HEDGES, JOHN I., WAYNE A. CLARK, AND GREGORY L. COWIE
Transcription
HEDGES, JOHN I., WAYNE A. CLARK, AND GREGORY L. COWIE
Limnol. Oceanogr., 33(5), 1988, 1137-l 152 0 1988, by the American Society of Limnology and Oceanography, Inc. Fluxes and reactivities of organic matter in a coastal marine bay John I. Hedges,’ Wayne A. Clark, and Gregory L. Cowie School of Oceanography, WB- 10, University of Washington, Seattle 98 195 Abstract Vertical fluxes of bulk particulate material, organic carbon, nitrogen, lignin-derived phenols, and neutral sugars through the water column and into surface sediments of Dabob Bay, Washington, were determined monthly for 1 yr by sediment trap deployments at 30, 60, and 90 m at a site 110 m deep. Vertical fluxes of sinking bulk particulate material in this marine bay were elevated during winter and increased in consistent proportion to sediment trap deployment depth throughout the year. Although annual average particle fluxes at 30 and 60 m bracketed the mean accumulation rate of the underlying sediment, the flux at 90 m was higher by a factor of 2 due to resuspension, horizontal advection, or both. The monthly fluxes of lignin-derived phenols paralleled those of total particulate material, indicating a common riverine origin. The annual average fluxes of vanillyl and cinnamyl phenols through the water column closely matched the corresponding accumulation rates in the underlying sediment, whereas about a third of the total syringyl phenol input was degraded at the watersediment interface. Although p-hydroxyacetophenone exhibited a stability typical of lignin-derived phenols, the distinctly higher reactivities (> 60% degradation) ofp-hydroxybenzaldehyde and p-hydroxybenzoic acid indicate a predominantly nonlignin source. On average, 60 and 70%, respectively, of the total particulate organic carbon and nitrogen and 65-75% of all neutral sugars settling through the midwater column were degraded at the watersediment interface. The elemental and carbohydrate composition of the degraded material was similar to that of local net plankton except for higher percentages of glucose and total neutral sugars. Land-derived organic material accounted for about one-third of the total organic carbon passing through the midwater column and two-thirds of the organic carbon accumulating in the underlying sediments. The amounts of plankton-derived organic matter sinking through the midwater column and being preserved in the sediments below corresponded to 14 and 3% of the annual mean primary productivity. Plankton-derived organic matter exhibited about 5 times the reactivity of local land-derived organic matter at the water-sediment interface of Dabob Bay and supported essentially all of the benthic respiration. Each year about 100 x 1015g of inorganic carbon is converted globally to plant biomass (Trabalka 1985). Less than 1% of this mass is ultimately preserved in marine sediments which constitute the only quantitatively significant organic carbon repository (Hunt 1979; Olson et al. 1985). Although ’ To whom correspondence should be sent. Acknowledgments This research was supported by NSF grants OCE 8219294 and OCE 84-21023. Contribution 1768 from the School of Oceanography, University of Washington. We thank Jeff Stem for elemental analyses, Coastal Science Laboratory for stable carbon isotope measurements, and Gray Drewry and Phil Crawford for operating research vessels. Roy Carpenter, Carl Lorenzen, Jan Downs, Fred Prahl, and Maria Vemet have cooperated in many ways to make this research possible. Reviews of this manuscript were provided by John Ertel, Karen Weliky, Susan Hamilton, Cindy Lee, and three anonymous reviewers. Michael Peterson helped with the final preparation of the manuscript. photosynthesis occurs essentially worldwide, - 85% of the preserved organic carbon is concentrated in fine-grained elastic sediments that are rapidly deposited along continental margins (Romankevich 1984; Baes et al. 1985). Thus, storage of organic matter in coastal sediments controls the passage of organic carbon from the biosphere to the geosphere (Mackenzie 198 1) and helps modulate the contemporaneous global carbon cycle (Broecker 1982; Walsh et al. 1981). From many studies in the coastal and open ocean it is evident that most organic matter is degradated in the water column or near the water-sediment interface (Suess 1980; Reimers and Suess 1983; Emerson 1985). There also is good evidence from both laboratory (Westrich and Berner 1984; Newell et al. 198 1) and field studies (Iturriaga 1979; Hargrave 1978; Lyons and Gaudette 1979) that the nature of organic matter strongly 1137 1138 Hedges et al. affects the rate and extent of degradation. The factors which control the fluxes and reactivities of different organic materials and the processes involved are not well understood, however, especially for coastal zones (Emerson and Hedges 1988). One impediment to research has been the problem of representatively sampling particulate organic fluxes in the highly advective water columns and rapidly mixed surface sediments typical of coastal regions. This largely technical problem often is compounded by distinct seasonal variations in the sources and transport of both marine and terrigenous organic materials. One means of defining the reactivities and fates of organic materials in the coastal ocean is by comparing annual average fluxes through the water column to the corresponding sediment accumulation rates. Almost all such studies of organic matter fluxes have involved bays or fjords of sufficient depth to reflect water column processes and proximate enough to be monitored on at least a monthly basis (e.g. Taguchi 1982; Bates et al. 1984). A series of such studies has already been carried out over the last decade in Dabob Bay, Washington (Prahl and Carpenter 1979; Bennett 1980; Prahl et al. 1980) an arm of Puget Sound (Fig. 1) that is removed from direct river runoff and similar in its biological and physical processes to the local continental shelf. This earlier work has provided a model for the present study as well as a background against which results of the present work can be viewed. We report here a study of the fluxes of bulk particulate material, organic carbon, nitrogen, neutral sugars, and lignin-derived phenols in the water column and through the water-sediment interface of Dabob Bay. Accumulation rates of these materials in sediment traps deployed at 30, 60, and 90 m in the 110-m water column have been determined on a monthly basis and compared to incorporation rates in the underlying sediments. Annual mean fluxes calculated from these data have been used to determine the relative reactivities of the different chemical species at the water-sediment interface and to calculate an organic matter budget for the bay. Fig. 1. Dabob Bay and greater Puget Sound. Experimental The study site and the analytical methods used here were described by Cowie and Hedges (19843) and in the preceding paper (Hedges et al. 1988). Briefly, Hg-poisoned sediment trap samples were collected about monthly (14 samples total) from 3 July 198 1 to 8 July 1982 at 30, 60, and 90 m in the 110-m water column in Dabob Bay. A 50cm-deep sediment box core and monthly net plankton samples were also collected. These samples were analyzed by previously described methods for organic carbon and nitrogen (Hedges and Stem 1984) neutral sugars (Cowie and Hedges 1984a), and lignin oxidation products (Hedges and Ertel 1982). The main symbols used here and their definitions are given in Table 1. Results Bulk particulate material fluxes- Sediment trap samples were recovered from all 14 deployments at 30 m and all but one (12 February-l 9 March 1982) of the 60-m and 90-m deployments, where the missing samples represent 9% of the 370-d study period. Reproducibility of measured vertical fluxes of bulk particulate material was evaluated Fluxes and reactions 1139 Table 1. Symbol definitions. MONTH JASONDJFMAMJJ oc tz : N)a a TCH,O R %D %VPD Organic carbon Inorganic carbon (carbonate) Atomic ratio of carbon to nitrogen Total yield (mg) of eight lignin-derived phenols per 100 mg of sample organic carbon (A = V + S + C) Total yield (mg) of aldosic sugars from 100 mg of sample organic carbon Ratio of the vertical flux through the water column at a given depth to the sediment accumulation rate Percentage of a particulate material that is degraded at the water-sediment interface Estimated percentage of the total organic carbon in a sample that exists in the form of vascular plant debris h B 2 16- Y !3 12- (4 M B B i 3 s 8-BBBBBT B 4-Y- M -Mj!mT T I 0 I BB ! M T B Y_ T I I B M M -TTMT I w---I I B B B B B M M B Y jjB BB TB 100 200 Bu g 200 by comparing matched pairs of individual traps that were located diagonally in the fourtrap arrays (Cowie and Hedges 19843). These comparisons were made at all three water depths for pairs of poisoned and unpoisoned traps from three early deployments (summer and fall 198 1) as well as for pairs of poisoned traps from one winter (15 January-l 2 February 1982) deployment. The measured fluxes were reproducible within an average of * 3% of the mean (6% variation between measured values, n = 12). An average variation of +6% was previously obtained with similar sediment traps deployed individually at 50-5 5-m water depth at the same study site (Bennett 1980). Table 2. Vertical fluxes of bulk particulate material for individual sediment trap deployments at 30, 60, and 90 m in Dabob Bay. Period (1981-1982) 3-21 Jul 21 Jul-20 Aug 20 Aug-15 Sep 15 Sep-15 Ott 15 Ott-10 Nov 10 Nov-16 Dee 16 Dee-1 5 Jan 15 Jan-12 Feb 12 Feb-19 Mar 19 Mar-2 Apr 2-23 Apr 23 Apr-14 May 14 May-10 Jun 10 Jun-8 Jul * Bulk compositional Bulk flux (g mm2 d-l) 30 m 3.60 1.26 2.33 2.48 1.46 8.34 3.04 3.15 3.64 4.19 2.65 1.55 1.45 3.71 data not available: 60 m 90 m 3.83 2.56 3.23 3.87 2.87 11.1 3.98 4.83 5.17* 5.77 3.76 2.34 2.04 4.26 estimation method 8.86 6.41 6.09 7.34 5.88 16.0 8.61 9.44 10.3* 10.3 7.16 7.38 4.43 7.10 given in text. Li 100 x ‘0 DAYS AFTER 1 JULY 300 400 198 1 Fig. 2. Plots vs. time of the vertical fluxes of (a) bulk particulate material and (b) particulate organic carbon (OC) into sediment traps deployed in Dabob Bay. The horizontal lines indicate the corresponding mean (? 1 SD) sediment burial fluxes as determined for 10 samples from the O-22-cm horizon of a sediment core taken at the study site. Abbreviations: T-top (30m) sediment trap; M-middle (60-m) sediment trap; B-bottom (90-m) sediment trap. No consistent trend in sampling reproducibility with water depth or month of collection was apparent. Over the yearlong sediment trap study, fluxes of total particulate matter varied from about 1 to 16 g m-2 d-l (Table 2). Fluxes were somewhat higher in winter (10 November 1981-12 February 1982) through early spring (12 February-2 April) and peaked during November-December 198 1 (Fig. 2a). Fluxes increased year-round with water depth during individual deployments, but maintained relatively uniform ratios at 30,60, and 90 m of about 1.O : 1.5 : 3.0 (Fig. 2a). Temporal and depth trends in vertical fluxes-The total particulate fluxes in Table 2 can be combined with the compositional data in table 4 of the previous paper (Hedges et al. 1988) to determine the vertical fluxes of organic carbon, nitrogen, lignin-derived phenols, and calcium carbonate at 30-, 60-, Hedges et al. 1140 MONTH - “: e w .% 2 g JASONDJFMAMJJ a15 - B 5- 8 2 c3 OO M T B T l0-M E . I M 25- T B M YbB T M Y : I I 100 T I I I 200 DAYS k 5 AFTER 1 300 1 JULY I , 400 198 1 Fig. 3. Plots vs. time of the vertical fluxes of (a) total vanillyl (I’) phenols and (b) glucose into sediment traps deployed in Dabob Bay. All details as in Fig. 2. and 90-m water depth during individual sediment trap deployments. Fluxes of individual neutral sugars can be similarly determined with the compositional data from table 3 of Cowie and Hedges (19843). Organic carbon fluxes ranged from about 100-5 50 mg C mP2 d-l and were low during the winter except for the November-December maximum in total particulate flux (Fig. 2). Vertical fluxes of organic carbon, like bulk particulate matter, increased with depth during individual deployments and were especially great at 90 m. Nitrogen fluxes followed essentially the same temporal and depth patterns. Lignin-derivedphenols all exhibited maximal vertical fluxes during the NovemberDecember peak in total particle input. Vanillyl phenol fluxes (Fig. 3a) ranged from about 0.2 to 13 mg m-2 d-l and were consistently elevated during winter ( 10 November-l 2 February). Measured vanillyl phenol fluxes increased with depth during every deployment in patterns that closely resembled those for bulk particulate material (Fig. 2a). Syringyl phenol fluxes consistently averaged about 25% of the contemporaneous vanillyl phenol fluxes and therefore exhibited essentially identical trends with season and depth. Cinnamyl phenol fluxes ranged from about 0.03 to 0.6 mg mm2 d-l and also increased with depth for all but one deployment. Cinnamyl phenol fluxes were more uniform seasonally than those of the other ligninderived phenols and were intermittently elevated in spring and summer 1982. Fluxes of p-hydroxyl compounds ranged from about 0.2 to 1.8 mg m-2 d-l and were greatest at 90 m during all but the first deployment. Neutral sugar fluxes (as represented by glucose, Fig. 3b) were low in winter months except for the November-December deployment. Glucose comprised about a third of the total neutral sugar flux year-round. Table 3. Annual average vertical fluxes (mg m-2 d-l) of selected particulate materials through the water column (30-, 60-, and 90-m depth) and within the surface sediments of Dabob Bay. Abbreviations: Bulk-total particulate material; MIN-mineral material (organic carbon- and calcite-free); OC-organic carbon; IC-inorganic carbon; N-total nitrogen; P-total p-hydroxy phenols; Po-p-hydroxyacetophenone; V-total vanillyl phenols; S-total syringyl phenols; C-total cinnamyl phenols; A-sum of V, S, and C, Sed-surface sediments (O-22-cm average); LX- lyxose; AR- arabinose; RI -ribose; XY - xylose; RH-rhamnose; FU - fucose; MN mannose; GA-galactose; GL-glucose; TcH~o-~~~ of all previous sugar fluxes. Annual data for sugar fluxes at 90 m are not available. Depth Bulk MIN 2,690 3,910 7,580 3,040 oc 186 224 386 82 IC N P PO V s C A 12.3 7.95 6.72 0.0 24.3 26.8 46.4 7.98 0.57 0.63 1.12 0.19 0.05 0.05 0.09 0.33 1.37 2.28 4.56 1.51 0.38 0.62 1.21 0.31 0.10 0.16 0.26 0.10 1.85 3.06 6.03 1.92 GL TCH~O 30 m 60 m 90 m Sed 3,170 4,430 8,410 3,200 LX AR RI XY 30 m 60 m Sed 0.35 0.51 0.13 1.78 2.30 0.81 1.02 1.15 0.30 2.47 3.18 0.91 RH 3.23 3.88 1.07 FTJ 2.52 2.90 0.78 MN 5.51 6.64 1.89 GA 5.74 7.00 1.80 12.6 13.7 3.42 35.2 41.2 11.1 Fluxes and reactions -i p Y fi ,M 12 10 8 i , 3 60 2 40 E g 20 DISCHARGE _____- PRECIPITATION (b 24 k E 2. lo- FROM THE NORTH o- g -lOE V-J-20 52 $ -30 -40’ 4.0 3 3.5 - 3z; 3.0 FROM THE SOUTH ’ ’ ’ ’ ’ ’ ’ (d) ki 2.5 k a” 2.0 5! la c 1.5 I l.O(!j more similar than for the lignin-derived phenols (e.g. Fig. 3). CaCOj (8.33 x IC) exceeded 2 wt% of the bulk particulate matter only in sediment trap samples collected during spring and summer. Highest vertical fluxes were observed in July and August 198 1 when CaCO, constituted -2O-25% of the sample mass. At these times carbonate fluxes decreased sharply with increasing water depth. CaCO, never exceeded 7% of the bulk particulate matter recovered from the 60- and 90-m traps. Discussion Bulk particulate matter dynamics- Temporal trends observed during 1981-1982 of 0 .P 1141 ’ ’ ’ ’ ’ ’ 300 100 200 DAYS AFlXR 1 JULY 1961 ’ I 1 400 Fig. 4. Plots vs. time of (a) the vertical flux of bulk particulate material at a depth of 60 m in Dabob Bay, (b) the daily precipitation at Quilcene, Washington (U.S. Weather Service data) and daily discharge of the Duckabush River (USGS Water Resour. Data for Washington), (c) daily average wind speed (north-south component) at West Point off Seattle (U.S. Weather Service data), and (d) daily mean differences in tidal extremes in Dabob Bay (NOAA Tide Tables). Glucose, mannose, rhamnose, and fucose exhibited particularly high vertical fluxes in March and April 1982 during the onset of the spring phytoplankton bloom. Although vertical fluxes were highest at 90 m for most sugars, ~I ~~, , fluxes at 30 and 90 m were tvnicallv elevated particulate matter fluxes during the winter and continuous input over the rest of the year (Fig. 2a) are similar to those reported by Bennett (1980) for a sediment trap study at the same site during 197719 7 8. The daily-weighted annual average flux of 4.43 g m-2 d-l obtained at 60 m in the present study (Table 3), however, is almost 50% greater than the value of 2.98 g m-2 d-l measured by Bennett with essentially identical sediment traps deployed at 50 m. Even excluding the large pulse observed in November-December 198 1, which accounted for 24% of the 198 l-l 982 total, the vertical flux of bulk particulate material (m 3.7 g mP2 d-l) measured at 60 m in the present study is 25% greater than that obtained by Bennett 4 yr earlier. Although the greater 198 l- 1982 average may be due in part to a deployment depth that was 10 m deeper (Fig. 2a), appreciable interannual differences in bulk particulate fluxes apparently occur in Dabob Bay. The bulk particulate material in the water column of Dabob Bay is largely (> 60%) aluminosilicates whose major elemental composition closely matches that of the Dosewallips River (Bennett 1980). Atmospheric dust precipitation is extremely small (~3 mg m-2 d-l) in this region (Windom 1969) and bank erosion is apparently minimal. Thus, the Dosewallips and other smaller rivers to the south, which drain into Hood Canal from the eastern Olympic highlands (Fig. I), appear to be the major ultimate sources of the suspended and sedimentary particles (Bennett 1980). 1142 Hedges et al. Given this predominant riverine origin, it is reasonable to expect that the measured particle fluxes in the water column of northern Dabob Bay might be related to local rainfall and river discharge, as well as stormdriven winds and water currents from the south. In fact, the November 198 1 pulse in bulk particle flux corresponded to a period of high rainfall after the onset of the local rainy season in September (Fig. 4a, b). The discharge of the Duckabush River (Fig. 1, the only gauged stream flowing into Hood Canal near Dabob Bay) closely paralleled rainfall at Quilcene (Fig. 4b), indicating that November was a period of regionally high rainfall and river discharge. Wind records taken at the Seattle weather station at West Point (Fig. 1) indicate that November was also a period of relatively strong and persistent southerly winds (Fig. 4~). Such winds could effectively lead to sediment resuspension and transport in Hood Canal and Dabob Bay (Ebbesmeyer et al. 1988) which are both narrow with long north-south trending fetches (Fig. 1). A strong association of sediment trapping rates with wind and water currents has been seen in other marine embayments that are removed from direct river influence (Ansell 1974; Webster et al. 1975; Taguchi 1982). Although high rainfall, river discharge, and southerly wind velocities in November 198 1 all favored sediment resuspension and transport in Hood Canal and Dabob Bay, these conditions were not unique to this period of sediment trap deployment. Even higher local rainfall and strong regional winds occurred during the December period of sediment trap deployment when only moderate vertical fluxes of bulk particulate materials were measured (Fig. 4). In addition, particle fluxes throughout the remainder of the winter were only moderately elevated over those measured during the preceding summer, at which time local rainfall was negligible and southerly winds were weak (Fig. 4). The sustained fluxes of particulate matter, even during the dry, calm summer months of 198 1 (Fig. 4), indicate that an additional continuous mechanism for sediment input must exist. According to Ben- nett (1980) this input likely results from resuspension of bottom sediments from the shallow perimeter of the bay by waves and tidal currents. In fact, maximal tidal excursions (measured as daily mean differences in height extremes) occurred in winter 198 l1982 (Fig. 4d), when associated strong tidal currents may have contributed to the November vertical flux pulse. Tidal excursions varied only moderately throughout the 198 1-1982 study period (Fig. 4d) but, as Bennett (1980) suggested, may well have provided the energy needed for sediment transport during the otherwise quiescent nonwinter months. Vertical trends in particle flux have not been previously studied in Dabob Bay with sediment trap deployments at different water depths. The consistent increase with depth in the measured fluxes of bulk particulate matter observed throughout 198 l-l 982 (Fig. 2a) indicate that particulate material does not simply originate in the surface (< 30 m) water and sink straight downward through the water column at the study site. Similar patterns of downward increasing fluxes have also been observed in sediment trap studies carried out in the central basin of Puget Sound (Bates et al. 1984; Baker 1984) and numerous other coastal (Taguchi 1982; Gardner et al. 1983) and open ocean (e.g. Fischer 1984; Dymond 1984) sites. One means of evaluating the depth trends in Dabob Bay is to compare the annual average particle fluxes in the water column to the local sediment accumulation rate. Based on 210Pb analyses of six different sediment cores, the average bulk sediment accumulation rate at the study site is 3.2OkO.61 g me2 d-l (Carpenter et al. 1985; Furlong 1986). In comparison, the time-weighted annual average fluxes of bulk particulate material measured during 198 1-1982 at 30, 60, and 90 m are 3.17,4.43, and 8.4 1 g me2 d-l (Table 3). These total fluxes can be better compared after subtracting the variable organic matter (N 2 x OC) and CaCO, (8.33 x IC) components (Hedges et al. 1988). The residual annual average “mineral” fluxes at 30 and 60 m of 2.69 and 3.91 g m-2 d-l bracket the average sedimentary mineral accumulation rate of 3.04 g mm2d-l. The mea- Fluxes and reactions sured flux of particulate mineral material at 90 m of 7.58 g m-2 d-l, however, exceeds this sediment accumulation rate by a factor >2. This result, plus the elevated concentrations of suspended particulate matter often observed by Bennett (1980) in the lower (> 85 m) water column of Dabob Bay, suggests that a major portion of the total mass collected by the sediment trap at 90 m may be resuspended or horizontally advected particles that are “trapped” in transit, but not efficiently incorporated in the underlying sediments. The observation that the measured fluxes at 30, 60, and 90 m are strongly proportional throughout the year regardless of overall magnitude (Fig. 2a) supports the conclusion of Bennett (1980) that much of the material collected at all depths has a common origin from the shallows of the bay. In contrast, local (deep water) resuspension should exhibit varying thresholds of upward penetration in the moderately stratified water column (Ebbesmeyer et al. 1988) and not be proportionally expressed year-round over the entire 30-90m depth interval. Both gravitational settling and gleaning by zooplankton may serve to progressively attenuate the near-surface component of a shallow lateral input, as has been observed in greater Puget Sound (Baker 1984). Lignin and elemental compositions of particulate organic material in all the sediment trap samples plot colinearly, but more closely resemble bottom sediment compositions at greater trapping depths (Hedges et al. 1988, their figure 6). This relationship suggests a single downward-intensifying source of bulk particulate material. The uniform organic compositions of sediment trap materials collected at all depths during winter months (Hedges et al. 1988) support the previous evidence that this source is shallow. The fact that the winter trap and surface sediment samples from the study site resemble each other so closely in all their organic characteristics (Hedges et al. 1988) may reflect their sequential derivation from common local river and shallow bay sources (Bennett 1980), rather than deep-water sediment resuspension alone. 1143 Average organicfluxes- To determine the annual average daily fluxes of different organic materials, we estimated the compositions of the missing 60- and 90-m trap samples from the 12 February-l 9 March period via interpolation with compositions for the bracketing deployments, using data from the continuous 30-m sample set to establish the fractional differences. The total particle fluxes at 60 and 90 m during the missing period were estimated from the corresponding 30-m flux with the previously discussed mean proportionality factors. These relatively small adjustments (< 10%) seem warranted by the consistent composition and flux trends observed at all depths during individual deployments (Hedges et al. 1988) and, in view of the high seasonal variability in organic compositions, were judged preferable to the error involved in omitting all flux data for this period. Before discussing the calculated annual average fluxes (Table 3), it is necessary to address the question of how representative was the 198 l-l 982 study period. In the case of lignin-bearing particles this question can be directly treated by comparing the mean mineral fluxes at 30 and 60 m to the average sediment accumulation rate. This approach is feasible because detrital minerals and lignin-bearing particles should be introduced from land via rivers in relatively constant proportion (Hedges and Mann 1979; Hedges et al. 1982, 1984). Fluxes at 90 m are excluded from this evaluation due to the likelihood of large “excess” contributions by resuspension and horizontal advection (previous discussion). The fact that the average annual lignin fluxes at 30 and 60 m are proportional to the corresponding mineral fluxes, which in turn bracket the annual mineral sedimentation rate (Fig. 5), indicates that the study period likely is representative in terms of terrigenous particle input. The good correspondence of these two independently determined fluxes also suggests that the water column values are not grossly in error due to sediment trap sampling biases (e.g. Butman et al. 1986). How representative the study period was for autochthonous organic matter input can be evaluated in relation to long-term mean 1144 Hedges et al. values at the site for primary production and the vertical fluxes of organic carbon, the latter of which is predominantly derived from plankton (Prahl et al. 1980; Hedges et al. 1988). Although primary production data are not available for July-December 198 1, the average for January-July 1982 is 980 g C m-2 d-l (J. Downs unpubl. data), as compared to an overall mean of 9 10 g C m-l d-l for the same monthly period during 1979-1985 (Welschmeyer 1982; Downs and Lorenzen 1985; J. Downs unpubl. data). Likewise, the annual average total organic carbon flux of 224 mg C m-2 d-l measured at 60 m in the present study (Table 3) is intermediate between the 1977-l 978 mean of 192 mg C m-2 d-l (Bennett 1980; Lorenzen et al. 198 1) and the average of 267 mg C m-2 d-l for 1982-1985 (Downs and Lorenzen 198 5). Therefore, all indications are that the 198 1-1982 study period also was typical in terms of the production and vertical flux of plankton-derived organic matter. On the basis of the previous results, it is reasonable to compare the annual average water column fluxes to the corresponding mean surface sediment (O-22 cm) accumulation rates (Table 3) in order to determine the extent of organic matter degradation at the water-sediment interface. Such a comparison is illustrated in Fig. 5 in the form of ratios of water column : sediment accumulation flux (R) at 30 and 60 m. The (O/00) correpercentages of degradation sponding to these ratios, %D = lOO( 1 1/R), are also given in Fig. 5. Consistent patterns between compound type and reactivity at the water-sediment interface are found (Fig. 5). For example, as a group, the unambiguously lignin-derived phenols are most efficiently transported from the upper water column into the compositionally uniform surface mixed layer of Dabob Bay sediments. The unadjusted flux ratios for total phenols from individual structural families range from 0.91 to 1.25 at 30 m, but increase with water depth in direct proportion to mineral flux and are consistently >2.5 for the 90-m samples. Normalization to mineral flux (previous discussion) removes most of this depth dependency and yields average mineral-nor- TRAP : SEDIMENT 5r FLUX RATIOS 180 4- E3 s E oc 2- &i ’ 01 MIN I Fig. 5. Ratios of the annual average fluxes of particulate materials through the upper (30 m) and mid (60 m) water column of Dabob Bay to the corresponding net burial rate in the underlying surface (O-22 cm) sediment. A ratio of 1 indicates conservative passage across the water-sediment interface, whereas ratios > 1 correspond to the percentages of degradation indicated on the right vertical axis. Ratios corresponding to the 60-m water column flux are underlined. Abbreviations as in Table 3. malized R values for all three sampling depths of 1. 1 + 0.1 for vanillyl and cinnamyl phenols and 1.5 +O. 1 for syringyl phenols. These calculations indicate that vanillyl and cinnamyl phenols are not appreciably degraded in the water column or at the water-sediment interface of Dabob Bay. In contrast, syringyl phenols, which exhibit remarkably constant proportions to vanillyl compounds in the water column (Hedges et al. 198 8), are measurably (30-40%) degraded at the water-sediment interface before ultimate burial. This result is in agreement with independent evidence for preferential syringyl phenol loss from angiosperm tissues degraded under both natural (Flaig 1964; Hedges et al. 1985; Ertel and Hedges 1984; Ertel et al. 1986; Haddad et al. 1988) and laboratory (Crawford 198 1; Hedges et al. in press) conditions. The evidence for conservative deposition of cinnamyl phenols is surprising because both ferulic and p-coumaric acid have unsaturated sidechains and are at least in part ester linked to relatively labile hemicellulose components of nonwoody vascular plant tissues (Atsushi et al. 1984; Smith and Hartley 1983). Preferential diagenetic loss of cinnamyl phenols has also been observed in Spartina detritus (Wilson et al. 1985; Fluxes and reactions Haddad et al. 1988) and inferred for the humification process in general (Ertel et al. 1984). Uniformly elevated acid : aldehyde ratios of the vascular plant debris in the sediment trap samples (Hedges et al. 1988), however, give evidence of microbial degradation before introduction, when especially labile lignin constituents may already have been lost. It appears that the lower cinnamyl : vanillyl phenol ratio of the sediment vs. the spring and summer trap samples (Hedges et al. 1988) results primarily from dilution by woody debris introduced during fall and winter and not from in situ diagenesis. In our previous paper (Hedges et al. 1988), we inferred, from the poor association between total vanillyl and total p-hydroxyl phenol concentrations in the sediment trap samples, that the latter compounds are predominantly plankton derived. This conclusion is strongly supported by the distinctly higher measured flux ratios (3.1-6.0) of the total p-hydroxyl phenols across the sediment-water interface (Fig. 5). Even with mineral normalization, these ratios do not decrease below 2.6 and thus correspond to a minimal degradation of 60%. The only exception to this overall trend is the relatively minor ketone member of this family, p -hydroxyacetophenone, which exhibits lower flux ratios similar to syringyl compounds. This phenol is not produced by the Cu0 oxidation of plankton (Hedges 1975) and covaries in the water column in direct proportion to the unambiguous lignin-derived phenols (Hedges et al. 1988) with which it apparently should be categorized. Thus, p-hydroxyacetophenone offers a possible means of distinguishing lignin vs. nonlignin sources of p-hydroxy Cu0 reaction products. The similar fluxes at 30 and 60 m of particulate organic carbon and nitrogen (Fig. 5) are to be expected because these two elements are associated primarily with plankton remains (Hedges et al. 1988) that originate in the upper water column and are vertically transported in fast-sinking zooplankton fecal pellets (Bennett 1980; Lorenzen et al. 1983; Downs and Lorenzen 1985). The mean flux ratios of organic carbon and nitrogen at 30 and 60 m of 2.5 and 1145 3.2 correspond to percentages of loss of 60 and 70%. Because these elements, in contrast to phenols and sugars, remain detectable throughout intermediate stages of degradation, they must be either completely solubilized or remineralized to disappear. Although the apparently higher percentage loss of organic nitrogen vs. carbon at the water-sediment interface (Fig. 5) is in agreement with trends generally seen for plankton degradation (Grill and Richards 1964; Suess and Miiller 1980), this conclusion cannot be confidently drawn from these data alone because the total flux ratios include a substantial portion (40-50%, cf. later discussion) of refractory organic material. All the neutral sugars have measured flux ratios in the upper water column that are consistently in the range of 2.2-4-O (Fig. 5) and, like ratios for OC and N, exhibit little sensitivity to water depth. The corresponding range of 5 5-75% sugar degradation includes the reactivity intervals for bulk organic carbon and nitrogen and suggests that the particulate polysaccharides sinking through the upper water column also are largely autochthonous. The same conclusion was drawn previously based on the aldose composition of the sediment trap samples (Cowie and Hedges 19843). The similar reactivities of the neutral sugars (Fig. 5) suggest little preferential degradation of individual compounds even though the overall extents of loss are large. The slightly higher reactivities of ribose and glucose, however, may be real because these two aldoses often occur, respectively, in relatively labile nucleotides and storage polysaccharides (Aspinall 1983). The reason for the consistently lower reactivity of arabinose (Fig. 5), which has many sources and biochemical forms, is unknown. Compositions of the labile componentThe colinear inverse relationship of nitrogen and lignin-derived phenols in all the sediment trap and core samples (figure 6 of Hedges et al. 1988) indicates that all these mixtures contain, in addition to autochthonous organic matter, a refractory component whose organic composition resembles local sediments. To determine the composition of the labile component alone, we must correct for this background sedimen- Hedges et al. 1146 [ SEDIMENT 80 m PLANKTON 70 t i VASCULAR 2 8 f50 50 2 40 0 iX iR FiI PLANTS iY IiH FiJ tiN dA & TCiI20 CONSTITUENTS Fig. 6. Annual mean weight percentages and total yields (mg per 100 mg OC) of neutral sugars (aldoses) in labile sediment trap material (30- and 60-m avg), surface sediment (0-22-cm interval), net plankton, and vascular plant debris in Dabob Bay. Abbreviations as in Table 3, and TcH20-total of aldose from 100 mg of organic carbon. tary fraction. This correction can be made for individual organic constituents by determining the mass ratio of that constituent to the refractory mineral component of the surface sediment, multiplying that ratio by the annual average mineral flux at a given water column depth, and subtracting that product from the corresponding total annual average flux (Gasith 1975; Bennett 1980; Taguchi 1982). The results of these calculations will not be reported for the 90-m samples or any of the lignin-derived phenols, both of which have such large refractory fractions (> 50%) that calculations based on differences become imprecise. The calculated annual average atomic C : N ratios of the labile organic material at 30 m (7.7) and 60 m (8.3) are somewhat higher than the value of 6.6 that was similarly determined for the midwater column at this site during 1977-l 978 (Bennett 1980). Our values also are greater than the range of 5.4-6.4 for “phytoplankton” (64-300 pm) samples collected during the study of Hedges et al. (1988) and the value of 6.6 suggested by Redfield et al. ( 1963) for “average marine plankton.” This difference likely is primarily due to preferential assimilation of nitrogen vs. carbon by herbivorous zooplankton (Prahl et al. 1980). In fact, Downs and Lorenzen ( 1985) reported an increase in atomic C: N from 6.1 for laboratorygrown Thalassiosira weissflogii to 7.5 for the fecal pellets of Calanus pacificus fed on this diatom. The laboratory feeding experiments of Landry et al. (1984) with these same two abundant organisms from Puget Sound gave elemental fractionations bracketing the above result. This interpretation is also consistent with field studies which indicate that the bulk of the vertical particle flux in Dabob Bay is as zooplankton fecal pellets (Shuman 1978; Bennett 1980; Downs and Lorenzen 198 5). The neutral sugar composition of the organic material being degraded can be similarly calculated and resembles that of the underlying surface (O-22 cm) sediment (Fig. 6) except for a lower percentage of arabinose, a higher percentage of glucose, and a greater total sugar yield. The composition of the labile aldose fraction also is similar to local plankton (Fig. 6) except for a higher percentage of rhamnose and lower percentages of ribose and glucose. The calculated yield of total neutral sugars from the labile organic fraction (TcH~O = 22 mg per 100 mg OC) is about twice that of local net plankton (TCH~O = 14 mg per 100 mg OC) but ~25% of the yield expected for the average mixture of vascular plant tissues found in the water column (Hedges et al. 1988, Fluxes and reactions (198 who late umn 0 50 loo 150 200 250 FLUX , mg rnm2d-’ Fig. 7. Uncorrected and normalized annual average fluxes of marine and terrestrial organic carbon through the upper water column (30 and 60 m) and into the surficial (O-22 cm) sediments of Dabob Bay. Normalized vertical fluxes of terrestrial organic carbon were calculated by multiplying the observed fluxes by corresponding normalized mineral fluxes, where a normalized mineral flux = (trapped mineral flux)/(net sedimentary mineral accumulation rate). The marine organic carbon fluxes were obtained by difference and are not normalized (see text fir discussion). TCH~O = 94 mg per 100 mg OC), which has a very different initial carbohydrate composition (Fig. 6). Thus, the carbohydrate compositions of the labile fraction are consistent with a mixture of predominantly planktonic origin that has been altered by preferential glucose loss and the introduction of rhamnose from some other upper water column source (Cowie and Hedges 19843). Organic matter budgets-Annual average budgets for bulk organic carbon and the corresponding nitrogen and major biochemical constituents of the Dabob Bay samples can be calculated by combining compositional data from our previous paper (Hedges et al. 1988) and Cowie and Hedges (1984b) with yearly water column fluxes (Fig. 2), sediment accumulation rates (Carpenter et al. 1985; Furlong 1986), and primary production estimates (Downs and Lorenzen 1985) for the study site. Previous budgets of this type have been published for bulk organic carbon by Bennett (1980) Lorenzen et al. 1147 l), and Downs and Lorenzen (1985) all measured annual average particuOC fluxes through the midwater col(50-60 m) at the Dabob Bay station. Average total organic carbon fluxes during 198 l-l 982 increased with depth from a value of 186 mg C m-* d-l at 30 m to 224 at 60 m and 386 at 90 m (Table 3). As previously discussed, downward increases in OC and total particulate fluxes are seen yearround (Fig. 2) and result primarily from greater resuspension or horizontal advection of sedimentary particles at depth. Nevertheless, the annual average total OC fluxes at 30 and 60 m measured during 198 l- 1982 agree well with corresponding values of 192 and 267 mg C me2 d-l previously measured in 50-60 m of water at the study site by Bennett (1980) and Downs and Lorenzen (1985), respectively. The sedimentary organic carbon accumulation rate of 82 mg C m-* d-l obtained in the present study is about half of the flux reported by Lorenzen et al. (198 l), but agrees well with a mean of 84 + 15 mg C m-* d-l obtained in three other studies (Bennett 1980; Prahl and Carpenter 1979; Furlong 1986). Thus, there is good general agreement among the annual average fluxes of total organic carbon measured at the Dabob Bay study sit2 over the last decade. A detailed organic carbon budget for 198 l1982 can be determined by separating the previous total organic carbon fluxes into marine and terrigenous components. The bulk terrigenous component can be estimated by dividing the annual average lignin phenol yields from the sediment trap and core samples (Table 3) by the yields estimated for local riverine organic matter (A = 3.5: Hedges et al. 1988). The calculated annual average fluxes of total terrigenous organic carbon increase sharply with depth in the water column from 53 mg C m-* d-l at 30 m to 87 and 172 at 60 and 90 m. As previously discussed, this increase is largely due to resuspended or horizontally advected (or both) sedimentary material as well as a somewhat greater than average input of particulate material during 198 l-l 982. Once normalized, based on the ratios of corresponding trap mineral fluxes to the average accumulation rate of mineral material 1148 Hedges et al. in the underlying sediment (Table 3) the fluxes of terrigenous organic carbon at 30, 60, and 90 m in the water column all fall within the range of 65 + 5 mg C m-* d-l (Fig. 7). In comparison, the net burial rate of organic carbon in the underlying sediment (O-22 cm) is 55 mg C m-* d-l. These calculations indicate that little if any degradation of terrigenous organic matter occurs between 30 and 90 m in the water column of Dabob Bay and agree with previous evidence (Fig. 5) for the high relative stability of the lignin component at the water-sediment interface where the time of exposure to degradation is much longer. This result, however, may not hold for the euphotic zone because the near-surface rate of terrigenous organic matter input and the ability of zooplankton to assimilate such material are both unknown. The implication from the flux data, that about 15% (m 10 mg m-* d-l) of the terrigenous organic carbon input to the water-sediment interface is remineralized, agrees with previous evidence (Fig. 5) that syringyl lignin is degraded within this zone. Vascular plant remains account for at most (if well preserved) about half of the allochthonous organic carbon passing through the water column and into the sediments of Dabob Bay (Hedges et al. 1988). On the basis of estimates that at least 40% of the original neutral sugar in this debris has been lost (Hedges et al. 1988) this lignin-based estimate for vascular plant material flux could well be too high by a factor of two or more. This uncertainty does not affect the previously calculated fluxes of total terrigenous organic carbon, but does mean that the fraction of total land-derived organic material that is present in forms other than chemically recognizable plant debris may considerably exceed 50%. As previously discussed, the marine component of the total organic carbon flux for 198 1-1982 should be typical of the longterm average. The average annual fluxes of marine-derived organic carbon at 30 m (133 mg C m-* d-l) and 60 m (137 mg C m-* d-l) (Fig. 7) were calculated from the difference between the total measured fluxes at those depths and the mineral-normalized terrigenous component (previous discussion). An accurate corresponding flux of marine-derived organic carbon at 90 m cannot be determined due to the large amount of resuspended material in the deep trap samples, which includes a marine component that does not vary in direct proportion to mineral content. The similar marine and organic carbon fluxes at 30 and 60 m, however, suggest that autochthonous particulate organic matter also is not appreciably degraded as it sinks through the water column of Dabob Bay. Minimal degradation is in fact to be expected from previously reported evidence that this vertical flux from the euphotic zone (< 30 m, J. Downs pers. comm.) is primarily in the form of large zooplankton fecal pellets (Bennett 1980; Downs and Lorenzen 1985; Welschmeyer 1982) which should traverse the 110-m water column in about a day (Lorenzen et al. 1983). It is reasonable to expect, therefore, that the effective net flux of autochthonous organic carbon through 90 m and to the floor of the bay is similar to the average flux of 135 mg C m-* d-l measured at 30 and 60 m. The average flux of total organic carbon through the water column of Dabob Bay, therefore, is -200 mg C m-* d-l. The above calculations indicate that 67% of the total organic carbon flux through the water column of Dabob Bay is marine derived, with the other 33% being of terrigenous (riverine) origin (Fig. 7). The 33% terrestrial estimate agrees well with the observation that about half of the annual average flux of organic carbon through the upper water column consists of refractory organic matter associated with sinking mineral material. This value also lies within the bounds of previous estimates that a major fraction of the bulk OC flux is not associated with plankton biomarkers such as chlorophyll pigments (Downs and Lorenzen 1985) and pristane (Prahl et al. 1980; Bennett 1980). The lignin-based estimates of the marine and terrigenous fluxes, therefore, are consistent with source distinctions based on various other independent tracers. The average annual nux of 135 mg m-* d-l of marine organic carbon through the water column of Dabob Bay represents 14% of the mean annual primary production of roughly 1,000 mg C m-2 d-l (Downs and Lorenzen 1985; J. Downs pers. comm.) at Fluxes and reactions this site. If this particle flux consists almost exclusively of recently egested zooplankton fecal pellets (Bennett 1980; Prahl et al. 1980; Downs and Lorenzen 198 5) that are not appreciably degraded as they sink through the upper water column, then the 86% difference between the primary production rate and the particulate OC flux should correspond approximately to the mean assimilation efficiency of the zooplankton community feeding in the euphotic zone above 30 m. In fact, Landry et al. (1984) have determined average assimilation efficiencies of 70-85% for the copepod, C. pacz$cus, which was acclimated in the laboratory to different concentrations of the diatom, T. weissjlogii. Although the estimated 86% annual average assimilation value for the natural plankton community may be elevated by carnivory, coprophagy, and other forms of secondary utilization, the overall agreement to laboratory results with locally abundant species is excellent. On the basis of an average sediment accumulation rate of 3.20 g m-* d-l (Carpenter et al. 1985; Furlong 1986), a mean %OC of 2.56, and an estimated 3 5% marine-derived origin (Hedges et al. 1988), the flux of preserved marine organic matter through the surface mixed layer of the underlying sediment is 29 mg C m-* d-l. This value corresponds to - 20% of the autochthonous organic carbon flux to the water-sediment interface and 3% of the mean annual primary production (Fig. 7). Thus, on average, about 105 mg m-* d-l of autochthonous organic carbon, equivalent to 10% of the primary production, is lost at the watersediment interface of Dabob Bay as a result of biological respiration. This calculated rate of marine carbon remineralization along with the previously estimated 10 mg m-* d-l of terrigenous carbon loss corresponds to a total of about 125 mg C m-* d-l of heterotrophic utilization at or near the water-sediment interface. If we assume a 0.80 molar relationship between carbon respired and O2 utilized (Pamatmat 197 l), this carbon flux is equivalent to an annual mean oxygen consumption rate of roughly 12 ml m-2 h-l. As a part of a summer oxygen budget for the deep waters of Dabob Bay, Christensen and Packard (1976) 1149 also calculated a mean benthic O2 consumption rate of 12 ml m-* h-l. This value is also similar to the rate of 13.6 ml m-2 h-l measured directly during summer at the study site by Bennett (1980) and to rates obtained in greater Puget Sound (Pamatmat and Banse 1969). Therefore the difference in the measured organic carbon fluxes for 198 1-1982 (Fig. 7) agree well with independently measured respiration rates within surface sediments of Dabob Bay. The efficiency of preservation of marine organic matter in the surface mixed zone of Dabob Bay sediments is only about a third of that which would be deduced from measurements of total organic carbon alone. About 80% of the vertical flux of marine organic matter through 30 and 60 m is degraded at the water-sediment interface, as compared to only about 15% (after mineral content normalization) of the terrigenous organic carbon in the same rain of particles. Even after suffering > 80% remineralization in the upper water column, the marine-derived organic matter reaching the watersediment interface is approximately a factor of five times more reactive than the accompanying terrigenous organic material. As a result of this reactivity difference, land-derived organic matter accounts for about twothirds of the total organic carbon in the surface sediments, but supports < 10% of the total respiration at the water-sediment interface. The benthos of Dabob Bay utilize almost exclusively autochthonous food sources. A detailed nitrogen budget can be worked out similarly. For example, the annual average fluxes of total particulate N through the upper water column and into the underlying surface sediment are 25.5 (30- and 60-m average) and 7.98 mg m-* d-l, respectively (Table 3). If we assume a mean atomic C : N ratio of 7.5 for plankton fecal remains (previous discussion), the marine components of the above total N fluxes can be resolved on the basis of the corresponding carbon fluxes. The resulting estimated fluxes of marine-derived N through the water column (20.8 mg m-* d-l) and across the water-sediment interface (4.5 mg m-* d-l) correspond to roughly 10 and 2% of the mean annual photosynthetic N production 1150 Hedges et al. (205 mg N m-* d-l) calculated from an average atomic C : N of 5.7 for local plankton (Hedges et al. 1988) and an annual average primary production rate of roughly 1,000 mg C m-* d-l (Downs and Lorenzen 1985; J. Downs pers. comm.). Plankton remains account for -80% of the total particulate nitrogen sinking through the upper water column and 55% of the total nitrogen accumulating in the O-22-cm sediment horizon. A total sugar budget based on an average TCH,O of 7.6 mg per 100 mg OC for Dabob Bay phytoplankton (Cowie and Hedges 1984b) indicates a total photosynthetic input of 76 mg sugar m-* d-l. The corresponding annual average flux through the upper water column is 38 mg sugar m-* d-l (average for 30 and 60 m), as compared to a mean sediment burial rate of 11 mg me2 d-l. Due both to uncertainty concerning the extent of sugar depletion in the vascular plant debris and to clear evidence for appreciable polysaccharide input from sources other than plankton and vascular land plants (Hedges et al. 1988) it is not possible to resolve the total neutral sugar fluxes into meaningful components. It is clear, however, that neutral sugars as a group have a comparatively high ratio of water column flux to primary production (R = 0.50) in comparison to their lability (-70% loss) at the water-sediment interface (Fig. 5). This quantitative result supports previous compositional evidence (Hedges et al. 1988) for an appreciable upper water column source of labile polysaccharides in addition to marine plankton. Downs and Lorenzen 1985; Furlong 1986; Furlong and Carpenter 1988), it appears that plankton-derived lipids (m 90% loss) and plant pigments (-99% loss) are even more reactive than carbohydrates toward degradation at the water-sediment interface. Although many of these reactivity differences have long been presumed (e.g. Lyons and Gaudette 1979; Westrich and Berner 1984) they have not been as clearly demonstrated for so large a grouping of organic substances as has been possible in Dabob Bay. The extreme diversity of these reactivities has important implications for optimal feeding strategies at the water-sediment interface, for interpretations of sedimentary organic matter records, and for biomarker applications. In the latter example it is clear that the degree to which a given organic “tracer” is representative for a wider class of organic materials of similar geographic or biological origin must be given careful scrutiny, lest a poor reactivity match between an organic fraction and its assigned biomarker lead to inaccurate conclusions concerning distributions and diagenetic fates. It is apparent also from even this preliminary study, that carbon cycling in coastal marine environments cannot be well understood without paying close attention to the sources of local organic materials and their reactions at the immediate water-sediment interface. 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