Patterns of carbon supply and distribution and oxygen renewal in two Alaskan fjords

Sedimentary Geology, 36 (1983) 93-115

93

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

P A T T E R N S OF CARBON SUPPLY A N D DISTRIBUTION A N D OXYGEN RENEWAL IN TWO ALASKAN F J O R D S *

D A V I D C. B U R R E L L

Institute of Marine Science, University of Alaska, Fairbanks, A K 99701 (U.S.A.) (Accepted for publicatian May 16, 1983)

ABSTRACT Burrell, D.C., 1983. Patterns of carbon supply and distribution and oxygen renewal in two Alaskan fjords. Sediment. Geol., 3 6 : 9 3 - 1 1 5 . The deep basin water of Resurrection Bay (a single-silled fjord at 60°N on the south-central Alaskan coast) is renewed each summer with water having a dissolved oxygen concentration > 4 ml 1- i. Prior to 1976 > 90% of the allochthonous organic carbon supply to the basin was from fish-processing waste. Oxygen concentrations at the bottom were reduced to around 1 ml 1 i during the winter, and Heggie and Burrell (1981) have computed that a quantity of carbon > 50% of the annual phytoplankton production (19 moles C m - 2 y r - J) was oxidized within the basin and near-surface sediments. Very little carbon (0.6 moles m -2 yr - I ) is removed via sediment burial in this estuary. Boca de Quadra Fjord is located at approximately 55°N adjacent to the Alaska-British Columbia border. The annual summer flushing sequence of the deep (365 m) central basin is basically the same as in Resurrection Bay. Estimated benthic respiration (around 8 moles C m - 2 y r - 1 ) may be supported by the in-fjord annual primary production ( > 12 moles C m -2 in 1980). The mean annual input of terrigenous particulate carbon is estimated to be of the same order of magnitude as the loss rate within the basin by sediment burial (9.0 moles C m 2 yr ~). The relatively high flux of allochthonous carbon into Boca de Quadra thus appears to consist predominantly of refractory material which does not create a significant oxygen demand within the basin.

INTRODUCTION

Only a few investigations of gross carbon flow within fjord estuaries have been attempted: those by Davies (1975) and Rosenberg et al. (1977) are notable recent examples. There has been no detailed study of carbon-oxygen budgets within any Alaskan fjord. As an initial exercise, major sources, pathways and reservoirs may be determined and transport and reaction rates estimated. This is the approach attempted here, with emphasis on the overall biogeochemical environment, rather than

* Contribution No. 521, Institute of Marine Science, University of Alaska, Fairbanks, Alaska. 0037-0738/83/$03.00

© 1983 Elsevier Science Publishers B.V.

94 on detailed biological components, and, even after multi-year investigations, many simplifications have necessarily been applied. The type of fjord considered here is a deep estuary characterized by one or more barrier sills which periodically restrict the free exchange of water. Within the basins defined by the sills, bottom water may potentially become anoxic-- seasonally, or on an irregular time-scale--where the input of labile carbon exceeds the local oxygen resupply rate. Such conditions, inimical to aerobic organisms, could occur naturally, or be induced by additional organic loading introduced by man. However, anoxic water has not been observed in any fjord examined to date along the southeast-south-central Alaskan coast. Conditions in two widely separated (5 ° lat.) sub-arctic fjords are described. Yhese provide contrasting environments, particularly with regard to a number of external functions such as seasonal river inflow, surface temperature-light regime, and the type of allochthonous sediment supplied. But the basic patterns of deep-water exchange, and the marked seasonality of properties, are common to both. LOCALITIES AND ENVIRONMENT Resurrection Bay (Fig. 1) is a single-silled fjord located at 60°N lat. on the Kenai Peninsula of south-central Alaska, and is some 6 - 8 km wide and 30 km long, opening directly onto the Gulf of Alaska. Sill and maximum basin depths are approximately 185 and 290 m, respectively. The Resurrection River enters at the head of the fjord adjacent to the town of Seward. The Boca de Quadra Fjord system (55°N; Fig. 2) is situated immediately north of the Alaska British Columbia boundary. This is a multi-silled complex, but the outer basin is well-mixed year-round. and interest here is focussed on the deep (around 365 m) central basin located up-fjord from the Kite Island sill (80 m depth). Two major rivers impinge oll this region of the fjord: the Keta River at the head, and the Marten River t i a Marten Arm. The distance from Kite Island to the mouth of the Keta River is approximately 35 kin, and the mean width of the fjord in this region is 2 3 kin. Schematic longitudinal sections down the center of both fjord basins on comparable scales are shown in Fig. 3. The volume of the central basin of Boca de Quadra (below sill height) is six times greater than the Resurrection Bay basin. Monthly flow rates (mean and range) from the Resurrection River over the period October 1965 June 1968 (U.S. Geological Survey, unpubl, data) are shown in Fig. 4A. Maximum precipitation and discharge in this region is in late summer fall when riverine discharge, which includes snow-field and glacier melt-water, is fifty times greater than that occurring in mid-winter (March). Mean volume influx via this river is, however, only approximately 2% of the tidal prism. Figure 4B shows volume flow data, October 1978 September 1981, for the Keta River at the head of Boca de Quadra Fjord (U.S. Geological Survey, 1979; unpublished data, 1982). The characteristic annual discharge pattern here is bimodal

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(as described in southern British Columbia; Pickard and Stanton, 1979), representing release of stored precipitation in the spring and maximum rainfall in the fall. Mean annual rainfall along the southeast Alaskan coast is around 2.5 m yr i. Steep, heavily forested terrain surrounds Boca de Quadra, and the combined watershed area is relatively small: approximately ten times the fjord surface area. Residence times for summer-fall precipitation within the catchment area are therefore short, and day-to-day fluctuations in the river flow may be very large (Burrell, 1982). The combined Marten and Keta River discharge is less than 50% of the total estimated mean freshwater addition to the marine fjord. Tides up to 6 m occur within the inlet and, as in the case of Resurrection Bay, the total freshwater volume flux constitutes < 5% of the tidal prism (Nebert and Burrell, 1981).

98 PROCEDURES Hydrographic parameters and dissolved oxygen distributions within Resurrection Bay were observed at approximately monthly intervals over the period November 1972 May 1975, and more irregularly between November 1977 May 1979. Sampling and analysis methods have been described by Heggie el al. (1977) and Heggie and Burrell (1981). Boca de Quadra Fjord and the Keta and Marten Rivers have been monitored at monthly intervals from September 1979 through October 1981. Dissolved oxygen data are for Winkler titration analysis of discrete samples (Carpenter. 1965) with a precision of +0.05 ml 1 ~. Separate aliquots of water were filtered through individual pairs of filters (sample plus blank) for total particulate load (47 m m diameter, 0.4/xm pore size Nuclepore polycarbonate membranes), and particulate organic carbon (POC) analysis (Gelman AE glass fiber filters). Particulate load filters were flushed with distilled water, frozen in individual petri containers onboard ship, and returned to the laboratory for analysis. POC filters were placed in 10 ml glass ampules, treated to remove inorganic carbon and sealed with persulfate added as the oxidant (Menzel and Vaccaro, 1964). Duplicate filtrations were performed for each parameter. In the laboratory the Nuclepore membranes were vacuum dried and weighed on an electrobalance (Cahn Model 4700) for total particulate load determinations. POC ampules were autoclaved at 135°C for 4 h and the evolved CO 2 measured using an IR analyzer (Beckman Model 215 B). Sediment samples for organic carbon analysis were freeze-dried, homogenized and treated with 1N HCI for 4 h in a 60°C water bath to remove inorganic carbon. After redrying, triplicate sub-samples were analyzed in a Leco carbon analyzer. Details of these procedures have been given by Burrell (1980). Sedimentation rates were determined for cores (Benthos model), collected in the fjord basins, bv the unsupported 21°pb dating procedure as described by Schell and Sugai (1980). After addition of a 2°SPo yield tracer, 2-3 g sediment aliquots were leached with concentrated acids. The Po isotopes were plated onto silver discs and ~-counted using Si surface barrier detectors. Phytoplankton productivity data were determined using the ~4C light and dark bottle technique with shipboard incubation. Water collected from each standard light-depth was added to two light bottles, with appropriate neutral density screening, and one dark bottle, which were inoculated with L4C-bicarbonate, placed in a flow-through incubator under natural light and cooled with surface seawater. After a 24 h incubation period, samples were filtered in the dark through 25 mm, 0.45 /zm Millipore filters and stored frozen in scintillation vials prior to counting in the laboratory by liquid scintillation. Dark bottle counts were subtracted from light bottle counts before calculation of uptake rates. In Boca de Quadra, shipboard potentiometric titrations were employed to determine the alkalinity at each light depth, and salinities were determined in the laboratory. Phytoplankton uptake rates were computed after correcting for salinity (determination of the appropriate acidity

99 constants using the equations of Gieskes, 1974). Details of the uptake procedures employed in Resurrection Bay have been given by Heggie et al. (1977). RESULTS Seasonal basin-water circulation

Basin-water replacement patterns within a number of Alaskan fjords have been discussed by Muench and Heggie (1978). In many examples--including the two estuaries described here--renewal of the deep basin water is driven by recurring seasonal cycles in the pressure field over the adjacent Gulf of Alaska (Royer, 1975). Heggie and Burrell (1981) have described influxes of dense water into the Resurrection Bay basin as a consequence of upwelling of Gulf waters onto the adjacent shelf through the summer. Influxes here may begin as early as April-May, with complete and multiple replacement of the sub-sill waters through September-October. Annual renewal patterns of the deep water within the central basin of Boca de Quadra (Fig. 3) are essentially similar (Burrell, 1980; Nebert and Burrell, 1981). The principal renewals of bottom water in this fjord also occur during the summer months (June-September). Periods of renewal of near-bottom water are shorter here than in Resurrection Bay largely because the barrier sill is at a shallower depth. Flushing of the deep basin does not occur through the winter-spring season. A pronounced pycnocline exists at the top of the basin. Below around 200 m the column is only very weakly stratified through the winter, but the mean density decreases due to vertical mixing with the water advected in at sill height. Seasonal dissolved oxygen distributions

The annual dissolved oxygen distribution pattern within the Resurrection Bay basin over the 1971-1975 period has been described by Heggie et al. (1977) and Heggie and Burrell (1981). The replacement water remaining at depth following the annual summer flushing events had an oxygen content of around 4 ml 1-1. Oxygen concentrations decreased from sill height towards the sediments, demonstrating the importance of near benthic remineralization processes. Continuing observations in this basin over the period 1977-1979 show the same seasonal oxygen maxima and minima patterns, but dissolved oxygen minima did not fall below 2 ml 1-L in the winter. Seasonal isopleths of oxygen distribution within the deep central basin of Boca de Quadra are shown in Fig. 5 for the period September 1979-October 1981. Oxygen concentrations at the bottom exceeded 4 ml 1-n during the summer 1980 replacement period, but were < 3.5 ml 1-1 through the 1981 flushing season. Winter minima were < 3.5 and < 3 ml 1- n in 1980 and 1981, respectively.

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Annual (1980) supply of riverine particulate carbon to Boca de Quadra The mean monthly flow rates of the Keta River into upper Boca de Quadra Fjord (Fig. 2) through June 1981 are shown in Fig. 4B; total discharge for 1980 was 8.0 × 108 m 3. Direct volume flow measurements are not available for the Marten River, but synthetic data (L. Bartos, pets. commun., 1980), computed using standard hydrologic parameters, and corrected for 1980 using the measured Keta data, show that the total discharge through 1980 into Marten Arm was approximately 22.4 × l0 s m 3. This particular year was characterized by greater than average flow ( 146% of the mean 1977-1981 discharge) in the final quarter. It is apparent that, in addition to the frequently extreme daily variations in volume discharge noted previously, there may also be large yearly differences, and fluxes computed for any given year may not easily be extrapolated. Estimates of the mean monthly inputs of total particulate sediment ( > 0.4 ~m) and particulate organic carbon (POC) for 1980 are given in Fig. 6A and B. It has been shown (VTN, 1982, unpubl, report) that total sediment load carried in suspension by the Keta River increases exponentially with increase in the volume flow. The mean monthly flux values shown in Fig. 6 have been computed from discrete-day data via a rating curve constructed using a set of October i 978 sediment load values (U.S. Forest Service, unpubl, report). No correction has been applied for cross-sectional variability since Naiman (1982), for similar climatic-zone rivers, has

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Pre-1975 allochthonous supply of carbon to Resurrection Bay The annual supply of riverine carbon into Resurrection Bay has not been monitored in detail. Water collected from the Resurrection River during early summer showed very low POC concentrations: < 50/~g C 1- ~. The mean annual flux into the basin from this source is estimated at 0.2 moles C m - 2 yr-1. Prior to 1976 there were additional major sources of anthropogenic carbon into Resurrection Bay. Over the period 1972-1975 it is computed that an annual mean of 5.6 × 105 kg wet weight of fish-processing waste was discharged in the center of the fjord basin. This waste consisted predominantly of coarse fragments and it may be assumed that most was transported to below sill height: a flux of 2.3 moles C m -2 y r - 1. Untreated urban sewage was effluxed contemporaneously into the head of the fjord. But the m a x i m u m carbon flux from this source--assuming all particulate material, and transport without loss into the b a s i n - - c o u l d have been only of the order of 0.16 moles C m - 2 y r - 1.

Phytoplankton primary production The phytoplankton bloom sequence in Boca de Quadra is similar to that previously described in Port Valdez (Prince William Sound, south-central Alaska) by

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Alexander and Chapman (1980). There is commonly a major spring bloom, largely diatomaceous (Skeletonema predominated in 1980; VTN, 1980), and a second much reduced peak in activity in the fall. Flagellate species dominate through the summer and fall. In the more northerly Resurrection Bay locality, the diatom bloom appeared later in the year, and no secondary bloom was observed. Figure 7 illustrates daily 14C uptake rates at the central basin station (BQ-9; Fig. 2) of Boca de Quadra through the 1980 season, and also within Resurrection Bay. Annual depth integrated values for the two years illustrated are 230 and 145 g C m 2 yr L respectively.

Seasonal distribution of particulates within Boca de Quadra A one-year (February 1980-February 1981) time series distribution pattern of particulate sediment through the central basin water column of Boca de Quadra (Fig. 8) shows two seasonal maxima within the surface zone. The spring peak approximates 100% by weight organic material (POM). This marks the peak phytoplankton density noted above. The fall maximum (approximately 80% POM) corresponds with the period of maximum riverine influx. Major increases in particulate sediment concentrations occur only in the near-surface zone, the result both of reaction and of enhanced stratification. The standing stock of particulates at mid-depths within the column is very low: < 0.5 mg 1- ~. Diurnal scouring and resuspension of deposited sediment is not observed within the deep basin. However, as noted previously, the stability of the water column below around 200 m is poor throughout the year, including through the winter "isolation" period, and recent sediment trap data indicate that resuspension of sediment may occur through the entire basin. There is also circumstantial evidence to suggest that episodic bed-load transport events generate more marked resuspen-

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sion signatures within the basin, as illustrated by the deep POC concentration maximum from October 1980 through February 1981 (Fig. 8B).

Sedimentation and burial of POC in Boca de Quadra At the head of Boca de Quadra and within Marten Arm, 21°pb dating profiles (S. Sugai, unpubl, data) indicate extensive episodic bed-load transport of the deposited sediments. Within the central basin (station BQ-9), however, particle-by-particle sedimentation has occurred, and cores retrieved in 1980 (13 cm) and in 1982 (38 cm) show sedimentation rates of 1.08 and 1.19 cm yr-1, respectively. At this station, the sediment POC content decreases downward from the interface to around the 7 - 8 cm horizon. The "equilibrium" POC concentration below this depth is 4.5 + 0.1% (dry weight; the inorganic C content is < 0.2%). DISCUSSION

Oxygen depletion and renewal in the deep basins The Resurrection Bay basin is well stratified through the oceanographic winter between renewal periods. Prior to 1976, oxygen was progressively depleted in the water adjacent to the bottom from around October through April of the following

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year (Fig. 9A). Minimum concentrations recorded were < 1.5 ml 1 ~. At the same time, oxygen concentrations at sill height increased by some 2 ml 1 ~, resulting in an increasingly large vertical gradient (positive upwards; Figs. 9B and 10), and hence diffusional resupply to the deep water. Mean concentrations of particulate carbon

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within the basin also decreased after around December• Typically by March-April, some 5 - 6 months after advective renewal (Fig. 9), the downward resupply of oxygen exceeded the consumption rate and near-bottom concentrations increased again prior to the new sequence of advective influxes (for example in May 1975; Fig. 10). After 1976, fish-processing waste was no longer discharged into this basin. The 1977-1979 oxygen distribution record (Fig. 9A) shows that a minimum of > 2 ml 1 i dissolved oxygen occurred much earlier (October), soon after the cessation of summer flushing. Other factors being equal, it may be computed that an approximate doubling of the pre-1976 anthropogenic carbon load should have resulted in local, but very temporary, formation of anoxic water adjacent to the sediment surface in late winter-spring. The basic annual trends in oxygen distribution at the bottom of the deep Boca de Quadra basin and at sill height (Fig. l lA) are similar to those described for Resurrection Bay, but there are important differences in detail. Most importantly, although the natural flux of allochthonous carbon into Boca de Quadra is very much greater than into the more northerly fjord (see discussion below), the maximum decrease in oxygen at the bottom from that present in the final summer replacement water is only some 0.5 ml 1-~. Consequently, the sill-to-sediment gradient in dissolved oxygen (Fig. l i B ) is much less than in Resurrection Bay. Figure 12 demonstrates (for the 1979-1980 winter season) that there is little change in oxygen concentrations at the bottom of the Boca de Quadra basin through the winter prior

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106 to advective renewal (June 1980 in the upper basin, with complete replacement by July), and that the column is near homogeneous in oxygen through this period below 200 m. 50

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Heggie and Burrell (1981) showed that, on an integrated annual basis, between 40-50% of the basin oxygen consumption occurred in the bottom 20 m of the water column and underlying surface sediments. Because the Boca de Quadra Basin is poorly stratified below 200 m, near-bottom respiration through the winter does not result in the development of a distinctive vertical gradient in oxygen below this horizon. As discussed below, it is believed that much of the carbon transported into the basin is refractory. Nevertheless, the less-pronounced oxygen consumption signature near the benthic boundary within this basin, as compared with Resurrection Bay, is explicable in terms of differing basin mixing patterns.

Seasonal distribution and transport of carbon Quantitative annual primary production within the central basin region of Boca de Quadra Fjord is dominated by the spring bloom, which is now known to vary significantly from year to year. Timing of the bloom is believed to be keyed primarily to the local light regime; but diatom density is a function of factors such as the stratification of the euphotic zone at that time. Based on carbon uptake measurements taken at station BQ-3 (Fig. 2) over the period M a r c h - M a y 1980 (VTN, 1980; A. Sugai, unpubl, data), it is considered that the bloom maximum at BQ-9 occurred in late March (as illustrated in Fig. 7), and recent results show that,

107

in 1982, the bloom at this locality peaked in early April. In 1981, however, the spring bloom was delayed until May, and preliminary computation of the depth-integrated phytoplankton carbon uptake for this year gives a value of around 115 g C m - 2. In the discussions that follow, the mean distribution of carbon within Boca de Quadra through calendar-year 1980 only is considered, and year-to-year variability should be noted. The 1976 data set for Resurrection Bay is, unfortunately, unique, and annual fluctuations here are unknown. The annual carbon uptake values determined for Boca de Quadra and Resurrection Bay appear to fall within the mean range reported for fjords in the northern hemisphere. Within Alaska, integrated uptake in Valdez Arm over a composite 1971-1972 period may be estimated at around 57 g C m -2 yr-1 from the data given by Goering et al. (1973). Wood et al. (1973) have measured 70 g C m 2 y r - i within a lower-latitude Scottish fjord, but fjords in southern British Columbia are generally more productive (Matthews and Heimdal, 1980). Lenz (1981) suggests that a value of around 150 g C m 2 yr-1 is about at the upper limit for Baltic sites. Figure 13 summarizes the 1980 seasonal distribution of carbon within Boca de Quadra Fjord. As noted previously (Fig. 8), concentration maxima in spring and fall in the near-surface zone correspond with autochthonous production, and the peak in A 1000 500

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I I I I I l [ l l l l l l l l l l l l l l l l t 0 t21 2 4 6 8 t 0 q21 2 4 6 t979 t980 198'1

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Fig. 13. Boca de Quadra; seasonal supply and distribution of particulate organic material, October 1 9 7 9 - A p r i l 1981: m e a n contents of Keta and M a r t e n Rivers, m e a n contents at the surface and b e l o w 100 m

at BQ-9.

A.

Particulate organic c a r b o n (/~g l

l). B.

Organic content (%) of particulate sediment.

108 river input, respectively. No direct carbon sedimentation measurements were attempted in 1980, but early results from a trap moored at 40 m (below the euphotic zone) within the central basin in 1981 show a larger downward flux of sediment in June than in July-August, increasing again in September October. Also, preliminary data show that the C / N ratios of the particulate organic material are at a minimum throughout the vertical fjord column through the summer, suggesting relatively rapid (order of months) sedimentation of autochthonous detritus. This is, however, the season of complex flushing of the deeper regions of the basin, and the vertical distribution of particulate material must be a function of the residence time of the water as well as the transit time through the column. Examination of seasonal standing stocks of particulate carbon (discrete samples) at depth can provide only very limited source and transport information, not least because the sampling procedure tends to discriminate against the less abundant larger particles. Time-series POC data at BQ-9 suggest that mean basin concentrations are highest in early winter, and lowest (and diluted more with aluminosilicates) through the summer. Slumping of sediment is known to occur at the head of the fjord, and bed-load transport appears to be prevalent in fall-early winter (Burrell, 1980). As noted previously, it is believed that the weak stratification of the deeper basin water through the winter season (Nebert, 1982) would permit low density organic particulate material to be carried towards the top of the basin. Surficial sediments at BQ-9. in the deepest portion of the central basin, are fine grained (silty-clay), compact (porosity < 9), and organic rich ( > 4% POC by weight). With such properties, a mechanically cohesive sediment would be expected: one not readily resuspended at moderate current shear.

Preliminary carbon budgets for the Resurrection Bay and Boca de Quadra Basins The major quantified sources of allochthonous carbon into Resurrection Bay prior to 1976 were the Resurrection River, fish-processing waste and urban sewage. Figure 14 shows these inputs schematically as fluxes (D, E and F in moles C m n yr i, respectively) into the basin, below the sill at a depth of 185 m. assuming no loss seaward outside the sill or reaction within the surface waters. This assumption is reasonable for the fish-processing waste. The urban-derived (from the town of Seward: Fig. 1) and riverine carbon transports cited are theoretical maxima entering the fjord, and a much reduced fraction of this material is likely to impact the basin. However, since fish-processing waste constituted > 90% of the total identified external carbon supply, the error here is insignificant in terms of the gross budget. Within the basin labile carbon is utilized by community respiration, and the more resistant fraction is lost from the system through burial with the sediments. Heggie and Burrell (1981) have computed mean annual oxygen consumption rates within the Resurrection Bay basin below and above a horizon 20 m above the bottom sediment boundary. These values (B~ and B 2, respectively) are incorporated in

109

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Fig. 14. Resurrection Bay; schematic of major c a r b o n t r a n s p o r t a n d process rates (moles C m - 2 y r - I ) . A = p h y t o p l a n k t o n ]4C uptake; B = oxygen utilization; C = s e d i m e n t burial; D = riverine supply; E = fish-processing waste; F = u r b a n sewage.

?

0.3

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Fig. 15. Boca de Q u a d r a ; s c h e m a t i c of m a j o r c a r b o n t r a n s p o r t a n d process rates (moles C m - 2 y r - t ) . A I = p h y t o p l a n k t o n 14C uptake; A 2 = z o o p l a n k t o n p r o d u c t i o n ; B = m a c r o b e n t h o s p r o d u c t i o n ; s e d i m e n t burial; D = riverine supply.

C =

I IU

Fig. 14 as equivalent carbon oxidation rates (using Redfield et al.'s, 1963, stoichiometry and assuming a respiratory quotient of unity). The rate of sediment carbon burial may be determined from the mass sedimentation rate and the mean carbon content remaining in the sediment below the reaction zone (Berner. 1971). A core taken from the deep basin gave a sedimentation rate of 0.16 cm yr t, and had a mean organic carbon content of 0.8% below 10 cm. The computed burial loss rate ( C of Fig. 15) within this basin is 0.6 moles C m 2 vr The computed mean annual removal rate of carbon via respiration and sediment burial from the pre-1976 Resurrection Bay basin was thus 13.3 moles m ~ vr I: the allochthonous influx was around 2.5 moles m 2 yr ~ There must have been additional terrigenous organic input from smaller streams and ground water run-off around the margins of the fjord. However, unless these sources provided carbon at a rate several orders of magnitude greater than that computed for the major Resurrection River, the input balance to the basin must be assigned, directly or indirectly, to autochthonous primary production. Annual depth-integrated phytoplankton productivity through the 1974--1975 period was 19 moles m -+ (A of Fig. 14), hence it would appear that between 50-60% of this material may have been transported, directly or indirectly, into the basin of this fjord. Figure 15 is a schematic of the presently identified major carbon fluxes and reactions into and within Boca de Quadra Fjord for calendar-year 1980. The computed macrofaunal productivity value (B) is derived from seasonal measuremerits of the infauna and epifauna within the central basin (VTN, 1980). A gross estimate of the benthic community respiration rate of around 8 moles (' m ~ yr L (RQ = 1) may be made using mean macro- and micro-benthos mass ratios published for a number of boreal estuaries, and the P / R ratio values determined by Ankar (1977) for a northern Baltic inlet. These data may not, of course, be directly compared with the benthic oxygen utilization numbers computed for Resurrection Bay (Fig. 14). As noted previously, the 1980 depth-integrated phytoplankton carbon uptake value determined for station BQ-9 (A~ of Fig. 15) is comparable with that for other years, and for other Alaskan fjords. Primary production is generally lower within the innermost basin at the head of the inlet, but chlorophyll a distributions seaward of the central basin remain approximately constant. Surface transport in the central region is tidally dominated, and there is no evidence to suggest significant mean advective losses or gains of phytoplankton biomass. Hence, if the fraction of autochthonous detritus surviving into the basin is comparable to that estimated for Resurrection Bay, energy derived from this source alone could support the secondary benthic production estimated for the deep basin. Peterson and Curtis (1980) have demonstrated that there is a proportionally greater transfer of primary produced energy to the benthos in high latitudes compared with more temperate regions. In 1980, the spring (late March) diatom bloom in Boca de Quadra preceded near-surface stratification (Burrell, 1982).

111

Zooplankton biomass was at a maximum in June (VTN, 1980; the zooplankton production rate given in Fig. 15 incorporates a mean P / B value determined for Bering Sea shelf water by Cooney, 1981). Under these conditions a major downward transfer of organic carbon could occur (Smetacek, 1980; Bodungen et al., 1981). Elmgren (1980) has recorded a 40-50% transfer of a spring diatom bloom--largely u n d e c o m p o s e d - - t o the sediments of a western Baltic inlet, and Theede (1981) suggests that as much as 60% of the relatively high primary production occurring in the Kiel Bight may reach the deeper sediments. These latter referenced Baltic localities are, however, shallower than the fjord basins described here, and only a small fraction of primary phytoplankton detritus would be expected to survive to the deep basin floor (circa 360 m) of Boca de Quadra. Downward transport via zooplankton fecal pellets is a commonly cited mechanism, and Honjo and Roman (1978) have observed that such agglomerates disintegrate more slowly, and hence sediment more rapidly, in cold water environments. Robb (1981) examined (via electron microscopy) particulate sediment sampled at discrete depths, and at the sediment surface (155 m), within the innermost basin of Boca de Quadra, and identified whole diatom tests and fecal pellets at all depths in April. However, the bulk of the biogenic particles recovered through the summer season consisted of fragments of tests, possibly from distintegrated pellets. The discrete sampling procedure used would have discriminated against the rarer larger particles. Chester and Larrance (198 l) believe that the major downward particulate flux in Cook Inlet (south-central Alaska) is in the form of zooplankton fecal pellets. Therriault et al. (1980) have shown that organic-rich particulate material is transported into the deep regions of the Saquenay Fjord (Quebec) with intrusions of near-surface water from seaward of the entrance sill. As described above, external water is advected into the deep central basin of Boca de Quadra each summer, but the source is from 100 m and below, and the mean particulate organic carbon content is < 50 /tg 1 1. This seasonal circulation is not believed to result in a significant net addition of carbon into the central basin. Intertidal zone productivity is an additional potential source which has not been evaluated. It is well known, for example, that areal macrophyte productivity may be an order of magnitude greater than phytoplankton (Seki, 1982). However, the combined intertidal areas adjacent to the Keta and Marten Rivers only amount to approximately 2% of the marine surface area up-fjord from the Kite Island sill. It has been frequently observed (e.g. Davies, 1975; Hargrave, 1978) that sedimented autochthonous carbon is rapidly (on the order of months) oxidized at the benthic interface, but that allochthonous riverine material is frequently more resistant (Johnson and Brinkhurst, 1971). Based on the sedimentation rates and carbon contents cited above for the sediment at BQ-9, it may be computed that 9.0 moles m -2 of carbon are lost annually via sediment burial (C of Fig. 15). It is apparent that there is a substantial input of refractory organic material into the deep basin. In calendar-year 1980 it has been estimated that in excess of 108 moles of

112

suspended particulate carbon were supplied to the upper regions of Boca de Quadra by the Keta and Marten Rivers. This constitutes an areal flux, spread uniformly over the deep floor of Marten Arm and the region up-fjord from Kite Island, of some 3.4 moles C m 2 yr ~ (D of Fig. 15). This supply rate--which is of POC carried in suspension by the r i v e r s - - m a y perhaps be increased to circa 5.3 moles m 2 yr ~ to accommodate additional input from the smaller lateral streams, based on computed catchment areas, and assuming the same mean POC concentrations as in the two principal rivers. Likens et al. (1977) have shown that, for an undisturbed forest system in northern New England, a sediment mass flux approximately equal to that carried in suspension, is also transported as bed load by the rivers. An equivalent factor cannot, unfortunately be easily determined for the Boca de Quadra rivers, although it is known, from dated core profiles (S. Sugai, unpubl, data) that organic-rich sediment, initially deposited at the head of the fjord inlets, is episodically redistributed down-fjord. As noted, bed-load transport is not observed in the deep central basin at BQ-9, but the primary, particle-by-particle sedimentary flux may be augmented, within the basin column, by material resuspended from areas closer to the head. Dissolved organic-carbon concentrations in the major rivers are around twenty times greater than the POC contents (Sugai and Burrell, unpubl. MS). and it is possible that precipitation of humic material is an important additional source of carbon to the sediments, as advocated by HOpner and Orliczek (1978). Whatever the source and mode of supply, lignin-derived humic material will be inert to further extensive degradation within the fjord (HOpner and Orliczek, 1978). Given a turnover time in the range of the 30 years determined by Seki (1982) for humic-rich sediments in an inlet in southern British Columbia, most of the terrigenous material entering Boca de Quadra must be sedimented and buried without generating a significant oxygen demand in the deep water. SUMMARY

Deep-water renewal patterns within two widely separated (5 ° lat.) Alaskan silled fjords are similar, being driven by large-scale seasonal pressure fields developed over the contiguous Gulf of Alaska. Basin water is replaced by denser water upwelled onto the shelf in spring and summer. On average, flushing commences earlier in the more northern fjord (Resurrection Bay) where the sill barrier is deeper: 180 versus 85 m in Boca de Quadra Fjord. From fall (September-October) through spring, the deep fjord basins are effectively advectively isolated. Benthic and basin oxygen utilization initially exceeds the resupply rate, and concentrations of dissolved oxygen above the sediment decrease. However, in Resurrection Bay near-bottom oxygen contents start to increase again in the spring (prior to advective renewal), and in Boca de Quadra, where stratification is very weak in the deep ( > 200 m) water. concentrations remain approximately constant (around 3 ml 1 ~ in 1979 1980), or

113 d e c r e a s e b y 0.5 ml 1- 1 or less t h r o u g h the winter. A t neither locality has b o t t o m - w a t e r a n o x i a been observed, n o r is it expected to occur u n d e r n a t u r a l conditions. In b o t h fjords b a s i n b e n t h i c p r o d u c t i v i t y (or oxygen utilization) is e s t i m a t e d at a r o u n d 6 0 - 7 0 % of the p h y t o p l a n k t o n c a r b o n u p t a k e rate, a n d a major, a n d relatively rapid, transfer of a u t o c h t h o n o u s c a r b o n to the b e n t h o s is indicated. Prior to 1976 there was an a d d i t i o n a l m a j o r flux ( a r o u n d 12% of the p r i m a r y p r o d u c t i o n rate) of labile organic waste into the R e s u r r e c t i o n Bay basin which resulted in much lower (from a r o u n d 3 to 2 ml 1 - ] ) dissolved oxygen contents in the b o t t o m water. T h e input (per unit area of b a s i n floor) of riverine p a r t i c u l a t e c a r b o n into Boca de Q u a d r a is of m a j o r i m p o r t a n c e , b u t is p o o r l y e s t i m a t e d because b o t h s u p p l y b y rivers a n d b e d - l o a d t r a n s p o r t within the fjord is p r e d o m i n a n t l y episodic and difficult to quantify. However, the m e a n a n n u a l riverine flux of P O C is e s t i m a t e d to be of the s a m e o r d e r of m a g n i t u d e as the mass rate of loss from the fjord system by s e d i m e n t b u r i a l ( a r o u n d 9.0 moles C m - 2 y r - ] within the deep central basin). It w o u l d a p p e a r that the n a t u r a l terrigenous P O C discharged into this southeast A l a s k a n fjord is largely resistant to further benthic d e g r a d a t i o n , a n d hence this flux does not result in a large a d d i t i o n a l utilization of oxygen within the basin. ACKNOWLEDGEMENTS

This work has been s u p p o r t e d b y the U n i t e d States Borax and C h e m i c a l C o r p o r a tion a n d the Pacific C o a s t M o l y b d e n u m C o m p a n y , a n d b y the U.S. D e p a r t m e n t of E n e r g y ( C o n t r a c t E(45-1)-2229). The practical help of Barry Spell, D o n n a W e i h s a n d F r a n k F l y n n is a c k n o w l e d g e d , also the masters a n d crew of the R / V " A c o n a " a n d M / V " R e d o u b t " . I a m grateful to D a v i d Heggie a n d Susan Sugai for critical comment. REFERENCES Alexander, V. and Chapman, T., 1980. Phytotoxicity. In: J.M. Colonell (Editor), Port Valdez, Alaska: Environmental Studies.' Institute of Marine Science, University of Alaska, Fairbanks, Alaska, Occas. Publ. No. 5, pp. 125-162. Ankar, S., 1977. The soft bottom ecosystem of the northern Baltic proper with special reference to the macrofauna. Contributions from the Ask6 Laboratory, University of Stockholm, Stockholm, 19: 1-62. Berner, R.A., 1971. Principles of Chemical Sedimentology. McGraw-Hill, New York, N.Y., 240 pp. Bodungen, G., Brrckel, K., Smetacek, V. and Zeitzschel, B., 1981. Growth and sedimentation of the phytoplankton spring bloom in the Bornholm Sea. Kiel. Meeresforsch., 5: 49. Burrell, D.C., 1980. Marine Environmental Studies in Boca de Quadra and Smeaton Bay: Chemical and Geochemical, 1980. Institute of Marine Science Report R82-2, University of Alaska, Fairbanks, Alaska, 272 pp. Burrell, D.C., 1982. The pre-impact biogeochemical environment of Boca de Quadra and Smeaton Bay fjords, Southeast Alaska. In: D.V. Ellis (Editor), Marine Tailings Disposal. Ann Arbor, Ann Arbor, Mich., pp. 311-340. Carpenter, J.H., 1965. The Chesapeake Bay Institute technique for the Winkler dissolved oxygen method. Limnol. Oceanogr., 10: 141.

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Chester, A.J. and Larrance, J.D., 1981. Composition and vertical flux of organic matter in a large Alaskan estuary. Estuaries, 4: 42. Cooney, R.T., 1981. Bering Sea zooplankton and micronekton communities with emphasis on annual production. In: D.W. Hood and J.A. Calder (Editors), The Eastern Bering Sea Shelf: Oceanography and Resources, Vol. 2. U.S. Dept. Commerce, Washington, D.C., pp. 947-974. Davies, J.M., 1975. Energy flow through the benthos in a Scottish sea loch. Mar. Biol.. 31: 353. Elmgren, R., 1980. Structures and dynamics of Baltic benthic communities with particular reference to the relationship between macro- and meiofauna. Kiel. Meeresforsch., 4: 324. Gieskes, J.M., 1974. The alkalinity--total carbon dioxide system in seawater. In: E.D. Goldberg (Editor), The Sea, Vol. 5. Wiley-Interscience, New York, N.Y., pp. 123 151. Goering, J.J., Shiels, W.E. and Patton, C.J., 1973. Primary production. In: D.W. Hood, W.E. Shiels and E.J. Kelley (Editors), Environmental Studies of Port Valdez. Institute of Marine Science, Occas. Publ. No. 3, University of Alaska, Fairbanks, Alaska, pp. 253-279. Hargrave, B.T., 1978. Seasonal changes in oxygen uptake by settled particulate matter and sediments in a marine bay. J. Fish. Res. Board Can., 35: t62l. Heggie, D.T., Boisseau, D.W. and Burrell, D.C., 1977. Hydrography, Nutrient Chemistry and Primary Productivity of Resurrection Bay, 1972-1975. Institute of Marine Science Report No. 77-9. University of Alaska, Fairbanks, Alaska, 111 pp. Heggie, D.T. and Burrell, D.C., 1981. Deep water renewals and oxygen consumption in an Alaskan fjord. Estuarine Coastal Shelf Science, 13: 83-99. Honjo, S. and Roman, M.R., 1978. Marine copepod fecal pellets: production, preservation and sedimentation. J. Mar. Res., 36: 45. H6pner, T. and Orliczek, C., 1978. Humic matter as a component of sediments in estuaries. In: Biogeochemistry of Estuarine Sediments. Unesco, Paris, pp. 70-74. Johnson, M.G. and Brinkhurst, R.O., 1971. Benthic community metabolism in Bay of Quinte and Lake Ontario. J. Fish. Res. Board Can., 28: 1715. Lenz, J., 1981. Phytoplankton standing stock and primary production in the western Baltic. Kiel. Meeresforsch., 5: 29. Likens, G.E., Bormann, F.H., Pierce, R.S., Eaton, J.S. and Johnson, N.M., 1977. Biogeochemistry of a Forested Ecosystem. Springer, New York. N.Y., 146 pp. Matthews, J.B.L. and Heimdal, B.R., 1980. Pelagic productivity and food chains in l]ord systems. In: tt.J. Freeland, D.M. Farmer and C.D. Levings (Editors), Fjord Oceanography. Plenum, New York. N.Y., pp. 377-378. Menzel, D.W. and Vaccaro, R.F., 1964. The measurement of dissolved organic and particulate carbon in seawater. Limnol. Oceanogr., 9: 138. Muench, R.D. and Heggie, D.T., 1978. Deepwater exchange in Alaskan sub-arctic fjords. In: B. Kjerfve (Editor), Estuarine Transport Processes. The Bell W. Baruch Library in Marine Science No. 7. University of South Carolina Press, S.C., pp. 239-268. Naiman, R.J., 1982. Characteristics of sediment and organic carbon export from pristine boreal forest watersheds. Can. J. Fish. Aquat. Sci., 39: 1699. Nebert. D.L., 1982. The circulation of the Smeaton Bay and Boca de Quadra fjord systems. In: D.V. Ellis (Editor), Marine Tailings Disposal. A n n Arbor, A n n Arbor, Mich., pp. 291-310. Nebert, D.L. and Burrell, D.C., 1981. Marine Environmental Studies in Boca de Quadra and Smeaton Bay: Physical Oceanography, 1980. Unpublished report to U.S. Borax and Chemical Corp., Institute of Marine Science, University of Alaska, Fairbanks, Alaska, 57 pp. Petersen, G.H. and Curtis, M.A., 1980. Differences in energy flow through major components of subarctic temperate and tropical marine shelf ecosystems. Dana, 1: 53. Pickard, G.L. and Stanton, B.R., 1979. Pacific Fjords: A Review of their Water Characteristics. Report No. 36, Department of Oceanography, University of British Columbia, B.C.

115 Redfield, A.C., Ketchum, B.H. and Richards, F.A., 1963. The influence of organisms on the composition of seawater. In: M.N. Hill (Editor), The Sea, Vol. 2. Wiley-Interscience, New York, N.Y., pp. 26-77. Robb, M.S., 1981. Composition and Manganese Association of Suspended Matter at the Head of a Southeast Alaska Fjord. Unpubl. Thesis, University of Alaska, Alaska, 185 pp. Rosenberg, R., Olson, J. and Olundh, E., 1977. Energy flow model of an oxygen-deficient estuary on the Swedish west coast. Mar. Biol., 47: 99. Royer, T.C., 1975. Seasonal variations of waters in the northern Gulf of Alaska. Deep-Sea Res., 71: 403. Schell, W.R. and Sugai, S., 1980. Radionuclides at the U.S. radioactive waste disposal site near the Farallom Islands. Health Phys., 39: 475. Seki, H., 1982. Organic Material in Aquatic Ecosystems. CRC Press, Boca Raton, Fla., 201 pp. Smetacek, V., 1980. Annual cycle of sedimentation in relation to plankton ecology, western Kiel Bight. Ophelia Suppl., 1: 65. Theede, H., 1981. Studies on the role of benthic animals of the western Baltic and the flow of energy and organic material. Kiel. Meeresforsch, 5: 434. Therriault, J.C., De Ladurantaye, R.. and Ingram, R..G., 1980. Particulate matter exchange processes between the St. Lawrence estuary and the Saguenay Fjord. In: H.J. Freeland, D.M. Farmer and C.D. Levings (Editors), Fjord Oceanography. Plenum, New York, N.Y., pp. 363-366. U.S. Geological Survey, 1979. Water Resources Data for Alaska. Water Data Report AK-78-1, U.S. Geological Survey, Anchorage, Alaska, 425 pp. VTN, 1980. Boca de Quadra Baseline Report. U.S. Borax and Chemical Corporation, Los Angeles, Calif. Wood, B.J.B., Tett, P.B. and Edwards, A., 1973. An introduction to the phytoplankton, primary production and relevant hydrography of Loch Etive. J. Ecol., 61: 569.