Beaufort Seas

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A numerical model of seasonal primary production within the Chukchi/Beaufort Seas John J. Walsha,, Dwight A. Dieterlea, Wieslaw Maslowskib, Jacqueline M. Grebmeierc, Terry E. Whitledged, Mikhail Flinte, Irina N. Sukhanovae, Nicholas Batesf, Glenn F. Cotag,{, Dean Stockwelld, S.B. Moranh, Dennis A. Hanselli, C. Peter McRoyd a

College of Marine Science, University of South Florida, St. Petersburg, FL 33701, USA Department of Oceanography, Naval Postgraduate School, Monterey, CA 93943, USA c Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996, USA d School of Fisheries and Ocean Sciences, University of Alaska, Fairbanks, AK 99775, USA e Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia f Bermuda Biological Station for Research, St. George’s, GE 01 Bermuda, USA g Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, VA 23508, USA h Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, USA i Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA b

Received 3 March 2004; accepted 21 September 2005

Abstract A coupled three-dimensional circulation and ecological model provided numerical analysis of daily carbon/nitrogen cycling by the planktonic and benthic components of western Arctic shelf/basin ecosystems during 2002, when extensive field data were obtained by American and Canadian ice-breakers. Seasonal model budgets of April–May, July–August, and September–October 2002 allowed both interpolation and extrapolation of these validation data, suggesting that the most productive shelf regime of the Chukchi/Beaufort Seas was that of summer. Yet, during this period of July–August, a combination of light-limitation and nutrient-limitation limited shelf-wide mean simulated net photosynthesis to only
709 mg C m2 day1 for shelf waters of o220 m depth. This modeled seasonal carbon fixation then accounted for
45% of the annual shelf primary production of 97.4 g C m2 yr1 Identification of the relative importance of natural control factors of light and nutrients by the coupled model provided insight into possible consequences of future global climatic changes at these high latitudes. The model’s seasonal penetration of relatively saline, nutrient-rich Anadyr Water of Pacific origin into the eastern Chukchi Sea replicated the time series of observed salinity fields. A similar fidelity of the simulated nitrate, silicate and dissolved inorganic carbon fields with the observed ones yielded an assessment of nutrient uptake and photosynthesis during a natural fertilization experiment. The simulated chlorophyll, dissolved organic carbon (DOC), and NH4 stocks also mimicked these shipboard observations. We found that the spring 2002 stocks of new nutrients were stripped by the end of summer, with little fall nutrient resupply by physical and biotic factors, when incident light waned. However, because of extensive ice cover and nutrient-poor upper waters within the Canadian Basin, the slope regions remained Corresponding author. Fax: +1 727 553 1189.

E-mail address: [email protected] (J.J. Walsh). Deceased.

{

0967-0645/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2005.09.009

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oligotrophic throughout the year, yielding a simulated annual net photosynthesis of
50 g C m2 yr1. We conclude that future ice cover retreat, without eutrophication, may have little impact on increased carbon sequestration within these high-latitude ecosystems. r 2005 Elsevier Ltd. All rights reserved.

1. Introduction Most field programs at latitudes of 4601N within the Bering/Chukchi/Beaufort Seas have been constrained by ice conditions, limiting both ice-breakers and conventional research vessels. For example, near the Alaskan coast at 701N during August 1778 ‘‘in shoal water, under a lee shorey the main body of ice y driving down upon us’’ halted studies that year (Cook and King, 1784). Fifty years earlier, Vitus Bering had reported that he ‘‘arrived in the latitude of 671180 y on the 15th of August 1728’’, where ‘‘y the land no longer extended to the north y from the Chukchi coast’’ (Coachman et al., 1975), but presumably ice then prevented much farther northward exploration. Given typical snow and ice thicknesses of 0.1 and 2.0 m, less than 0.2% of the incident photosynthetically active radiation (PAR) would have reached the ocean’s surface, such that light limitation may be the major factor in curtailing photosynthesis of some Arctic phytoplankton populations within the underlying water column. In subsequent years, chemical observations were mainly made here during other ice-free times of August 1922–1924 (Sverdrup, 1929), 1934 (Barnes and Thompson, 1938), 1937–1938 (Goodman et al., 1942), 1963 (Codispoti and Richards, 1968), 1968–1970 (Kinney et al., 1970; Hufford and Husby, 1970), 1983, 1985–1989 (McRoy, 1993), 1990 (Cooper et al., 1997), 1993–1994 (Cota et al., 1996; Weingartner et al., 1998; Wheeler et al., 1997), 2000 (Belicka et al., 2002), and 2002. These recent data all depict nutrient-depleted surface and bottom summer conditions of o1 mmol NO3 kg–1 within Alaska Coastal Water (ACW) during August on the eastern side (Fig. 1) of Bering Strait (Walsh et al., 1989). Given a half-saturation constant of 0.9 mmol NO3 kg1 for nitrate uptake by phytoplankton, their protein synthesis would have been
50% of the maximal value at such nutrient stocks. Thus, nutrient limitation may also be the major factor in regulating primary production of some regions within Arctic Seas to the north of Bering Strait. By comparison, however, in the Anadyr Water (AW) column as much as 20–25 mmol NO3 kg1

were instead found during August 1934–2002 on the western side of the Strait (Fig. 1), depending upon the intensity of upwelling. Were these east-west gradients of summer nutrients within Bering Strait a consequence of differences of downstream—in space and time—supply, or utilization? For example, our prior numerical analysis of August– September nutrient and plankton dynamics of the Chukchi/Beaufort Seas (Walsh et al., 2004) had successfully mimicked previous observations of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC) in these waters. The major failing of the model was our inability to replicate fall nutrient observations at the edge of the Canadian Basin, because of unknown winter–spring initial conditions within Bering Strait. Here, we use minor revisions of the same ecological model (Walsh et al., 2004), combined with physical forcings in 2002 of a circulation model (Maslowski and Lipscomb, 2003; Maslowski et al., 2004) and boundary conditions at Bering Strait over March–October 2002 (Fig. 2), to explore the relative roles of light and nutrients in controlling the present and future amounts of photosynthesis and protein formation by these Arctic phytoplankton. Based upon model interpretation of the extensive validation data obtained during 2002 (Fig. 2), we then evaluate the seasonal contributions of their primary production to annual utilization by the rest of the food web of the Chukchi/Beaufort Seas. A few spring observations were made earlier within ACW of o50 m depth during April 1980 in the southeastern Bering Sea (Fig. 2; Whitledge et al., 1986), during April 1979 in Bering Strait (Horner and Wencker, 1980), and during April 1972, 1981, and 1987 in the Beaufort Sea (Horner, 1972; Schell et al., 1982; Aagaard et al., 1988). They all suggested winter accumulations of only 5–10 mmol NO3 kg1 (Fig. 3B). Other March observations of ACW at Point Barrow in the western Beaufort Sea during 1971 (Horner, 1972) and in the Bering Sea, during 2001 by the ice-breaker Polar Star (Clement et al., 2004) around St. Lawrence Island (Fig. 3A) and during 2002 off Little Diomede Island (Cooper et al., 2005a), indeed found
10 mmol NO3 kg1

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Fig. 1. A schematic of the water mass distributions of recently upwelled, salty and nutrient-rich Anadyr Water (AW) and relict riverimpacted, fresher and nutrient-depleted Alaska Coastal Water (ACW) within the Bering and Chukchi Seas. Both water masses are of Pacific origin.

Fig. 2. Orthogonal cruise tracks within the Arctic basins of the submarines Pargo (’) and Cavalla (m) during March 1991 and 1995, in relation both to stations (K) of the ice-breakers Polar Star/Healy, the research vessel Thomas Thompson, and at Little Diomede Island during March 1980, 2001, 2002, April 1979,1980,1999, 2000, May–August 2002, a moored time series of nitrate during 2001 and 2002 (n), and to boundaries (–) of the coupled circulation and ecological models.

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Fig. 3. The composite near-bottom distributions of nitrate (mmol kg1) in the Bering/Chukchi Seas during (A) winter of March 1972, 1980, 1991, 2001, and 2002 and (B) spring of April 1972, 1979, 1980, 1981, 1987, 1999, 2000 and May 2002. Locations of these stations are shown in Fig. 2.

within surface and bottom waters. Previously as much as
20 mmol NO3 kg1 were then observed (Clement et al., 2004) within near-bottom AW to the southwest of St. Lawrence Island (Fig. 3A). Additional under-ice sampling by submarines during the SCICEX program (Fig. 2) confirmed these different winter nutrient initial conditions of ACW and AW, sampled by ice breakers. For example, a section of the USS Pargo, parallel to the International Date Line (the closed square symbol of Fig. 2), found that subsurface AW stocks of 415 mmol NO3 kg1 extended northwards, from the shelf-break of the Chukchi Sea to at least 851N in the Makarov Basin, during March 1991 (Figs. 4A–C). Within another orthogonal submarine section of the USS Cavalla in March 1995 (the closed triangle symbol of Fig. 2), the width of this AW plume of 415 mmol NO3 kg1 was
300 km in the Canadian Basin between 1651W and 1751W (Figs. 4D–F), with only
5 mmol NO3 kg1 found at a depth of 60 m in ACW, between 1401W and 1601W on the Beaufort slope. We believe that when sufficient light conditions favor phytoplankton growth and inhibit nitrifying bacteria, nitrate stocks of ACW are quickly stripped to o0.1 mmol

NO3 kg1 within both near-bottom and near-surface waters of shallow regions of the southeastern Chukchi Sea (Fig. 3B). As much as
10 mg chl l1 of phytoplankton biomass were formed in April 1979 (Horner and Wencker, 1980)—as during August within the northern Chukchi Sea (Walsh et al., 2004). Similar nutrient depletion in the southeast Bering Sea (Fig. 5) fuels the spring blooms of 410 mg chl l1 in this region as well (Walsh and McRoy, 1986). In contrast, both prolonged ice cover within the western Beaufort Sea and some lateral advection presumably allowed 410.0 mmol NO3 kg1 to accumulate off Point Barrow, Alaska in April 1971–1973, when algal stocks of o1 mg chl l1 prevailed there over the
5-m water column (Horner, 1972). Consequently, a diatom-like PN (Particulate Nitrogen)/chl (chlorophyll) ratio (mmol/ mg) of 0.6 (Walsh et al., 1978) and a depletion of at least 10 mmol NO3 kg1 (Figs. 3–5) implied that 16 mg chl l1 of ‘‘new’’ spring production must have been consumed and/or exported by the food web of nearshore Alaskan waters each year. Initial grazing may usually be minimal, since
12 mg chl l1 and
0.5 mg phaeopigments l1 were left behind in the

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Fig. 4. The winter distributions of the inorganic nutrients (mmol kg1) phosphate, nitrate, and silicate sampled by submarine over the upper 300 m of the water column of the Chukchi Sea and Canadian Basin during March 1991 (A–C) and at a nominal depth of 60 m along the Beaufort, Chukchi and East Siberian slopes during March 1995 (D–F). Locations of these sections are shown in Fig. 2. The m and dashed line of the first three panels is the sampling depth and location of the Cavalla section within the orthogonal one of the Pargo, while the dashed line of the last three panels is the position of Pargo within the along-slope sampling of Cavalla.

Fig. 5. Nitrate time series (mmol kg1), from moored near-surface sensors, at a depth of
5 m on the 70-m isobath of the southeastern Bering Sea during 2001 (—) and 2002 (- - -). Location of the moored array is shown as the symbol in Fig. 2.

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water column near Bering Strait during April 1979 (Horner and Wencker, 1980). Given the potential future trajectory of the Arctic ecosystem to an icefree state (Overpeck et al., 2005), a set of unanswered questions is: (1) does light or nutrients control primary production to the north of Bering Strait and (2) how much of the carbohydrate and protein synthesis of the phytoplankton is passed up the marine food web to higher trophic levels? The problem of missing or neglected spring production in annual estimates of food web yield in ice-covered environments would be exacerbated if winter nitrification and cross-shelf mixing of deepsea nutrients stocks of Pacific origin (Whitledge and Luchin, 1999) replenished those of ACW to the same higher levels of AW. Despite recent continuous time series of winter nitrate stocks during 2001 and 2002, measured by moored nutrient sensors at a depth of
5 m (Fig. 5) above the
70-m isobath of the Southeast Bering Sea (the open triangle symbol of Fig. 2), record gaps still prevent us from an empirical assessment of the actual amount and fate of spring primary production—both in AW and ACW. Indeed, the seasonal variation, and thus the annual amount, of present phytoplankton growth and consumption in the western Arctic shelf and slope ecosystems remain unknown. They are thus the subjects of our simulation analyses over an 8-month period of March–October 2002, when three shelf-break interactions (SBI) cruises (Fig. 2) of the USCG ice breakers, Polar Star and Healy, provided spatial snapshots of validation data during 5/5–6/15, 7/15–8/13, and 7/16–8/26 of 2002. Additional validation data were available from observations (Lee and Whitledge, 2005) in the Canadian Basin (Fig. 1), aboard the Canadian ice breaker Louis St. Laurent, to the north of 741N and between 1301W and 1601W, during August–September 2002.

and 2000. Here, we place these prior simulated estimates of fall primary production within the context of annual carbon fixation and consumption during spring, summer, and fall of 2002. This was a year of strong northward flows through Bering Strait (Fig. 6) and minimal ice extent (Fig. 7). The ice cover at the end of summer during 2002 was the smallest of the previous 20 years, monitored by satellite since 1982 (Overpeck et al., 2005). At each 15-min time step of the coupled physical/ ecological model, the computed depth-dependent fields of light (blue-green and red wavelengths) and nutrient (nitrate, ammonium, silicate, and DIC stocks) were used to assess the relative limitation of the gross growth rates of the phytoplankton community (diatoms, microflagellates, and colonial prymnesiophytes). With these growth terms, the model’s different phytoplankton losses of respiration, excretion, grazing, settling, and decomposition provided estimates of the standing stocks of chlorophyll and the associated algal stocks of particulate carbon, nitrogen, and silicon left behind in the water column—for comparison with the limited SBI observations over time and space. Concomitant field observations of: DIC provided an estimate of the model’s fidelity of phytoplankton respiration losses—as well as those of bacteria and zooplankton; of DOC for the algal excretion losses—as well as ‘‘sloppy feeding’’ by zooplankton

2. Methods A previous three-dimensional, coupled biophysical model of circulation impacts (Maslowski et al., 2004) on the plankton and benthos dynamics (Walsh et al., 2004) in the Chukchi/Beaufort Seas considered interannual variations of late summer/ early fall utilization of nutrients during August– September 1980 and 1989. Using the many observations taken during August of 1963–2000 as initial conditions, the results of this model matched most of the available validation data of September 1997

Fig. 6. The 2002, northward transport (Sv) through Bering Strait in relation to the mean shelf (o200 m) temperature (1C) and monthly average ice cover (%) over the model domain for March–October, 2002.

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Fig. 7. Monthly average ice cover over the Bering/Chukchi/Beaufort Seas during March–October (A–H) 2002.

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and the benthos; of particulate radioisotopes for the settling losses of phytoplankton—as well as those of copepod and protozoan fecal pellets; and of ammonium for the decomposition losses via ammonifying bacteria. By difference, the last phytoplankton loss was that of grazing by the herbivores, which were a source of ammonium, DOC, and settling debris. We believe that physical forcing (Roach et al., 1995; Chapman and Walsh, 1993) and subsequent primary production and nutrient cycling during 2002 were similar to those of 1989. The simulated summer/fall photosynthesis in 1989 was perhaps 2-fold larger than that of 1980, as a result of more light penetration over a greater area of these shelves (Walsh et al., 2004). Here in this work, the satellite observations of ice conditions for 2002 (Cavalieri et al., 2003) were imposed as monthly ice cover (Fig. 7) at 9-km spatial resolution, from 1701W in the East Siberian Sea to 1301W in the Beaufort Sea, and from the Bering Strait boundary conditions (Table 1) to the 2000-m isobath of the Canadian Basin. The ecological model was nested within the larger spatial domain of a Pan-Arctic circulation model (Maslowski and Lipscomb, 2003; Maslowski et al., 2004) over all Arctic seas, with the same resolution of a 1/121 horizontal grid and a 45-layer vertical grid. The vertical resolution was highest near the surface, with 15 layers of minimal thickness of 5 m and maximal thickness of 20 m within the upper 200 m of the water column. Here, we report the results of the model from both the shelves and a
75 km-wide region of the upper slope, shoreward of the permanent pack ice (Fig. 6). Details of our current modification of the prior ecological model (Walsh et al., 2004) are presented in Appendix A. Briefly, it consisted of 16 biochemical explicit state variables: nitrate, ammonium, silicate, DIC, monomeric and macromolecular DOC, ammonifying bacteria, microflagellates, colonial prymnesiophytes, diatoms, fecal pellets of copepods and protozoans, sediment detritus of siliceous and non-siliceous origin, as well as interstitial DIC and silicate. Colored dissolved organic matter (CDOM), dissolved organic nitrogen (DON), nitrifying bacteria, adult copepods, and bioturbating macrobenthos were all implicit state variables. Their interactions were forced by physical factors: of wind stress; water motion and mixing; temperature; salinity; ice cover; and incident light, including ultraviolet photolysis of CDOM and PAR

for photosynthesis. Basically, we exploited the in situ observations: of physical and chemical hydrographic properties; phytoplankton elemental stocks and species composition; particle fallout; and benthos recycling during the Healy cruises in 2002 to estimate the daily fluxes of carbon, nitrogen, and silicon between the autotrophic and heterotrophic components of the food web. In the present version of the model, a reduced grazing stress was now imposed on the diatoms. The daily ingestion ration of copepods increased with greater chlorophyll concentrations to a maximum of 10% of herbivore body carbon per day at a chlorophyll stock of 10 mg chl l1. Such a greater influx of phytodetritus to the sediments led to a more active benthos in our simulations. Their catabolism in the model was now a function of both chlorophyll and fecal pellet content of the overlying water column, rather than as a previous constant (Walsh et al., 2004). In terms of carbon, they released 90% of the sediment efflux as macromolecular DOC, and 10% as DIC (Walsh and Dieterle, 1994; Cooper et al., 2005b). Among the modified physical factors, the circulation model was the same (Maslowski and Lipscomb, 2003; Maslowski et al., 2004; Walsh et al., 2004), but vertical exchange processes differed. The horizontal flow fields were again calculated ‘‘off line’’, i.e. the results of prior computations of the circulation model were used as input to the ecological model that had a numerical time step of 900 s over the three-dimensional grid. The timedependent wind speed at 10 m above the sea surface (Kanamitsu et al., 2002) was to calculate piston velocity for air–sea exchange of carbon dioxide, rather than a previous constant value of 5 m s1. The oceanic surface mixed layer depends in a complicated way on the surface wind stress and upper-layer density gradients (Large et al., 1994), which we did not attempt to simulate. Instead, we used a simple prescription of a 20-m surface mixed layer, with a vertical eddy diffusivity, Kh, of 10 cm2 s1 in open water regions of ice concentrations of o10% cover. It was a constant Kh of 1 cm2 s1 elsewhere. Across the sediment water interface, the temperature-dependent bioturbation coefficient (Table 2) remained the same. But, the pore water diffusion coefficient was 1000-fold greater than that of our prior 1989 case (Walsh et al., 2004), to explore possible impacts of the macrofaunal benthos on release of ammonium and DOC (Cooper et al., 2005b).

1 May

5 5 5 6 15

0.5 0.5 0.5 0.2 0.0

Chlorophyll

20 [0.5] 2.5 [2.5] 55 [10.0] 2000 [2000] 10.0 [0.5] 1.0 [0.5] 0.5 [2.0] 10.0 [5.0] 90 [60] 1.0 [2.0]

1 Aug

0.5 0.5 0.5 0.5 0.1

Copepods

20 [0.5] 2.5 [1.5] 55 [10.0] 2000 [2000] 10.0 [0.5] 1.0 [0.5] 0.5 [2.0] 10.0 [5.0] 90 [60] 1.0 [2.0]

1 Sep

2 2 2 2 0.2

Protozoans

20 [0.5] 2.5 [0.5] 55 [10.0] 2000 [2000] 10.0 [0.5] 1.0 [0.5 0.5 [2.0] 10.0 [5.0] 90 [60] 1.0 [2.0]

1 Oct

0.003 0.003 0.003 0.003 0.003

Pellets

20 [0.5] 1.0 [1.0] 55 [10.0] 2025 [2010] 1.0 [0.5] ] 1.0 [0.5] 0.5 [2.0] 5.0 [1.0] 90 [60] 1.0 [2.0]

1 Nov

60 60 60 60 60

DOC

2 2 2 0.8 0.8

Bact-

The boundary conditions of slope waters within the Canadian basin are instead time-invariant, but change with depth over the greater water column. All units are mmol kg1 of N, Si, and C, except mg chl l1 of the total phytoplankton biomass of pigments. At the southern boundary, the functional group per cent composition of total phytoplankton biomass in AW is 50/49/1 of diatoms, microflagellates, and Phaeocystis during March 1–June 30 and 85/5/10 after July 1. Within the ACW, the imposed seasonal shift of phytoplankton dominance at Bering Strait is 50/49/1 during March 1–June 30 and 10/85/5 afterwards. At the northern boundary of the polar basin, it remains at 85/5/10 over the water column. The brackets denote properties of ACW.

0.5 0.5 0.5 0.1 0.1

2125 2125 2125 2150 2200

0.5 0.5 0.5 9.0 15.0

0 30 65 220 4220

Bering Strait] 20 [0.5] 2.5 [2.5] 55 [10.0] 2000 [2000] 10.0 [5.0] 1.0 [0.5] 0.5 [2.0] 10.0 [5.0] 80 [60] 0.9 [2.0]

1 Jul

DIC

[ACW of eastern 20 [4.0] 0.5 [1.0] 55 [21.5] 2050 [2000] 2.0 [10.0] 0.5 [0.5] 0.5 [2.0] 5.0 [2.0] 70 [60] 0.8 [2.0]

1 Jun

Northern boundary: Canadian Basin slope waters Depth (m) NO3 NH4 SiO4

Southern boundary: AW of western Bering Strait Nitrate 20 [10] 20 [8.0] 0.1 [0.5] 0.2 [0.5] NH4 Silicate 55 [27.5] 55 [25.5] DIC 2100 [2035] 2100 [2035] Phyto0.1 [0.1] 0.5 [2.0] Cop0.5 [0.5] 0.5 [0.5] Proto0.5 [2.0] 0.5 [2.0] Pellets 0.1 [0.1] 1.0 [1.0] DOC 60 [60] 60 [60] Bact0.5 [2.0] 0.6 [2.0]

1 April

Table 1 Time-dependent, vertically uniform boundary conditions of the plankton component of the coupled models within distinct water masses (Anadyr Water—AW and Alaska Coastal Water—ACW) on the western (cross-sectional area of 2.26 km2) and eastern (cross-sectional area of 1.52 km2) sides of shallow (
30 m) Bering Strait during 2002

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Table 2 Model parameters ci gi gi;NO3 gi;NH4 gi;SIO4 ei ci L(t, z) Lis Im Ip Rb ts D p cb m g3 g4 b Zi Zc,m gd z gm gfGm gpGm Pps wd wp wf wzd wzf ac,m,b fc,m,b [1ac,m,b,fc,m,b] Grmini Grlayerc,m kiNO3 kiNH4 kiSiO4 kDOC kNIT kb (w, i, s) kr (w, i, s) kc km k (a) ki ky z (i, s, a) X1 X2 lm Kb Km Kh (C/N)r (C/Si)r (C/N)b (C/CHL)i

[0.70,0.70,0.76] e0633T — — — — [0.10, 0.05, 0.05] [0.04, 0.04, 0.40] — [45, 35, 25] — — 0.5 — — 3.14y 0:008e0:092T 0.002+0.005T — — 0.5(g3+g4) [0.0, 0.0, 0.0] [0.0, 0.0] 2 [0.01Pd(a)] 0.1 z(Pf+Pp) — — 0.9Pp 0.025P2d 0.025P2p 0.5 100.0 30.0 [0.325, 0.60, 0.20] [0.50, 0.0, 0.50] — [0.05, 0.05, 0.05] [200, 100] [0.9, 0.9, 0.9] [0.2, 0.2, 0.2] [1.15, 0.0, 0.0] 0.83 0.10 [0.0438, 2, 15] [0.40, 4.0, 35] — — 0.38 [0.024, 0.048, 0.012] 47.0  106 [2.0,0.1,0.01] — 0.20 50 3:5  104 e0:092T b 3.5  107 1.0–10.0  104 (100/15) (100/15) (100/20) [45, 50, 125]

Maximum growth rate as a function of temperature, T (day1) Algal growth as a function of light and nutrients (day1) Algal uptake rate, nitrate (day1) Algal uptake rate, ammonium (day1) Algal uptake rate, silicate (day1) Algal respiration rate ( percent of gi) Algal excretion of DOC1 (percent of gi) Photosynthetic active radiation in the water column (W m2) Saturation (optimal) light intensity for growth (W m2) Maximum noon radiation (W m2) Daily mean PAR (W m2) Fraction of surface PAR in the blue-green wavelengths Time since sunrise (h) Photoperiod (h) Maximum bacterial growth rate as a function of T (h1) Bacterial mortality rate as a function of temperature (h1) Bacterial uptake rate of DOC1 (day1) Bacterial uptake rate of DOC2 (day1) Bacterial respiration (day1) Algal lysis to DOC2 (percent algal biomass per day) Fecal pellet lysis (percent fecal pellet biomass per day) Grazing loss of diatoms(percent copepod body carbon per day) Protozoan grazing coefficient (mmol C kg1 day)1 Protozoan daily ration (% protozoan body carbon per day) Microflagellate grazing by protozoans (mmol C kg1 day1) Phaeocystis grazing by protozoans (mmol C kg1 day1) Single-cell Phaeocystis concentration (mmol C kg1) Diatom settling velocity(m day1). Pd in mg chl l1 Phaeocystis settling velocity(m day1). Pp in mg chl l1 Microflagellate settling velocity (m day1) Settling velocity, siliceous fecal pellets (m day1) Settling velocity, non-siliceous fecal pellets (m day1) POC respired by copepods, protozoans and carnivores (%) DOC2 from sloppy copepods, protozoans and carnivores (%) Fecal pellet production from herbivores and carnivores Grazing threshold (mg chl l1) Maximum extent of copepod/protozoan grazers (m) Half-saturation constant for nitrate uptake (mmol N kg1) Half-saturation constant for ammonium uptake (mmol N kg1) Half-saturation constant for silicate uptake (mmol Si kg1) Half-saturation constant for DOC1,2 uptake (mmol C kg1) Half-saturation constant for nitrification (mmol N kg1) Attenuation of blue-green light (m1) in seawater, ice, snow Attenuation of red light (m1) in seawater, ice, and snow, Attenuation coefficient of chlorophyll (m1) Attenuation coefficient of CDOC (m1) Attenuation coefficient of ice algae (m1) Specific algal attenuation coefficient (m2 (mg chl)1) Specific CDOC attenuation coefficient [m2 (mg DOC)1] Thickness of ice, snow, and ice algae layers(m) Nitrification rate (mmol N kg1 s1) Photolytic loss of DOC2 to DOC1 (mmol DOC kg1 h1) Maximum degradation rate sediment particulate carbon (day1) Bioturbation coefficient and bottom temperature Tb (m2 s1) Porewater diffusion coefficient (m2 s1) Water column eddy diffusivity (m2 s1) Redfield molar C/N ratio in all organic matter except bacteria Redfield molar C/Si ratio of diatoms and siliceous fecal pellets Molar C/N ratio of bacteria Particulate organic carbon/chlorophyll ratio (mg/mg)

Subscript i is d, f, p for diatoms, microflagellates, and Phaeocystis.

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3. Results 3.1. Salinity fields The observed salinity isopleths of 432.5 at a depth of 30 m delineated Bering Sea AW on the outer Chukchi shelf, north of 701N. They suggested that shelf waters of Pacific origin must have been advected eastward of 1601W, between May (Fig. 8A) and August (Fig. 8B) 2002. Similar salinity fields were computed at a depth of 30 m

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by the circulation model at the end of July 2002, i.e. on Julian Day 212. They depicted eastward-tending tongues of 32.6 salinity (Fig. 8D) between Wrangel Island and Point Barrow, Alaska, in response to simulated eastward flows of 5 cm s1, or 5 km day1. The computed salinity field at depth of 30 m by the end of May 2002 (Fig. 8C), i.e. on Julian Day 151, indicated that AW of 32.6 was restricted to a region mainly west of 1601W. Thus, the model’s threedimensional salt fields replicated the spring (Fig. 8A) and summer (Fig. 8B) salinity observations. During

Fig. 8. The observed 2002 fields of salinity at a depth of
30 m during (A) 8 May–12 June and (B) 18 July–21 August in relation to computed salinity at the same depth within the Chukchi/Beaufort Seas on (C) 31 May and (D) 31 July. The July–August stations with the & symbols around respective Polar Star (J) and Healy (K) observations were used to estimate the mean summer properties of Anadyr Water within the eastern Chukchi Sea.

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some spring seasons, moreover, these penetrations of AW of 32.6 salinity and of o0.5 1C temperature, with associated nutrient stocks of 435 mmol SiO4 kg1 and 410 mmol NO3 kg1 were found as far east as 1531W on the eastern Beaufort shelf, during April 1987 (Aagaard et al., 1988). 3.2. Nutrient fields A mean salinity of 32.6 was observed at a depth of 30-m during 20 July–19 August 2002 (Table 3) over 10 Polar Sea/Healy stations within those AW plumes that penetrated into the eastern Chukchi Sea [indicated by the open square symbols of Fig. 8B]. Furthermore, within these observed 2002 AW plumes of 32.6 salinity, the nitrate stocks at a depth of 30 m remained the same during spring and summer (Figs. 9A and B). The small temporal change of nitrate suggested mainly physical transport, with little biotic utilization during transit. Why? Over a distance of
800 km, between the western side of Bering Strait and about 72.51N, 162.51W in the northeastern Chukchi Sea, the simulated speed of
5 km day1 suggested that a water parcel sampled here at the end of July would have exited Bering Strait 5 months earlier, by mid-February 2002. The ice cover was large (Figs. 6 and 7) and incident radiation was low. At the 10 Polar Sea/ Healy stations, the nutrients at 30 m were still unutilized means of 36.6 mmol SIO4 kg1 and 12.2 mmol NO3 kg1 during mid-August within the eastern Chukchi Sea (Table 3).

The plankton model’s mean specific algal attenuation coefficient was 0.028 m2 (mg chl)1 for the three functional groups of phytoplankton (Table 2). Although no pigment observations were made on the Polar Sea cruise in 2002, this specific attenuation value and a mean observed chlorophyll stock of 5.75 mg chl m3 for the five Healy stations (Table 3) suggested an euphotic zone of, at most,
30-m depth, in the absence of other water and CDOM attenuation. Similar shallow depths of 1% penetration of the surface light were sometimes found on the Chukchi shelf during the 2002 Healy cruises. Initial phytoplankton stocks of 0.1–0.5 mg chl m3 (Table 1) instead would have yielded a much deeper euphotic zone of 4100 m, of course, ignoring once again attenuation by water and CDOM. In the coupled model, these summer aphotic fields of nitrate at the bottom of the euphotic zone (Fig. 9B) were indeed replicated at the end of July 2002 (Fig. 9D). Little nutrient removal was effected by the model’s light-limited phytoplankton at this depth of 30 m within the northeastern Chukchi Sea at
72.51N, 162.51W. Note that the gross growth rates of the phytoplankton community within the euphotic zone were limited more by light, than by nutrients, over 98% of the shelf region during April–May 2002 (Table 4). In contrast, during September–October 2002, 34% of the phytoplankton on the shelf was limited by nutrients (or 66% by light in Table 4). The observed spring 30-m fields of nitrate (Fig. 9A), left behind in this region by earlier seasonal productivity events to the south of Bering Strait, were reproduced by the model as well at the

Table 3 Mean properties of summer Anadyr Water during 2002 in the eastern Chukchi Sea Stations

Polar Star 9 10 11 47 53 Healy 10 11 25 39 40 Mean

Date

Salinity 30 m

Nitrate

Ammonium

Silicate

Chlorophyll

0m

30 m

0m

30 m

0m

30 m

0m

30 m

7/10/02 7/21/02 7/21/02 8/4/02 8/5/02

33.12 32.45 32.25 32.78 32.67

0.81 3.73 0.07 0.00 0.00

14.67 14.35 14.35 12.92 9.65

— — — — —

— — — — —

36.65 34.35 31.40 12.10 7.45

48.85 48.14 45.80 30.80 30.30

— — — — —

— — — — —

7/21/02 7/21/02 8/2/02 8/18/02 8/19/02

32.34 32.48 32.98 32.46 32.61 32.61

0.11 0.12 0.00 0.09 0.04 0.49

10.85 10.97 14.84 16.41 3.05 12.21

0.39 0.49 0.84 0.68 0.66 0.61

0.51 1.78 1.18 2.22 0.95 1.33

21.20 12.89 15.17 3.93 3.07 17.82

30.80 31.18 40.41 49.65 10.10 36.60

2.43 2.88 0.42 0.21 1.79 1.54

32.34 13.35 1.03 1.40 1.66 9.95

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Fig. 9. The observed 2002 fields of nitrate (mmol NO3 kg1) at a depth of
30 m during (A) 6 May–12 June and (B) 18 July–21 August in relation to the computed fields of nitrate (mmol NO3 kg1) at depths of (C, D)
30 m and (E, F) 2.5 m on 31 May and 31 July.

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Table 4 Seasonal carbon (mg C m2 day1) budgets for the Chukchi/Beaufort Seas during 2002, nitrification in units of mmol N m2 day1. The simulated shelf annual net primary production was then
97.4 g C m2 year1, whereas that of the slope was
50.0 g C m2 year1

Shelves [slopes] Net photosynthesis Percent of annual F-ratio of primary production Percent of region light limited

January– March

April–May

June

July–August

September– October

November— December

2.5 0.2

9.9 [0.7] 0.6 0.98 98 [98]

279.7 8.6

709.1 [292.5] 45.1 0.64 83 [89]

578.3 [454.6] 36.2 0.44 66 [71]

146.6 9.1

Air– sea exchange Atmospheric sink Atmospheric source Grazing losses

0.0 [0.0] 7.4 [70.2] 4.1 [5.0]

173.8 [0.0] 0.0 [18.4] 141.9 [30.1]

341.4 [121.9] 0.0 [0.0] 160.1 [118.7]

Net particle influxes to bottom Phytodetritus Fecal pellets

3.7 [1.7] 7.8 [11.7]

320.2 [1.7] 69.8 [15.0]

382.0 [1.7] 100.9 [25.1]

DOC sources Phyto exudates Sloppy grazing Messy predation Sediments Carbon fixation of ammonifying bacterioplankton Nitrification Microbial loop shunt to metazoans

0.6 [1.7] 0.2 [0.0] 13.3 [101.9] 10.0 [10.0] 76.8 [354.3] 0.65 26.6 [203.9]

58.1 [36.8] 12.9 [1.7] 53.0 [45.1] 342.7 [15.0] 361.5 [267.4] 0.88 105.8 [90.3]

56.9 [56.8] 21.5 [15.0] 64.3 [76.9] 437.9 [21.6] 452.4 [453.0] 1.28 153.1 [155.4]

Respiratory costs Microalgae Bacteria Herbivores Carnivores Sediments

1.0 [0.0] 47.9 [222.3] 2.3 [3.3] 5.3 [40.1] 1.2 [1.7]

67.6 [26.7] 226.0 [167.1] 78.0 [16.7] 21.1 [18.4] 38.1 [1.7]

55.5 [158.8] 282.7 [282.5] 84.1 [31.7] 30.7 [31.7] 48.7 [1.7]

The results of the model in slope waters are enclosed by the bracket symbols for each month.

end of May 2002 (Fig. 9D). Consequently, the simulated primary production over this region of the Chukchi/Beaufort shelves was less than 200 mg C m2 day1 by the end of both May (Fig. 10A) and July (Fig. 10B) 2002. In contrast, farther south and west at the same depth of 30 m within the central Chukchi Sea at
701N, 167.51W, the observed nitrate stocks were depleted by
15 mmol NO3 kg1 between May and August 2002 (Figs. 9A and B). Over a shorter travel distance of
500 km, between the western side of Bering Strait and the central Chukchi Sea, the same simulated speed of
5 km day1 suggested that a water parcel sampled here at the end of July would have exited Bering Strait by mid-April. Resulting ice cover was less (Figs. 6 and 7) and incident radiation was greater than two months earlier. Here in the model, consequently the same amount of
15 mmol NO3 kg1 of the model’s near surface nitrate pool

was removed (Figs. 9E and F), when only 83% of the shelf phytoplankton community was lightlimited during July–August 2002 (Table 4). A Redfield C/N molar utilization ratio of 6.7 for balanced phytoplankton growth implied that the near-surface depletion of inorganic carbon also should have been at least
100 mmol DIC kg1 during this time interval in 2002, assuming little regeneration of CO2. The observed DIC stocks indeed exhibited a net removal of
100 mmol DIC kg1 during this period (Figs. 11A and B). Above the 50-m isobath in the south central Chukchi Sea, the same depletion of
100 mmol DIC kg1 was simulated at a depth of 2.5 m between 31 May and 31 July 2002 (Figs. 11C and D). There, the net carbon fixation of the model was
3 mmol C l1 day1 (
36 mg C m3 day1) by the end of July (Fig. 10B). When averaged over all of the Chukchi/Beaufort shelves of o220 m depth during

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April–May 2002, however, the model results suggested that these regions were just a weak CO2 source to the atmosphere in spring of 7.4 mg C m2 day1 (Table 4). Only the southern Chukchi Sea and the western Beaufort Sea had partial pressures of CO2 within the model’s surface sea water of o365 matm, i.e. that of the atmospheric value (Table 2), by 31 May 2002 (Fig. 10E). Consequently, over most of the shelves, carbon dioxide outgassed to the model’s atmosphere. At shallow bottom depths on the shelf, growth of the model’s phytoplankton at a depth of 2.5 m within the southeastern Chukchi Sea stripped the initial surface nitrate stocks of
10 mmol NO3 kg1 of AW in May–June 2002 (Fig. 9E) to o0.1 mmol NO3 kg1 by July–August (Fig. 9F). Such a seasonal removal of new nutrients was similar to that within the upper 10 m of the water column off Point Barrow, between April and September 1971–1973 (Horner, 1972; Alexander et al., 1974). Diatoms were the spring dominants of the phytoplankton community in 2002 (Fig. 12A), contributing as much as 80% of the observed total carbon biomass of microalgae within surface waters. Consequently, silica depletion was found in the euphotic zone between May (Fig. 13A) and August (Fig. 13B) 2002 as well. Diatoms were then the model’s dominant functional group in surface waters (Fig. 12B) as well. When they were abundant, the simulated N/SiO uptake processes proceeded at a N/Si molar removal ratio (Table 2) of 1/1 for diatoms [Brzezinski, 1985]. Not surprisingly, the model’s silicate pool at
72.51N, 162.51W was thus also stripped of 10 mmol SiO4 kg1 by 31 July 2002 at a depth of 2.5 m (Fig. 13D). 3.3. Phytoplankton fields

Fig. 10. The computed depth integrals of net primary production (mg C m2 day1) on (A) 31 May, (B) 31 July, and (C) 31 October 2002.

In previous models when diatom populations were the dominant phytoplankton of spring blooms, the observed PN/chl ratio (mmol/mg) was 0.6 (Walsh et al., 1978). Thus, a total phytoplankton standing stock of at least 10 mmol PN kg1, inferred from both the observed and simulated silicate and nitrate depletions within the euphotic zone, should have amounted to an equivalent amount of 16.6 mg chl l1 within the upper
30 m by 21 August 2002. In the eastern Chukchi Sea during July–August 2002, however, only 2 mg chl l1 were found within nearsurface waters (Fig. 14B). Instead,
10 mg chl l1 was present at a depth of 30 m (Fig. 14D), suggesting the export of phytoplankton to at least

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Fig. 11. The observed 2002 near-surface fields of DIC (mmol DIC kg1) during (A) 6 May–12 June and (B) 18 July–21 August in relation to the computed (C, D) stocks of DIC and (E, F) partial pressures (matm) of carbon dioxide (pCO2) within surface sea water on 31 May and 31 July.

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Fig. 12. The (A) observed 2002 near-surface diatom contribution (percent of total algal carbon biomass) to phytoplankton species composition during 8 May–12 June in relation to the computed dominance of phytoplankton functional groups of diatoms, prymnesiophytes, and microflagellates, at depths of 2.5 and 30 m on (B, C) 31 May and (D, E) 31 July.

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Fig. 13. Observed 2002 fields of near-surface silicate (mmol SiO4 kg1) during (A) 8 May–12 June and (B) 18 July–21 August in relation to the computed fields of silicate at a depth of 2.5 m on (C, D) 31 May and 31 July.

the bottom of the euphotic zone. Furthermore, an observed depth integral of 410 mg chl m2 over the upper cm of surface sediments in July–August (Fig. 15A), or an equivalent stock of 4100 mg chl l1, suggests that most of the expected summer chlorophyll biomass survived descent to the seabottom during 2002. It is notable that just north of Bering Strait on 31 July 2002, the simulated nearbottom phytoplankton stocks at a depth of
30 m were a maximum of 134 mg chl l1 (Fig. 15D), approximating the underlying amount of chlorophyll measured in surficial sediments. As a consequence of the model’s phytoplankton growth processes of photosynthesis and protein formation and their immediate daily loss processes via (1) respiration, (2) excretion, (3) grazing, and (4)

settling, the net accumulations of mid-summer microalgal biomass left throughout the water column were computed as chlorophyll stocks of three functional groups of diatoms, microflagellates, and colonial prymnesiophytes over time and space. Their sums within near-surface waters (Fig. 14F), at the bottom of the euphotic zone (Fig. 14H, and in the sediments (Fig. 15D) then replicated the sparse field observations of algal stocks measured during July–August 2002 (Figs. 14B, D and 15B). Some of the higher expected chlorophyll stocks of 16.6 mg chl l1, derived from the above nutrient budgets and inferred from satellite imagery (Wang et al., 2005), were indeed simulated in near-surface waters of the northwestern Chukchi Sea (Fig. 14F), in a region where the Healy and Polar Star did not

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sample (Fig. 14B). When the model’s predictions and field validation data were coincident at a depth of 25 m in the south central Chukchi Sea, moreover, both suggested microalgal stocks of 410 mg chl l1 (Figs. 14D and H), comparable to those of satellite estimates (Wang et al., 2005). We concluded that the growth and combined loss processes of the coupled model were reasonable estimates of physical and ecological processes, allowing further dissection of the separate sinks of phytoplankton production. Thus far, the replicated DIC fields also indicated that phytoplankton respiration losses, along with those of the bacteria and zooplankton, were correctly estimated by the model.

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4. Dissolved organic matter fields In evaluation of the model’s five phytoplankton loss processes (respiration, excretion, grazing, settling, and decomposition), the observed estimate of particle efflux at a depth of 50 m (Fig. 16B) also was mimicked correctly by the model’s combined simulated fall out of phytodetritus and zooplankton fecal pellets of 440 mmol C m2 day1 at a depth of 50 m on 31 July 2002 (Fig. 16C). Phytoplankton excretion—the second loss process in our study, was a source of DOC in the model. DOC was considered available from inefficient grazing by pelagic herbivores, particle utilization and

Fig. 14. The observed 2002 total phytoplankton biomass (mg chl l1) at depths of (A, B) near-surface and (C, D) 30 m during 8 May–12 June and 18 July–21 August in relation to the computed fields of pigments (mg chl l1) at depths of (E, F) 2.5 m and (G, H) 30 m on 31 May and 31 July.

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Fig. 14. (Continued)

decomposition in the sediments. Bacterial utilization was a sink of DOC (see Appendix A). DOC was released in the model (Figs. 17C and D) from inefficient grazing by copepods and protozoans, as well as from decomposition of phytodetritus and fecal pellets within surficial sediments; both sources also served as indirect assessments of the third phytoplankton loss process of grazing. We noted particular fidelity of predicted and observed stocks of 80 mm DOC kg1 at a depth of 30 m near the end of July 2002, within the northeastern Chukchi Sea (Figs. 17B and D). To estimate the grazing loss, we must first consider the longer time scale of near-bottom remineralization of particulate detritus.

4.1. Recycled nitrogen fields Upon decomposition of settling PON/POC influxes within the sediment layer, both ammonium and DOC were released to the model’s overlying water column. The simulated summer stocks of recycled nitrogen at a depth of 30 m (Fig. 18D) within the northeastern Chukchi Sea were similar to those of 2.0 mmol NH4 kg1, observed at the end of July 2002 (Fig. 18B). Furthermore, the larger simulated values of 5.6 mmol NH4 kg1 along the Siberian coast (Fig. 18D) were now only
2-fold greater than those of maxima there in the 1989 case of our previous model, when the molecular diffusion of ammonium within surficial sediments was much less

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Fig. 15. The observed 2002 stocks of micro algal biomass (mg chl m2) within the upper cm of surficial sediments during (A) 8 May–12 June and (B) 18 July–21 August in relation to mean computed fields of bottom debris (mmol C kg1) of phytodetritus and fecal pellets of copepod, protozoan, and carnivorous origin on (C) 31 May and (D) 31 July.

(Walsh et al., 2004). Recall that in this present formulation, the benthos was more active, recycling detritus as a function of the phytoplankton and fecal pellets in the near-bottom water column. A much larger interstitial diffusion rate was imposed as well. It would appear that fall out of plankton debris from the euphotic zone may set a limit on release of recycled nutrients into the near-bottom water column, rather than the physical and biochemical activities of the underlying sediment biota. Given daily estimates of respiration, excretion, settling, and decomposition rates of the phytoplankton, we then obtained their grazing losses from seasonal budgets that integrated over possible time lags of primary and secondary production.

4.2. Seasonal carbon budgets Based on the fidelity of the model results with validation data collected in 2002, we developed seasonal carbon budgets to assess the relative importance of both unknown spring/fall events of carbon fixation and their sequestration. We then compared them to the more well-studied summer ones (Table 4). For example,
8.5% of the negligible mean spring gross production of
10.8 mg C m2 day1 (Fig. 19A) within the light-limited water column was recycled via phytoplankton respiration to DIC. The net production on the shelf was then
9.9 mg C m2 day1, compared to 0.7 mg C m2 day1 on the upper slope (Table 4). This provided an

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Fig. 16. The observed 2002 effluxes of plankton (mmol POC m2 day1) at a depth of
50 m during (A) 17 May–2 June and (B) 21 July–16 August in relation to the (C, D) computed effluxes of phytodetritus and fecal pellets (mmol POC m2 day1) at the end of May and July 2002.

assessment of the first loss process, such that we could also estimate the food available to the rest of the marine food web, and eventually the possible yield to humans. During April–May 2002,
41% of the daily net carbon production of the model was consumed by herbivores (Fig. 19A). Another 5.8% of the particulate increment was lost to DOC. We estimate that 34.5% of net photosynthesis was then deposited in the sediments, amounting to a phytodetrital influx of 3.7 mg C m2 day1, exclusive of the fecal pellet influx of 7.8 mg C m2 day1. Despite the reduced grazing stress of this version of the model compared to our previous formulation (Walsh et al., 2004), a settling flux derived from fecal pellets of

herbivore and carnivore origin was twice that of phytodetritus. With respect to an annual net photosynthesis of 97.4 g C m2 yr1 (Table 4), the spring primary production of particulate matter of 0.6 g C m2 was only 0.6% of the annual potential yield to higher trophic levels on the Chukchi/ Beaufort shelves. About 41% of the phytoplankton net growth was passed up the food web through the pelagic herbivores (Fig. 19A). Because little net storage of spring POC effluxes occurred within the surficial sediments of the model,
87% of the POC supply was released as DOC, providing 10.0 mg DOC m2 day1 to the water column (Fig. 19A). This amounted to
42% of the pelagic bacteria’s supply of substrate, with the rest

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Fig. 17. The observed 2002 stocks of dissolved organic carbon (mmol DOC kg1) at a depth of
30 m during (A) 8 May–12 June and (B) 18 July–21 August in relation to the mean computed fields of the sum of monomeric and macromolecular DOC at the same depth on (C) 31 May and (D) 31 July.

supplied from water-column sources of DOC. Because the bacterial demand for DOC was larger than the daily supply, DOC stocks then declined and the bacteria became carbon-limited. Instead, the DOC stocks increased in the water column during summer (Fig. 19B) and fall (Fig. 19C). Furthermore, since all DOC sources (sediments, phytoplankton excretion, and sloppy feeding of pelagic herbivores and carnivores) were a small amount of 24.1 mg C m2 day1 during spring (Fig. 19A), compared to 466.7 mg C m2 day1 in summer (Fig. 19B) and 592.8 mg C m2 day1 in fall (Fig. 19C), ammonium did not initially accumulate in the model’s spring euphotic zone. Thus, little

recycled nitrogen was available to phytoplankton, such that the f-ratio (Eppley and Peterson, 1980) of the simulated primary production, i.e. the ratio of nitrate-based production to total production derived from assimilation of nitrate and recycled nitrogen, was highest at 0.98 during April–May 2002 (Table 4). By the end of July, most of the nitrate at a depth of
30 m in the southeastern Chukchi Sea had been stripped in the model, but ample supplies of new nitrogen were left behind in the northeastern portion, where the ice cover had now sufficiently receded (Fig. 7) to alleviate light limitation there. During July–August, 83% of the phytoplankton

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Fig. 18. The observed 2002 fields of ammonium (mmol NH4 kg1) at a depth of
30 m during (A) 8 May–12 June and (B) 18 July–21 August in relation to the computed fields of ammonium at the same depth on (C) 31 May and (D) 31 July.

populations on the shelf were light-limited, compared to 98% in spring and 66% in fall (Table 4). Accordingly, over all of the Beaufort/Chukchi shelves, the mean net photosynthesis was a seasonal maximum of 709.1 mg C m2 day1 during July– August 2002 (Table 4), amounting to
45% of the annual primary production. This lead to 20% of the phytoplankton net growth passed up the food web through the pelagic herbivores (Fig. 19B). The shelves were now an intermediate sink for atmospheric CO2 of 173.8 mg C m2 day1 (Fig. 19B; Table 4), despite the larger amounts of algal, bacterial, metazoan, and benthic respiration. The concomitant return of regenerated nitrogen to the water column and ensuing phytoplankton utiliza-

tion during summer photosynthesis yielded an intermediate f-ratio of 0.64 during July–August (Table 4). In this seasonal case, less new production was associated with a smaller yield to the rest of the food web, compared to the spring trophodynamics in the water column, such that carbon sequestration was in the form of more sediment debris—not increments of potential fish yield. Unlike the seasonal resupply of
14 mmol NO3 kg1 to the upper water column of the shelf, after fall overturn in the upstream Southeast Bering Sea (Fig. 5) the observed nitrate stocks within surface waters on the eastern Chukchi Sea near 157.91W were o0.5 mmol NO3 kg1 in September (Murata and Takizawa, 2003). Similarly, within the

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eastern Beaufort Sea, the measured shelf stocks of new nitrogen remained o0.5 mmol NO3 kg1 in October near 1501W (Aagaard et al., 1988). By November, these observed stocks remained only
1.2 mmol NO3 kg1, with a continued overwinter accumulation from nitrification and advective supplies yielding an April stock of
4.8 mmol NO3 kg1 (Fig. 4) during 1982 at the 10-m isobath, near 1501W (Schell et al., 1982). Such field estimates of a nitrification rate of 0.05–0.15 mmol NO3 kg1 day1 in the Beaufort Sea (Schell et al., 1982) were thus maximal ones, since they included the advective resupply of nitrate as well. During August 1972, smaller ammonium oxidation rates of 0.02–0.05 mmol NO2+NO3 kg1 day1 were indeed observed within the oxygenated, upper 25 m of the isolated Skan Bay, adjacent to the SBS (Hattori et al., 1978). The model’s nitrification rate, X1, in the water column was thus expressed by
 NH4 X 1 ¼ 0:04 , kNIT þ NH4

Fig. 19. Daily mean (A) spring (April–May), (B) summer (July–August), and (C) fall (September–October) carbon budgets (mg C m2 day1) for the Chukchi/Beaufort shelves of o220 m depth. The numbers in brackets [ ] are the air–sea fluxes, while the herbivore- and carnivore-regulated fluxes between the model pools of the sum of functional groups of microalgae, bacteria, water column detritus, DOC, DIC, and sediment debris are denoted, respectively, by H and C.

where 0.04 mmol N kg1 day1 was the maximum rate (Walsh and Dieterle, 1994). Despite this slow daily rate of nitrification, it was larger over the water column than the simulated advective resupply of ‘‘new nitrogen’’ on the Chukchi/Beaufort shelves. We found a mean ammonium oxidation rate of 0.88 mmol N m2 day1 (Table 4) compared to a nitrate transport of 0.65 mmol NO3 m2 day1 in April–May and 1.28 NO3 m2 day1 in July–August 2002. Consequently, there were few new nutrients left by summer’s end seen in actual observations , or our model, to fuel fall primary production (Fig. 10C) under seasonal adverse light conditions of extensive ice cover (Fig. 7). Yet, 2002 was a year of unusually large open water conditions on the Beaufort shelf, such that the mean simulated daily carbon fixation was then 578.3 mg C m2 day1 in September– October (Fig. 19C), or 36.2% of the annual amount of photosynthesis (Table 4). Then, 28.0% of the phytoplankton net growth, with an f-ratio of 0.44 (Table 4), was passed up the shelf food web through the pelagic herbivores (Fig. 19C). In slope waters, an equivalent simulated amount of 454.6 mg C m2 day1 was computed within
75 km of the shelfbreak, where 29% of the phytoplankton populations were not light-limited (Table 4). There, the model’s estimates were 10-fold more than the maximum fluxes of 26.8 mg C m2 day1, observed with duel labels of 13C and 15N during

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August–September 2002. However, these incubations were made within ice-covered and nutrientpoor (
0.02 mmol NO3 l1) surface waters of the Canada Basin (Lee and Whitledge, 2005). Future retreat of ice cover, without eutrophication, may thus have little impact on carbon sequestration within these high-latitude ecosystems.

5. Discussion Our numerical analysis of carbon/nitrogen/silicon cycling by planktonic and benthic components of present light-limited and/or nutrient-limited regimes of western Arctic shelf/basin ecosystems may provide insight into possible consequences of future global climatic changes at these high latitudes. Within 75 km of the shelf-break, as much as 29% of the model’s slope phytoplankton populations were already nutrient-limited during September– October 2002 (Table 4). During the preceding April–May 2002, 98% of the phytoplankton were light-limited. Seasonal penetration of relatively saline, nutrientrich AW into the eastern Chukchi Sea, as shown by the 2002 time series of observed and simulated salinity fields, provided a natural fertilization experiment of more oligotrophic Arctic waters, typified by those of the Canadian Basin and ACW in the Bering and Beaufort Seas. We were able to replicate the sparse observations of tempo-spatial fields of spring and fall nitrate, ammonium, silicate, DIC, total chlorophyll, and total DOC stocks, as well as estimates of particle efflux at a depth of 50 m. Given that we do not really know, from either the SBI observations or from moored nutrient sensors, when nutrient depletion and carbon fixation started and stopped within the Chukchi/Beaufort Seas during 2002, one must question how realistic are the model budgets. Furthermore, another highlatitude ecosystem of similar March initial conditions of
15 mmol NO3 kg1 was sampled every 2–3 h, with no spring record gaps (Fig. 5) during March–May of 2000, 2001, and 2002 (Fig. 20). This time series above the 250-m isobath of the upstream Gulf of Alaska (591250 N, 149130 W) portrayed rapid episodes of spring nutrient removal at a depth of
11 m that would probably be missed by conventional shipboard sampling programs. How significant was our under sampling during the 2002 field programs of the three ice breakers?

Fig. 20. Nitrate time series, from moored near-surface sensors at a depth of
11 m on the 250-m isobath, within deeper regions of the upstream Gulf of Alaska (59 1250 N, 1491030 W) during 2000 (&), 2001 (&), and 2002 (&).

Within some coastal ecosystems adjacent to marine laboratories, however, such as either Auke Bay with the facilities of NOAA/NMSF and UAF Center for Fisheries and Ocean Sciences or Point Barrow with the Naval Arctic Research Laboratory, daily sampling rates were obtained to capture some features of high-frequency variability of Arctic plankton dynamics. Prior to initiation of the APPRISE daily time series during 1985–1989 (Fig. 21), for example, a composite of the seasonal stocks of nitrate within the euphotic zone above the 40-m isobath of Auke Bay, Alaska (581220 N, 1341400 W) during 1963–1967 had been obtained (Bruce et al., 1977). Nutrient cycling was reevaluated from August 1976 to August 1977 (Coyle and Shirley, 1990), with respect to the subsequent 43 h depth integrals of net photosynthesis measured there by radiocarbon methodology (Williamson, 1978). These authors obtained an annual estimate of net photosynthesis of
400 g C m2 yr1. At a similar latitude, however, less frequent 14C measurements led to an estimate of only
165 g C m2 yr1 for the Southeastern Bering Sea during 1979–1981 (Walsh and McRoy, 1986), compared to
97 g C m2 yr1 farther north within the Chukchi/Beaufort Seas (Table 4). Yet, another composite of the seasonal nutrient time cycle within the euphotic zone (Whitledge et al., 1986) above the deeper 70-m isobath (571N, 1651W) of the Bering Sea was similar to that for Auke Bay (Fig. 21C)—at least, in terms of nitrate. Both shipboard nutrient time series in Auke Bay and the southern Bering Sea began with well-mixed water columns and March nitrate stocks of 415 mmol NO3 kg1, before phytoplankton utilization stripped them to o1 mmol NO3 kg1 within the May euphotic zones. Since the minimal seasonal temperature of surface waters in Auke Bay was

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Fig. 21. A composite of (A) the seasonal stocks of nitrate within the euphotic zone on the 40-m isobath, within shallower regions of the upstream Auke Bay, Alaska (581220 N, 1341400 W) during 1963–1967, in relation to the (B) daily depth integral of net photosynthesis during 1976–1977 (after Coyle and Shirley, 1990) and (C) the spring stocks of nitrate at a depth of 10 m during 1985–1989.

3–4 1C in March, unlike o0 1C for the same month in the Southeastern Bering Sea (Coachman, 1986), ice cover was not a major factor in the former system for light reduction, compared to those of low sun angle, cloudiness, and amount of incident radiation (Ziemann et al., 1990a). Thus, alleviation of light limitation was not a likely factor in promoting the greater total annual carbon fixation within Auke Bay. Once the ambient light regime was favorable for onset of photosynthesis, however, rapid depletion of the spring stocks of nitrate occurred in Auke Bay at a depth of 10 m during March–June 1985–1989 (Ziemann et al., 1990a). Concurrent 14C incubations of the ambient phytoplankton community suggested that the mean ‘‘new’’ spring (April–June) production of the
10m euphotic zone was 125 g C m2 (Ziemann et al., 1990b). It might have accounted for at most
30% of the annual production, since spring ammonium stocks of 1–5 mmol NH4 kg1 also were found at a depth of 10 m (Ziemann et al., 1990a). If no other nitrate stocks were utilized during the rest of the year in this shallow shelf ecosystem, the annual f-ratio would have instead been 0.30—a typical value of most shelves (Walsh, 1991). Other likely candidates for recycled nitrogen supplies in Auke Bay were, of course, urea (Demanche, 1974) and

free amino acids (Schell, 1974) of the DON pool. Summer storms (Iverson et al., 1974) provided episodic intervals of nitrate injection from the aphotic zone stocks of Auke Bay (Fig. 21), as also inferred from the continuous records of such daily increments at similar latitude in deeper regions of the Gulf of Alaska (Fig. 20). Near-bottom stocks would have been injected within the euphotic zone as well. In contrast, once seasonal stratification ensued within the Southeast Bering Sea, the near-bottom summer ammonium stocks of the aphotic zone were instead much larger than
1 mmol NH4 kg1 found within the overlying water column (Whitledge et al., 1986). June accumulations of recycled nitrogen there were as much as 13 mmol NH4 kg1 at the 75-m isobath, about twice those simulated within AW along the Siberian coast in 2002 (Fig. 18D). One must question if the shallow regions of ACW in the Chukchi and Beaufort Seas are more like Auke Bay, the southeast Bering Sea, or even less productive than the overall shelf mean of
97 g C m2 yr1 (Table 4). Adopting the same approach here as Coyle and Shirley (1990), we used the daily duration of daylight hours for Point Barrow, Alaska and 66 estimates of 14C -assessed photosynthesis made

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Fig. 22. The near-surface seasonal stocks of (A) nitrate and (B) chlorophyll within the euphotic zone on the 6-m isobath within shallower regions of the downstream Point Barrow, Alaska within the western Beaufort Sea (711200 N, 1561410 W) during 1971 (J), 1972 (n), and 1973 (K).

there during 1971–1973 (Horner, 1972; Alexander et al., 1974), to obtain an April–June total production of 4.8 g C m2. When the nitrate stocks were depleted (Fig. 22), however, spring ammonium stocks of 2–6 mmol NH4 kg1 also were found here (Horner, 1972). A ‘‘new’’ nitrate-based production of at most
32% of the annual carbon fixation of 15.3 g C m2 yr1 was thus estimated by us for these shallow waters of
6 m depth in the western Beaufort Sea. Farther east in the Beaufort Sea, other 14C data, euphotic zone depths, nutrient budgets, and observations of ice retreat all suggested that the total annual primary production also may be of same order of 10–40 g C m2 yr1 (Schell et al., 1982). We thus concluded that our simulated estimate of annual net carbon fixation over all of the Chukchi/Beaufort shelves of
97 g C m2 yr1 did not suffer from undersampling—either at the beginning of the April–May spring bloom, which only accounted for 0.2% of annual primary production, or after the fall bloom during September–October, when 36.2% occurred (Table 4). Present and past estimates of the Beaufort shelf primary production were
10% of those at lower latitudes within the Bering Sea and Auke Bay (Walsh and McRoy, 1986; Coyle and Shirley, 1990), however. Part of the low Arctic carbon fixation on the shelves east of Cape Barrow resulted from light limitation, of course, but the winter initial conditions of these waters were surprisingly oligotrophic (Fig. 3). This shelf ecosystem is evidently dependent upon advection of nutrient supplies from
900 km away on the western side of Bering Strait (Fig. 1) to at least 1531W on the Beaufort shelf (Aagaard et al., 1988). Like waters of the Beaufort slope ecosystem (Lee and Whitledge, 2005), those of the shelf food web may not benefit from increased light availability in the future from ice retreat unless nutrients

are added as well. In future scenarios, we will thus explore the importance of nutrient loadings from the East Siberian Sea (Codispoti and Richards, 1968), for comparison with those of Bering Strait (Barnes and Thompson, 1938).

Acknowledgments This analysis was funded by Grants OPP-0124864 to JJW, OPP-0124943 to WM, OPP-0124917 to SBM, and OPP-0125049 to GC, from the National Science Foundation as part of the joint SBI project ‘‘Collaborative research: carbon cycling in the Chukchi and Beaufort Seas—field and modeling studies’’. Additional support was made to TEW and DS from the National Oceanic and Atmospheric Administration as part of the ‘‘Southeast Bering Sea carrying capacity (SEBSCC)’’ project, with award # NA67RJ0147 from the North Pacific Research Board. Support for analysis of the satellite imagery was supplied to GC from NASA NAG5-10528. Other services and funds were made available to us from the United States Coast Guard and the Office of Naval Research. We also thank our present colleagues of the SBI program, whose many hours at sea provided some of the data analyzed in this study and served as the basis for other contributions to this special issue. Similarly, past colleagues: Vera Alexander, Lawrence Coachman, John Goering, Donald Hood, Rita Horner, and Donald Schell generated both field data and hypotheses over 40 years of historical perspective, providing an important background for our present observations and interpretations. In particular, we dedicate this paper to the late Glenn Cota. Finally, JJW thanks the University of South Florida for a sabbatical leave to undertake such a synthesis.

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Appendix A A.1. State equations of the ecological model The governing equations of the circulation model have been published (Maslowski and Lipscomb, 2003; Maslowski et al., 2004). Similarly, the ecological model given here is in most respects the same as published in Walsh et al.(2004). The current version included a surface mixed layer, with some modifications to the sediment layer formulation and the zooplankton grazing rate. For the set of ice parameters in this and the previous version, algal growth was found to be negligible below the ice cover. It was thus set to zero in the current version, when ice was present. The coupled model was composed of an upper water column module and a benthic one. The water column module had 39 layers of variable thickness, with a maximum bottom depth of 4500 m, while the benthic module consisted of a well-mixed 1-cm thick sediment layer. The model now had 18 explicit biochemical state variables: nitrate, ammonium, silicate, DIC, diatoms, microflagellates, Phaeocystis, copepod and protozoan fecal pellets, grazer stocks of copepods and protists, Gc and Gm, macromolecular and monomeric DOC2,1, bacterioplankton, detrital sediment carbon of non-siliceous and siliceous origin, Sp and Sps, and interstitial DIC and silicate. Within the water column, the tempo-spatial distributions of nutrients and phytoplankton were constrained by either the computed u, v, w components of flow, or the mean layer flow, ud, vd, in the case of migratory herbivores. Some estimates of Kh for vertical mixing, of ice thickness, and of the blue and red light forms of incident radiation also were imposed. The processes affecting their change over time, t, and space, x, y, z, in the water column were described by the following equations, where the parameter values are given in Table 2: qPd q ¼ TrðPd Þ  ðwd Pd Þ qz qt þ ðgd  ed  cd  Zd ÞPd  gd Gc ,

ð1Þ

qPf q ¼ TrðPf Þ  ðwf Pf Þ qz qt þ ðgf  ef  cf  Zf ÞPf  gf G m ,

ð2Þ

qPp q ¼ TrðPp Þ  ðwp Pp Þ qz qt þ ðgp  ep  cp  Zp ÞPp  gp Gm ,

ð3Þ

3569

qB ðC=NÞb ¼ TrðBÞ þ ðg þ g4 ÞB  ðb þ mÞB, qt ðC=NÞr 3

(4)

X qDOC1 ¼ TrðDOC1 Þ þ c i  g3 B þ X 2 , qt i

(5)

qDOC2 ¼ TrðDOC2 Þ þ Zc Z c þ Zm Z m þ fc gd G c qt þ fm ðgp þ gf ÞG m þ fb mB  g4 B  X 2 , ð6Þ qZ c q ¼ TrðZ c Þ  ðwzc Z c Þ qz qt þ ð1  ac  fc Þgd G c  Zc Z c ,

ð7Þ

qZ m q ¼ TrðZ m Þ  ðwzm Z m Þ qz qt þ ð1  am  fm Þðgp þ gf ÞG m þ ð1  ab  fb ÞmB  Zm Z m , qNH4 ¼ TrðNH4 Þ þ ðN=CÞr qt "  ac gd G c þ am ðgp þ gf ÞG m 

ð8Þ

X

# giNH4 Pi

i

þ ðN=CÞb ½b þ ab m þ ½ðN=CÞb  ðN=CÞr  ½ð1  ab  fb Þ þ fb mB  X 1 , X qNO3 ¼ TrðNO3 Þ  ðN=CÞr giNO3 Pi þ X 1 , qt i qSiO4 ¼ TrðSiO4 Þ þ ðSi=CÞr qt ½ðac þ fc Þgd Gc þ ðZd  gd ÞPd þ Zc Zc ,

ð9Þ (10)

ð11Þ

X qDIC ¼ TrðDICÞ þ ðei  gi ÞPi þ ac gd G c qt i þ am ðgp þ gf ÞGm þ Bðb þ ab mÞ þ Bðg3 þ g4 Þð1  ½ðC=NÞb =ðC=NÞr Þ,

ð12Þ

qG c qud G c qvd G c ¼  , qt qx qy

(13)

qG m qud G m qvd G m ¼  , qt qx qy

(14a)

where Pd, Pf, Pp are diatoms, microflagellates, and Phaeocystis, respectively, B is bacteria, DOC1, DOC2 are monomer and macromolecular DOC, Zc, Zm siliceous and non-siliceous fecal pellets, NH4, NO3, SIO4 are ammonium, nitrate, and silicate, respectively,

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DIC is dissolved inorganic carbon, Gc, Gm are copepods and protozoans, respectively. The ‘‘Tr(..)’’ terms represented physical advective and diffusive transport:   q q q TrðAÞ ¼  ðuAÞ þ ðvAÞ þ ðwAÞ qx qy qz
 q qA þ Kh , ð14bÞ qz qz where the last term was the vertical diffusive transport and A was any of the above state variables, except Gc and Gm. A no-flux condition was used along solid coastal boundaries—the horizontal velocities u, v, and mean layer flows ud, vd there were zero. The vertical velocity, w, was set to zero at the bottom of the water column. Advection through the air–sea interface was also zero. Because of the strong implicit horizontal diffusion in the numerical algorithm for advective transport, explicit horizontal turbulent mixing was ignored. The coupled model did consider the vertical component of mixing, affected by wind, water motion, and organisms. For example, the vertical eddy diffusivity, Kh, was set to a background value of 1 cm2 s1, except within open water of ice concentrations o10%, where it was 10 cm2 s1 over the top 20 m of the water column to emulate the development of a surface mixed layer. The settling velocities for diatoms and Phaeocystis, wd and wp, were functions of their respective biomass, whereas wf for microflagellates and wzc, wzm, for copepod and protozoan fecal pellets were constants (Table 2). Subscript ‘‘r’’ was the Redfield ratio in all organic matter, except bacteria, denoted by subscript ‘‘b’’ (Table 2). Such parameter values determined, of course, the outcome of competition in this second Arctic adaptation of our previous model of similar Antarctic phytoplankton communities (Walsh et al., 2001). We selected values, based upon a prior sensitivity analysis of over 100 simulation cases of the one-dimensional form of our ‘‘polar’’ model. Drawing upon extensive field observations of similar genera in the Southern Ocean, for example, we started with an observed light-saturation intensity, Is (Table 2), of 25 W PAR m2 (
110 mE m2 s1) for near-ice stocks of Phaeocystis in the Ross Sea (Palmisano et al., 1986). Here they out competed diatoms within deep surface mixed layers of the water column (Arrigo et al., 2003). Furthermore,

their cultures yielded maximum net growth rates of
0.2 day1 at 0.0 1C (Verity et al., 1991), as much as 40% excretion rates (Reid et al., 1990), and similar shade-adapted Is (Matrai et al., 1995). Since sun-adapted diatom communities, with higher Is, were instead typical of shallow polar surface mixed layers of 20–40 m thickness in the Arctic (Platt et al., 1982), we instead used 45 W PAR m2 for this group—as found for the total phytoplankton community of the Barents Sea (Rey, 1991) and employed in our prior models of the Bering/Chukchi Seas (Walsh and Dieterle, 1994; Walsh et al., 1997). When Cryptomonas was the dominant phytoplankton in the Bransfield Strait, however, an intermediate Is of
35 W m2 was observed (Figueiras et al., 1994), compared to higher values of Is assumed for the model’s diatoms and lower ones for Phaeocystis (Table 2). We realized that all polar phytoplankton were capable of light-adaptation, with a wide range in realized Is, e.g., much higher values of
53–66 W m2 have been found for other culture studies (Verity et al., 1991) and field populations (Cota et al., 1994) of Phaeocystis. Indeed, similar net realized growth rates of both diatoms and Phaeocystis in the Barents Sea (Vernet et al., 1998) suggested that their different loss rates would instead determine the outcome of their competition in the ‘real world’ as well as in our model. Most diatoms have relatively low excretion rates of
4% (Reid et al., 1990), but their respiration rates may be as much as
19% of the daily gross production at
1.5 1C in mesocosms (Keller and Riebesell, 1989). We assumed a conservative respiration rate of 10% for diatoms, and lower values of respiration for the more shadeadapted flagellates and prymnesiophytes in this analysis (Table 2). The algal gross growth terms of Eqs. (1)–(3) and (9)–(12), were given by 
  Lðt; zÞ Lðt; zÞ gi ¼ min ci exp 1  ; gi;n ; gi;SiO4 , Lis Lis (15) "

gi;N

# NO3 NH4 ¼ ci max ; , kNO3 þ NO3 kNH4 þ NH4

gi;SiO4 ¼ ci

SiO4 , kSiO4 þ SiO4

(16)

(17)

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where subscript ‘‘i’’ was d, f, or p for diatoms, microflagellates, and Phaeocystis, respectively. The maximum growth rates of each functional group, ci, were functions of temperature (Eppley, 1972). The half-saturation constants, kiNO3 , kiNH4 , and kiSiO4 , for the three groups are given in Table 2, as is the Si/C ratio of diatoms (Brzezinski, 1985). Ice may overly only a fraction of the 9-km wide model grid cells. Thus, the light-regulated component of phytoplankton growth within any model grid cell was calculated as the weighted averaged of their growth in open water, multiplied by the percentage of open water in the grid cell, and their growth under ice, multiplied by the percentage of ice cover. In open water, the fraction of surface PAR penetrating to depth, z, was described (Fasham et al., 1983; Taylor et al., 1991) by   I w ðzÞ ¼ Rb eðkb ðwÞþkm Þz þ ð1  Rb Þekr ðwÞz , (18) where Rb was the fraction of surface light in the blue-green wavelengths. The attenuation coefficients km and kc were computed as Z ky z km ¼ ðDOC1 þ DOC2 Þ dz, (19) z 0 Z 1 z kc ¼ ðkd Pd þ kf Pf þ kp Pp Þ dz (20) z 0 with the values of the other attenuation coefficients, and Rb, given in Table 2. Ice was assumed to be capped by a layer of snow, with a layer of ice algae at its undersurface (Smith, 1995). The fraction of light penetrating to the water interface was given by  I w ðzÞ ¼ Rb e½kb ðsÞzðsÞþkb ðiÞzðiÞ  ð21Þ þð1  Rb Þe½kr ðsÞzðsÞþkr ðiÞzðiÞ ekðaÞzðaÞ ; where the attenuation coefficient for ice algae, k (a), was 0.38 m1 and z (a) ¼ 1 cm (Smith, 1995). The attenuation coefficients for blue-green and red light in ice and snow were 2.15 and 4.35 m1, respectively (Wadhams, 2000). Snow and ice thicknesses were set at 2.0 and 0.1 m, respectively. Given this set of snow/ice parameters, only 0.2% of surface PAR passed through to the surface of the model’s water column, when covered by ice, such that net photosynthesis was ignored under these conditions. The monthly average PAR, Ip, was first prescribed as a function of latitude and longitude, using 50% of the NCEP reanalysis (Kanamitsu et al., 2002)

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monthly mean downward surface shortwave fluxes and a surface albedo of 7% (Payne, 1972). These light constraints were interpolated to the model grid from the coarser 2.51 resolution of the NCEP data set. Monthly ice fields for 2002 were also interpolated from averages of the DMSP SSM/I daily polar gridded sea-ice concentrations (Cavalieri et al., 2003) at a spatial resolution of 25 km. Light at the surface was also a function of time of day. It was next calculated, assuming a sinusoidal distribution over the photoperiod as Lðt; 0Þ ¼ I m sinðpts =DÞ,

(22)

where ts was time since sunrise, D was the photoperiod , 0ots/Do1, and I m ¼ 24I p ðp=2DÞ. The light field at depth, z, in each algal growth term was Lðt; zÞ ¼ Lðt; 0ÞI w for open water, and Lðt; zÞ ¼ Lðt; 0ÞI w I ice under ice. Grazer stocks, Gc and Gm, changed only through advection by the mean layer flow, ud, vd, since their biological growth and loss processes were assumed to be in balance, as they migrated daily over the water column. The rate at which diatoms were grazed, gd, (Table 2), was twice the daily ration (as percent body carbon) of copepods, Gc. We assumed that 50% of the prey encountered by copepods was lost as DOC during ‘‘sloppy feeding’’. Copepod grazing was restricted to the upper 200-m of the model’s water column. The daily ingestion ration of copepods increased with greater chlorophyll concentrations to a maximum of 10% of herbivore body carbon per day at a chlorophyll stock of 10 mg chl l1. Protozoans, Gm, grazed both microflagellates and single-cell Phaeocystis (Pps). In these scenarios, Pps was assumed to be 90% of the total Phaeocystis biomass to impose maximal grazing losses on the prymnesiophytes (Walsh et al., 2001). The daily food ration of protozoans was a function of prey availability, gm ¼ zðPf þ Pps Þ, where z was in units of mmol kg1 day1. They grazed the most abundant food first, and then the least abundant prey item to satisfy their metabolic requirements. Protozoan grazing stress was restricted to the upper 100-m of the water column. No grazing by protozoans, nor by copepods, was imposed on algal concentrations of less than 0.05 mg chl l1 of each algal group to establish prey refugia. The bacterial growth terms, g3, g4, of Eqs. (4), (7) and (8) were g3 ¼ c b

DOC1 , kDOC þ DOC1

(23)

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g4 ¼ 0:4cb

DOC2 , kDOC þ DOC2

(24)

where the half-saturation constant kDOC was 0.83 mmol kg1 (Table 2). The maximum bacterial growth rate, cb, was a function of temperature, like bacterial mortality, m (Table 2). Only 40% of the macromolecular DOC2 was considered labile (Walsh and Dieterle, 1994). The photolysis, X2, of DOC2 to more usable DOC1 was a zero order rate term, applied within the upper 5 m of the water column, in open water during daylight hours (Table 2). The bacterial nitrification rate, X1, in the water column was expressed by
 NH4 X 1 ¼ 0:04 , (25) kNIT þ NH4 where 0.04 mmol N kg1 day1 was the maximum rate (Walsh and Dieterle, 1994). In the sediments, benthic nitrification was assumed to be 20% of the DON released from PON sources of the settling phytodetritus and fecal pellets that survived descent though the water column. Within the sediment layer, the state variables of non-siliceous and siliceous particulate sediment carbon, Sp and Sps, were described by qS p;ps q2 S p;ps ¼ Kb  lS p;ps , qt qz2s

(26,27)

where Kb, the bioturbation coefficient, was a function of bottom temperature (Walsh and Dieterle, 1994), supplied from the physical model. Since benthic biomass and catabolism were strongly correlated with the chlorophyll content of the overlying water column (Grebmeier et al., 1988), the decay coefficient, l, in Eqs. (26) and (27) was a function of total algal and fecal pellet concentration at the bottom of the water column: P P i Pi ðdÞ þ j Z j ðdÞ P P l ðday1 Þ ¼ lm , (28) 1 þ i Pi ðdÞ þ j Z j ðdÞ where lm was the maximum rate (Table 2), and ‘‘(d)’’ denoted the lowest layer of the water column and subscript ‘‘i’’ was d, f, or p for diatoms, microflagellates, and Phaeocystis, respectively, subscript ‘‘j’’ for siliceous and non-siliceous fecal pellets, respectively. The last two state variables of sediment DIC and silicate were governed by qDs q2 D s ¼ Km þ lSp þ lps , qt qz2s

(29)

qDSiO4 q2 DSiO4 ¼ Km þ lps , qt qz2s

(30)

where Km was the porewater molecular diffusivity (Table 2). As described in the next section on vertical boundary conditions, dissolved recycled nitrogen, i.e. ammonium and nitrate, in the sediments were implicit variables, assuming a Redfield C/N ratio of source Ds+DOC/DON. At the start of the simulation on 1 March 2002, the sediment module was assumed to be in equilibrium with the overlying water column, such that sediment particulate carbon and silicate were Sp ¼ Sp(w) and Sps ¼ Sps(w) and dissolved carbon and silicon were Ds ¼ DOC2(w), DSIO4 ¼ SIO4(w). A.2. Boundary conditions in z The settling velocities of Eqs. (1)–(3), (7) and (8) were identically zero at the air–sea and watersediment interfaces. The diffusive fluxes were also set to zero at the air–sea interface in Eqs. (1)–(11), whereas for Eq. (12),
 qDIC Kh ¼ 1:11  105 W 10 m C CO2 qz 0   ðpCO2 Þair  ðpCO2 Þ0 , ð31Þ where W10 m was the monthly average 10-m wind speed obtained from the NCEP data set. Based on prior Arctic measurements (Fanning and Torres, 1991), the piston velocity for gas transfer through ice was assumed to be 25% of the open-water value, such that the total flux was a weighted average over open water and under ice cover. The model’s partial pressure of CO2 in air (pCO2)air was constant at 365 matm, similar to that observed in September 2000 (Murata and Takizawa, 2003). In the top layer of the water column (pCO2)0, as well as the solubility of CO2 in seawater, C CO2 , was calculated (Peng et al., 1987) as a function of temperature and salinity. For the alkalinity, we assumed a constant value of 2200 meq kg1 over the model domain, which corresponded to a salinity of 31.82, if alkalinity were related (Murata and Takizawa, 2003) to salinity, S, by 2419.7 (S/35). A global relationship of salinity and alkalinity (Millero et al., 1998) was initially employed, because it matched the Arctic Ocean Section data quite well (Swift et al.,1997). Surface salinity of the physical model ranged over
18.5 to
32.5, however, such that the calculated pCO2 values were out of range. We obtained more reasonable results, by assuming a

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constant salinity of
32 for our pCO2 calculations. A salinity of 32 yields an alkalinity of about 2200 from prior observations (Murata and Takizawa, 2003; Fransson et al., 2001). At the water-sediment interface, the boundary conditions for Eqs. (2), (3) and (8) were Kh

qA qSp ½Sp ðwÞ  S p  AðdÞ ¼ Kb , ¼ Kb qz 0:5½dzðwÞ þ dzðsÞ S p ðwÞ qzs (32234)

where A was either Pf, Pp, or Zm. Here, A(d) was the value in the bottom layer of the water column, while S p ðwÞ ¼ ðPf ðdÞ þ Pp ðdÞ þ Z m ðdÞÞ, dz(w) was the thickness of the bottom layer in the water column, and dz(s) was the thickness of the sediment layer. Similarly, for Eqs. (2) and (7): Kh

qA qSps ½Sps ðwÞ  Sps  AðdÞ ¼ Kb , ¼ Kb qz 0:5½dzðwÞ þ dzðsÞ Sps ðwÞ @zs (35,36)

where A was Pd or Zc. Particulate material was constrained to move from the water column to the sediments via mixing from bioturbation, Kb, as a function of the model’s bottom temperature (Table 2). Accordingly, at simulated shelf temperatures of 1.8 to+6.3 1C, the biotic inward transport of debris on the Chukchi shelf was always at a bioturbation rate of 40.1 cm2 yr1 (Smith et al., 2003), while particulate fluxes directed out of the sediment were zero, i.e. ignoring resuspension events (Moran et al., 2005). Sediment DOC was assumed to be macromolecular DOC2, such that the flux of monomeric DOC1 from the sediments was thus zero. The DOC2 flux for the lower boundary condition of Eq. (6) was Kh

qDOC2 qDs ½DOC2 ðwÞ  Ds  , ¼ Km ¼ Km 0:5½dzðwÞ þ dzðsÞ qz qzs (37)

where DOC2(w) was again the concentration of DOC2 in the bottom layer of the water column. The bottom condition on silicate of Eq. (11) was similarly Kh

qSiO4 qDSiO4 ½SiO4 ðwÞ  DSiO4  . ¼ Km ¼ Km 0:5½dzðwÞ þ dzðsÞ qz qzs (38)

The microbial utilization of Ds was partitioned (Walsh and Dieterle, 1994) as 10% respired to CO2, with 90% released as DOC2. The DIC flux at the

sediment-water interface for Eq. (12) was
 qDIC 0:1 qDOC2 ¼ Kh . Kh qz 0:9 qz

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(39)

Assuming a Redfield balance of nitrogen to carbon atoms, the lower boundary conditions of Eqs. (9) and (10) were also described by   qNH4 qDIC Kh ¼ f s ðNH4 Þ ðN=CÞr K h , (40) qz qz Kh

  qNO3 qDIC ¼ f s ðNO3 Þ ðN=CÞr K h , qz qz

(41)

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