Experimental research on the impact of Corbicula fluminea on DIN exchange at a tidal flat sediment-water interface

Chinese Journal of Oceanology and Limnology Vol. 25 No. 4, P. 434-443, 2007 DOI: 10.1007/s00343-007-0434-9

Experimental research on the impact of Corbicula fluminea on DIN exchange at a tidal flat sediment-water interface*

LIU Jie (刘杰), CHEN Zhenlou (陈振楼)**, XU Shiyuan (许世远), ZHENG Xiangmin (郑祥民) (Ministry of Education Key Laboratory of Geo-information Science, East China Normal University, Shanghai 200062)

Received Jan. 9, 2006; revision accepted Mar. 3, 2007

Abstract Based on a simulative experiment and a comparison analysis, the effect of bivalve Corbicula fluminea activity on sediment-water exchange of dissolved inorganic nitrogen (DIN) is studied. The areas included three intertidal flat sites of the Changjiang (Yangtze) River estuary in China. The interface exchange flux of ammonium, nitrate and nitrite in the short experiment (6 h) was −46.4–40, − 74.8–929.1 and 2.5–14.6 µmol/(m2·h), respectively. It was found that the burrowing activities of C. fluminea increased +

4NH and -3NO release from sediments to overlying water in the short-term

experiment. During long-term incubation, +4NH and -

3NO released in turn from the sediments. At the beginning of incubation, bioturbation by C. fluminea could accelerate +

4NH release from sediments 2–17 times in different sites, resulting in stronger nitrification and increased -

3NO concentrations in the overlying water. Sediment profile analysis post-incubation shows that organic matter mineralization and sediment-water +

4NH exchange had been stimulated by C. fluminea bioturbation and bioirrigation during the experiment. Therefore, C. fluminea activities such as excretion, burrowing, irrigation and turbation can effectively alter nitrogen dynamics and accelerate and stimulate nitrogen exchange and cycling at the sediment-water interface.

Keyword: Corbicula fluminea; sediment-water interface; DIN; impact; Changjiang (Yangtze) River estuary

1 INTRODUCTION An estuary is widely identified as one of the most

valuable and vulnerable habitats (Jikells, 1998) and is an intersection between the river and the sea. Its many particular characteristics, include powerful tidal and riverine hydrodynamic processes, frequent sediment transportation and material exchange, acute physical and chemical gradients, and various species of organisms. The sediment-water interface is an important environmental borderline where physical, chemical and biological characters vary sharply as chemical transportation and exchange take place (Wu et al., 1996). In estuarine and coastal zones, macrofauna is a key constituent of the interface. Acting as one of the most important environmental factors, macrofauna has extensive effects on the processes of substance, energy, and information flows and exchanges at the interface in various sites (Ye, 1997). Through a series of physicochemical and microbial processes, nitrogen from riverine input or pelagic sedimentation

participates in complex biogeochemical cycling in estuarine and coastal benthic systems. In these courses, the nitrogen load estuarine ecosystem will either decline because of burial and benthic denitrification or increase as a result of organic debris mineralization and nutrient regeneration. Therefore, the above-mentioned processes are key factors regulating coastal and estuarine eutrophications. The macrofaunal activities near a sediment-water interface regulate nutrient dynamic largely, so that the whole nitrogen biogeochemical cycling process becomes even more complex and dynamic. Studying macrofaunal influences on these cycling processes and their mechanisms has outstanding theoretical and practical significance to the modulation and remediation of coastal eutrophications.

It has been widely accepted that macrofaunal activities around sediment-water interfaces exert * Supported by NSFC(No40173030, 40701164) and Science and Technology Committee of Shanghai (No 04DZ19301) ** Corresponding author: [email protected]

No.4 LIU et al.: Experimental research on the impact of C. fluminea on DIN exchange 435

outstanding influence upon nutrient dynamics and biogeochemical cycling. Researches on the ecological and environmental effects of benthic animals have been the emphases and hotspots with interdisciplinary characteristics in the past decades. From a burrow segment to the whole marine ecosystem, researches on benthic faunal effects have acquired predominant development on a macro and micro level. Numerous systematic studies have stressed important roles of macrofauna on nitrogenous exchanges and biogeochemical processes in coastal and estuarine marine sediments. These researches focused on the effects and mechanisms in the processes of organic nitrogen remineralization (Mayer et al., 1996; Sun et al., 1999), nitrification (Pelegri and Blackburn, 1994; Hansen and Kristensen, 1998) and denitrification (Svensson, 1997; Tuominen et al., 1999). Many advanced technologies such as radio tracer methods and microelectrodes have been used (Herbert, 1999) and models have been established (Gilbert et al., 2003; Furukawa et al., 2001). However, corresponding researches are quite lacking due to the delay of and financial limitations to correlative studies. Reports about macrofaunal effects can hardly be seen in Chinese publications. The impacts of biological activities of C. fluminea, an aboriginal keystone bivalve species in the Changjiang (Yangtze) River estuary, on the inorganic nitrogen exchange and biogeochemical processes in the island and coastal intertidal wetland sediment-water interface is discussed in this paper.

Fig.1 Sampling sites in Changjiang (Yangtze) River estuary

2 MATERIALS AND METHODS 2.1 Study site

The annual mean air temperature of the Changjiang estuary region is about 16°C (Shen, 2001). The average salinity range of this estuarine water body is 0.21–5 ppt (PSU). At present, the Changjiang estuary coastal wetlands consist of some inshore or island coastwise intertidal wetlands (e.g., Chongming east beach and Nanhui east beaches), estuarine sandbank wetlands (e.g., Jiuduan sandbank) and inshore beach at the head of Changxing Island (Cheng, 1998). The total area of coastal wetland in Changjiang estuary is about 40 470 hm2 (Yuan, 2001). A tidal flat near Xupu Town and Gulu Town in the south bank of the estuary and a Chongming east beach tidal flat are chosen as our sample sites to represent upstream, downstream and island tidal flats, respectively. The three sites are marked as “XP”, “GL”, and “CM” (CM2 as middle intertidal flat and CM3 as low intertidal flat) in this paper.

2.2 Experimental macrofaunal species

In terms of body size, benthic organisms are classified as microbenthos, meiobenthos and macrobenthos (Herman et al., 1999). Based on recent investigations (Yuan and Lu, 2002), there are a total of 68 macrofauna species in the Changjiang estuarine wetland. Among the dominant species are mollusca Corbicula flumine, Stenothyra glabra, crustacea Ilyrlax deschampsi, and Sesarma denaan. It is investigated that the bivalve Corbicula fluminea is the most predominant and extensive species in the tidal flat ecosystem of the Changjiang estuary. Therefore, it is chosen for this research.

2.3 Sampling

Sediment cores, seawater, and bivalves Corbicula fluminea used in core incubations were sampled in November 2002. Parameters of salinity, temperature, dissolved oxygen concentration, pH and conductivity of water column were measured, and the abundance of bivalves in tidal flats was investigated in-situ. Undisturbed sediment cores (25 cm long) were collected in a small area (50 cm×50 cm) by hand with 50 cm long, 8 cm i.d. perspex tubes, with each tube sealed with rubber plugs before they were carried to the laboratory. 9 cores were taken from each sampling station. Seawater for incubation was sampled at high tide; water was filtered through a sieve to remove plankton. Certain intact specimens of C. fluminea were collected simultaneously for use in the experiment. All the samples were carried immediately to the laboratory for incubation after sampling.

CHIN. J. OCEANOL. LIMNOL., 25(4), 2007 Vol.25 436

2.4 Pre-treatment of experiment

Sediment cores were kept in a dark glass box (40 cm×40 cm×70 cm) submerged in local seawater for acclimation in the laboratory. During acclimation, each core was aerated continuously to keep overlying water columns oxygen-saturated. After 12 hours’ acclimation under laboratory conditions, nine cores of each station were treated by the following procedures:

Six cores were defaunated by purging overlying water with N2 for 1 h before the cores were sealed with rubber stoppers and stored for 24 h in light-free conditions. The anoxia induced by this procedure is known to force the infaunas to the sediment surface (Andersen and Kristensen, 1992). All aboriginal macrofauna were then removed from the sediment surface, and these cores returned to oxic conditions in the incubation system for 24 h. The other untreated three cores were kept in the incubation system as natural controls (C-cores). Just before the flux measurement experiment, pre-weighed C. fluminea were added to three defaunated cores as bioturbated cores (B-cores), while the other three defaunated cores were considered as non- bioturbated control cores (D-cores). Despite its predominance in the Changjiang estuarine tidal flat, biomass of C. fluminea is quite small and unevenly distributed. Thus, the addition quantity of C. fluminea was defined by the maximal biomass distribution of each sample site; 9 g, 12 g and 3 g C. fluminea were added into every B-core of XP, GL and CM respectively.

2.5 Experiment operation

The incubation experiment was performed short-term and long-term. Overlying water of the core chamber was adjusted to 20 cm at the beginning of the short-term experiment. Nine initial water samples (40 ml) were pumped by an injector before Corbicula fluminea were added into B-cores of each sample station. The intensity of aeration was adjusted to drive the circulation of overlying water but under a resuspension limit. Samples were collected every 2 hours in short-term incubation (6 h) and filtered through a 0.45-μm sieve soon after

being pumped into injectors and then frozen immediately for later analysis. All the cores were submerged in seawater again for acclimation after final sampling for the short-term experiment, and long-term experiment began after 24 h acclimation. Water samples were performed at 0 d (initial sampling day) 3, 6, 9, 14 and 29 d. The sampling operation was the same as in the short-term one. Sediment cores were cut into 1 or 2 cm intervals to a 10-cm depth after long-term incubation. Subsamples of the sediment slices and the initial sediment mixtures were examined for water content, specific density, and organic content. The remaining sediment was used for pore water extraction by centrifugation (3 000 r/m, 10 min). All sediment core treatments were performed under N2 pressure.

2.6 Chemical analysis

The concentrations of -3NO , -

2NO and NH4+

were analyzed by zinc-cadmium reductive method, sulfanilamide and N-(1-naphthyl) ethylene diamine dihydrochloride colorimetry and sodium hypobromate oxidation method respectively. Water content was calculated from weight loss after drying (110°C, 6 h). Organic content was determined as loss on ignition at 500°C for 6 h.

Net DIN fluxes across the sediment-water interface were calculated from the concentration change between initial and final samples and were determined according to Aller et al. (1985)

F=∑Vt(Ct-Ct-1)A-1T-1 (1)

where F is the nitrogen flux (μmol/(m2h) and Vt the total volume of overlying water at time t in the Perspex chamber(l). A is the surface sediment of exchange (m2), while Ct and Ct-1 are the dissolved inorganic nitrogen concentrations at time t and t-1.

3 RESULTS 3.1 Sediments and overlying seawater characteristics of sampling spots

Table 1 shows the physical and chemical parameters of the overlying water and sediments in all sampling spots. The overlying water salinity values of all spots are in quite a low level because

Table 1 Main physical and chemical parameters of water and sediment

Temp. DO NH4+ NO2

- NO3- Biomass Organic content Sampling site

(°C) Sal.

(mg/L) PH

(μmol/L) (μmol/L) (μmol/L) (g/m2) (%) GL 15.5 0.5 8.7 7.98 6.8 3.25 101.53 329.6 3.6 XP 17.4 0.1 8.49 8.28 2.184 0.36 81.95 157.34 2.5

CM2/CM3 17.9 0.3 8.13 7.56 2.45 0.1 79 97.03/9.49 2.3／1.5


the freshwater flow of the Changjiang River is dominant in the estuary area during this season. The dissolved oxygen values are high. Nitrate is a predominant component of the dissolved inorganic nitrogen (DIN) in the overlying water of most sites with a concentration ratio of more than 90%, and the DIN concentration rank is NO3

- >NH4+ >NO2

-. This indicates that DIN composition characters are controlled by the fluvial stream and adjacent seawater because of their composition resemblance (Lin et al., 1995). The maximal Corbicula fluminea biomass is observed in GL, while the minimal one in CM3; the porosity and the organic content (LOI) of surface sediment (0–4cm) show the same order among sites.

3.2 DIN flux in short time incubations

Fig.2 shows the comparison of the DIN fluxes in three types of cores of short-term incubations. Similar to overlying water concentrations, the nitrate fluxes are the biggest, followed by the ammonium and nitrite ones. The nitrite flux values among sites are from −2.5 to 14.6 µmol/(m2·h); there is no coherent comparison of B-core and the other two control cores (D-core and C-core). The ammonium fluxes exhibit mainly sediment uptake with a range of −46.4–40 µmol/(m2·h). Compared with the other two types of control cores, B-core shows additional upward ammonium flux from sediment, thus resulting in more ammonium releases or less uptakes. Nitrate in most of cores release from the sediment and the maximal value appears in the GL experiment (B-core, 929.1 µmol/(m2·h)). It is shown evidently that nitrate release from B-core is

stronger than in D-core and in C-core at all sites.

3.3 Temporal pattern of DIN concentration in long time incubation

After one month’s observation of overlying water DIN change in light-free incubations, we found similar patterns among all site experiments. A notable ammonium release from sediment occurred first, resulting in an ammonium concentration climax in the 3rd or 6th day of the incubation (Fig.3a1, b1, c1, d1). The exchange of nitrate shows only a minimal uptake to sediments at initial stages, then the dissolved nitrate content tends to increase rapidly, but the increasing rate becomes mild afterward (Fig.3a2, b2, c2, d2). Nitrite concentration in overlying water kept a rather low level and had a decreasing trend throughout the incubation (Fig.3a3, b3, c3, d3).

On account of the difference in sediment quality, bivalve individual, and some other factors of incubation, the role of Corbicula fluminea activities on overlying water DIN concentration change has a distinct response. As to nitrite, most B-cores present a fall-rise-fall change pattern, while D-cores and C-cores show a generally declining trend. Ammonia of all cores reach their top ammonium concentrations in the beginning phase. Compared with the control cores, bioturbated cores have higher peak values during incubation periods. Similarly, the release of nitrate from sediments in B-core is stronger.

Fig.2 Fluxes of DIN in short-term experiment


Fig.3 DIN concentration change profile in long time experiment

3.4 Pore water DIN profile in core sediments

It can be seen in Fig.4 that pore water DIN profile patterns are generally similar whether affected by bioturbation or not. Ammonium concentrations increase with depth, nitrate concentrations decrease sharply in the surface layer and nitrite peak concentrations appear around the subsurface. The activities of Corbicula fluminea exert an outstanding influence on the profile of solutes in pore water. Ammonium content is markedly lower in B-cores than in defaunated cores. Furthermore, ammonium concentrations stay at almost a certain level in

uppermost sediments (0–2cm) or elevate slightly in B-core instead of zooming in D-core (Fig.4a1, b1). The distribution of pore water nitrate content shows a quick decline (reduction > 90%) in surface layers derived from strong denitrifications in sediment (Fig.4a2, b2). Nitrate concentrations in overlying water of B-cores are much higher than those in D-cores, and pore water nitrate concentrations in upmost layers are higher than those of D-cores. Nitrite in pore water is relatively low and its vertical distribution is characterized by an accumulation in the subsurface layer (Fig.4a3, b3).


Fig.4 Vertical profile of DIN in sediment pore water

4 DISCUSSIONS

Microcosm incubation systems in the laboratory have become a popular method to simulate natural exchange at environmental interfaces in the last few decades, and it is widely applied in researches on the impacts of benthic alga and macrofauna on sediment biogeochemical processes and solute fluxes (Tuominen et al., 1999; Conley and Johnstone, 1995; Liu et al., 1999). In the present study, we want to reveal the impacts of burrowing activities and inhabitation of bivalve Corbicula fluminea on DIN exchange inn tidal flat sediment-water interface through a series of microcosm incubations. We also discuss the influences of Corbicula fluminea bioirrigations on nitrogen dynamics with a comparison of pore water DIN profile patterns when bioturbation is present or absent.

4.1 Impacts of Corbicula fluminea burrowing activities on the short-term sediment-water DIN flux

Measuring the DIN flux in the experiment can help investigators find out DIN exchange behaviors and physical, chemical and microbial processes of nitrogen. The Changjiang estuary seawater has a high concentration of ammonium. In the fall, oxygen can penetrate strongly into surface sediments to stimulate nitrification. Therefore,

sediments act as the “fate” of dissolved ammonium. When macrofauna Corbicula fluminea were added onto the sediment surface at the beginning of the short-term experiment, they dug with their feet and were then buried into the sediment in several minutes. All the burrowing processes took place over a short time period and violently bioturbated the sediment-water interface, then destroyed the powerful oxidation layer at the sediment surface. In this case, relatively dense ammonium in sediment took the chance to avoid being nitrified and quickly went beyond the interface layer and entered the water column. Corbicula fluminea are surface and suspended deposit feeders that can capture organic detritus by siphoning; after digestion the leftover detritus and excreta are ejected to overlying water (Nakamura and Kerciku, 2000). These residues were more prone to decompose than intact sediment so that the digestion and body metabolism of Corbicula fluminea also accelerated decomposition processes of organic matter and ammonification. The two points above are the main reasons why the B-cores have a larger release of ammonium.

Nitrate is the largest component in DIN of the Changjiang estuary seawater, so there are very large nitrate fluxes in our experiments. It is evidently shown in Fig.2 that the activities of Corbicula fluminea enhanced nitrate efflux from sediments. This observation should be mainly owed to the improvement of nitrification in sediments by Corbicula fluminea. This promoting function mainly proceeds from the following mechanisms: A number of physicochemical and biological factors are important in regulating nitrifying activity in coastal marine sediments. These include temperature, ammonium concentration, O2 tension, pH, salinity, presence of inhibitory compounds, light, macrofaunal activity, and presence of macrophyte roots (Herbert, 1999). Firstly, the nitrification process needs abundant available ammonium, and the metabolism of Corbicula fluminea can support mass available ammonium. The irrigation activites of Corbicula fluminea can stimulate solute transport and exchange between sediment and water column, which also effectively increase the ammonium transport coefficient and availability. Secondly, activities of Corbicula fluminea can increase the oxygen penetration capability as well as the available sediment-water contact surface area and the volume of the oxic layer (Fry, 1982). Thus, the activity range of nitrifying bacteria is increased, and the nitrification conditions are improved. Nitrite acts


as an interim compound in the cause of the nitrogen redox process. Its fluxes are introduced by small releases from sediments and no outstanding patterns are found.

4.2 Impacts of Corbicula fluminea inhabitation and bioturbation on the long-term sediment-water DIN cycling

In the course of short-term incubation, the ammonium in overlying water was gradually consumed by nitrification. However, at the beginning of the long-term experiment, ammonium concentration increased, which indicated the “ammonium source” function of sediment. Table 2 shows the average fluxes of ammonium at initial stages of the long-term experiment (0–6 d). It can be obviously seen that the ammonium effluxes in B-cores are higher than in D-cores and in C-cores. The ammonium flux values of B-core in GL, XP, and CM2 are 17.3, 2.1, and 2.6 times those of D-core respectively. There was very high ammonium concentration in sediment pore water. For example, ammonium concentration in GL sediment pore water was 6 times more than that of overlying water; most of interstitial ammonium would be oxidated when passing through a powerful oxidation layer of sediment surface in the case of aeration. Thus, the overlying water ammonium concentrations of D-cores and C-cores have relatively low variations (Fig.3). However, the bioturbation destroyed the oxidation mantle so as to enhance ammonium release. The long-term

experiment in CM2 and CM3 were considered as parallel experiments except for the only difference in the sediment characters. Sediments of CM2 have bigger nitrogen storage because of its higher content of organic matter, smaller grain size and presence of some remnants of rotten sea grass root. In about one month incubation, the same inhabitation and bioturbation factors at two sites resulted in diverse concentration change response: the concentration climax of CM2 B-core appeared earlier than CM3 and the peak value of CM2 was approximately 8 μM, more than the 5 μM of CM3. Consequently, there was more ammonium release in CM2.

Although the activities of Corbicula fluminea did not intensively burrow sediment surface in the long-term experiment phase, long time inhabitation effectively developed the ammonium release. First, in spite of starvation, Corbicula fluminea still excreted metabolite continually, which was prone to generate ammonium. Second, rhythmic bioirrigation of Corbicula fluminea resembled the function of a water pump, which actively improved solute exchange of overlying water and pore water and thus helped ammonium diffusion. Finally, activities of Corbicula fluminea increased mineralization and decomposition rates of organic matter in sediments and ultimately enhanced ammonium efflux. These stimulative mechanisms made B-core have a sharp concentration elevation at initial incubation stage when nitrification in sediment is not very strong.

Table 2 Interface exchange of ammonia in early days of long-term incubation (µmol/(m2·d))

GL XP CM2 CM3 B-core 229.07±95.53 194.69±134.26 214.81±38.06 33.18±20.88 D-core 13.21±6.90 91.83±69.73 82.56±52.03 -18.68±8.73 C-core -7.87±5.48 128.84±88.53 76.34±37.43 -28.31±6.66

Nitrification and denitrification are key processes in nitrogen cycling (Soonmo and Samantha, 2001). A number of studies have shown that nitrification and denitrification rates can be synchronously increased in the presence of burrowing macrofauna (Kristensen, 1988; Pelegri and Blackburn, 1994; Svensson, 1997). In the experiment, nitrate concentration started to increase greatly several days after incubation. This followed ammonium descending inflexion, which reflected the growth of nitrifying bacteria in sediment (Kristensen and Blackburn, 1987). Nitrifying bacteria is a kind of microbe with low multiplication (Kaplan, 1983), which can explain the large nitrate release after

several days. From Table 3, there are much more nitrate fluxes

out of sediments in B-cores than in control cores in the middle period of long-term incubation. In XP and CM2 experiments, B-cores nitrate effluxes are 2-5 times those of D-cores and C-cores. The difference of nitrate exchange between bioturbated cores and control cores is distinct in the GL experiment: In D-core and C-core, strong denitrification resisted nitrification all square which maintained a certain nitrate concentration in overlying water. However, bioturbation broke this balance and resulted in a huge release of nitrate. In parallel experiments of CM2 and CM3, B-core


nitrate release of CM2 sediment is about 2 times more than that of CM3 only because of different sediment characters. Nitrifying bacteria are obligate aerobes and thus depth distribution of nitrifying bacteria is ultimately constrained by the limits of downward O2 diffusion, which is typically 1–6.5 mm (Herbert, 1999; Revsbech et al., 1980), by enhancement of sediment-water interface square and oxic sediment volume. The long time inhabitation and bioturbation of Corbicula fluminea helped nitrifying bacteria cast off the sediment surface localization to improve the intension and extension

of nitrification activities in sediment. At the same time, abundant ammonium excreted by Corbicula fluminea provided higher substrate availability, which supplied nitrification strongly. The above-mentioned mechanism can explain how Corbicula fluminea activities improved sediment nitrification processes and nitrate release in B-cores of all sites. In the later phase of long-term incubation, increasing nitrate concentration stimulated denitrification in sediments. More efficient nitrate removal later resulted in a mild increasing slope of nitrate curve.

Table 3 Interface exchange of nitrate in long time incubation (µmol/(m2·d))

GL XP CM2 CM3 6–14d 14–29d 6–9d 9–29d 6–19d 19–29d 3–9d 9–29d B-core 2085.46±796.46 310.68±122.55 2085.14±449.02 688.10±211.52 539.76±127.07 168.98±166.90 567.93±299.27 -0.88±171.47D-core 118.03±114.68 -22.58±52.73 494.95±178.45 55.56±41.96 264.46±92.98 16.23±42.30 379.46±218.91 -174.45±84.99 C-core -40.45±54.13 48.68±24.18 1741.21±458.26 132.82±71.93 304.88±151.97 74.79±142.48 369.31±156.62 -90.84±48.42

4.3 Impacts of Corbicula fluminea activities on intertidal sediment pore water DIN vertical distributions

Most organic matter and detritus subsiding to the sediment surface will be inevitably mineralized and decomposed in the process of early sediment diagenesis. Bioturbation can supply a deep sediment layer more electron acceptors such as oxygen for mineralization and thereby stimulate sediment mineralization and ammonification. The increased mineralization and ammonium liberation in the presence of C. fluminea would increase the pore water ammonium concentration. However, ammonium concentrations in pore water in B-core were obviously lower because bioturbation of C. fluminea “diluted” relatively high ammonium concentration in sediment and reduced the concentration gradient between pore water and overlying water, eventually enhancing solute exchange rate around the interface.

It is popular to use a Fichian molecular model of diffusion to assess the flux across the sediment-water interface of a constituent dissolved in sediment pore water (Berner, 1976). The ratio of measured flux to the molecular diffusion flux calculated using Fich’s 1st law could be used as an indication of how much bioirrigation enhances fluxes (Mortimer, et al., 1999). In this paper, we take the GL experiment as an example to assess this influence. Measured nitrogen fluxes (Fm) across the sediment-water interface were worked out from the concentration difference between initial and final

samples of B-cores according to Aller et al. (1985) (Eq.1).

Calculated flux according to the formulation of Berner (1976) is expressed as:

Fc=-φDs∂Ci/∂x (2)

Ds= D0/φ0f (3)

where Fc is the diffusive flux in µmol/(m2·d), φ the sediment porosity in the upper part (2 cm) of the sediment expressed as a percentage, ∂Ci/∂x the concentration gradient of dissolved ammonium in μmol/(ml·cm), and Ds the whole sediment diffusion coefficient (cm2/s) expressed by Krom and Berner (1980). In equation 3 D0 is the diffusion coefficient for ammonium (17.6×10-6 cm2/s) in water at infinite dilution, φ0 is the average sediment porosity, and f is the modified formation factor of Krom and Berner (1980). The equation was corrected for viscosity and deviation from the Archie relation (F=φ-2; Manheim, 1970).

In general, the ratio of measured flux to the molecular diffusion flux is higher for the site with denser macrofaunal communities. Callender and Hammond (1982) found that nutrient fluxes measured in situ with benthic chambers may be 1-10 times the flux calculated from pore water nutrient profiles. Clavero et al. (1991) found that the ratio, measured as flux/diffusion flux of phosphate, increased from 1.77 to 5.85. Mortimer et al. calculated the ratio values to be from 1–25 when they studied ammonium exchange of six sites in the Humber estuary in the presence of various macrofaunal bioirrigation. In the GL experiment the ratio of measured ammonium flux to calculated


ammonium flux (Fm/Fc) is 13.8, a quite big value because of high density of Corbicula fluminea in incubation, which explained the distinct influence of bioturbation on ammonium interface exchange. This diluting effect transferred downward gradually so that the pool of ammonium in upper sediment was cut down. After the long-term incubation, the bioturbation of C. fluminea obviously reduced dissolved ammonium pool of upper sediment (1–6 cm); compared with D-core, pools of B-cores had a reduction of 20% and 25% in GL and XP respectively.

Nitrate in marine sediment pore water typically distributes in 1-5 cm depth with maximal concentration near the sediment subsurface when nitrogen load in overlying water is low (Henriksen et al., 1980; Tuominen et al., 1999). In this study there was a quite high nitrate concentration in overlying water from accumulation in long-term incubation so that nitrate concentration profiles presented a sharply decreasing trend from overlying water to interface pore water and subsurface pore water. Oxygen is required for each of the two major oxidation steps in nitrification. In the course of nitrification the second step, i.e., nitrite oxidation is more sensitive to low oxygen tension, which results in nitrite accumulation when there is inadequate oxygen in the subsurface layer (Henriksen, 1988). However, in the course of nitrogen reduction, nitrite is a kind of middle product that would also accumulate with increasing oxygen tense (Tiedje, 1988). Therefore, both nitrification and denitrification can induce high pore water concentration of nitrite in the case of low oxygen tense in the subsurface of sediments, and the coupling degree of nitrification and denitrification in this layer is very high.

5 CONCLUSIONS Excretion and burrowing activities of Corbicula

fluminea in a short-term experiment can increase +4NH release from sediments. Environmental

factors controlling nitrifying activities are changed by Corbicula fluminea so that nitrifications are stimulated and nitrate release to overlying water occurred more within bioturbated sediments.

+4NH and -

3NO released in turn from the sediments and -

2NO concentration decreased gradually in overlying water in our long-term closed incubation (30d). Long-term bioturbation of Corbicula fluminea can facilitate +

4NH release from the sediments and strongly improve nitrification within the sediments to increase NO3

- accumulative concentration in overlying water.

By analyzing sediment organic matter content and pore water DIN profiles, it is deduced that the actions of Corbicula fluminea can stimulate organic matter mineralization and +

4NH exchange around the sediment-water interface of intertidal flats.

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