Manganese and iron distributions off central California influenced by upwelling and shelf width

Marine Chemistry 95 (2005) 235 – 254 www.elsevier.com/locate/marchem

Manganese and iron distributions off central California influenced by upwelling and shelf width Zanna Chasea,*, Kenneth S. Johnsonb, Virginia A. Elrodb, Joshua N. Plantb, Steve E. Fitzwaterb, Lisa Pickella,1, Carol M. Sakamotob a

Oregon State University, College of Oceanic and Atmospheric Sciences, Ocean Administration Building #104, Corvallis, OR 97331-5503, United States b Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, United States Received 14 April 2004; received in revised form 28 September 2004; accepted 29 September 2004 Available online 29 December 2004

Abstract In July 2002, a combination of underway mapping and discrete profiles revealed significant along-shore variability in the concentrations of manganese and iron in the vicinity of Monterey Bay, California. Both metals had lower concentrations in surface waters south of Monterey Bay, where the shelf is about 2.5 km wide, than north of Monterey Bay, where the shelf is about 10 km wide. During non-upwelling conditions over the northern broad shelf, dissolvable iron concentrations measured underway in surface waters reached 3.5 nmol L 1 and dissolved manganese reached 25 nmol L 1. In contrast, during nonupwelling conditions over the southern narrow shelf, dissolvable iron concentrations in surface waters were less than 1 nmol L 1 and dissolved manganese concentrations were less than 5 nmol L 1. A pair of vertical profiles at 1000 m water depth collected during an upwelling event showed dissolved manganese concentrations of 10 decreasing to 2 nmol L 1, and dissolvable iron concentrations of 12–20 nmol L 1 in the upper 100 m in the north, compared to 3.5–2 nmol L 1 Mn and ~0.6 nmol L 1 Fe in the upper 100 m in the south, suggesting the effect of shelf width influences the chemistry of waters beyond the shelf. These observations are consistent with current understanding of the mechanism of iron supply to coastal upwelling systems: Iron from shelf sediments, predominantly associated with particles greater than 20 Am, is brought to the surface during upwelling conditions. We hypothesize that manganese oxides are brought to the surface with upwelling and are then reduced to dissolved manganese, perhaps by photoreduction, following a lag after upwelling. Greater phytoplankton biomass, primary productivity, and nutrient drawdown were observed over the broad shelf, consistent with the greater supply of iron. Incubation experiments conducted 20 km offshore in both regions, during a period of wind relaxation, confirm the potential of these sites to become limited by iron. There was no additional growth response when copper, manganese or cobalt was added in addition to iron. The growth response of surface water incubated with bottom sediment (4 nmol L 1 dissolvable Fe) was slightly greater than in control incubations, but less than in the presence of 4 nmol L 1 dissolved

* Corresponding author. Tel.: +1 541 737 5192; fax: +1 541 737 2064. E-mail address: [email protected] (Z. Chase). 1 Now at School of Marine Sciences, University of Maine, Orono, ME, 04469, United States. 0304-4203/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2004.09.006

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iron. This may indicate that dissolvable iron is not as bioavailable as dissolved iron, although the influence of additional inhibitory elements in the sediment cannot be ruled out. D 2004 Elsevier B.V. All rights reserved. Keywords: Iron; Manganese; Upwelling; Coastal; California

1. Introduction There has been a growing interest in the distribution of iron in coastal waters following the recognition that iron may regulate ecosystem properties (e.g. community composition and nutrient consumption ratios) and productivity in coastal upwelling regions (Bruland et al., 2001; Hutchins et al., 1998; Johnson et al., 2001). Shelf sediments are the main source of iron to these systems (Johnson et al., 1999) and the availability of iron therefore varies with the availability of sediment, i.e. with the width of the continental shelf (Bruland et

al., 2001). Questions remain, however, about the interaction between wind forcing and iron delivery over the short term. Macronutrient concentrations and biological activity can respond rapidly to changes in the wind (e.g. MacIsaac et al., 1985; Service et al., 1998) but less is known about the response of micronutrients. A recent intensive study of the chemical evolution of upwelling plumes off An˜o Nuevo, central California (Fitzwater et al., 2003), found a rapid increase and subsequent decrease in particulate and dissolvable iron in response to the initiation and ageing of an upwelling event.

Fig. 1. Mean July SeaWiFS chlorophyll from 1998–2003 using 1.1 km resolution level 2 data calculated using the standard OC4 v4 algorithm via SeaDAS.

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Relatively little is known about the behavior of other bioactive metals in coastal upwelling settings and their potential effect on phytoplankton growth and community structure. In combination with high copper concentrations, manganese may be limiting in some upwelling situations (Sunda et al., 1981). The main sources of manganese to coastal waters are from river runoff (Bender et al., 1977; Jones and Murray, 1985) and a dissolved manganese flux from margin sediments (Johnson et al., 1992; Trefry and Presley, 1982). The dynamics of manganese oxide formation and reductive dissolution play an important role in determining dissolved manganese distributions in surface waters (Schoemann et al., 1998; Sunda and Huntsman, 1988; Sunda et al., 1983). It is not known whether coastal manganese concentrations, like iron, are linked to the width of the continental shelf. Copper and cobalt are also required biologically, and while their total concentration is not expected to be limiting in coastal areas complexation by strong organic ligands could reduce their availability. The objective of this study was to compare the biogeochemical properties of the upwelling centers north and south of Monterey Bay, California off An˜o Nuevo and Point Sur, respectively. The width of the shelf differs substantially between these two sites, being about 10 km in the north and 2.5 km in the south (defined by the 150 m isobath). There is also a sharp contrast in summertime chlorophyll concentrations at these two sites, with persistently lower levels found south of Monterey Bay (Fig. 1). We concentrate here on the distributions of iron and manganese during upwelling and downwelling episodes that occurred over a 10-day period in July 2002. We also report results of a series of incubation experiments to test the effect of added Fe, Mn, Cu, Co and bottom sediments on phytoplankton growth.

2. Methods 2.1. Sample collection Fieldwork for the MBARI Upwelling Science Experiment (MUSE) II took place aboard the R/V Western Flyer from 10–16 July 2002. Sampling was

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concentrated near the upwelling centers off An˜o Nuevo Point, north of Monterey Bay, and off Point Sur, south of the Bay (Fig. 2). There was a focus on the temporal evolution of an upwelling plume at the Point Sur site. The general approach was to conduct mapping surveys at night and to occupy CTD stations during the day. While the ship traveled at 9–10 km h 1, nearsurface (~2 m) seawater for trace metal analysis was continuously pumped into a class-100 flow bench in the main shipboard laboratory through acid-cleaned polyethylene tubing attached to a towed fish. For iron and manganese analysis two water streams were drawn from the bulk flow (~5 L min 1) at a rate of ~8 ml min 1 using small peristaltic pumps, and then passed through acid-leached, 20 Am, inline polyethylene filters to remove large particulate matter. Vertical profile samples for iron and manganese analysis were collected in 2.5 L bottles with Teflon coated interior surfaces (Ocean Test Equipment) mounted in a small Rosette modified for trace metal work (Johnson et al., 2003). Unfiltered samples were collected from the Rosette in rigorously cleaned (3 N HCl followed by 2 N HNO3), 125 ml LDPE bottles. The samples were refrigerated (less than 6 h) until analysis. Unfiltered nutrient samples were stored frozen until analysis on shore. 2.2. Analytical methods Iron (Fe(III)) was measured by Flow Injection Analysis (FIA) with chemiluminescence detection (Obata et al., 1993, as modified by Johnson et al. 2003). The 20 Am-filtered seawater stream was acidified inline to pH 3.4 for 1 minute prior to analysis for iron. We therefore refer to the iron measured by this method as ddissolvableT. This method does not detect Fe(II). An iron measurement was made approximately every 5 min. Dissolvable iron in the unfiltered Rosette samples was analyzed similarly after a 1 minute in-line acidification to pH 3.4. Manganese (Mn(II)) was measured by FIA with chemiluminescence detection (Chapin et al., 1991, as modified by von Langen et al., 1997) with further modifications to take advantage of the 10-port valve

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Fig. 2. Location of stations where vertical profiles of iron and manganese were collected. Metal enrichment experiments were performed at stations 1 and 4. Also shown is the location of MBARI mooring M1 and the NDBC weather buoy (B) 46042. The box off Big Sur indicates the location used for the time series analysis presented in the text and in Fig. 3. The 100, 500 and 1000 m depth contours are indicated.

system developed for iron FIA (Johnson et al., 2003). Manganese measurements were made on seawater that was not acidified. This method detects dissolved manganese but not particulate manganese, including manganese oxides (von Langen et al., 1997). Surface water nutrient concentrations (nitrate+nitrite and silicic acid) were determined underway once per minute using an Alpkem RFA analyzer (Sakamoto et al., 1996). Nitrate, phosphate and silicic acid concentrations from vertical profiles were determined at MBARI using an Alpkem RFA analyzer (Sakamoto et al., 1990). If samples are thawed carefully, silicic acid concentrations determined in frozen samples are not compromised by polymerization artifacts, partic-

ularly when concentrations are less than 65 Amol L 1 (Sakamoto et al., 1990), as were almost all of the samples analyzed here. The mole fraction of carbon dioxide was determined underway as described in Friederich et al. (1995). Chlorophyll a was determined fluorometrically at sea with a Turner Designs Model 10-005 R fluorometer. Chlorophyll a was also determined in several samples by High Pressure Liquid Chromatography (HPLC) at the Center for Hydro-Optics and Remote Sensing, San Diego State University Foundation, under the SIMBIOS program. Sample volume was 540 ml. Carbon fixation (primary productivity) was estimated using on-deck incuba-

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tions with 14C as described by Pennington and Chavez (2000). 2.3. Metal enrichment experiments Near-surface water for two incubation experiments was collected using the clean pumping system (see dsample collectionT, above). Seawater was initially collected in one 30 L carboy then homogenized and dispensed into acid-cleaned 1 L polycarbonate bottles. There were four replicate bottles for each treatment, including the control. At the northern incubation site treatments consisted of iron (+4 nmol L 1) and iron (+4 nmol L 1) and manganese (+8 nmol L 1). At the southern site, treatments were: iron (+4 nmol L 1); iron (+4 nmol L 1) and manganese (+8 nmol L 1); iron (+4 nmol L 1) and copper (+1 nmol L 1); iron (+4 nmol L 1) and cobalt (+0.1 nmol L 1); copper (+1 nmol L 1) and manganese (+8 nmol L 1); and sediment. Metals were added (b100 AL per 1000 ml incubation bottle) from acidified stock solutions (2 ml L 1 6 N HCl) made by diluting AA standards (Fisher). Sediment was collected from the ROV Tiburon on June 29, 2002 from a depth of 100 m, south of Big Sur (35.18N, 120.88W). The sediment was dried for 24 h at 60 8C (in order to weigh it), and 1000 mg of sediment was resuspended in 50 ml of low iron seawater for 2 h just prior to addition to the incubation bottles. Aliquots of this resuspension were transferred to the incubation bottles to achieve sediment amendments of 2 mg L 1, which is typical of the concentrations of suspended particulate matter found in the An˜o Nuevo upwelling plume (Fitzwater et al., 2003). Samples were incubated on deck in a non-shaded polycarbonate incubator cooled by the ship’s seawater system. An initial sample was collected from the 30 L carboy for chlorophyll and nutrient analyses. Subsequent sampling from incubation bottles was done every 24 h. Sampling of the four bottles was done such that for each day after day 1, samples were taken from two bottles, one of which had not been previously opened. Each bottle was sampled twice during the course of the experiment, which lasted 5 days. Sampling was done in a class-100 flow bench to prevent trace metal contamination.

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3. Results and discussion Observations began on 10 July 2002, during a period of wind reversal and relaxation. Winds had been blowing strongly from the north, so favorable for upwelling, for the week prior to the cruise (according to data from NBDC Buoy 46042). Southward wind stress increased rapidly beginning 13 July, and reached a maximum on 14 July of almost 0.21 N m 2, and then remained southward at roughly 0.1 N m 2 until just before the end of the cruise (see Fig. 3A). Surface water temperature and nutrient distributions responded to this change in wind forcing as expected, at both the northern and southern sites: SST cooled by about 1 8C (Figs. 4A and 5A), and surface nitrate concentrations increased by about 3 Amol L 1 (Figs. 4B and 5B). The mole fraction of CO2 at the surface increased by about 100 ppm at both sites (Figs. 4C and 5C). Surface water silicic acid (Si) concentrations were similar at both sites after the upwelling event (16F8 Amol L 1), but before the event Si concentrations at the southern site were significantly lower than at the northern site (4 versus 11 Amol L 1; Figs. 4D and 5D). 3.1. Iron concentrations Dissolvable iron concentrations were consistently close to 0.7 nmol L 1 at the southern site before upwelling began on 13 July. Iron concentrations were on average higher at the northern site, before upwelling started, with a maximum of 3.5 nmol L 1 very close to shore near An˜o Nuevo Point (Fig. 6A). After upwelling began iron concentrations at the southern site were again relatively uniform but on average about two times higher than observed on the first survey, before the upwelling event. Dissolvable iron concentrations close to 6 nmol L 1 were measured at the northern site after upwelling (Fig. 6C), near An˜o Nuevo. A direct comparison with the pre-upwelling survey in the north is difficult, however, because the first survey did not extend as far north as the area where the highest iron concentrations were observed during the second survey. The area that was sampled on both surveys— the dUT closest to Monterey Bay on July 15 (Fig. 6)— shows no significant difference in iron between the two surveys (1.3F0.9 to 1.7F0.9 nmol L 1).

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Fig. 3. A time series of surface water properties and wind forcing during MUSE II. A—The upwelling rate for 368N and 1228W from the Pacific Fisheries Environmental Laboratory (thin line) and the alongshore wind stress at 36.758N and 122.428W as measured by the NBDC buoy 46042 (thick line). B–F (black circles)—Temperature, xCO2, nitrate, silicic acid, and dissolvable iron in surface waters from a dboxT just off Big Sur (see Fig. 2) measured from the ship during MUSE II. B and D (black line)—Temperature and nitrate recorded at the surface at the MBARI mooring in Monterey Bay (M1). The nitrate record is from an optical nitrate sensor. Time is GMT.

Surface water measurements of the upwelling indicators temperature and salinity show clearly that upwelling occurs both in the An˜o Nuevo region and in the Point Sur region. Whereas underway dissolvable iron concentrations off An˜o Nuevo reach 6 nmol L 1 after peak upwelling, off Point Sur they reach no more than 1.5 nmol L 1 even during peak upwelling (Fig. 7). The same pattern is observed in the surface discrete samples: 15.7 and 11.5 nmol L 1 Fe at stations 19 and 20 (North) and 8.2 and 5.3 nmol L 1 Fe at stations 16 and 17 (South). This basic distribution of surface water iron appears to be a robust feature. In August 2000, dissolvable iron concentrations as high as 6.5 nmol L 1 were measured in the An˜o Nuevo upwelling plume (Fitz-

water et al., 2003). Similarly, in July 1995, labile dissolved iron concentrations near shore at 37.98N were about three times higher than labile dissolved iron concentrations near shore at 368N (Bruland et al., 2001). The consistently higher iron concentrations north of Monterey Bay can be linked directly to the greater surface area of continental shelf there relative to south of Monterey Bay. Shelf sediments are the primary source of iron to surface waters along the central Californian coast (Fitzwater et al., 2003; Johnson et al., 1999). Previous work has hypothesized that shelf iron is mobilized through bottom boundary layer turbulence during downwelling and then reaches the surface when deep isopycnals intersect the surface

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Fig. 4. The distribution of temperature, nitrate, CO2 (xCO2; the mole-fraction of CO2 in air, ppm) and silicic acid in surface waters on 10 July (data north of 36.58N) and 12 July (data south of 36.58N), 2002, during a period of wind relaxation. The mean and standard deviation of the measurements is indicated on each map. Depth contours are as in Fig. 2.

during upwelling (Chase et al., 2002; Fitzwater et al., 2003; Johnson et al., 2001). The vast majority (at least 80%) of the upwelled iron is associated with particles (Bruland and Rue, 2001; Bruland et al., 2001; Fitzwater et al., 2003; Johnson et al., 2001). Observations during MUSE II confirm various aspects of this hypothesis. Vertical profiles over the shelf all show dissolvable iron concentrations increasing significantly towards the seafloor (Fig. 8 and Table 1), consistent with shelf sediments being a source of iron to overlying waters. Vertical profiles for trace metal analysis (Figs. 8 and 9) were collected from a total of 17 stations; 14 were from the southern site and 3 from the northern site (Table 1). Stations 1

(north), 2, 4, 5, 6, 7, 8, 9, 12, 13, 14 and 15 (south) were occupied before the upwelling event and stations 16, 17, 18 (south), and 19 and 20 (north) were occupied during the upwelling event. Several profiles collected over the shelf both to the north and south, after the onset of upwelling-favorable winds, show iron concentrations of 5–10 nmol L 1 uniformly throughout the water column (Fig. 9 and Table 1), demonstrating that upwelling brings deep (~50 m), benthic-sourced iron to the surface. Note that dissolved oxygen profiles (not shown) were similar at sites north and south of Monterey Bay. Concentrations decreased from about 250 Amol L 1 at the surface to 50–100 Amol L 1 at 200 m. The trend towards higher

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Fig. 5. The distribution of temperature, nitrate, CO2 (xCO2; the mole-fraction of CO2 in air) and silicic acid in surface waters on 14–15 July (data north of 36.58N) and 15–16 July (data south of 36.58N), 2002, during a period of upwelling-favorable winds. The mean and standard deviation of the measurements is indicated on each map. Depth contours are as in Fig. 2.

iron concentrations in denser, more recently upwelled surface waters (Fig. 7A) is also consistent with a shelf source of iron. Upwelling over the shelf, be it a narrow or a broad shelf, can rapidly produce elevated iron concentrations in surface waters very near shore. Previous work in the California upwelling system has emphasized the importance of particulate iron both as it contributes to total iron concentrations and as a source of dissolved iron (Bruland et al., 2001; Fitzwater et al., 2003; Johnson et al., 2001). Although particulate iron was not measured directly in this study, indirect evidence confirms the importance of particulate iron as a source of dissolvable iron to

surface waters. By comparing dissolvable iron measured in the surface Rosette sample, which was not filtered, with dissolvable iron measured by the flowthrough system at the same time, which was passed through a 20 Am filter, we get an indication of the amount of easily leachable particulate iron greater than 20 Am (Fig. 10). This is an imperfect measure, because the cutoff size of the filter undoubtedly decreases as the filter becomes clogged. However, from this analysis it is clear that a substantial portion of the dissolvable iron is derived from large particles (N20 Am), particularly when the total iron concentration is high.

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Fig. 6. Dissolvable iron and manganese concentrations in surface waters during MUSE II. The mean and standard deviation of the measurements is indicated on each map. Depth contours are as in Fig. 2.

A pair of vertical profiles (Fig. 9) at stations 18 (south) and 20 (north), collected after the onset of upwelling, highlights the difference between the two regions north and south of Monterey Bay. Both stations are comparable distances from shore and have comparable bottom depths (~1000 m). The dissolvable iron concentration at station 20 is 12–20 nmol L 1 in the upper 75 m and decreases to a minimum of 10 nmol L 1 at 100 m, whereas at station 18, the dissolvable iron concentration is 0.6 nmol L 1 at the surface and increases to a maximum of less than 10 nmol L 1 at 100 m. These differences in dissolvable iron are consistent with a shelf-iron source. That is, the elevated concentrations in the

northern profile (wide shelf) are restricted to the upper 100 m, or approximately the depth of the shelf. These profiles also suggest the dshelf effectT extends beyond the shelf, such that these sites at the 1000 m isobath reflect the difference in shelf width in their general vicinity. We argue above, as others have, that the source of this shelf iron is resuspended sediments. Another potentially important source of iron from continental shelves is the flux of dissolved iron from sediments driven by the oxidation of organic carbon. The average iron flux from Monterey Bay sediments, measured with benthic landers, is 5 Amol Fe m 2 day 1 (Berelson et al., 2003; Elrod et al., 2004). On average, the shelf (to the 150 m isobath) is about four times

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Fig. 7. Relationship between chemical properties and density from underway measurements of surface waters. Data from north and south of 36.58N are indicated by grey and black dots, respectively. Samples from within Monterey Bay are excluded.

wider to the north of Monterey Bay (10 km) than to the south (2.5 km). The difference in integrated iron values between stations 18 and 20 is 1200 Amol Fe m 2. If the difference is due only to the flux of dissolved iron, then the difference represents the cumulative effect of 2 months (60=(1200/5)/4) of benthic flux from the four times greater shelf area to the north. Even considering the maximum benthic iron flux of 14 Amol Fe m 2 day 1 observed by Elrod et al. (2004), it would still take 21 days to account for the difference between the two sites. Even in relatively dretentiveT regions of upwelling systems, water typically resides on the shelf not longer than 8 days (Graham and Largier, 1997). Thus the diffusive flux cannot have a major impact on dissolvable iron concentrations that reach values greater than 5 nmol L 1 following weeklong upwelling events (Fig. 7). Sediment resuspension, as discussed above, must deliver iron at a faster rate over the short term. Despite the relatively modest impact of the dissolved iron flux on the high dissolvable iron concentrations that are observed after upwelling, the dissolved flux may still have a significant biogeo-

chemical impact. For one, it is almost certainly more bioavailable than particulate iron. Iron in the dissolved phase will likely have a longer residence time than the particulate fraction, because the latter appears to be dominated by particles N20 Am, which will settle rapidly (Fitzwater et al., 2003). As a result, the offshore transport of bioavailable iron at concentrations b1 nmol L 1 over 100’s of km (Johnson et al., 2003) may be significantly impacted by the dissolved benthic flux. 3.2. Temporal variability In this study we observed significant changes in both surface and subsurface iron and manganese concentrations in response to short-term changes in wind stress. During MUSE I (August 2000) an upwelling event was tracked off An˜o Nuevo, from initiation through to relaxation. Surface water measurements during the plume initiation phase documented an initial increase in dissolvable iron at a rate of 3 nmol L 1 day 1 and increases in nitrate and silicic acid of 8 and 11 Amol day 1, respectively

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dissolvable iron, at a rate of 59 nmol L 1 day 1 (Fitzwater et al., 2003). In 2002 we observed the initiation and early stages of an upwelling event off Point Sur. In order to better compare this event with that observed off An˜o Nuevo in 2000 we have calculated the rate of change of various parameters for a box of 0.10.058 centered just off Point Sur (~121.938W; 36.308N; Fig. 2). We find that between 13 July 15:00 and 14 July 19:00, temperature decreased, and salinity, Mn, Fe and nutrients increased at the rates given in Table 2 and shown in Fig. 3. Note that this figure includes some data not displayed in Figs. 4–6. When the iron input rate is calculated from unfiltered surface Rosette samples collected from stations 6 and 16 (12 and 14 July) a much higher rate of 3 nmol L 1 day 1 is found compared to the value computed from the underway surface samples (0.19 nmol L 1 day 1), which were filtered through a 20 Am filter. This observation again demonstrates the importance of particulate iron for the input of iron to surface waters during upwelling. Based on the slower rates of nitrate (70% slower) and silicic acid (60% slower) increase observed during 2002 we conclude that this upwelling event was slightly less intense than that observed off An˜o Nuevo in 2000. In contrast to the ~65% slower macronutrient input rate in 2002 versus 2000, the dissolvable iron input rate (0.19 nmol L 1 day 1 from underway measurements) was 91% slower at Point Sur (2002) compared to An˜o Nuevo (2000). Although combining data from different seasons, this comparison provides further support for the idea that the greater shelf width off An˜o Nuevo contributes to faster rates of iron input to surface waters during upwelling. Fig. 8. Examples of profiles of dissolvable iron [nmol L 1], dissolved manganese [nmol L 1], nitrate [Amol L 1] and primary productivity [mg C m 3 day 1] collected 10–12 July, before the onset of strong upwelling-favorable winds. Profiles from north of 36.58N are in white and profiles from south of 36.58N are in black. Note the different depth scale used in the productivity panel.

(Fitzwater et al., 2003). As the plume aged, dissolvable iron in surface waters decreased at a rate of 1.7 nmol L 1 day 1, nitrate decreased by 5 Amol L 1 day 1 and silicic acid by 5.7 Amol L 1 day 1. Total particulate iron decreased much faster than

3.3. Manganese concentrations Surface manganese concentrations at the northern site were 2–3 times higher than at the southern site, both before and after the upwelling event (Fig. 6B,D). A maximum manganese concentration of 28 nmol L 1 was measured before upwelling off An˜o Nuevo (Fig. 6B). In contrast to the iron profiles, manganese profiles collected before the upwelling event generally show a near-surface concentration maximum and no indication of an increase towards the seafloor (Fig. 8, Table 1). Stations 18 and 20, occupied during upwelling, show a strong contrast in manganese

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Table 1 Temperature, salinity, dissolvable iron, dissolved manganese and nutrient data from vertical profiles collected during MUSE II Station

Depth [m]

Temp. [ C]

Salinity

Iron [nmol L 1]

Manganese [nmol L 1]

Phosphate [Amol L 1]

Silicic acid [Amol L 1]

Nitrate [Amol L 1]

Station 1 36.748N/122.208W 10 July 20h18 1.2 12.66 33.55 6.0 12.58 33.57 11.2 12.24 33.60 21.1 12.02 33.53 31.3 12.03 33.67 41.0 11.62 33.76 60.8 9.68 33.65 77.6 9.55 33.84 98.6 9.30 33.96 148.9 8.70 34.02 199.5 8.45 34.09 495.4 6.11 34.23

0.6 0.4 0.5 3.3 1.1 1.5 0.9 1.7 4.5 3.3 3.1 2.9

ND ND ND ND ND ND ND ND ND ND ND ND

0.9 1.0 0.9 1.0 1.0 1.2 1.6 1.8 1.9 2.0 2.2 2.9

6.4 6.7 7.1 8.0 11.3 15.6 23.0 26.6 30.8 35.3 39.9 72.7

9.8 10.0 10.7 11.6 12.6 14.9 23.2 24.5 27.5 29.7 31.4 39.9

Station 2 36.338N/121.958W 11 July 13h15 0.8 11.08 33.76 4.1 11.11 33.76 9.8 10.87 33.76 13.5 10.57 33.74 19.9 10.28 33.74 24.6 10.19 33.76 29.6 10.18 33.77 33.4 10.17 33.78 39.8 10.05 33.82 44.3 9.97 33.85 49.5 9.93 33.87 57.3 9.90 33.88

1.0 0.8 1.1 1.2 1.2 1.4 1.5 2.0 5.1 6.5 8.3 9.7

3.5 3.5 3.8 3.6 3.3 3.3 3.1 2.9 3.5 3.6 3.7 3.8

1.4 1.4 1.4 1.5 1.6 1.6 1.6 1.7 1.7 1.8 1.9 1.9

18.4 18.5 19.3 19.6 21.8 22.3 22.6 22.7 25.1 26.0 27.3 27.2

18.5 18.2 19.0 19.7 21.4 21.6 22.0 22.1 23.3 23.6 24.0 24.4

Station 4 36.338N/122.208W 11 July 19h00 1.0 13.09 33.53 5.0 12.97 33.56 10.7 12.37 33.57 20.2 11.04 33.60 31.3 10.92 33.64 40.7 10.73 33.71 60.8 9.87 33.75 81.1 9.11 33.77 99.8 8.96 33.84 149.4 8.38 34.01 198.8 7.79 34.02 496.1 6.27 34.24

1.3 0.5 0.7 0.6 0.6 0.7 0.8 1.0 1.4 1.3 1.2 8.0

ND ND ND ND ND ND ND ND ND ND ND ND

0.7 0.7 0.7 1.0 1.2 1.5 1.7 1.9 2.1 2.2 2.2 3.0

0.9 0.8 1.3 3.1 9.7 18.6 27.0 30.4 32.5 38.2 44.2 78.1

6.7 6.8 7.3 10.3 14.0 19.1 25.3 27.6 29.0 31.2 31.8 39.7

Station 5 36.338N/122.418W 11 July 21h15 0.5 13.73 33.46 5.0 13.57 33.47 10.7 13.10 33.49 20.4 12.27 33.60 31.0 11.48 33.67 40.0 10.39 33.69 59.9 9.18 33.78 79.5 8.94 33.87 100.5 8.91 33.93

0.7 0.4 0.4 0.7 0.7 1.1 1.1 1.6 2.0

ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND

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Table 1 (continued) Iron [nmol L 1]

Manganese [nmol L 1]

Phosphate [Amol L 1]

Silicic acid [Amol L 1]

Nitrate [Amol L 1]

Station 5 36.338N/122.418W 11 July 21h15 148.9 8.38 34.03 199.5 7.60 34.05 495.9 5.99 34.27

2.5 1.7 ND

ND ND ND

ND ND ND

ND ND ND

ND ND ND

Station 6 36.218N/121.798W 12 July 11h30 0.3 11.87 33.80 8.6 10.89 33.80 19.2 10.67 33.80 29.6 10.42 33.84 51.1 10.00 33.87

2.3 4.6 3.4 6.3 17.4

4.0 3.7 3.2 3.4 3.5

1.2 1.4 1.5 1.6 1.8

16.9 18.5 20.0 22.9 26.8

14.7 16.5 18.9 20.5 23.5

Station 7 36.248N/121.868W 12 July 12h45 0.3 12.18 33.79 8.8 12.32 33.80 19.4 10.67 33.79 28.7 10.50 33.81 48.3 10.13 33.86

4.8 4.7 3.1 5.2 12.9

4.3 4.9 2.9 3.0 3.2

1.0 1.0 1.5 1.6 1.7

14.0 14.1 19.4 21.1 25.3

10.8 10.9 18.7 19.7 22.3

Station 8 36.268N/121.918W 12 July 14h45 0.3 12.25 33.73 8.8 11.44 33.78 18.3 10.59 33.81 28.7 10.37 33.84 49.7 10.19 33.86

1.6 3.0 3.6 7.4 9.5

3.7 4.2 3.6 3.8 3.6

1.1 1.1 1.5 1.6 1.7

13.3 14.9 20.3 22.8 24.9

13.7 13.1 19.2 20.5 22.0

Station 9 36.228N/121.998W 12 July 16h00 0.5 13.14 33.49 4.8 12.90 33.50 9.3 12.75 33.53 20.2 10.99 33.60 29.6 10.42 33.66 39.1 10.09 33.72 49.9 10.10 33.73 58.7 10.07 33.75 78.1 9.64 33.89 100.5 9.45 33.93

0.7 0.5 0.3 0.4 0.7 1.3 1.4 1.8 9.2 14.7

ND ND ND ND ND ND ND ND ND ND

0.8 0.9 0.9 1.3 1.5 1.6 1.7 1.7 1.9 2.0

3.5 3.8 3.7 13.9 19.0 22.3 22.4 23.2 30.6 32.4

8.6 9.3 9.8 16.6 19.9 22.0 22.1 22.5 25.7 26.9

Station 12 36.258N/122.038W 13 July 11h45 2.0 12.92 33.57 10.7 12.30 33.64 20.2 11.45 33.72 39.3 9.59 33.71 59.9 9.44 33.86 80.2 9.34 33.91

1.6 1.7 1.8 2.4 4.3 6.3

3.6 4.0 4.0 3.4 3.1 3.1

1.1 1.1 1.2 1.7 1.9 1.9

11.1 11.5 12.9 24.4 28.8 30.7

14.5 14.6 15.5 24.1 26.3 27.2

Station 13 36.328N/122.068W 13 July 13h00 1.2 12.39 33.62 9.0 11.43 33.70 20.2 10.36 33.80 39.3 10.04 33.83 59.9 9.76 33.87 79.2 9.53 33.90

1.8 3.3 7.9 11.9 14.0 14.7

3.8 4.1 4.2 3.8 3.9 3.7

1.1 1.2 1.6 1.8 1.9 1.9

11.3 14.8 22.6 25.9 28.5 30.5

14.1 16.1 21.1 23.4 25.1 26.9

Station

Depth [m]

Temp. [ C]

Salinity

(continued on next page)

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Z. Chase et al. / Marine Chemistry 95 (2005) 235–254

Table 1 (continued) Iron [nmol L 1]

Manganese [nmol L 1]

Phosphate [Amol L 1]

Silicic acid [Amol L 1]

Nitrate [Amol L 1]

Station 14 36.408N/122.088W 13 July 16h00 0.5 13.18 33.54 9.8 13.14 33.54 19.2 12.60 33.54 39.3 10.43 33.71 60.3 9.72 33.83 79.7 9.72 33.88

1.1 0.8 0.6 1.2 3.7 6.3

3.6 4.4 4.1 3.5 3.0 3.6

0.9 0.8 1.0 1.5 1.8 1.8

3.3 3.4 4.7 20.0 27.2 28.7

8.9 8.9 10.1 20.0 24.7 25.4

Station 15 36.498N/122.078W 13 July 18h00 2.0 13.60 33.56 7.9 12.22 33.55 19.4 11.42 33.59 39.1 10.57 33.64 59.6 10.03 33.79 79.9 9.54 33.87

0.8 0.9 0.8 1.0 2.1 3.4

4.8 5.0 4.8 3.6 3.3 3.1

0.8 1.0 1.2 1.4 1.6 1.8

1.9 5.6 11.9 18.5 24.1 28.6

7.0 10.2 13.9 19.1 22.5 26.3

Station 16 36.228N/121.848W 14 July 10h15 0.5 11.13 33.77 8.6 11.10 33.78 19.4 11.07 33.78 39.3 10.86 33.79 60.1 10.23 33.82 79.7 10.14 33.86

8.2 8.9 10.0 7.3 5.8 9.2

ND ND ND ND ND ND

1.2 1.3 1.3 1.5 1.7 1.8

19.9 19.9 20.0 21.5 24.5 27.0

16.3 16.4 16.6 17.8 21.7 22.6

Station 17 36.448N/121.958W 14 July 13h30 0.5 11.11 33.76 9.8 11.09 33.76 19.2 10.80 33.79 41.0 10.51 33.83 60.3 10.31 33.84 70.5 10.24 33.84

5.3 5.3 5.1 6.8 7.8 8.6

4.2 3.6 3.6 3.3 3.6 3.1

1.4 1.4 1.5 1.6 1.7 1.6

18.1 18.0 19.6 21.6 23.6 24.3

16.6 16.8 18.0 19.8 21.2 21.7

Station 18 36.388N/121.148W 14 July 15h30 0.0 12.26 33.59 4.1 12.26 33.60 9.8 12.25 33.59 20.4 11.64 33.65 29.8 10.85 33.72 39.3 10.31 33.76 58.4 9.77 33.83 79.0 9.64 33.88 98.9 9.30 33.96 148.7 8.71 34.07 198.1 8.18 34.13 498.9 6.18 34.22

0.6 0.5 0.4 0.8 1.2 2.7 2.7 6.6 6.8 5.9 6.4 6.5

3.0 3.5 3.7 3.5 3.5 3.3 3.0 2.9 2.4 1.9 1.7 1.8

1.1 1.1 1.1 1.2 1.5 1.7 1.9 1.9 2.0 2.4 2.2 2.9

8.3 8.2 8.4 12.0 18.3 22.6 27.1 28.7 32.3 44.6 38.5 73.3

12.8 12.9 12.8 14.9 18.7 21.6 24.6 26.2 28.3 32.6 31.0 39.0

Station 19 37.018N/122.348W 15 July 10h30 1.0 11.72 33.88 5.0 11.59 33.86 11.2 10.76 33.90 16.1 10.11 33.90 21.1 9.98 33.90 31.0 9.72 33.91

15.7 32.0 10.9 11.5 11.4 12.5

6.7 7.9 8.8 7.2 7.0 6.7

1.3 1.4 1.9 1.9 1.9 2.0

16.0 18.6 28.4 31.0 31.0 33.7

14.5 15.9 22.1 24.2 24.7 25.7

Station

Depth [m]

Temp. [ C]

Salinity

Z. Chase et al. / Marine Chemistry 95 (2005) 235–254

249

Table 1 (continued) Station

Depth [m]

Temp. [ C]

Salinity

Station 20 36.818N/122.308W 15 July 13h00 0.3 11.77 33.82 5.3 11.75 33.82 10.7 11.73 33.82 20.2 11.53 33.82 29.8 11.42 33.82 39.1 11.24 33.82 59.9 10.18 33.85 80.7 9.23 33.93 99.1 9.10 33.95 199.9 8.20 34.04 496.3 6.03 34.19

Iron [nmol L 1]

Manganese [nmol L 1]

Phosphate [Amol L 1]

Silicic acid [Amol L 1]

Nitrate [Amol L 1]

11.5 17.0 19.6 14.1 18.5 13.2 13.1 21.5 8.3 5.4 4.4

10.3 10.4 9.5 7.8 7.6 5.4 ND 3.9 1.9 1.1 1.7

1.2 1.2 1.2 1.3 1.2 1.6 1.8 2.0 2.0 2.4 3.0

12.9 13.1 13.3 15.3 15.9 17.1 24.9 33.0 31.6 38.1 72.2

11.6 11.8 12.0 13.5 14.0 14.8 21.5 28.2 28.3 30.2 39.3

Metal enrichment experiments were performed at stations 1 and 4. Times are local. ND=not determined.

concentration similar to that seen for iron. The manganese concentration at station 20 decreases almost linearly from a surface maximum of about 10 nmol L 1 to a minimum of 2 nmol L 1 by 100 m (Fig. 9), whereas at station 18, manganese decreases from a subsurface (10 m) maximum of 3.5 nmol L 1 to the same minimum of 2 nmol L 1 by 100 m. The contrast between surface manganese concentrations off An˜o Nuevo and Point Sur is in fact slightly greater than the contrast in iron concentrations, particularly before upwelling (3.4 times higher Mn in the north and 2.1 times higher Fe; Fig. 6). A study of manganese and iron cycling in the North Sea (Schoemann et al., 1998) suggested high manganese concentrations may be associated with elevated primary productivity, which promotes higher rates of benthic remineralization of manganese. Indeed, high rates of dissolved manganese efflux were measured from sediments in Monterey Bay (Berelson et al., 2003). In our study, however, manganese profiles do not show the pronounced increase towards the seafloor evident in the iron profiles, suggesting either a minimal benthic source of dissolved manganese or re-oxidation of benthic derived manganese upon contact with the oxygenated watercolumn. It is therefore not immediately obvious why manganese concentrations are higher to the north over the broad, more productive shelf. One possibility is that although the sediment is not a source of dissolved manganese, it may be a source of particulate manganese, including manganese oxides (MnOx), which are not detected by our methods (von Langen et al., 1997). When this

MnOx is brought to the surface during upwelling it may be photoreduced to the dissolved form (Sunda et al., 1983) which we measured. The near-surface maximum evident in many of the manganese profiles is consistent with this explanation. In particular, the dissolved manganese at station 20 (after upwelling) decreases linearly with depth, which is consistent with the signal being derived from photochemical reduction of manganese oxides that were brought to the surface with upwelling. Photoinhibition of bacterial manganese oxidation near the surface may also contribute to the surface maximum in dissolved manganese (Sunda and Huntsman, 1988). Interestingly, manganese and iron appear anticorrelated on the second survey of the northern site. The highest manganese concentrations were measured at the southern portion of the survey, whereas the highest dissolvable iron concentrations were measured at the northern portion of the survey (Fig. 6B,D). We believe this is due to differences in upwelling strength at the two sites. Based on the higher nutrient and CO2 levels and the lower SST to the north, it appears upwelling was more intense in the northern part of the survey than to the south at the time of sampling. The higher manganese concentration at the site with less intense upwelling (the south) is consistent with the discussion above concerning the reduction of manganese oxides. That is, if MnOx introduced to surface waters during upwelling and must be reduced before it is detected, then concentrations of dissolved manganese should be lowest in recently upwelled waters, and should increase with time following upwelling as the

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What is unclear is whether the much higher dissolved manganese concentrations observed north of Monterey Bay are the result of higher rates of input of sediment-derived MnOx, or the result of faster rates of MnOx reduction. Both factors may contribute. Rates of MnOx input may be greater in the north due to the same shelf-width effect we observe for iron. In addition, faster rates of primary production north of the Bay (Figs. 1 and 8), and associated higher rates of carbon oxidation at the seafloor, may promote faster rates of manganese reduction in the sediments (Berelson et al., 2003; Schoemann et al., 1998). Manganese reduction rates in surface waters may also be faster to the north. Preliminary results using a cDOM (colored dissolved organic matter) fluorescence sensor (WetLabs WSCD) towed during mapping sessions indicate roughly twofold greater cDOM concentrations north of Monterey Bay than to the south, particularly after the upwelling event. Higher concentrations of cDOM to the north may support faster rates of MnOx reduction in surface waters (e.g. Bertino and Zepp, 1991) either photochemically via the production of H2O2 (Spokes and Liss, 1995), or directly via thermal reactions. This difference in cDOM may be due to either the greater phytoplankton biomass to the North (Fig. 1), or to a shelf-width effect, since humic compounds are usually highly enriched in coastal sediments. Thus potentially two factors contribute to

Fig. 9. Examples of profiles of dissolvable iron [nmol L 1], dissolved manganese [nmol L 1], nitrate [Amol L 1] and primary productivity [mg C m 3 day 1] collected 14–15 July after the onset of strong upwelling-favorable winds. Profiles from north of 36.58N are in white and profiles from south of 36.58N are in black. Note the different depth scale used in the productivity panel.

waters are exposed to sunlight and reduction (and inhibition from oxidation) occurs. Dissolvable iron concentrations, in contrast, decrease rapidly following upwelling (Fitzwater et al., 2003). Thus we would predict that as an upwelling plume ages the ratio of dissolved manganese to dissolved iron will increase. It is possible that iron is also photochemically reduced at the surface after upwelling; however, even if Fe(II) persisted it would not be detected by our methods.

Fig. 10. Relationship between dissolvable iron measured underway (filtered Fe), which passed through a 20 Am filter, and dissolvable iron measured in water collected by surface bottles of the trace metal clean rosette (unfiltered Fe), which was not filtered. The relationship is expressed as the fraction of easily dissolvable iron associated with particles greater than 20 Am (unfiltered–filtered), as a function of total dissolvable iron (unfiltered Fe).

Z. Chase et al. / Marine Chemistry 95 (2005) 235–254 Table 2 Rate of change of surface water properties between 13 July and 14 July, 2002 Rate of change [C day 1]

Property [C] a

1

Iron (TMR Fe) [nmol L ] Manganese [nmol L 1] SST [8C] Salinity Nitrate [Amol L 1] Silicic acid [Amol L 1]

0.19 (3.01)a 0.52 0.55 0.07 2.35 4.63

Unless otherwise indicated measured values are from underway mapping. a TMR Fe refers to a rate of change calculated from surface water iron measured in vertical profiles from stations 6 and 16, July 12 and 14. See Fig. 2 for locations.

the higher dissolved manganese concentrations north of Monterey Bay: Greater input of MnOx, due to the greater shelf area and dissolved manganese efflux, and faster rates of MnOx reduction in surface waters, due to the greater concentration of cDOM. The combination of these two factors may explain why manganese distributions show an even greater north versus south contrast than do iron concentrations. 3.4. Implications for biology We performed a series of simple incubation experiments to investigate some of the biological effects of iron and other metals in this region. Similar experiments have been done by Hutchins et al. (1998), who showed that iron stimulates phytoplankton growth off Point Sur, but not to the north, over the broad shelf. In addition to iron, we tested the response to added manganese, copper and cobalt. We also attempted to simulate iron fertilization during coastal upwelling by performing an incubation with added sediment. Incubations were performed using near-surface water collected at stations 1 (north) and 4 (south). Underway dissolvable Fe measured upon arrival at the stations was 0.6 nmol L 1 at station 1 and 0.7 nmol L 1 at station 4. Water for the incubation experiments was drawn from the underway pump, which had its inlet at least 2 m below the surface. This may explain the discrepancy between underway dissolvable Fe and dissolvable Fe from the surface-most Rosette sample at these two stations (Table 1). The difference may also be because the underway samples were first filtered through a 20 Am filter (see Fig. 10). Initial manganese

251

estimated from underway mapping was 7.3 nmol L 1 at station 1 and 4 nmol L 1 at station 4. In all incubation experiments, bottles with added iron achieved significantly higher levels of chlorophyll a than the unamended bottles that served as controls (Fig. 11). At the Point Sur site, bottles with added iron also achieved greater nitrate consumption than the control bottles. Firme et al. (2003) introduced the Iron Limitation Index (ILI), a useful metric for comparing iron manipulation experiments. The ILIchl, for example, is the ratio of chlorophyll increase in the incubation receiving iron to the increase over the same period in the control. Expressed in this way, for day 3 of the incubations, all treatments showed a positive response, or ILIN1. Furthermore, the ILIchl from the southern station treatments with iron was about twice as large as for the northern station (Fig. 12). The magnitude of the ILIs observed in this experiment for iron alone are at the high end of those observed in this region in 1998 by Firme et al. (2003). Differences between treatments receiving Fe alone and Mn, Co and Cu in addition to Fe were insignificant or small when fluorometric chlorophyll was used as the metric (Fig. 12), suggesting iron is the primary limiting micronutrient in this system. However, ILIchl based on HPLC-determined chlorophyll a was particularly high in the Fe+Cu and Fe+Co treatments. We do not have an explanation for this discrepancy but caution that the HPLC results are not replicated. Chlorophyll accumulation and nitrate consumption in the bottles that received sediment additions were significantly less than in those with 4 nmol L 1 of dissolved iron added, but only slightly more than the controls with nothing added, although the difference was statistically significant (Fig. 11). We measured dissolvable iron concentrations of 5.7 and 3.0 nmol L 1 in two of the sediment-amended bottles prior to incubation. The total iron concentration in the sediment-amended bottles was estimated to be ~200 nmol L 1 based on measurements of particulate iron concentrations in the An˜o Nuevo upwelling center in 2000 (Fitzwater et al., 2003). Because the concentration of dissolvable iron in the sediment-amended samples was roughly equivalent to the concentration of dissolved iron in the Fe-amended samples (4 nmol L 1 added +~0.7 nmol L 1 ambient), yet the growth response was significantly less, it appears the operationally defined ddissolvable ironT overestimates bio-

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Fig. 11. Incubation experiments conducted at stations 1 (northern site) and 4 (southern site). Error bars represent the range of duplicate incubations. Initial conditions are listed in Table 1.

logically available iron, at least over the short term. Some caution is warranted in interpreting these results, however, because (1) the added sediment contained constituents other than Fe, possibly including toxic trace metals which may have had an inhibitory effect on all or some of the phytoplankton community and (2) drying the sediment may have reduced its bioavailability. Both sites, north and south of Monterey Bay, responded to iron addition, an observation that appears at odds with our argument that populations north of the Bay are iron replete and those to the south are iron limited. In fact, iron concentrations were not that different at the two sites (i.e. 0.6 and 0.7 nmol L 1 at the northern and southern sites, respectively). This concentration is close to the upper limit at which iron limitation was observed in this region in June 1998 (Firme et al., 2003). It is probable that at this distance from shore, the communities at both sites at times

experience some degree of iron stress, particularly towards the end of an interval between upwelling events. However, given the large difference in iron concentrations north and south of the Bay (e.g. Figs. 6 and 9), we expect to see ambient biological differences between the two upwelling centers. An average of satellite-derived July chlorophyll (SeaWiFS) for 1998– 2003 shows clearly the persistently higher chlorophyll biomass north of Monterey Bay (Fig. 1). Consistent with the satellite climatology, chlorophyll measurements during the cruise show higher concentrations in the An˜o Nuevo upwelling center than the Point Sur region, both before and after the upwelling event (data not shown). More importantly, significantly higher rates of primary productivity were measured at station 20 (north) compared to station 18 (south), consistent with the markedly higher dissolvable iron and manganese concentrations at station 20 (Fig. 9). Macronutrient concentrations were the same at both sites and

Z. Chase et al. / Marine Chemistry 95 (2005) 235–254

Fig. 12. Response to incubation with metals compared to response in the control treatment after 3 days, expressed as an bIron limitation IndexQ (after Firme et al., 2003), which is the ratio of the chlorophyll increase in the treatment to the increase in the control. Fluorometric chlorophyll a was determined on duplicate incubation samples and includes error bars, whereas HPLC chlorophyll a was determined on single samples and therefore does not include error bars.

cannot explain this difference in primary productivity. At the nearshore stations (17 and 19), where dissolvable iron concentrations were high (N5 nmol L 1) at both sites, productivity was in fact greater at the station in the south, where dissolvable iron levels were slightly lower (Fig. 9). Macronutrient and CO2 fields mapped during MUSE II show further imprints of the contrast in iron supply north and south of Monterey Bay. In the north, nitrate is depleted to near zero, whereas in the south, close to 7 Amol L 1 nitrate remains in waters of the same density (Fig. 7C). Likewise, CO2 is as low as 220 ppm in the north, whereas in the south the lowest measured values was just under 400 ppm (Fig. 7D). These data strongly suggest that low iron concentrations limit the uptake of carbon and nitrogen in surface waters south of Monterey Bay, more so than to the north of the Bay.

4. Conclusions In the upwelling system of central California the amount of iron and manganese present in surface waters is greater where the shelf is wide (north of

253

Monterey Bay) than where it is narrow (south of Monterey Bay). Manganese displays even greater enrichment over the broad shelf than does iron. Iron concentrations in both systems increase rapidly in response to the onset of upwelling-favorable winds. Our observations are consistent with current understanding of the mechanism of iron supply to coastal upwelling systems; iron from shelf sediments, predominantly associated with particles greater than 20 Am, is brought to the surface during upwelling conditions. This iron is rapidly lost from the water column by biological uptake and particulate settling, such that after a period of wind relaxation phytoplankton growth within 20 km from shore responds to artificial iron addition even in a region with a broad shelf. The greater manganese concentrations in surface waters over the region of broad shelf are more difficult to explain given the present dataset. The sediments may well be a source of particulate manganese, as they are a source of particulate iron. We hypothesize that manganese oxides are brought to the surface with upwelling and are then reduced to dissolved manganese in surface waters following a lag after upwelling. It is also possible that rates of solubilization of particulate manganese are higher north of Monterey Bay, perhaps due to the greater concentration of dissolved organic matter. Several lines of evidence—satellite climatology of chlorophyll, ship-board measurements of primary productivity, and the distribution of macronutrients and CO2—indicate a strong and persistent enhancement of biological activity in the region receiving higher iron inputs. Biogeochemical models of the coastal zone will need to account for such regional variability in iron supply.

Acknowledgements We thank the captain and crew of the Research Vessel Western Flyer. We thank Gernot Friederich and Peter Walz for nutrient and xCO2 measurements, Peter Strutton for primary productivity measurements, Victor Kuwahara for coordinating the HPLC samples, Sierra Senyak for assistance at sea and Patrick McEnaney for help in preparing Fig. 1. This work was supported by the David and Lucile Packard Foundation.

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