (Fig. 7) control the fraction of new production that is exported from the euphotic zone by convective mixing or as fast sinking particles. There are a number of other data sets supporting the importance of transient blooms (i.e. blooms that develop under conditions of transient stratification), which may be used to quantify the contribution of convective mixing to particle export. Data from OWS India (601N, 201W, Williams and Robinson, 1973; Williams, 1974; Williams and Hopkins, 1975, 1976) show elevated chl-a concentrations down to about 150 m during April of 1971–1974. Observations from the 1991 experiment of the MLML-program give evidence for the importance of convective transport down to about 80–100 m at the same site (Stramska et al., 1995), but observations during 1989 show a more typical spring bloom development that was merely restricted to the upper 50 m (Dickey et al., 1994). These and other studies (see the introduction)

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show that transient blooms may be more frequent and that convective transport may be more important for particle export of organic matter than generally believed. If blooms develop under weakly stratified conditions, they are sensitive to perturbations by diurnal variations of heat flux and mixed layer depth, storms or other cooling events (Woods and Barkmann, 1986; Gardner et al., 1993; Stramska et al., 1995; Gardner, 1997; this study). This has a number of implications. Mixing to greater depth will reduce net community production during the time of mixing. Convective mixing will furthermore export biogenic particles from the euphotic zone to deeper layers, but on the other hand supply new nutrients from below the productive layer (Gardner et al., 1993, 1995; Townsend et al., 1994; Koeve, 2001; this study). The latter will increase seasonal new production. Since, particle concentration is decreased and bloom development is retarded, however, the partitioning between export to the deep ocean by aggregates (Riebesell and Wolf-Gladrow, 1992) and transfer of matter to higher trophic levels (Peinert et al., 1989) may be altered, complicating predictions about the net effect of storms on the sequestering of carbon to the deep ocean.

Acknowledgements We would like to thank captain and crew of FS Meteor cruise 21 for excellent seamanship and support of our work. Our colleagues A. Dettmer, . S. Bohm and S. Podewski were most helpful during the work on board and later shore-based . analyses. S. Podewski, T. Korner and D. SchulzBull (all IfM Kiel) provided unpublished data. This paper benefited from discussions with colleagues from the former ‘Feedback’-Working Group at IfM. Comments by W. Gardner, H. Ducklow, P. K.ahler, J. Waniek and two anonymous reviewers helped to clarify the manuscript. This work is a contribution to the German JGOFS Synthesis and Modelling Effort and was supported by grants of the Minister of Research and Technology (BMBF-BEO 03F0202A and D).

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