Organic carbon and nitrogen in sediments and in resuspended sediments of Venice Lagoon: Relationships with PCB contamination

Volume 22/Number

1l/November

1991 0025-326X/91 $5.00+0.00 © 1991 Pergamon Press pie

Marine I'ollution Bulletin, Volume 22, No. 11. pp. 543-547, 1991. Printed in Great Britain

Organic Carbon and Nitrogen in Sediments and in Resuspended Sediments of Venice Lagoon: Relationships with PCB Contamination C. CALVO*, M. GRASSO* and G. G A R D E N G H I t

*Department of Environmental Sciences, University of Venice, Calle Larga S. Marta, 2137, 30123 Venezia; t Department of Experimental and Evolutionistic Biology, University of Bologna, Via S. Giacomo, 9, 40100 Bologna, Italy

The distributions of organic carbon and nitrogen in sediments and in resuspended sediments obtained by resuspension experiments on undisturbed sediment cores from Venice Lagoon, were investigated in July 1987. Shear stresses which can be produced by wind waves during storms were used. Organic carbon ranged 8.6-27.3 mg g-i in resuspended sediments and 5.214.6 mg g-t in sediments. Organic nitrogen ranged 0.95-3.70 mg g-t in resuspended sediments and 0.300.92 mg g-! in sediments. At every station, resuspended sediments had a C/N atomic ratio lower than that of the corresponding underlying sediment. This is ascribed to a lower decomposition degree of organic matter in resuspended sediments. The composition of organic matter appears to play an important role in determining the accumulation of PCBs in resuspended sediments. The surprisingly high level of these contaminants in resuspended sediments at a large distance from the contamination source may be ascribed to a contribution of animal materials.

Surface sediments may be resuspended and redistributed by the action of waves and currents (Lick, 1986; Ziegler et al., 1987, 1988). As these phenomena erode the topmost layers of the sediment column, resuspended sediments contain recently deposited organic matter. In shallow water bodies (i.e. a few meters deep), this matter may berich in easily degradable fractions, as debris from dead marine organisms may rapidly settle onto the sediment surface. So, through the processes of sediment resuspension and redistribution, organic matter at the sediment-water interface may be a significant source of easily remineralizable materials. In this paper we report on the elemental (C, N) composition of organic matter in surface sediments and in resuspended sediments obtained by resuspension experiments carried out on undisturbed sediment cores

from Venice Lagoon, in July 1987. The PCB contamination of these samples (Raccanelli et al., 1989) is discussed in relation to the distribution of organic carbon and nitrogen. S t u d y area Venice Lagoon (area 500 km 2) is a shallow (average depth about 1 m) marine embayment (Fig. 1) with an average tidal excursion of 0.6 m (Pirazzoli, 1974). It is connected to to the Adriatic Sea through three port entrances. As these entrances are relatively narrow and open at all times, the Venice Lagoon can be classified as restricted lagoon (Kjerfve, 1986). Tidal currents flow through the lagoon entrances up to 2.69 m see -1 at 1 m from the bottom (Tosi, 1970), with decreasing energy toward shallow internal areas, where they have surface velocities of 0.13-0.40 m see -1 or less (DAlpaos and Di Silvio, 1979). Surface sediments of the Lagoon (top about 6 cm) are essentially fine grained: The clay + silt fraction (<62 ~tm) prevails in the largest lagoon area whereas, with the exception of occasional mollusc shells, particles >0.5 mm in diameter are absent (Barillari, 1978, 1981; Barillari and Rosso, 1975). Mineralogical studies on surface sediments in the central lagoon basin (Menegazzo Vitturi et al., 1989) reveal that carbonates and quartz are the most important mineralogical components. During spring-summer, Venice Lagoon is a highly productive system (Facco et al., 1986; Sfriso et al., 1988), as a consequence of large nutrient inputs (Bernardi et al., 1986) and favourable climatic conditions. Organic carbon content of surface sediments ranges from values around 0.5% to 4.5% near the inner border of the Lagoon (Facco, 1983). On the basis of the features of the Lagoon, five representative sampling sites were selected for the present study. Stations 1C, 2C, and 3C are located in the central, most polluted part of the Lagoon. The main 543

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pollution source is the industrial zone of Porto Marghera. Station 4N, located near the mouth of Canale Silone, in the northern basin, is characterized by relatively high freshwater input (salinity up to 20.3%0; Calvo, 1989). Station 5S, which is located in the southern basin, is characterized by its large distance from contaminant sources. The average water depth at the sampling sites is from 0.5 to 1.5 m.

Materials and Methods

Devicefor sediment resuspension The device for sediment resuspension was assembled and calibrated at the University of California (Department of Mechanical and Environmental Engineering) in Santa Barbara (Tsai and Lick, 1986; Ziegler et al., 1987). This device consists of a horizontal plexiglass grid (a disk with h. 0.6 cm, d. 11 cm, porosity 42.8%) which oscillates vertically with a determined frequency inside a cylindrical chamber containing the sediment core with the overlying natural water. The sediment chamber is the same coring tube used in sampling. The 544

grid oscillation into the water create the turbulence for sediment resuspension. For natural sediments the concentration of suspended particles increases for the first few minutes until it reaches a relatively steady value. The shear stress applied to the sediment surface was indirectly determined by comparing the concentration of resuspended sediment with the concentration obtained in an annular flume with a known bottom shear stress. Due to the small dimensions of the entire apparatus (w. 32 cm, 1. 63 cm, h. 82 cm), the resuspension device can be used easily on board a ship, minimizing the possible alterations of sediment stratification due to transport and storage of the cores.

Sampling, resuspension experiments and analytical procedures The resuspension experiments were carried out in the field on board of a small laboratory-ship during the third week of July 1987. These experiments correspond to shear stresses of 6.6 and 9.0 dynes cm-2. It is possible that the reproducibility of these stresses is

Volume 2 2 / N u m b e r 11/November 1991

lower during the cruise than in the ashore calibration laboratory, due to non-optimal conditions in the field. The shear stresses used in our experiments are somewhat considerable and correspond to wind waves occurring during storms, which can mobilize significant amounts of sediments in shallow systems (Ziegler et al., 1988). The undisturbed sediment cores along with overlying water were sampled with a plexiglass coring tube (i.d. 11.7 cm, h. 30.5 cm) and processed within a few hours. After a shaking time of 10 min, a 50 ml sample of suspension was withdrawn with a syringe to determine the amount of resuspended sediment. Then the resuspension device was switched off and the suspension collected immediately into a beaker. The solid matter from these experiments was isolated by centrifugation and freeze-dried before analysis. For the determination of resuspended sediment concentration, the 50 ml suspension samples were filtered through 0.45 Ixm preweighed filters, washed with distilled water, freezedried, and finally reweighed. From the processed cores, 50-100 g samples of the top 4-5 cm sediment layer (approximate due to irregularity of the surface) were taken and mixed together in order to obtain an average sample which was freeze-dried. The analysis of organic carbon (OC) both in sediments and in resuspended sediments was performed by a Perkin-Elmer 240B Elemental Analyzer (precision 2.3%), after treatment of the sample with 1 N HC1 and drying at 105°C. For determining total Kjeldahl nitrogen (TKN), samples were digested with HzSO 4 (15 ml, 98%)-K z S O 4 (10 g)--CuO (50 mg) mixture in a Biichi mod. 425 digestor. Ammonia liberated was distilled into standard solutions of H2SO4 by a Biichi 315 steam distillation unit and the amount of acid not neutralized was titrated with standard NaOH solutions. Standard deviation of this procedure obtained in duplicate analyses of sediments was lower than 3% (Calvo, 1989). It is known that TKN is the sum of organic-N and ammonia-N in the sample. Exchangeable ammonia nitrogen (EAN) was determined by extraction with KCI solution. Samples were shaken with a neutral 2N KCI solution and after filtration through 0.45 ~tm HA Millipore filters, dissolved ammonia was analysed by a modification (precision 2.7%) of the phenol-hypochlorite method (Strickland and Parsons, 1972). As alumina silicates capable of fixing ammonium appear to be scarcely important in Venice Lagoon sediments (Menegazzo Vitturi et al., 1989), fixed ammonium was not determined in our samples. Organic nitrogen (ON) was calculated by difference between TKN and EAN.

obtained with the two stresses are very similar. In both cases the highest amount of resuspended sediment was obtained at station 2C; the lowest at station 3C. So it is possible to determine spatial differences in sediment resuspension properties in complex environments such as Venice Lagoon, provided that sufficient numbers of replicate resuspension experiments at each location are averaged (7 to 11 in this study). These results give us confidence that the sediment surface was not drastically perturbed during sampling and transport of the cores. From the amount of resuspended sediment and from sedimentation rates, we can estimate the thickness and the age of the resuspended layer (Calvo et al., 1991). The thickness ranges from 0.06 to 0.25 mm, whereas its age at stations 1C and 4N, where sedimentation rates are known, is from 5 to 18 days. These values indicate that the resuspended sediment layer corresponds to an extremely short deposition time. Since we saw no significant movement of deep (older) sediment layers during the resuspension experiment, resuspended sediments may contain significant amounts of freshly deposited materials. The thickness of the resuspended layer is orders of magnitude lower than that of the corresponding sediment sampled, whereas its OC and ON contents (Table 2) differ by less than one order of magnitude from those of the underlying sediment. Therefore, the composition of the sediment after processing is practically identical to that of the initial (not processed) core. TABLE 1 Mean concentrations (m) of resuspended sediment.

Station 1C 2C 3C 4N 5S

355 640 279 404 477

12 61 86 37 56

Higher stresst m SD mg1-1 % (n)

(9) (9) (11) (9) (8)

729 1182 403 457 796

56 71 83 54 78

(8) (8) (10) (7) (8)

*Shear stress = 6.6 dynes cm -2. tShear stress = 9.0 dynes cm -2. SD = standard deviation. n = Number of cores processed.

TABLE 2 Concentrations (mg g - ' dry wt) of Organic Carbon (OC), Organic Nitrogen (ON) and Exchangeable A m m o n i a Nitrogen (EAN) in resuspended sediments and in sediments.

Station 1C 2C 3C 4N 5S

Results and Discussion Mean concentrations of resuspended sediment (Table 1) range from 279 to 640 mg 1-~ with the lower stress and from 403 to 1182 mg 1-1 with the higher one. Despite the relatively high standard deviations (1286%), the trends of resuspended sediment amounts

Lower stress* m SD m g l -I % (n)

1C

2c 3c 4N 5S

Resuspended sediment Lower stress Higher stress OC OC 22.5 9.4 8.6 27.3 26.2

25.2 13.7 10.2 20.9 26.2

Sediment OC 14.6 5.2 5.6 12.7 11.7

ON

EAN

ON

EAN

ON

EAN

2.89 1.58 2.34 3.70 3.09

0.027 0.018 0.011 0.040 0.020

2.10 1.00 0.95 3.38 3.27

0.023 0.010 0.013 0.020 0.012

0.92 0.30 I).45 0.72 0.89

0.023 0.003 0.008 0.004 0.035

545

Marine Pollution Bulletin

The OC content of the sediment at station 1C (14.6 mg g-t) is comparable to that obtained in July 1982 (14 mg g-l; Facco, 1983), indicating that no remarkable changes in the depositional environment of the central Lagoon have occurred during recent years. The OC content both of the sediment and of the resuspended sediments of station 1C is higher than that of the corresponding samples of station 2C and 3C. This is reasonable since station 1C is characterized by the highest phytoplankton biomass (Facco et al., 1986), and that organic matter can easily settle due to the reduced water circulation within this area. On the basis of these conclusions, and considering the higher phytoplankton biomass and lower currents at station 2C with respect to station 3C, we expected higher values of OC at station 2C than at station 3C. Surprisingly, no or scarcely significant differences were detected between the OC values at these two stations. For ON we have a more distinct contrast. In fact, at station 2C the ON content both of the sediment (0.30 mg g-l) and of the resuspended sediment obtained with the lower stress (1.58 mg g-l) is lower than that of the equivalent samples of station 3C (0.45 and 2.34 mg g-l, respectively). These facts suggest the presence of particular organic components at station 3C. Differences in composition of organic matter among stations in the central Lagoon are revealed by the C/N (OC/ON) atomic ratios (Fig. 2). The lowest values are at station 3C. Particularly, resuspended matter obtained at station 3C with the lower stress shows a C/N atomic ratio of 4.3, which suggests the presence of materials related to animal organisms and/or bacteria. In fact, these organisms, being rich in proteins, have particularly high organic nitrogen contents and may show C/N atomic ratios in the order of 4-5 (see Miiller, 1977, for a review). On the other hand, the deposition of algal debris at this station is hindered by the strong tidal current. At station 4N and 5S, both OC and ON values are close or higher to the highest ones detected in the central Lagoon. The strong presence of organic matter at station 4N is probably a consequence of the contribution of organic materials and nutrients from land run-off (Bernardi et al., 1986). The high values of OC and ON at station 5S are reasonably due to the deposition of organic debris from a large population of macroalgae (essentially Ulva rigida C. Ag.). Several debris of macroalgae were noted in resuspended sediments at this station. In Fig. 2 we note that at all stations the C/N atomic ratio in sediments is higher than in the corresponding resuspended sediments. This can be ascribed to diagenesis effects: Since sediments are older than resuspended sediments, their higher C/N atomic ratios appear to be a consequence of the preferential remineralization of organic nitrogen with respect to organic carbon occurring in marine sediments (Klump and Martens, 1983). The possibility that materials with particularly high C/N atomic ratios were accumulated in sediments during previous seasons may be excluded. In fact, data of C and N in sediments obtained during seasonal samplings (Calvo, 1989), reveal that the C/N 546

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Fig. 2 C/N (OC/ON) atomic ratio in resuspended sediments and in sediments.

atomic ratios are lower than the corresponding values measured in July 1987. Concerning EAN (Table 2), we note its poor importance both in sediments and in resuspended sediments. This appears to confirm the scarce presence of negatively charged clay minerals (Rosenfeld, 1979). Polychlorinated biphenyls (PCBs) are typical hydrophobic contaminants. In aquatic environments they are largely associated with organic matter (Eisenreich et al., 1980). In the Venetian area the presence of PCBs is essentially localized in the industrial zone of Porto Marghera, where many industrial transformers contain PCB mixtures as dielectric fluids. Among common benthic organisms of Venice Lagoon, higher amounts of total PCBs are accumulated in benthic animals (molluscs and crustaceans) than in benthic algae (Fossato, 1982; Pavoni et al., 1990). Analyses of total PCBs in sediments and resuspended sediments obtained in the same sets of resuspension experiments considered in this paper, have been already reported (Raccanelli et al., 1989). PCB concentrations were lower in sediments (3-16 ng g-l) than in resuspended sediments (30-280 ng g-l). The areal distribution of PCBs reveals quite surprising aspects, which are n o t discussed in the paper just mentioned. Particularly, PCB concentration in the resuspended sediment at station 3C was comparable (higher stress) or even higher (for the lower stress) than in the equivalent sample of station 1C, which is the sampling site nearest to the polluted zone of Porto Marghera. As PCBs are essentially bound to organic matter, it is useful to calculate the PCB amounts per unit of OC. PCBs/OC values (~tg g-l) in benthic animals (molluscs and crustaceans) are in the range 2,1-56.9 (Fossato, 1982). In resuspended sediments, PCBs/OC values (Table 3) range from 1 ~tg g-i (station 4N) to 32 ~tg g-1 (station 3C) with the lower stress and from 1 ~tg g-l (station 4N) to 11 ~g g-i (station 3C) with the higher one. Considering resuspended sediments obtained with the lower stress at the stations in the central Lagoon, we note that the highest PCBs/OC value (32 txg g-l) is that of station 3C, though this station is located at the

Volume 22/Number 11/November 1991 Barillari, A. (1978). Prime notizie sulla distribuzione dei sedimenti superficiali nel bacino centrale della Laguna di Venezia. Atti delr lstituto Veneto di Scienze, Lettere ed Arti 136, 125-134. Barillari, A. (1981). Distribuzione dei sedimenti superficiali nel bacino Resuspended sediment meridionale della Laguna di Venezia. Atti deWlstituto Veneto di Lower stress Higher stress Sediment Scienze, Lettere ed Arti 139, 87-109. Station PCBs/OC PCBs/OC PCBs/OC Barillari, A. & Rosso, A. (1975). Prime notizie sulla distribuzione dei sedimenti superficiali del bacino settentrionale della Laguna Veneta. 1C 6 5 1.1 Memorie di Biogeografia Adriatica 9, 13-32. 2C 14 4 1.4 Bernardi, S., Cecchi, R., Costa, E, Ghermandi, G. & Vazzoler, S. 3C 32 11 1.1 (1986). Trasferimento di acqua dolce e di inquinanti nella Laguna di 4N 1 1 0.4 Venezia. lnquinamento (1/2), 46-64. 5S 4 2 0.2 Calvo, C. (1989). Biogeochimica delrinteffaccia acqua-sedimento nella Laguna di Venezia. Search Doctorate thesis. University of Venice. *Data from Raccanelli et al. (1989). Calvo, C., Donazzolo, R., Guidi, E & Orio, A. A. (1991). Heavy metal pollution studies by resuspension experiments in Venice Lagoon. Accepted for publication in Wat. Res. highest distance from the industrial zone of Porto L. & Di Silvio, G. (1979). Le correnti di marea nella laguna di Marghera. This sample has a C/N atomic ratio of 4.3, D~lpaos, Venezia. Presentazione dei risulati, pp. 41-46. Istituto di Idraulica which is compatible with animal type organic matter. dell'Universit~ di Padova. The fact that PCBs are particularly accumulated in Eisenreich, S. J., Hollod, G. J., Johnson, T. C. & Evans, J. (1980). Polychlorinated biphenyl and other microcontaminant-sediment benthic animals, with PCBs/OC values up to 56.9, interactions in Lake Superior. In Contaminants and Sediments (R. A. appears to be a reasonable explanation for the high Baker, ed.), pp. 67-94. Ann Arbor Science. level of PCBs at station 3C. With the increase of the Facco, S. (1983). Studio deUe distribuzioni verticali delle specie eutrofizzanti ed inquinanti nei sedimenti lagunari. Tesi di Laurea C/N atomic ratio (stations 2C and 1C in succession), Universit~ di Venezia. the relative contribution of animal matter appears to Facco, S., Degobbis, D., Sfriso, A. & Orio, A. A. (1986). Space and time variability of nutrients in the Venice Lagoon. In Estuarine decrease: To this fact are reasonably due the decreasing Variability (D. A. Wolfe, ed.), pp. 307-318. Academic Press, New PCBs/OC values from station 3C towards internal York. areas of the central Lagoon. Also with the higher stress • Fossato, V. (1982). Etude des hydrocarbures chlorrs dans l'environnement de la lagune de Venise. V/e~Journ~es Etud. Pollutions, 465we note a similar, although less marked, trend. 468. C.I.E.S.M., Cannes. In resuspended sediments at stations 4N and 5S Kjerfve, B. (1986). Comparative oceanography of coastal lagoons. In lower PCBs/OC values were detected with respect to Estuarine Variability (D. A. Wolfe, ed.), pp. 63-81. Academic Press, New York. the central area. This is reasonably a consequence of the J. V. & Martens, C. S. (1983). Benthic nitrogen regeneration. In large distance from the polluted zone of Porto Klump, Nitrogen in the Marine Environment (E. J. Carpenter & D. G. Marghera and of the relatively scarce contribution of Capone, eds), pp. 411-457. Academic Press, New York. animal matter, suggested by C/N atomic ratios higher Lick, W. (1986). Modeling the transport of fine-grained sediments in aquatic systems. Sci. Tot. Environ. 55, 219-228. than 4 - 5 . Menegazzo Vitturi, L., Molinaroli, E., Pistolato, M. & Rampazzo, G. At every station, lower PCBs/OC values were (1989). Sediment properties and their influence on the geochemical composition in the Lagoon of Venice. Bollettino di Oceanologia detected in sediments with respect to the corresponding Teorica ed Applicata 7, 191-205. resuspended sediments. This fact may be ascribed to M/iller, E J. (1977). C/N ratios in Pacific deep-sea sediments: Effect of release of contaminants to the water column (where inorganic ammonium and organic nitrogen compounds sorbed by clays. Geochim. Cosmochim. Acta 41,765-776. they are dispersed by currents), in consequence of the Pavoni, B., Calvo, C., Sfriso, A. & Orio, A. A. (1990). Time trend of decomposition of organic matter occurring in sediPCB concentrations in surface sediments from a hypertrophic, ments. macroalgae populated area of the lagoon of Venice. Sci. Tot. Environ. 91, 13-21. Pirazzoli, E (1974). Dati storici sul livello medio di mare in Venezia. Atti Accademia Scienze Istituto Bologna 18, 124-148. Raccanelli, S., Pavoni, B., Marcomini, A. & Orio, A. A. (1989). PolyConclusions chlorinated biphenyl pollution caused by resuspension of surface sediments in the Lagoon of Venice. Sci. Tot. Environ. 79, 111-123. Although further investigation is necessary to clarify Rosenfeld, J. K. (1979). Ammonium adsorption in nearshore anoxic the origin of organic matter in resuspended sediments, sediments. Limnol. Oceanogr. 24, 356-364. the presence of scarcely decomposed materials is Sfriso, A., Pavoni, B., Marcomini, A. & Orio, A. A. (1988). Annual variations of nutrients in the lagoon of Venice. Mar. Pollut. Bull. 19, indicated by the large amount of organic nitrogen in 54-60. comparison to the sediment organic matter. Reasonable Strickland, J. D. H. & Parsons, T. R. (1972). A Practical Handbook of Seawater Analyses. Fisheries Reseach Board, Canada, Ottawa. explanations for the distribution of PCBs can be provided on the basis of the elemental composition of Tosi, R. (1970). Ricerche sul regime delle correnti di marea nei canaliporto della Laguna du Venezia. A t t i e Memorie dell'Accademia organic matter, and they suggest the importance of the Patavina di Scienze, Lettere ed Arti 82,383-407. nature of this matter in determining the accumulation of Tsai, C.-H. & Lick, W. (1986). A portable device for measuring sediment resuspension. J. GreatLakes Res. 12,314-321. PCBs. Ziegler, C. K., Tsai, C.-H. & Lick, W. (1987). The resuspension, deposition, and transport of sediments in the Venice Lagoon. UCSB report. The authors are grateful to Dr. A. Sfriso and R. Donazzolo for Ziegler, C. K., Tsai, C.-H. & Lick, W. (1988). Transport of sediments in technical help. We also gratefully acknowledge the financial support the Venice Lagoon. In 3rd Int. Conf. on Environmental Contamifrom the Ministero della Ricerca Scientifica. nation, Venice, pp. 336-338. TABLE 3

Concentrations of PCBs* per unit of OC (PCBs/OC, ~tg g-l).

547