[developments in geochemistry] diversity of environmental biogeochemistry volume 6 || behaviour of...

BEHAVIOUR OF DISSOLVED ORGANIC MATTER IN PORE WATERS OF NEAR-SHORE MARINE SEDIMENTS

J. FAGANELII* and G.J. HERNDL2

1Marine Research Centre, 66330 Piran, Yugoslavia 2lnstitute for Zoology, University of Vienna, A-1090 Vienna, Austria

ABSTRACT

Pore water samples, collected from short sediment cores in the Gulf of Trieste over a yearly cycle, were analysed for dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DIP), as well as ammonia and phosphate. The concentrations of DOC, DON and DIP in pore waters were up to 10-fold higher than in the overlying water. The concentrations of DOC increased with sediment depth, while DON and DIP showed no trend. Pore water DIM concentrations were highest in late summer. This is thought to be caused by a higher microbial activity and restricted upward fluxes towards the overlying water because of a strong pycnocline just above the bottom. A mean DOC:DON:DOP ratio (atomic) of 35:9:1 was computed. Low DOC:DON ratios, rang-ing between 2-4, were taken to indicate the predominance of proteinaceous matter.

The benthic fluxes of DOC, DON and DIP, measured in parallel with 02 consumption in situ with benthic chambers, showed in general a net influx, except for DOC in the period of high pore water DOC content and benthic 02 consumption when strong DOC fluxes out of the sediment prevailed. The bulk of DOC was found to be refractory or decomposed in anoxic conditions. In contrast to DOC behaviour, DON and DIP are more reac-tive involving aerobic and anaerobic pathways of decomposition. The DOC benthic fluxes were strongly temperate dependent, and the high apparent activation energy computed indicated a significant proportion of a high molecular weight fraction of DOC, while DON and DIP fluxes were not directly temperature depend-ent, probably because of a random fluctuating supply of degradable sedimentary organic matter and oxidants. Constructing the budgets of C, N and P, including the DOC, DON and DIP benthic fluxes, it was found that the total N and P inputs are finally all utilized in benthic primary production while the balance of C remains obscure.

Keywords: dissolved organic matter, benthic fluxes, oxygen consumption, pore water consumption, Northern Adriatic

INTRODUCTION

Dissolved organic matter (DOM) in pore water is produced during early diagenesis of organic matter in recent marine sedi-ments (Krom and Sholkovitz, 1977). High concentrations of DOM in pore waters of dif-ferent marine sediments as compared to

overlying water (Lyons et al., 1979; 1980; 1982; Whelan, 1974; Watson et al., 1985) suggest that bacterial metabolic processes play an important role in its production (Barcelona, 1980) . Furthermore, D O M is thought to be an intermediate in the forma-tion of humicmaterial and kerogen (Nissen-baum and Kaplan, 1972) and at high con-

* Corresponding author

158 ORGANIC MATTER IN SEDIMENT PORE WATERS

TABLE 1

Basic sedimentological and geochemical data for sampling points MA, E and F (0-5 cm)

Sampling point/ depth (m) MA (15) E (8) F (20)

1Grain size %< 64 mm 98

94 1Carbonate % 22

35

Org. C o/o 1.38

0.99 Tot. N % 0.12

o.11

Tot. P (ppm) 268

205 Org. R (ppm) 84

14

C:N (atomic) 13.3

1o.5 C:N:P (atomic 439:33:1

1888:180:1

d13C (%o) —20.4 —23.0 2Bacterial No.

C 108 (g sed.dry wt.)-1 6.6 2.6

36 65 0.67 0.08 214 64 9.8 280:29:1 —22.4

3.5

10gorelec et a1., in preparation; 2Herndl et a1., 1987.

centrations it can inhibit CaCO3 precipita-tion (Berner et al., 1970) and rhodocrosite nucleation (Holdren, 1977) in pore waters.

Despite the biogeochemical importance of DOM in pore waters, the spatial and tem-poral distribution and the role of DOM in pore waters and at the sediment—water interface is at present rather unclear. A detailed knowledge of DOM content and reactivity in pore waters and at the sedi-ment—water interface would be important in understanding the mechanism of early diagenesis of sedimentary organic matter, formation of humates and kerogen, and the impact of pore water DOM on DOM dis-tribution and reactivity in the sediment—water interface and overlying water.

The present paper deals primarily with the spatial and seasonal variations of DOM content, in terms of dissolved organic carb-on (DOC), nitrogen (DON) and phosphorus (DIP), in surficial near-shore marine sedi-ments of the Gulf of Trieste (Northern Adriatic). We also present the results of in

situ measurements of DIM benthic fluxes over a yearly cycle and the reactivity of DIM at the sediment—water interface.

MATERIALS AND METHODS

The present study was conducted in the central and southern part of the Gulf of Trieste, the northernmost and shallowest part of the Adriatic Sea (Fig. 1). The maxi-mal depth of the Gulf does not exceed 30 m. The waters of the Gulf in general remain permanently oxygenated throughout the water column, except during the late sum-mer when the strong pycnocline is located just above the bottom (approx. 1 m) result-ing in bottom waters being depleted in oxygen. The near-shore sediments of the Gulf consist principally of clayey silt, while the sediment of the central part of the Gulf is composed of silt and silty sand (Table 1). The coarse fraction is mostly composed of foraminifers and skeletons of moluscs. The carbonate content and the grain-size com-

ORGANIC MATTER IN SEDIMENT PORE WATERS 159

Fig. 1. Sampling points in the Gulf of Trieste (Northern Adriatic).

position decrease from the central part of the Gulf towards the shore. The mineral composition of the surficial sediments is fairly uniform. Quartz prevails over clay minerals, carbonates and feldspar. The

authigenic minerals are aragonite and pyr-ite, the latter being an indication of a reduc-ing environment in the sediment, which usually appears a few centimeters below the surface.


Sediment core collections and benthic flux measurements were performed in three locations in the Gulf (Fig. 1), situated in the Bay of Piran (sampling points MA and E) and in the central part of the Gulf (sampling point F) from September 1985 to December 1986, approximately bimonthly. Sediment samples were taken by SCUBA diving, inserting a glass tube (6 cm in dia-meter) directly into the sediment. Approxi-mately 100 ml of sea water above the sedi-ment surface (supernatant water) was capped and about a 30 cm long core stop-pered, stored upright and transported to the laboratory within 4 hours of collection. They were then cut into 2 cm segments in the surface and 5 cm segments below which were successively used for extraction of pore waters. The extraction was performed by high speed centrifugation at 15,000 rpm for about 15-20 min. The residual sediment samples were freeze-dried, homogenized and used for organic C, N and P analyses.

Measurements of in situ benthic fluxes of DOC, DON, DIP, NH4+ and P043- and sediment 02 consumption were performed in duplicate using Plexiglas benthic cham-bers covering an area of 0.07 m2 of sediment and containing 10.5 1 of sea water. Each deployment lasted about 24 hours, and each chamber was sampled two or three times during this period. Water samples were withdrawn from the chamber with a 50 ml syringe after stirring the enclosed water to ensure homogenous distribution of dis-solved compounds. Care was taken to avoid disturbing the sediment during stirring. Sediment 02 consumption was measured in parallel using a battery-driven UW-respi-ration set equipped with polarographic 02

sensors (Svoboda and Ott, 1984). Before chemical analyses all water sam-

ples were filtered through Sartorius mem-

brane filters of 0.2 mm pore size, previously leached with organic-free distilled water to minimize contamination of the samples. Supernatant water and overlying water from the benthic chambers were analysed directly, whereas pore waters required about 1:10 dilution prior to analysis. All water analyses were performed within 24 hours of sample collection.

The filtrates were analysed for DOC using a total organic carbon analyser (Be-ckman Tocamaster, Mod. 915-B) at a com-bustion temperature of 950°C. DON and DIP in filtrates were determined with the wet oxidation method of Valderrama (1981) using potassium persulphate as an oxidant. DON and DIP were calculated from the difference between the total dissolved N and the sum of NO3, NO2- and NH4+, and between total P and P043-, respectively, using standard methods (Grasshoff, 1976) . The precision of the analytical methods for DOC, DON and DIP were ±4%, ±3% and ±2%, respectively.

Organic C in freeze-dried sediment samples was determined by the wet com-bustion method of Gaudette et al. (1974), and total N by the Dumas method (Keeney and Bremner, (1967). Total P was deter-mined by ignition and inorganic P by ex-traction with 1 N HC1 (Aspila et al., 1976). Organic P was taken as the difference be-tween total and inorganic P. For isotopic C analyses of sedimentary organic matter the samples were washed in 3 N HC1, ignited to CO2 and the isotopic C composition deter-mined using a Varian MAT 250 Mass spec-trometer. Isotope ratios are reported using d13C notation, with values referenced to the PDB standard. The precision of the meas-urements of sedimentary organic C, N and

3 P were ±5%, and those of d1 C ±0.2%.

ORGANIC MATTER IN SEDIMENT PORE WATERS

161

RESULTS

Composition of pore water DOM and of solid phase

Pore water concentrations of D O C and DON at all sampling points ranged around 200 and 3000 mM respectively, while those of DIP ranged between 2 and 200 mM. The inter-site differences in pore water DIM composition were minimal. The pore water contents of DOC, DON and DIP were ap-proximately up to 10 times higher than those determined in the supernatant waters, and also in the coastal water col-umns of the Gulf of Trieste (Faganeli and Herndl, in press). The depth distribution of DOC contents exhibited, with few excep-tions, a general pattern of increase with depth, while DON and DIP contents showed no consistent variation profiles, as shown for sampling points MA and F in Figs. 2 and 3. Considering the temporal variations of pore water DIM contents, the highest DOC, DON and DIP contents were regularly found in late summer and aut-umn (September-November). The mean DIC:DIN:DOP ratio (atomic), computed for all three sampling points and averaged over the sediment depth, was approximate-ly 35:9:1. The DOC:DON ratios were low, varying mostly in the range between 2-4, thus reflecting the high DON content in these pore waters.

Pore water concentrations of NH4+ and P043-, measured in parallel with DIM con-tents at all sampling points, varied between approx. 20 and 350 mM and between approx. 20-20 mM, respectively. Again, the inter-site differences between the three sampling points were minimal. With few exceptions, the concentrations of NH4+ increased with depth, while those of P043- showed an ir-

regular depth related pattern, as shown in Figs. 2 and 3 for sampling points MA and F. The highest values of NH4+ and P043- were found in late summer and autumn (Septem-ber-November), in parallel with DOC, DON and DIP maxima in pore waters. NH4+/PO43- ratios (atomic), averaged over sediment depth, were high, mostly exceed-ing 30.

The surficial sediment (0-5 cm) at the sampling point MA contained the highest values of organic C and total N, and total and organic P, in comparison with the contents at the sampling points E and F (Table 1). d13C values at the sampling points varied in the range between -20.4 and -23.0%0 (Table 1).

DOM fluxes across the sediment-water interface and the sediment oxygen consumption

The results of the fluxes of DOC, DON and DIP across the sediment-water interface and of sediment 02 consumption for all sampling points are presented in Figs. 4 and 5. The DOC and DON benthic fluxes ranged from -336.7 to +290.0 mM m-2d-1 and between -4.85 and +14.13 mM m-2d-1, respectively, and those for DIP between -1.71 and ±2.05 mM m 2d-1. The temporal variations of in situ benthic fluxes of DOC, DON and DIP at all sampling points showed net influxes, except in the period of high pore water DIM content in the late summer (September) when fluxes out of the sediment prevail. The temporal fluctuations are presented in Fig. 6 for sampling point MA and identical seasonal dynamics were observed at the sampling points E and F. The benthic fluxes of DOC, DON and DIP at all sampling points, integrated over a year, showed distinct inter-site differences. Thus,

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We found that only the DOC benthic fluxes were directly temperate dependent. Using an apparent Arrhenius function of the form:

1n J= Ej/RT+ b

where J is the flux, R is the universal gas constant, T the temperature in K and Ea the apparent activation energy. We described the DOC flux (400) at all sampling points and assuming first order rate kinetics, in the form:

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Y = sgnJDOC ' In I JDOC I , because the DOC fluxes across the sediment water interface were positive and negative, where

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Ea for the DOC flux is thus 104 kcal mol-1. The results of the sediment 02 consump-

tion measurements at all sampling points ranged between 7.68 and 39.90 mM 02 m 2 d-1. The highest values were found in late summer (September) in parallel with the highest pore water DOM content, as it is seen for sampling point MA in Fig. 6. The annual 02 consumption rate, computed for sampling point MA amounted to 6.95 M 02 m 2y 1, and that for sampling point F was nearly equal (7.85 M 02 m 2y1) but lower than that from sampling point E (9.64 M 02 m2y1). Sedi-ment 02 consumption at all sampling points appeared to be temperature dependent. Ap-plication of the Arrhenius equation to 02 con-sumption gave the relation:

lnJ02 = -5922.98/T + 23.55, r2 = 0.64, n = 22.

The computed Ea for 02 consumption was 12 kcal mol-1.

DISCUSSION AND CONCLUSIONS

High DOC, DON and DIP contents found in the pore waters of the Gulf of Trieste in comparison with sea water column (Faganeli and Herndl, in press) sug-gest that pore water DIM is of autoch-tonous origin produced by microbial degra-dation of sedimentary organic matter and probably also to a lesser extent by release of photosynthetic products of benthic primary producers, especially algal mats, whose production was estimated to be about 3 M C m-2y 1, and 0.8 M N m-2y 1 (Herndl et al., 1987). Sedimentary organic matter in the Gulf was found to be prevalently mar-ine, produced by pelagic and benthic produ-cers, as indicated by the marine C : N: P ratio and d13C values of sedimentary organic matter . (Faganeli et al., in press). Further-more, the low DOC/total alkalinity ratio, approximating a value of 1, which was cal-culated from the total alkalinity data of Faganeli et al.(1987), would suggest that DOC is produced almost entirely by sedi-mentary microbial processes (Lyons et al., 1982) .

The concentrations of DOC found in the surficial sediments of the Gulf were similar to those reported from near-shore sedim-ents where little or no sulphate reduction is proceeding (Krom and Sholkovitz, 1977; Lyons et al., 1979; Lyons et a1.,1980). Nevertheless, the surf"icial sediment of the Gulf below a few cm (approx. 5 cm) is anoxic, and no distinct decrease of NO3 and 5042-

, but an increase of alkalinity, NH4+ and P043- was observed (Faganeli et a1., 1987). This would not be expected since distinct zones of bacterial decomposition in pelagic sediments were described: 02 > N03 > 5042- (Bender et al., 1977). The intense ir- rigation by benthic fauna mainly

— Doc

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polychaetes as the most important group in the area causes a partial reoxidation of reduced compounds (Aller, 1982). Increas-ing pore water DOC content with depth con-firms the findings of various authors (Stari-kova, 1970; Lyons et al., 1979) that in oxic marine sediments the DOC contents re-main nearly constant while in anoxic sedi-ments they increase along the core. The in-creasing DOM contents with depth in anoxic pore waters has been attributed to an increase in concentration of the high molecular weight fraction of DOM, contain-ing a large proportion of humic substances, mainly fulvic acid (Krim and Sholkovitz, 1977; Lyons et al., 1979; Paxeus et al., in press). The low molecular weight fraction appears to be present at constant con-centration in both oxic and anoxic environ-ments, decomposing by bacterial metabo-lism and condensing to high molecular weight DOM-humic substances and mela-noidines (Krim and Sholkovitz, 1977; Paxeus et al., in press) . Paxeus et al. (in press) have reported that in situ release of humic substances from oxic sediments is negligible, probably due to adsorption and coprecipitation with Fe00H, while in an-oxic sediments a significant flux from pore water into the overlying water exists. The high Ea of DOC fluxes at the sediment—waten interface in the Gulf of Trieste could be explained, bearing in mind that a sig-nificant proportion of pore water DOM con-sists of humates, by the low mobility of humic substances with effective diffusion coefficient De < 6 • 10-6 cm2 s-1 (Paxeus et al., in press) and exchange between oxic and anoxic conditions in the sediment. Similar De (-5.6 • 10-6 cm2 s-1) was obtained for DOC fluxes determined experimentally by diaphragm—cell method between pore water and distilled water (Mackin, 1986) .

The high DOC benthic efflux found in late summer (September) is a consequence of higher bacterial decomposition, indicated by high 02 benthic consumption and high pore water NH4+ and R043- contents. In this period the low DOC:DON ratio (-2, atomic) in the uppermost part of the sedi-ment indicates the predominance of pro-teinaceous DOM which is rapidly decom-posed, as indicated by the increasing DOC:DON ratio with depth. The DOC released is accumulated under the pycno-cline, normally present in that period 1-2 m above the bottom (Faganeli et al., 1985).

Using the plots of DOC, DON and DIP vs. benthic 02 consumption we found that these fluxes were not significantly linked to 02 consumption (Figs. 4, 5). The majority of pore water DOC is resistant towards micro-bial oxidative degradation (perhaps high molecular weight DOM), or decomposition processes other than oxidation take place at the sediment—water interface of the Gulf. Among them, microbial oxidation using 113 , Mn(IV) and Fe00H, and especially anaerobic fermentation and 5042- reduc-tion are probably operative, since anaerobic mineralization could also proceed during oxic conditions. Facultative anaerobes use 02 during oxic conditions but turn quickly to other electron acceptors when 02 is de-pleted. Degobbis et al. (1986) suggested denitrification in surficial sediments of the Northern Adriatic to be an important pro-cess affecting the C and N budget. In the other hand, high NH4+/P043 ratios ob-served in pore waters were attributed to 13043- precipitation as phosphate minerals, e.g., apatite, struvite (Faganeli et al., 1987). However, sediment 02 consumption also comprises nitrification and sulphide oxida-tion, as well as oxidation of other reduced compounds (Fe(II), Mn(II)), and Jorgensen


(1980) estimated that about 5-10% and up to 50% of 02 consumption were involved in nitrification and sulphide oxidation, respec-tively, in the surficial sediments of Limfjor-den (Denmark). Also, the role of benthic in-fauna in the sediment-water exchange of 02 in coastal sediments could not be neg-lected (Andersen and Helder, 1987). Our es-timated Q10 for 02 consumption was about 2 and thus close to those reported for other coastal sediments (Revsbech et al., 1980). The anaerobic route of decomposition of DOM, makes the stoichiometry of decom-position difficult to resolve (Nixon, 1981) .

DON and DOP appeared to be more reac-tive than DOC at the sediment-water inter-face since higher DON and DOP benthic in-fluxes and NH4+ and 1)043- effluxes were measured simultaneously in the period of late summer-autumn in parallel with en-hanced benthic 02 consumption. The inter-site differences observed between DOM fluxes, calculated on an annual basis, could be explained by differences in bioturbation and local hydrodynamical conditions, lead-ing to differences in pore water-overlying water exchange. The basic sedimentary properties of all three sampling points were fairly similar.

We tried to elucidate the importance of DOC, DON and DOP benthic fluxes in the C, N and P budgets of the coastal marine region, e.g., Gulf of Trieste, using the ap-proach where the amount of component entering the system (C; ) is approximated by the sum of amount lost (Coot) and the amount retained in the system (Cret):

cmn = Cout + Cret, (Martens and Klump, 1983).

In these estimations we used the measured benthic fluxes of DOC, DON and DOP and the data of NH4+ and 13043- ben-

thic fluxes (Herndl et al., in press), calcu-lated on an annual basis to approximate steady state conditions, and the sedimen-tation rate of POM. This estimation is possi-ble only for the sampling point MA where the data of POC, PON and POP sedimen-tation rates, corrected for resuspension measured over a yearly cycle, exist (Faganeli, in press). The total input of N and P to the sediment amounted to about 0.55 M m-2y 1 and 0.08 M m-2y 1, respec-tively, in a N:P ratio (atomic) of about 6.8. Total N and P input is finally all utilized in the benthic primary production, which is in excellent agreement with that independ-ently measured by Herndl et al. (in press), since the yearly averaged fluxes of NH4+ and 1'043- were oriented towards the sedi-ment. On the other hand, the budget of C remains unclear. The fluxes of total CO2 were not measured and the steady state conditions in DOC flux annual measure-ments were probably not obtained. High DOC benthic efflux, calculated on an an-nual bases, derives from some occasionally high effluxes (late summer) while in the other periods they were oriented towards the sediment (Table 2) .

More research efforts towards detailed pore water DOM chemical analysis and the reactivity of individual compounds are re-quired to obtain a better insight into the be-haviour of a complex mixture of dissolved organic compounds in pore -waters and at the sediment-water interface. Working in situ with benthic chambers offers an excel-lent opportunity to assess the mobility and biogeochemical fate of dissolved organic compounds in marine sediments, and also contributes to a better understanding of C, N and P cycling in the marine environment.


TABLE 2

Mass balance of C, N, and P, calculated on an annual basis, for the sampling point MA (M m-2 y)

C N

R

Sedimentation

1.8 0.20

0.02 Benthic fluxes

org. +29.6 -0.34 -0.06 inorg. ? -0.01 -0.002

Input to the sediment ? -0.55 -0.08

? = unknown.

REFERENCES

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