Continental-Shelf Sediment As a Primary Source of Iron for Coastal
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letters to nature Acknowledgements. We thank T. P.Barnett for discussion and M. MuÈnnich for help with data processing. surface were as high as 25 mM. Surface salinities measured along the This work was sponsored by the German government under its programme `KlimavariabilitaÈt und 12 Signalanalyse' and by the European Union through its `SINTEX' programme. The climate model transect were .32.8, which is typical of the CCS in this region . The integrations were performed at the Deutsches Klimarechenzentrum. shipboard measurements of dissolvable iron at stations within Correspondence and requests for materials should be addressed to M.L. (e-mail: [email protected]). 100 km of the coast averaged 6 nM, but values as high as 20 nM were observed near the core of the upwelling system at AnÄo Nuevo, which feeds Monterey Bay13. In the open ocean waters more than 200 km offshore (west of 124.28 W), the dissolvable iron concen- tration was 0:2 6 0:1 mean 6 standard deviation nM. High Continental-shelf sediment chlorophyll concentrations developed near the coast under these conditions (Fig. 1c). as a primary source of iron Wind conditions that favoured upwelling were rare during the El NinÄo conditions of late winter and early spring of 1998. High-wind for coastal phytoplankton events were primarily from the south (Fig. 2c), which would have suppressed offshore transport, and the surface ocean was nearly 3 8C Kenneth S. Johnson*², Francisco P. Chavez² & Gernot E. Friederich² * Moss Landing Marine Laboratories, PO Box 450, Moss Landing, March 1997 March 1998 California 95039, USA 25 ² Monterey Bay Aquarium Research Institute, PO Box 628, Moss Landing, Monterey Bay 2 34 California 95039, USA 20 S 33 ......................................................................................................................... 15 Fe 1 Fe The availability of iron, an essential nutrient, controls rates of 32 Salinity phytoplankton primary productivity in the open-ocean, upwel- 10 1,2 0 31 ling ecosystems of the equatorial Paci®c . Upwelling injects large -124 -123 amounts of macronutrients into the euphotic zone of eastern 5 ab boundary currents, such as the California Current System (CCS), iron (nM) Dissolvable where iron can become the limiting factor on productivity3,4. Iron 0 addition to samples from some areas of the CCS has been shown to 30 5,6 Monterey Bay increase rates of biomass production , but the processes that 25 control iron availability in these systems remain poorly under- 20 M) stood. Here we report measurements of dissolvable iron (that is, µ dissolved plus leachable iron at pH 3) in transects across the CCS 15 in March of 1997 and 1998. We found high concentrations of iron Nitrate ( Nitrate 10 in 1997 during strong upwelling conditions. During the 1998 El NinÄo, the concentration of dissolvable iron in surface waters was 5 cd low, even though that year was marked by high river ¯ow and low 0 offshore salinity. These results indicate that the primary source of 4 iron in the CCS is resuspension of particles in the benthic boundary layer, followed by upwelling of this iron-rich water, ) 3 rather than direct riverine input. This source of iron must be an –1 g l essential but variable component of the high productivity found µ 2 in upwelling ecosystems. Upwelling along eastern boundary currents sustains some 11% of the global ocean primary production7 and 50% of the global ®sh 1 production8. The waters that come to the surface in these systems ( Chlorophyll ef are enriched in macronutrients. However, there is no corresponding 0 -126 -124 -122 -124 -122 subsurface enrichment of iron in the offshore areas that are the Longitude (degrees west) Longitude (degrees west) source of the upwelled waters3. Consequently, there must be addi- tional sources of iron at the continental margin to sustain elevated Figure 1 Dissolvable iron, nitrate and chlorophyll on transects across the CCS. rates of primary production. Large concentration gradients of Measurements were made from Moss Landing in Monterey Bay (36.808 N, 9 10 dissolved and particulate iron across both eastern and western 121.798 W) to a point 390 km offshore (35.078 N, 125.578 W) on cruise S197 (3±9 ocean boundaries demonstrate that ocean margins are important March 1997) and 317 km offshore (35.468 N,124.908 W) on cruise S298 (March 18± sources of iron. It has been hypothesized that elevated iron con- 23 1998). The inset to b shows iron concentration on an expanded scale and the centrations in coastal regions may arise from riverine input, salinity on S298, which is anomalously low over the inner 150 km. Dissolvable iron 9 resuspended sediment or atmospheric deposition , but so far we was measured by ¯ow-injection analysis with chemiluminescence detection at 6- do not know the relative importance of these mechanisms or how min intervals24. Sea water was sampled with an all-te¯on pump system towed at they vary over time. 1±2 m depth. Nitrate in this sample stream was determined colorimetrically We have made measurements of dissolvable iron, nitrate and aboard ship using an automated continuous-¯ow analysis system25. Sea water chlorophyll in March 1997 (cruise S197) and March 1998 (cruise from the pump was passed through a 10-mm polypropylene ®lter and acidi®ed to S298) at a depth of 2 m along transects from Monterey Bay in a pH 3 for ,0.5 min before iron analysis. Laboratory experiments demonstrate no southwesterly direction across the CCS (Fig. 1). Conditions in difference between the amount of iron desorbed from particles at pH 3 in 1 min March of 1997 were representative of typical spring upwelling, and in .6h26, indicating that we detected dissolved plus all absorbed iron. The with strong winds from the northwest (Fig. 2c). A sharp drop in dissolvable iron/nitrate relationship in the data was similar to that reported surface temperature, driven by strong winds that favoured upwel- previously for Monterey Bay9. Chlorophyll was determined by extracting the ling, was recorded just before the S197 cruise at the MBARI M1 pigment and measuring ¯uorescence in a shore-based laboratory. The nitrate 11 mooring in Monterey Bay (Fig. 2a). Temperatures remained low data near the upwelling front at 1248 W on S197 were obtained on the inshore leg throughout the cruise and dissolved nitrate concentrations near the of the cruise to replace data missing on the offshore transect. © 1999 Macmillan Magazines Ltd NATURE | VOL 398 | 22 APRIL 1999 | www.nature.com 697 letters to nature a b c Figure 2 Daily average values of temperature, salinity and north windspeed. S298 cruises are shown. Data are available from the MBARI database located on Parameters were measured from 1 January 1997 to 30 May 1998 on the M1 the Internet at www.mbari.org. mooring in Monterey Bay (36.738 N, 122.018 W). The time spans of the S197 and warmer than in 1997 (Fig. 2a). Rainfall during the winter of 1997/98 Although these low-salinity events clearly carry elevated iron was double the annual average at meteorological stations through- (Fig. 1b, inset), the concentration increase of ,1 nM is extremely out the central California region. Salinity across the inner 150 km of modest when compared with upwelled water. Large amounts of the this transect was less than 32.8 to depths of 10±15 m (Figs 1b, 2b), dissolved (,0.4 mm) iron in rivers are present mainly as colloids. which is anomalously low12. Drifting buoys drogued with 10-m These colloids aggregate as salinity increases and settle out in holey socks of the WOCE design and equipped with ARGOS satellite estuaries, which prevents most riverine iron from reaching offshore transmitters were deployed at three stations along the cruise track. waters15. Atmospheric deposition also seems to be a relatively minor The inner two drifters deployed near 122.58 and 123.88 W moved source as offshore winds were more frequent during 1998 when continuously south for the next month at an average speed of dissolvable iron concentrations were lowest. 10 cm s-1. The mean speed increased to 20 cm s-1 when the wind was The iron source in the upwelled water does not appear to from the north. The drifter tracks show that the low salinities must originate from diffusion of dissolved Fe from shelf sediments. have been produced by out¯ow from the Sacramento/San Joaquin Measurements of the ¯ux of dissolved iron across the sediment± River system that has been carried south in the CCS. These two water interface on the continental shelf in Monterey Bay, using rivers, which discharge 100 km to the north of the transect through benthic ¯ux chambers16,17, average 5 6 3 mmol Fe m 2 2 d 2 1 (stan- San Francisco Bay, combine to form one of the 60 largest systems in dard error, n 11; K.S.J., unpublished results). If the shelf has a the world. The dissolvable iron concentrations in the offshore mean depth of 50 m, then the ¯ux of dissolved iron can add only waters were slightly higher (0:4 6 0:1 nM) than the values recorded 0.1 nM Fe d-1, which is an insigni®cant amount. in 1997, whereas values in the river-in¯uenced inner 100 km of the Vertical pro®les of dissolvable iron were collected along the transect had an average value of 0:9 6 0:2 nM (Fig. 1b). Little transect during the S298 cruise to assess the amount of dissolvable chlorophyll was found in this system except in plumes of low- iron in the subsurface waters (Fig. 3a). The concentrations of salinity water at 122.88 W and 121.88 W (Fig. 1f).