Structure of Haida Eddies and Their Transport of Nutrient from Coastal Margins into the NE Pacific Ocean

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1 Journal of Oceanography, Vol. 58, pp. 715 to 723, 2002 Structure of Haida Eddies and Their Transport of Nutrient from Coastal Margins into the NE Pacific Ocean FRANK WHITNEY* and MARIE ROBERT Institute of Ocean Sciences, Fisheries and Oceans Canada, Sidney, B.C., V8L 4B2, Canada (Received 30 October 2001; in revised form 10 February 2002; accepted 12 March 2002) Anticyclonic mesoscale eddies form near shore each winter in the Gulf of Alaska. One site near the Queen Charlotte Islands is shown to produce eddies that transport from 3000 to 6000 km 3 of coastal water up to 1000 km westward. Eddies carry shelf nutrients either into the high nutrient, low chlorophyll waters of the NE Pacific, or in a more southerly direction into seasonally nitrate depleted waters. A large eddy sampled in summer 1998 was found to have elevated particulate levels on its perimeter. Nitrate supplied to the euphotic zone by this eddy during its natal summer is estimated to be three times greater than the usual seasonal nutrient transport in the Gulf of Alaska. Keywords: Anticyclonic eddies, NE Pacific, nutrients, nitrate, silicate, POC. 1. Introduction In the Gulf of Alaska, anticyclonic mesoscale eddies have been found in hydrographic data for several decades (Tabata, 1982). The Sitka eddy that Tabata described contained a warm, fresh core of water within the halocline that depressed isopycnals at depths of more than 1000 m. Until satellite altimetry became available, these eddies could not be tracked moving away from the coast into the open ocean. Unlike warm core eddies of the Gulf Stream and Kuroshio, they do not carry a surface temperature signal that can be followed much beyond their formation period. However, near the British Columbia (B.C.) and Alaska coasts warm cores are detected by satellite radiometry during brief cloud-free periods when eddies are still near the coast in late winter (Thomson and Gower, 1998). When the Colorado Center for Astrodynamics Research (CCAR) began posting near-real-time altimetry maps in 1998, it became possible to direct research vessels to individual eddies. These altimetry maps show there is an annual production of eddies along the B.C. and Alaska coasts. Eddies typically form in late winter, primarily in two distinct areas (adjacent to Sitka, Alaska, and the Queen Charlotte Islands), detach from the coast by March, then move westward into the Gulf of Alaska (Crawford and Whitney, 1999; Crawford, 2002). They can persist for several years, travelling as much as 1000 km from their origin in this time. These eddies provide a means of transporting coastal * Corresponding author. whitneyf@pac.dfo-mpo.gc.ca Copyright The Oceanographic Society of Japan. waters, which contain low salinity water, heat, nutrients and plankton, from continental margins into the open ocean. Many studies have described the importance of both anticyclonic and cyclonic mesoscale eddies in enhancing nutrient transport into the euphotic zone (e.g. Yentsch and Phinney, 1985; Franks et al., 1986; Falkowski et al., 1991; McGillicuddy et al., 1998; Martin and Richards, 2001; Zhang et al., 2001). Each of these discussions focussed on the enhancement of vertical nutrient transport due to processes within an eddy. To date, transport of nutrients away from coastal areas by eddies has not been shown to be an important process. Herein, we provide a physical description of Haida eddies which form off the coast of the Queen Charlotte Islands, based on eddies produced in 1995, 1998 and From sequential sampling of the intense 1998 eddy, we estimate transport of coastal water and nutrients to open ocean and loss rate of nutrients from the eddy over time. 2. Methods Eddies were surveyed by chance in August 1995 and 1998 on time-series cruises along Line P to Ocean Station Papa (Fig. 1; Whitney and Freeland, 1999). In February 1999, Haida-1998 was resurveyed on a single transect and in June 1999, two stations in the eddy were sampled to full ocean depth. From several surveys of Haida eddies, we estimate that using satellite altimetry (TOPEX/Poseidon and ERS satellites) to locate the center of an eddy is accurate within 25 km. Additional surveys were conducted in 2000 and 2001 in a program established to study eddy dynamics. Measurements included 715

2 temperature (T) and salinity (S) using a Seabird 911plus CTD mounted on a 24 bottle rosette, light transmission by a Sea Tech transmissometer that was interfaced with the CTD, salinity by Guildline Portasal, and major nutrients (nitrate, phosphate and silicate) by modified Technicon procedures using an autoanalyzer (Barwell- Clarke and Whitney, 1996). Data are available in archives at the Marine Environmental Data Service or from the authors. Seawater densities were calculated as sigma-theta only for deep casts in February 1999 where potential temperature departs significantly from temperature, and as sigma-t for shallow casts where potential temperature corrections are trivial. 3. Results Each year from 1993 to 2001, the period for which winter TOPEX/Poseidon satellite altimetry is available, anticyclonic eddies have formed along the coast of the Queen Charlotte Islands. Tracks of the dominant eddy each year show that these water masses move away from the coast in north-westerly to southerly directions (Fig. 1). Eddies that take the more northerly route cross a series of seamounts, Bowie Seamount (53 18 N, W, 25 m below surface) being the shallowest. Haida-2000 lingered near Bowie Seamount from May to October 2000 before moving seaward. North-westward (1996, 1999 and 2000) and westward (1993, 1994, 1997) tracking eddies Fig. 1. Tracks of Haida eddies which formed off the coast of the Queen Charlotte Islands (QCI) from 1993 to 2001 as discerned from satellite altimetry. Boundaries of the HNLC waters (from Whitney and Welch, 2002) and the Hecate Strait (HS), Queen Charlotte Sound (QCS) area are demarked. Note the location of seamounts between the tracks of H-96 and H-00, of which Bowie is the largest. Also, stations Papa (+) and P16 ( ) are shown. Fig. 2. Temperature (a) and salinity (b) sections through 1995, 1998 and 2000 Haida eddies. 716 F. Whitney and M. Robert

3 leave coastal waters and enter the high nutrient-low chlorophyll (HNLC) realm of the North Pacific (Whitney and Welch, 2002) within their first year. Haida-1997 travelled almost due west, to within 50 km of Station Papa before being lost to satellite altimetry in June This is the most persistent eddy that we have followed (duration of 3.5 years). In 1995 and 1998, eddies with strong altimetry signals (max. 27 and 30 cm, respectively) took southerly routes, remaining in nitrate depleted waters through spring and summer. Fig. 3. Density difference (sigma-theta) between core and surrounding waters of Haida-1998 in February Anticyclonic Haida eddies contain a core of warm, low salinity water that depresses isothermal surfaces in and below the halocline and dome them near the ocean surface when thermal stratification is well established in summer (Fig. 2). Isotherms are deepened by more than 200 m in Haida-1995, 1998 and 2000, the 5 C isotherm being deflected downward by 230 m in Haida In this eddy, the 9 C surface is domed by 30 m, which results in the spread between 5 and 9 C isotherms being 260 m greater in the eddy than in surrounding waters. This spreading of isothermal surfaces to depths of 500 to 600 m defines the core waters of Haida eddies. However, buoyant eddy water continues to depress isopycnal surfaces to much greater depth. In February 1999, CTD profiles to the ocean bottom through Haida-1998 showed a density difference between eddy and surrounding waters to a depth of 2500 m (Fig. 3). Below this, sigma-theta differences were consistently ~0.003 which is nearing the precision of our measurements. Both the 1995 and 1998 eddies were sampled near station P16 along Line P. At salinities between 32.9 and 33.9 (base of the mixed layer to the base of eddy core waters), eddies were 1 to 2 C warmer than either adjacent waters (Figs. 2(a) and (b)) or those found at station P16 in September 2000 (Fig. 4(a)). Haida 2000 took a more northerly path into open ocean (Fig. 1) where its core waters were also ~1 C warmer (Figs. 2(a) and (b)) than the surrounding ocean. The core T of Haida eddies reflects conditions that existed along the B.C. coast during Fig. 4. (a) Temperature-salinity plot of three Haida eddy stations sampled in August/September and a reference cast at station P16 in September Density surfaces of sigma-t 25.6 and 26.8 are indicated as dotted lines. (b) Nitrate-salinity plot of the same three eddy stations ( 1995, , 2000) with northern (open diamonds) and southern (open circles) reference stations. Structure of Haida Eddies and Their Transport of Nutrient from Coastal Margins into the NE Pacific Ocean 717

4 Fig. 5. Nitrate and silicate vs. salinity in core waters of Haida-1998 over time. Reference stations include those of Hecate Strait which appears to be the source of elevated nutrient in Haida eddies and a southern station with low nutrient levels. Shown as a heavy dashed line is the estimated time zero (Est. T = 0) nitrate concentration of eddy Haida eddy formation (Crawford, 2002). Haida-1998 formed when the strong El Niño resulted in record warm temperatures along the coast of B.C. (Whitney and Welch, 2002), and consequently contains the warmest core temperatures of these three eddies. Coastal temperatures were cooler through the winter of 2000 and this is reflected in Haida-2000 waters. However, similar nutrient levels are found in core eddy waters during their natal summers. Sampling of the 1995, 1998 and 2000 Haida eddies in summer shows that, at the same age, each eddy had a similar linear nitratesalinity relationship below the summer mixed layer (~32.4) over a range of salinities up to 33.9 (Fig. 4(b)). Each eddy also experienced nitrate depletion in surface waters following spring phytoplankton growth. Haida eddies can appear either nutrient enriched or impoverished when compared with ambient waters of similar salinity. If an eddy heads southward as in 1995 and 1998, then its core waters are relatively nutrient rich by as much as 10 µm nitrate (Fig. 4(b)) and 15 µm silicate (data not shown) over the salinity range 32.7 to However, eddies that travel westward into HNLC waters of the Gulf of Alaska appear slightly nutrient poor. Haida-1998 was sampled three times during its first year and a half at sea. Between the September, February and June surveys, a steady decline in nutrient levels of core waters was observed (Fig. 5). Initial nutrient concentrations of this eddy are estimated from data collected in its formation area over the past several years. Sampling in Hecate Strait in June 1997 (Fig. 5) and in core waters of Haida-2001 during its formation period in February 2001 (data not shown) give us confidence that we are conservatively estimating initial levels in Haida Because this eddy travelled south into relatively nutrient poor waters, it continued to lose nitrate and silicate over the observation period. By June 1999, nutrient levels in the upper ocean were the same inside and outside the eddy, although deeper waters (salinity 32.9 to 33.9) still contained excess nitrate and silicate. A survey through Haida-1998 during February 1999 shows that winter mixing penetrated well into its core waters (Fig. 6). During the previous summer, surface waters had warmed to >17 C, providing a thermal cap over the eddy (Fig. 2). Cooling and storm mixing through fall and winter removed this cap, resulting in eddy ventilation to 140 m. As a result of this deep mixing, surface waters across the eddy in February 1999 contain a slight warm, fresh and nutrient poor (0.2 µm NO 3 and 0.4 µm Si) signal compared with the surrounding ocean (Fig. 6). During this time, the 25.6 isopycnal surface, which is approximately the density of the base of the winter mixed layer, rose from 180 m to 150 m, then to 100 m by June. Haida-1998 was not sampled in spring when highest phytoplankton growth rates would be expected. By September 1998, nitrate was depleted throughout the mixed layer to a depth of m. Within the seasonal thermocline, however, nutrient levels increased. This gave rise to a subsurface chlorophyll and transmissivity maximum at the base of the euphotic zone. Light transmission can be used to estimate particulate organic carbon (POC) 718 F. Whitney and M. Robert

5 with some confidence (Bishop, 1999). These measurements show that there are elevated levels of POC at the edges of Haida-1998, with maxima >5 µmol/l being found at depths of between 40 and 50 m about 50 km from the eddy center on the north, south and west edges (Fig. 7). With the coarse spacing of stations on the east-west survey (70 km), we assume that the eastern eddy edge was not sampled. These maxima occur over a depth range, but on a single isopycnal surface of sigma-t POC in the eddy center shows no subsurface maximum, suggesting waters are mixed to a depth greater than the euphotic zone. Waters away from the influence of this eddy show weaker subsurface POC maxima. Integrating POC to 100 m yields values of 200 to 230 mmol m 2 at the eddy edge stations, 150 mmol m 2 in the eddy center, and 160 to 190 mmol m 2 in surrounding waters. The additional ~50 mmol C m 2 at edge stations is concentrated in a layer ~25 m deep (Fig. 7) which yields an average increase of ~2 mmol m Discussion Fig. 6. West to east survey of Haida-1998 in February The top panel shows density structure (sigma-t) in the upper 500 m and the lower panel plots surface temperature (T) and salinity (S) on this section. 4.1 Haida structure Haida eddies are typical of anticyclonic eddies found in other oceanic realms (Nelson et al., 1985; Tomosada, 1986; Pingree and Le Cann, 1992; Rogachev et al., 1996; Zhang et al., 2001; Savidge and Williams, 2001), in that they contain a core of buoyant water which distorts the density field in their vicinity. However, unlike anticyclonic eddies which form along western boundaries of oceans, Haida eddies transport coastal waters as much as Fig. 7. POC concentrations with depth (a) and density (b) at selected stations around Haida-1998 in August POC is estimated from light transmission data using formulae of Bishop (1999). Structure of Haida Eddies and Their Transport of Nutrient from Coastal Margins into the NE Pacific Ocean 719

6 Table 1. Nutrient transport by and loss from Haida-1998 in surface waters (S < 32.9) and pycnocline waters (S = 32.9 to 33.9). Data are taken from profiles near the center of Haida-1998 (Sep., Feb. and Jun.) and from source waters (estimate for Mar. 98 from Fig. 5). Average winter (W) and summer (S) nutrient concentrations from stations P16 and P (Whitney and Freeland, 1999) provide an estimate of typical nutrient uptake rates. Loss rates of nutrients and the silicate to nitrate uptake ratio are calculated for surface layers km into the open ocean (Fig. 1). Such eddy transport has also been observed from the European coast in the North Atlantic (Pingree and Le Cann, 1992), although these are smaller eddies which are less persistent. Similar to Haida eddies are those that form further north along the North American coast near Sitka, Alaska (Tabata, 1982). A Sitka eddy surveyed in 1961 had core waters 0.5 C warmer than the surrounding ocean to a salinity of Haida eddies likewise contain a core 1 to 2 C warmer than the offshore waters near its formation site, to a salinity of 33.9 and a depth of ~500 m, suggesting both of these eddies form by similar processes. Haida eddies apparently form as a result of outflow of shelf waters around the southern end of the Queen Charlotte Islands (Crawford et al., 2002). Nutrient data from three eddies and the west coast of B.C. confirm that shelf waters are needed to elevate nitrate and silicate levels above those found along the continental slope of B.C. (Fig. 4; unpublished data). The maximum salinity of 33.9 at which we see temperature and nutrient anomalies in Haida eddies (Fig. 4) is found in bottom waters of Queen Charlotte Sound and Hecate Strait. Several anticyclonic eddies form each winter along the British Columbia and Alaska coasts (Crawford and Whitney, 1999) then head into the Gulf of Alaska in spring. There is some suggestion that the intensity of eddy production is dependent on El Niño conditions when coastal waters are warmer (Meyers and Basu, 1999). For example, the 1998 Haida eddy formed during one of the strongest El Niños of the 20th century, creating the most intense eddy (sea level elevation >30 cm) we have seen off the B.C. coast since TOPEX/Poseidon satellite altimetry began in To understand the importance of offshore eddy transport on redistribution of water properties and biota in the Gulf of Alaska, we must estimate the volume of coastal water that eddies carry. Haida eddies contain waters with anomalous T/S and nutrient/s properties to a depth of ~500 m (Figs. 2 and 4). Eddy radii of 75 km to 125 km yield cylindrical volumes of 9,000 to 25,000 km 3. However, only a portion of this water can be considered of shelf origin. Using T/S and nutrient/s properties independently, we estimated the volume of coastal waters by assuming that the eddy core contained pure source water to the depth of its anomalous signal. Both estimates are based on an eddy shape that is approximately conical, so that the volumes of the eddy can be visualized as a series of nested cones. The percent difference between core (100%) and surrounding waters (0%) was computed throughout the eddy and volumes of coastal water were calculated: coastal volume = V V where V n = the volume of water with n percent coastal characteristics. These estimates show that Haida-1998 contained 5,000 to 6,000 km 3 of coastal water when sampled in September 1998 and Haida-2000 carried ~3,000 km 3 of coastally derived water in June Of the coastal water in Haida-1998 (using the lower volume estimate), 1000 km 3 had a salinity <32.9 and 4000 km 3 was in the range The area of Hecate Strait and Queen Charlotte Sound is approximately 55,000 km 2 and total volume ~6,800 km 3 (M. Foreman, pers. comm., from detailed bathymetry used in his circulation model of this region). This is not much 720 F. Whitney and M. Robert

7 Fig. 8. Density structure (sigma-t) of Haida-1998 in September 1998, with estimates of the degree of isopycnal rebound between September 1998 and June 1999 for the 25.4 layer which sits near the base of the winter mixed layer, and 26.8 layer which is the maximum density at which anomalous water properties are seen. Outward-directed arrows show the path of water loss from the eddy core. greater than eddy volumes. We envision a flow through this coastal region that persists over several months to create Haida eddies. En route, waters would be enriched with nutrients (and warmed) while crossing the large area of continental shelf. We expect that this offshore flow would affect the reproductive success of any organisms that were planktonic between November and February. 4.2 Nutrient transport in Haida-1998 Evidence we have collected from our surveys suggest that nutrient loss from Haida eddies occurs through several processes. There is undoubtedly a spring bloom brought about by increased surface layer stratification and solar radiation some time shortly after these eddies leave the coast, since sampling in June 2000 found Haida-2000 nitrate depleted in the upper 30 m. Subsequently, nutrients are supplied from below, possibly due to frictional decay of the eddy (Franks et al., 1986), storm events (Woodward and Rees, 2001; Zhang et al., 2001), and other processes by which vertical mixing is enhanced (e.g. Yentsch and Phinney, 1985; Martin and Richards, 2001). The presence of a biomass ring around Haida-1998 in summer, on a single density layer, suggests that nutrient supply along density surfaces increases productivity at the base of the euphotic zone. Through winter, storms mix the fresh surface layers in these eddies, so that by February 1999, Haida-1998 was ventilated to a depth of 140 m (Fig. 8). Over the next few months, isopycnal surfaces rose in this eddy, and by June 1999 it no longer contained any excess volume of water of salinity <32.9. A sea level altimetry signal persisted one year longer. We estimate the amount of macro-nutrient that is carried from shore and released from Haida-1998 either into the euphotic zone or surrounding waters over time (Table 1). Such an estimate is not relevant for eddies that track west to northwest since they enter the relatively nutrient rich waters of the Gulf of Alaska (Figs. 1 and 4). However, micro-nutrient transport should then become important. The HNLC waters of the NE Pacific are known to be iron and zinc depleted, elements which coastal areas contain in abundance (Martin et al., 1989; Boyd et al., 1996; Nishioka et al., 2001; Lohan et al., 2002). Nutrient loss rates are calculated in eddy waters of salinity <32.9 that supply the mixed layer (approximately equal to sigma-t of 25.6), and those of salinity 32.9 to 33.9 (sigma-t ~25.6 to 26.8) that deposit nutrients into the pycnocline where they will be available to the upper ocean over time. Since phosphate recycles much the same as nitrate ( NO 3 : PO 4 of 16:1 is consistent in this study) and has not been implicated as a controlling nutrient in the subarctic Pacific, transport of nitrate and silicate only are calculated. These results show a persistent decline in nitrate and silicate both through the growing season and in winter when storms expose eddy core waters to surface storms. Through summer, the removal of 1.0 mmol NO 3 m 2 from the eddy center, when averaged over the observation period (180 d), results in a loss rate of 5.6 mmol m 2 d 1. This is ~3 times greater than the seasonal nitrate uptake rate estimated at Station Papa or at station P16 near where Haida-1995 and 1998 were sampled in late summer (Wong et al., 1995; Whitney et al., 1998; Whitney and Freeland, 1999). Nutrient loss from eddy waters continues through the following winter and spring, so that low salinity waters have similar nutrient levels to the surrounding ocean by June 1999 (Fig. 5). At depth, Haida-98 still retained a core of nutrient rich water with nitrate levels 3 µm higher than ambient, which presumably continued to enrich the pycnocline with coastal nutrients as the eddy decayed. The Si:N uptake ratio of 3.1 observed over the summer period in Haida-1998 is much higher than the ratio of 1.0 measured by Brzezinski (1985) for cultured marine diatoms. Such proportionally high silicate utilization suggests that diatom growth in eddies is strong and that nitrogen is efficiently recycled to stimulate further silica uptake. Takeda (1998) observed high silicate utilization in low iron conditions, but Si:N ratios under iron stressed conditions at Station Papa result in seasonal Si:N uptake Structure of Haida Eddies and Their Transport of Nutrient from Coastal Margins into the NE Pacific Ocean 721

8 rates of only 1.5 (Whitney and Freeland, 1999). The low Si concentrations observed in surface waters of Haida in summer (Fig. 5) are a common feature of recent Haida eddies (unpublished data), and are reminiscent of the silicate depleted waters observed three times in the 1970s at Station P (Wong and Matear, 1999). Enhanced levels of POC at the edge of Haida-1998 in September 1998 (Fig. 7) show that nutrients were not being supplied uniformly to the base of the euphotic zone over the area of the eddy. Perhaps, as Yentsch and Phinney (1985) suggested, the high velocity perimeter of the eddy becomes the area of maximum nutrient supply. Commercial squid catches were obtained at the edges of Haida in the summer of 1998 and not in adjacent waters (I. Perry, pers. comm.) which supports the notion that this eddy generated a very productive ring at its periphery. The net effect of Haida eddies can be viewed as one of offshore transport of materials contained in coastal waters, which increases the productivity of open ocean, likely at the expense of the formation region. These eddies may offer an explanation for low silicate (Wong and Matear, 1999), high chlorophyll (Boyd et al., 1998) and high sedimentation events (Wong et al., 1999) observed over the past several decades in the HNLC waters of the Gulf of Alaska. Acknowledgements Thanks to many samplers and analysts for their diligence at sea, and to the crew of the John P Tully for valued assistance. Funding was provided by the Department of Fisheries and Oceans through their Ocean Climate Program and Strategic Science Fund. Robert Leben of Colorado Center for Astrodynamics Research developed and maintains the web site used to display near-real-time satellite elevation contours. References Barwell-Clarke, J. and F. Whitney (1996): Institute of Ocean Sciences nutrient methods and analysis. Can. Tech. Rep. Hydrogr. Ocean Sci., 182, vi + 43 pp. Bishop, J. K. B. (1999): Transmissometer measurements of POC. Deep-Sea Res. I, 46, Boyd, P. W., D. L. Mugli, D. E. Varela, R. H. Goldblatt, R. Chretien, K. J. Orians and P. J. Harrison (1996): In vitro iron enrichment experiments in the NE subarctic Pacific. Mar. Ecol. Prog. Ser., 136, Boyd, P. W., C. S. Wong, J. Merrill, F. Whitney, J. Snow, P. J. Harrison and J. Gower (1998): Atmospheric iron supply and enchanced vertical carbon flux in the NE subarctic Pacific is there a connection? Global Biogeochem. Cycles, 12, Brzezinski, M. A. (1985): The Si:C:N ratio of marine diatoms: interspecific variability and the effect of some environmental variables. J. Phycol., 21, Crawford, W. R. (2002): Physical characteristics of Haida Eddies. J. Oceanogr., 58, this issue, Crawford, W. R. and F. A. Whitney (1999): Mesoscale eddy aswirl with data in Gulf of Alaska. EOS, 80, 365, 370. Crawford, W. R., J. Y. Cherniawsky, M. G. G. Foreman and J. F. R. Gower (2002): Formation of the Haida-1998 oceanic eddy. J. Geophys. Res. (in press). Falkowski, P. G., D. Ziemann, Z. Kolber and P. K. Bienfang (1991): Role of eddy pumping in enhancing primary productivity in the ocean. Nature, 352, Franks, P. J. S., J. S. Wroblewski and G. R. Flierl (1986): Prediction of phytoplankton growth in response to the frictional decay of a warm-core ring. J. Geophys. Res., 91, Lohan, M. C., P. J. Statham and D. W. Crawford (2002): Dissolved total zinc in surface waters of the subarctic North East Pacific in summer and winter. Deep-Sea Res. II (in press). Martin, A. P. and K. J. Richards (2001): Mechanisms for vertical nutrient transport within a North Atlantic mesoscale eddy. Deep-Sea Res. II, 48, Martin, J. H., R. M. Gordon, S. Fitzwater and W. W. Brokenow (1989): Vertex: phytoplankton/iron studies in the Gulf of Alaska. Deep-Sea Res., 30, McGillicuddy, D. J., A. R. Robinson, D. A. Siegel, H. W. Jannasch, R. Johnson, T. D. Dickey, J. McNeil, A. F. Michaels and A. H. Knap (1998): Influence of mesoscale eddies on new production in the Sargasso Sea. Nature, 394, Meyers, S. D. and S. Basu (1999): Eddies in the eastern Gulf of Alaska from TOPEX/POSEIDON altimetry. J. Geophys. Res., 104, Nelson, D. M., H. W. Ducklow, G. L. Hitchcock, M. A. Brzezinski, T. J. Colwes, C. Garside, R. W. Gould, T. M. Joyce, C. Langdon, J. J. McCarthy and C. S. Yentsch (1985): Distribution and composition of biogenic particulate matter in a Gulf Stream warm-core ring. Deep-Sea Res., 32, Nishioka, J., S. Takeda, C. S. Wong and W. K. Johnson (2001): Size-fractionated iron concentrations in the northeast Pacific Ocean: distribution of soluble and small colloidal iron. Mar. Chem., 74, Pingree, R. D. and B. Le Cann (1992): Three anticyclonic Slope Water Oceanic eddies (SWODDIES) in the southern Bay of Biscay in Deep-Sea Res., 39, Rogachev, K. A., P. Y. Tishchenko, G. Y. Pavlova and A. S. Bychkov (1996): The influence of fresh-core rings on chemical concentrations (CO 2, PO 4, O 2, alkalinity, and ph) in the western subarctic Pacific Ocean. J. Geophys. Res., 101, Savidge, G. and P. J. le B. Williams (2001): The PRIME 1996 cruise: an overview. Deep-Sea Res. II, 48, Tabata, S. (1982): The anticyclonic, baroclinic eddy off Sitka, Alaska, in the Northeast Pacific Ocean. J. Phys. Oceanogr., 12, Takeda, S. (1998): Influence of iron availability on nutrient consumption ratio of diatoms in oceanic waters. Nature, 393, Thomson, R. E. and J. F. R. Gower (1998): A basin-scale oceanic instability event in the Gulf of Alaska. J. Geophys. Res., 103, Tomosada, A. (1986): Generation and decay of Kuroshio warm- 722 F. Whitney and M. Robert

9 core rings. Deep-Sea Res., 33, Whitney, F. A. and H. J. Freeland (1999): Variability in upperocean properties in the NE Pacific Ocean. Deep-Sea Res. II, 46, Whitney, F. A. and D. W. Welch (2002): Impact of the El Niño and 1999 La Niña on nutrient supply in the Gulf of Alaska. Prog. Oceanogr. (in press). Whitney, F. A., C. S. Wong and P. W. Boyd (1998): Interannual variability in nitrate supply to surface waters of the Northeast Pacific Ocean. Mar. Ecol. Prog. Ser., 170, Wong, C. S. and R. J. Matear (1999): Sporadic silicate limitation of phytoplankton productivity in the subarctic NE Pacific. Deep-Sea Res. II, 46, Wong, C. S., F. A. Whitney, K. Iseki, J. S. Page and J. Zeng (1995): Analysis of trends in primary productivity and chlorophyll-a over two decades at Ocean Station P (50 N, 145 W) in the Subarctic Northeast Pacific Ocean. In Climate Change and Northern Fish Populations, ed. by R. J. Beamish, Can. J. Fish. Aquat. Sci., 121, Wong. C. S., F. A. Whitney, D. Crawford, K. Iseki, R. J. Matear, W. K. Johnson, J. S. Page and D. Timothy (1999): Seasonal and interannual variability in particle fluxes of carbon, nitrogen and silicate form time-series sediment traps at Ocean Station P, : relationship to changes in subarctic primary productivity. Deep-Sea Res. II, 46, Woodward, E. M. S. and A. P. Rees (2001): Nutrient distribution in an anticyclonic eddy in the northeast Atlantic Ocean, with reference to nanomolar ammonium concentrations. Deep-Sea Res. II, 40, Yentsch, C. S. and D. A. Phinney (1985): Rotary motions and convection as a means of regulating primary production in warm core rings. J. Geophys. Res., 90, Zhang, J. Z., R. Wannikhof and K. Lee (2001): Enhanced new production observed from the diurnal cycle of nitrate in an oligotrophic anticyclonic eddy. Geophys. Res. Lett., 28, Structure of Haida Eddies and Their Transport of Nutrient from Coastal Margins into the NE Pacific Ocean 723