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sediment dynamics
Estuarine, Coastal and Shelf Science 56 (2003) 943–956
Effect of changes in water salinity on ammonium, calcium, dissolved inorganic carbon and influence on water/sediment dynamics P. Lo´pez Department of Ecology, University of Barcelona, Avda Diagonal 645, E-08028 Barcelona, Spain Received 14 May 2001; received in revised form 22 April 2002; accepted 25 April 2002
Abstract The effect of a sudden increase in salinity from 10 to 37 in porewater concentration and the benthic fluxes of ammonium, calcium and dissolved inorganic carbon were studied in sediments of a small coastal lagoon, the Albufera d’Es Grau (Minorca Island, Spain). The temporal effects of the changes in salinity were examined over 17 days using a single diffusion-reaction model and a + mass-balance approach. After the salinity change, NH+ 4 -flux to the water and Ca-flux toward sediments increased (NH4 -flux: 5000– 2 1 2 1 2 1 3000 lmol m d in seawater and 600/250 lmol m d in brackish water; Ca-flux: 40/76 meq m d at S ¼ 37 and 13/ 10 meq m2 d1 at S ¼ 10); however, later NH+ 4 -flux decreased in seawater, reaching values lower than in brackish water. In contrast, Ca-flux presented similar values in both conditions. The fluxes of dissolved inorganic carbon, which were constant at S ¼ 10 (55/45 mmol m2 d1), increased during the experiment at S ¼ 37 (from 30 mmol m2 d1 immediately after salinity increase to 60 mmol m2 d1 after 17 days). 2+ In brackish conditions, NH+ fluxes were consistent with a single diffusion-reaction model that assumes a zero-order 4 and Ca + reaction for NH4 production and a first-order reaction for Ca2+ production. In seawater, this model explained the Ca-flux observed, but did not account for the high initial flux of NH+ 4. The mass balance for 17 days indicated a higher retention of NH+ 4 in porewater in the littoral station in seawater conditions (9.5 mmol m2 at S ¼ 37 and 1.6 mmol m2 at S ¼ 10) and a significant reduction in the water consumption at both sites (5 mmol m2 at S ¼ 37; 35/23 mmol m2 at S ¼ 10). In contrast, accumulation of dissolved inorganic carbon in porewater was lower in seawater incubations (10/1 meq m2 at S ¼ 37; 50/90 meq m2 at S ¼ 10) and was linked to a higher efflux of CO2 to the atmosphere, because of calcium carbonate precipitation in water (675/500 meq m2). These results indicate that in‘reased salinity in shallow coastal waters could play a major role in the global carbon cycle. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: benthic fluxes; coastal lagoons; porewater; diffusion-reaction model; carbon balance; ammonium balance
1. Introduction In many coastal lagoons sudden changes in salinity can occur because of seawater input from tidal exchange or through man-regulated channels. Moreover, the rise in seawater level caused by climatic changes may increase the salinity of such coastal water bodies. The effect of these changes on the benthic flux of nutrients from sediments, however, remains unclear. The role of sediments as a source of nutrients to the overlying water is well documented in shallow coastal
E-mail address: [email protected] (P. Lo´pez).
areas, benthic fluxes being determined by multiple processes. Diffusion-controlled transport of the products of organic matter decomposition is the key mechanism that determines porewater concentration and vertical distribution of ammonium and dissolved inorganic carbon (e.g. Cermelj, Bertuzzi, & Faganeli, 1997; Ullman & Aller, 1989) and thus their benthic fluxes (Aller & Mackin, 1989; Berner, 1977). Other factors like bioturbation (Aller, 1994; Aller & Aller, 1998; Barbanti, Ceccherelli, Frascari, Reggiani, & Rosso, 1992; Hammond et al., 1985), sediment structure (Vidal & Morguı´ , 2000), macrophyte community (Bartoli, Cattadori, Giordani, & Viaroli, 1996; Viaroli et al., 1996), benthic microalgae (Bertuzzi, Faganeli, Welker,
0272-7714/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0272-7714(02)00299-8
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& Brambati, 1997) and resuspension of sediments (Sondergaard, Kristensen, & Jeppesen, 1992; Spagnoli & Bergamini, 1997) also affect the extent of the transfer of chemicals across the water–sediment interface. Salinity affects not only the chemical reactions that occur in sediments, like precipitation/dissolution of CaCO3 (Stumm & Morgan, 1981) and the NH+ 4 adsorption capacity of clays (Rysgaard, Thastum, Dalsgaard, Christensen, & Sloth, 1999), but also the sediment structure—through the formation of a viscous layer by flocculation of colloids (Ayukai & Wolanski, 1997). In addition, drastic changes in water salinity may induce physiological stress on living organisms and alter the biotic activity of the macrofauna (Cheung, 1997; Rosas et al., 1999) and microbial community (Rysgaard et al., 1999). Consequently, changes in benthic fluxes that occur after a sudden increase in salinity may be due to a complex web of interactions, thereby making it difficult to predict these changes from a theoretical basis. The best approach to the question is from an experimental basis. Because of the complexity of the processes that determine nutrient release from sediments, comparison between porewater profiles and direct measurement of benthic fluxes is preferable to a single approach (e.g. Barbanti et al., 1992; Gomez-Parra & Forja, 1993). The ÔAlbufera d’Es GrauÕ is a small mesohaline coastal lagoon connected to the sea by a narrow channel with a floodgate that regulates the flux between seawater and lagoon water. The system prevents seawater from flowing into the lagoon, thereby avoiding summer stratification, which caused a serious dystrophic crisis in 1983 (Pretus, 1989). In recent years, controlled
seawater supply to the lagoon has been proposed as a mechanism of preventing the desiccation of shallow areas caused by the high loss of water by evaporation during severely dry summers. To evaluate the impact of this measure it is necessary to establish the effect of salinity on nutrient release from sediments and consequently on the trophic dynamics of the lagoon. The results of a preliminary experiment, which studied the changes in sediment/water fluxes of ammonium and dissolved inorganic carbon caused by the replacement of the natural lagoon water (salinity: 10) by seawater (salinity: 37) are presented here. The aim of this study was not only to draw up guidelines for the adequate management of the Albufera system, but also to improve knowledge of the effect of salinity changes on nutrient fluxes in such coastal areas.
2. Material and methods 2.1. Study area The Albufera d’Es Grau is a small lagoon located on the northeastern coast of the island of Minorca (Fig. 1). The watershed is mainly formed of carboniferous sandstones and has a maximum length of 1700 m, maximum width of 880 m and maximum depth of 3 m, with a mean depth of 1.37 m and a volume of 1 hm3. Since the lagoon is connected to the sea through a narrow, sinuous channel about 300 m long, the role of seawater exchange in the turnover of water mass is quantitatively of minor importance. The lagoon also has low anthropogenic inputs, and the cycle of nutrients is
Fig. 1. Map of Albufera d’Es Grau showing locations at which sediment samples were collected for experiments.
P. Lo´pez / Estuarine, Coastal and Shelf Science 56 (2003) 943–956 Table 1 Annual trends in water characteristics in the Albufera d’Es Grau Station 1
Station 6
(A) Mean annual values of selected water variables (standard deviation in brackets) 23.2 (6.4) 23.0 (5.6) Temperature ( C) pH 8.95 (0.41) 8.41 (0.07) O2 (mg l1) 10.16 (2.27) 7.85 (1.66) O2 (%Saturation) 125 (33) 96 (12) Salinity 11.9 (2.1) 11.6 (2.1) Alkalinity (meq l1) 3.55 (1.37) 3.50 (1.36) Ca2+ (meq l1) 7.94 (0.57) 7.64 (0.65) SRP (PO43) (lM) 0.136 (0.069) 0.121 (0.063) 13.140 (11.15) 13.066 (9.713) NH+ 4 (lM) NO3+NO2 (lM) 1.913 (0.950) 2.611 (1.915) (B) Characteristics of superficial sediments Porosity 0.670–0.762 Mean ; (lm) 49.90 LOI (%) 5.70–12.71 1.78 Al (mmol g1) 0.467 Fe (mmol g1) Ca (mmol g1) 4.43 C/N atomsa 13–18 b Abundance of Nereis ind. m2 1000–1500
0.604–0.777 25.03 5.26–13.97 2.22 0.498 3.74 9–17 350–650
a
Values from Lo´pez, Lluch, Vidal, & Morguı´ , 1996. Abundance of Nereis is a rough estimate from the number of living Nereis observed at the end of the experiment. b
strongly associated with the annual cycle of phytoplankton and macrophytic population (Pretus, 1989). Some annual trends in water characteristics are given in Table 1A. Sediments are fine-grained and have a high content of aluminum (>1.75 mmol g1 dw) and iron (>0.45 mmol g1 dw) and a high C/N ratio (18) (Table 1B). The lagoon is inhabited by a dense population of Ficopomatus enigmaticus (Fauvel), whose debris are abundant in sediments. Nereis diversicolor is the main polychaete of bottom macrofauna. Macrophytic vegetation was present in the littoral area (<1.5 m), with Potamogeton pectinatus and Ruppia cirrosa as the main species at the time of the experiment. 2.2. Experimental procedure Experiments were performed in July (month proposed for seawater input). Sediment cores with a diameter of 6 cm were collected from two sites free of macrophytes: one (S1) representative of the littoral area (1 m in depth) and the other (S6) representative of the central basin (3 m in depth) (Fig. 1). Undisturbed cores were taken to the laboratory within 24 h of collection and analyzed as follows. (A) One core from each station was sliced into 1-cm thick sections under N2 atmosphere, pH was measured and porewater extracted by centrifugation (15 min at 3500 rpm). The water obtained was immediately analyzed for alkalinity and nutrients,
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and samples for metal determinations were acidified with HNO3 and kept at 4 C until analysis. Previous results indicated that variability of porewater analysis in Albufera was lower than 5%. (B) Six cores from each station were incubated in the dark in a continuous flow system, as described by Andersen and Jensen (1992) and Jensen, Mortensen, Andersen, Rasmusen, and Jensen (1995). Filtered seawater in experimental conditions (three cores) and filtered deep-lagoon water in control conditions (three cores) were circulated between each core (headspace ca. 12 cm) and a reservoir, the surface sediment/water volume ratio being 10 cm2 l1. Reservoir water was aerated to maintain oxygen saturation as in natural conditions (see Table 1A). Continuous circulation of water ensured oxic conditions in the overlying water and prevented gradient attenuation throughout the experiment. Sediment–water exchange of nutrients was measured at intervals (on days 1, 9 and 17) by stopping the water flow for 20 h and gently aerating headspace water to mix the water and to maintain air saturation. Water samples for analysis were taken at the beginning and end of 20-h incubations, and flux rates were calculated from the change in concentrations. Blank incubations (i.e. changes in water without contact with sediment) were performed simultaneously. (C) After the last incubation (day 17), cores incubated with seawater and deep lagoon water were treated as indicated earlier in (A). Dissolved compounds were analyzed after filtration through GF/F glass fiber filters. Alkalinity was analyzed by Gran titration (Talling, 1973), NH+ 4 by the standard method described by Grasshoff, Ehrhardt, and Kremling (1983) and Ca2+ by inductively coupled plasma (OES), the detection limits being 0.5 lM for NH+ 4 and 1 mg l1 for Ca. The concentrations of dissolved inorganic carbon and carbonate were calculated from alkalinity and pH values following Millero (1995). The saturation index with respect to calcium carbonate 0 was calculated as: log X ¼ logð½CO3 ½Ca=Kara Þ where [CO3] and [Ca] are the concentrations of carbonate and 0 calcium, respectively, and Kara is the apparent solubility constant of aragonite for the salinity and temperature observed. 3. Results In control incubations, water concentrations of NH+ 4 decreased to values near zero at the end of the experiments, whereas DIC slightly increased. No significant changes were observed in the concentration of Ca2+. In marine experiments, NH+ 4 and DIC increased with time and Ca2+ showed a significant decrease (Table 2).
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Table 2 2+ and DIC and pH in the incubation Concentrations of NH+ 4 , Ca water at the initial conditions and on days 9 and 17 Control
Seawater
S1 Mean
S6 SD
Mean
S1 SD
Mean
S6 SD
Mean
SD
NH+ 4
(lM) Initial 24.260 1.630 22.500 1.610 6.840 0.460 6.840 0.460 9-day 13.310 2.060 20.100 0.660 52.000 1.233 49.000 2.000 17-day 6.350 2.380 1.600 0.200 20.000 1.000 10.000 2.000
Ca (meq l1) Initial 8.980 0.210 9-day 8.570 0.100 17-day 8.920 0.130
8.570 0.010 23.220 0.940 23.220 0.940 8.320 0.090 21.740 0.380 21.900 0.900 8.330 0.100 20.500 0.220 20.900 1.000
DIC (mM) Initial 4.673 0.132 9-day 5.343 0.107 17-day 5.701 0.066
4.534 0.069 5.219 0.059 5.562 0.053
2.931 0.065 3.177 0.160 3.534 0.039
2.931 0.065 3.426 0.007 3.560 0.021
pH Initial 9-day 17-day
8.643 0.009 8.520 0.002 8.541 0.002
8.150 0.042 8.055 0.006 8.191 0.006
8.150 0.042 8.055 0.001 8.189 0.001
8.596 0.039 8.513 0.002 8.528 0.029
In porewater (Figs. 2 and 3), the concentration of Ca2+ increased at the end of the experiment, the highest increases occurring in marine incubations. The pattern of variation for DIC concentrations differed: in control sediments a significant increase with time was observed, while in seawater cores concentrations were nearly constant. Porewater NH+ 4 showed slight differences between the two study sites: at S1, the increase in marine incubations was much higher than in controls, whereas at S6 the magnitude of the NH+ 4 increase was similar in the two experimental conditions. The main mechanism that controls the concentration of calcium in porewater is the equilibrium with respect to calcium carbonate (aragonite or calcite). Water and porewater were always supersaturated with respect to calcite and aragonite (Fig. 4). In control sediments, supersaturation increased in porewater at the end of the experiment, indicating the presence of factors that prevent calcium carbonate precipitation. In contrast,
Fig. 2. Porewater profiles of pH, dissolved inorganic carbon, calcium, magnesium, ammonium and phosphate in station S1. Circles: initial concentrations; squares: final concentrations in control incubations; triangles: final concentrations in seawater incubations.
P. Lo´pez / Estuarine, Coastal and Shelf Science 56 (2003) 943–956
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Fig. 3. As Fig. 2, but in station S6.
in marine sediments, supersaturation decreased in superficial porewater (top 3 cm). The evolution of sediment/water fluxes is shown in Figs. 5–7. In control incubations, the NH+ 4 -flux rose from 600/250 lmol m2 d1 (S1 and S6, respectively) on the first day to 1100/400 lmol m2 d1 on days 9 and 17 (Fig. 5A). Blank flux indicated a net consumption in water that was linearly correlated with water NH+ 4 1 concentration (NH+ ¼ [NH+ 4 consumption lM d 4 ] 0.29+1.08, p < 0:009). In marine incubations, the NH+ 4 flux was very high on the first day (5000/3000 lmol m2 d1) and then decreased to values lower than those observed in control experiments on days 9 and 17 (Fig. 5B). Blank flux indicated a net production in water on the first days, which then became a net consumption on the final days of the experiment (NHþ 4 consumption lM d1 ¼ 2:91 lnðTIMEÞ þ 6:76, p < 0:003).
In the control experiment, the Ca-flux varied from 10 meq m2 d1 on the first day to values near zero on days 9 and 17 (Fig. 6A), whereas in seawater this flux varied from 40 to 2 meq m2 d1 at S1 and from 70 to 10 meq m2 d1 at S6 (Fig. 6B). Blank flux was undetectable in control incubations and showed an exponential decay with time in marine conditions (S1: y ¼ 0:35 expð0:1299 TIME) p < 0:0001; S6: y ¼ 0:43 expð0:1919 TIME) p < 0:00001). The sediment–water flux of DIC remained close to 45–55 mmol m2 d1 throughout the control experiment (Fig. 7A) and increased almost linearly with time in marine incubations (Fig. 7B). Blank fluxes showed an exponential decay with time in both conditions (Control—S1: y ¼ 0:119 expð0:277 TIME) p < 0:0001; S6: y ¼ 0:065 expð0:210 TIME) p < 0:0001; Seawater— S1 and S6: y ¼ 0:033 expð0:117 TIME) p < 0:009).
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P. Lo´pez / Estuarine, Coastal and Shelf Science 56 (2003) 943–956
Fig. 4. Vertical profiles of saturation index with respect to aragonite. Black symbols: initial conditions; white symbols: final conditions.
3.1. Diffusion-reaction model The sediment–water fluxes of Ca2+ and NH+ 4 , as well as the changes in water and porewater concentrations, were first analyzed using a single diffusion model. The values observed were compared with time evolution of diffusive fluxes and final porewater and water concentrations estimated from the initial concentrations, assuming the following diffusion-reaction model: Js ¼ Ds /ðdC=dzÞ
(cm2 d1), dCT/dt, the change in water concentration with time, /, porosity (cm3 cm3), S/V, the sediment surface/water volume ratio (m2 l1), z, the depth (cm) and R (lM d1 or meq l1 d1) is the production rate in sediments. The parameter R is constant if the substance is produced by a zero-order reaction (for example, NH+ 4 ) and R ¼ KðC Ceq Þ (where K is a constant and Ceq (meq 11) is the apparent equilibrium constant of the reaction) if it is produced by a first-order reaction. Values of Ds, R, K and Ceq were tested to obtain the same final concentrations (after 17 days) in water and porewater as those observed in the experiments (Table 3). Finally, C, a function that describes the removal of the substance from water, was deduced from blank incubations. The values predicted are shown as solid lines in Figs. 5 and 6. In control sediments, the temporal variation of NH+ 4 flux and the final concentrations observed in water and porewater were consistent with a diffusion-reaction model that assumes a zero-order reaction: dC=dt ¼ dJ=dz þ R and includes water consumption (Fig. 5A; Table 3). The values of Ds used in the model (6 and 4 cm2 d1) were three or fivefold the molecular diffusion 2 1 coefficient of NH+ at 25 C for U ¼ 0:77 4 (1.014 cm d (Li & Gregory, 1974)), which is consistent with the density of Nereis diversicolor observed in the cores (Table 1B). The same model was applied to the seawater incubation by assuming that the final flux was the steady-state flux (i.e. that the total production rate of 1 NH+ in S1 and 4 in sediments, R, was 28 and 10 lM d S6, respectively). When these R values and the water production were included in the diffusion-reaction model, a Ds value around 2 cm2 d1 was needed to adjust the final flux predictions to those observed, however, the model could not explain the high flux observed in the first incubation nor the final concentrations (Fig. 5B; Table 3). The temporal variation of Ca-flux and the final water and porewater concentrations were consistent with a diffusion-reaction model that assumes a first-order reaction: dC=dt ¼ dJ=dz þ KðC Ceq Þ. In control conditions, the model was a reliable predictor of the observed values when Ceq was considered not to be constant with time but to linearly vary from 4 to around 10 meq l1 (Fig. 6A; Table 3). In seawater, an apparent equilibrium concentration of 20 meq l1 and water removal depending on time allowed good agreement between the observed and predicted values (Fig. 6B; Table 3).
dCs =dt ¼ Ds ðd2 C=dz2 Þ þ R 3.2. Mass balance dCT =dt ¼ /Ds ðdC=dzÞðS=VÞ C ðAller & Mackin,1989Þ where Js is the water/sediment flux (lmol m2 d1 or meq m2 d1), dCs/dt, the change in porewater concentration with time, Ds, the apparent diffusion coefficient
The sediment–water fluxes and changes in water and porewater concentrations were also analyzed from a mass-balance approach, which was calculated as follows for the whole experimental period.
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Fig. 5. Mean and standard error of measured benthic fluxes of NH4+. (A) Control incubations; (B) seawater incubations. White symbols: S1; black symbols: S6. Solid lines indicate the temporal variation predicted from a diffusion-reaction model (see text for more explanation).
1. DCw: change observed in the water mass, calculated as the difference between initial and final concentrations in water incubation. 2. DCpw: change observed in the porewater, calculated as the difference between initial and final concentrations in porewater. P 3. Js: total amount of the substance released (or incorporated) by sediments, calculated by adjusting
the data of observed fluxes as a function of time by the polynomic equation that best fits the data ðp < 0:005Þ. This equation was then integrated for 17 P days to obtain the total flux. 4. Jb: total amount of the substance released (or incorporated) in water P by organisms or suspended matter, calculated as Js, but using the blank fluxes instead of the sediment/water fluxes.
Fig. 6. Mean and standard error of measured benthic fluxes of calcium. (A) Control incubations; (B) seawater incubations. White symbols: S1; black symbols: S6. Solid lines indicate the temporal variation predicted from a diffusion-reaction model (see text for more explanation).
P. Lo´pez / Estuarine, Coastal and Shelf Science 56 (2003) 943–956
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Fig. 7. Mean and standard error of measured benthic fluxes of dissolved inorganic carbon. (A) Control incubations; (B) seawater incubations. White symbols: S1; black symbols: S6.
All these values were expressed in mmol m2 by using the porosity and the S/V ratio for comparison purposes (Table 4). P P In a closed P system, DCw should be J s J b, whereas DCpw+ Js will be the total production in sediments by organic matter decomposition or solid phase dissolution (or the total amount precipitated as solid phase when negative). The whole balance of NH+ 4 indicates that consumption in water throughout the control experiment exceeded the input from sediments by diffusion in both stations, leading to the observed decrease in water concentration (Fig. 8; Table 4). This acted as a driving
force in maintaining a high flux from sediments and only a small fraction (10% in station 1, 26% in station 6) of the NH+ 4 produced in sediments remained in porewater. In contrast, in seawater, the net consumption in water was lower than the input from sediment, which determined the increase in water at the end of the experiment (Table 4). Moreover, the production rate in sediments did not reach the total amount of NH+ 4 accumulated in porewater and released to the water,
Table 3 Parameters in the equations that describe the time evolution of benthic fluxes and final concentrations predicted and observed in water and porewater (see text for explanation)
70 200 1540 1314
7 20 250 300
Ds
R K
Ceq
Cw obs Cw pre Cpw obs Cpw pre
Ca2+ S1 control S6 control S1 seawater S6 seawater
6 4 2 1.75
37 19 28 10
— — — —
6.4 1.6 20.2 10.1
NH4+ S1 control S6 control S1 seawater S6 seawater
20.90 17.42 7.46 1.79
2.78 1.34 0.57 0.26
1.2 1.2 1.2 1.2
— 0.75 3.5–11 8.92 0.75 4.0–10 8.33 0.75 21.5 20.5 0.75 19.5 20.9
DIC S1 control S6 control S1 seawater S6 seawater
1299 857 342 372
77 57 22 37
NH+ 4 S1 S6 S1 S6
control control seawater seawater
Ca2+ S1 control S6 control S1 seawater S6 seawater
Table 4 Components of the mass balance (see text for explanation) for the total duration of experiment (see text for explanation) P P DCw DCpw Jb Js
— — — —
6.7 5.9 12.1 2.7 8.91 8.48 20.93 20.89
71 76 241 82 10.09 9.69 21.61 19.84
82 64 180 77 10.08 9.11 21.57 19.91
Cw obs, final concentration observed in water; Cw pre, final concentration in water predicted from the model; Cpw obs, final concentration observed in porewater; Cpw pre, final concentration in porewater predicted from the model.
66 90 599 576 1.6 2.7 9.5 2.9 46 89 10 1
n.d. n.d. 1270 80 1082 50
73 85 142 333
35.14 22.46 5.24 5.24
2.17 1.95 0.68 0.68
15.72 1.98 7.45 2.10 8.95 3.04
431 226 135 135
50 98 25 25
935 765 834 739
4 15 18 60
100 85 70 57
and DIC and in Quantities are expressed in mmol m2 for NH4 P meq m2 for dissolved calcium. Negative values of Js indicate flux P from water to sediment. Negative values of Jb indicate consumption in water.
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Fig. 8. Mass balance of NH4 for the whole period of experiments. Values in mmol m2. Numbers in brackets were added in order to match the balance.
with a difference of 5 and 3 mmol m2 in S1 and S6, respectively. These differences were close to the fluxes observed on the first day, which could not be explained by the single diffusion-reaction model. The integrated flux of calcium during the control experiment accounted for 70 to 85 meq m2 (in S1 and S6, respectively), which was slightly higher than the increase in porewater content (Fig. 9; Table 4). Thus, only a small fraction of calcium was removed from solution to the solid phase, a finding that is consistent with the increase of the apparent equilibrium concentration used in the diffusion-reaction model. This supports the hypothesis that some factors inhibited the precipitation of calcium carbonate in the sediments. The integrated flux was also consistent with the diminution observed in water at S1. In contrast, the decrease of Ca2+ in water was twice the integrated flux at S6 (Fig. 9; Table 4), although no Ca2+ removal was observed in blank incubations. This loss of about 100 meq m2 in water will be discussed on the basis of dissolved inorganic carbon changes. In seawater, the estimated total flux toward sediments accounted for 142 and 330 meq m2 (in S1 and S6, respectively), values which were notably lower than the increase observed in porewater content (Fig. 9; Table 4). This was in agreement with the dissolution of calcium carbonate
from the solid phase, which could be expected from the change in the apparent equilibrium constant caused by the increase in salinity. Removal of Ca2+ from water by diffusion toward sediment was much lower at both stations than removal through precipitation (estimated from blank incubations), the sum of both mechanisms being close to the decrease observed in water (Fig. 9; Table 4). Significant differences were observed in the mass balance of DIC between the two study sites in the two experimental conditions. In control incubations, the total amount of DIC released to the water at S1 (around 1000 mmol m2) accounted for more than 80% of the increase observed in DIC in water (Fig. 10; Table 4). The remaining 400 mmol m2 were provided by respiration in water (estimated from the blank flux), which followed an exponential decay. However, the increase observed in water at S6 was lower than the sum of the flux from sediments and the water respiration (Fig. 10; Table 4). The lack of about 100 mmol m2 of DIC in water agrees with the lack of Ca2+ (around 100 meq m2=50 mmol m2) observed at this station, and indicates the precipitation of 50 mmol m2 of calcium carbonate by the reaction: Ca2þ þ 2HCO 3 $ CaCO3 þ CO2 . Precipitation was not observed in the blank flux, probably because calcium
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Fig. 9. Mass balance of calcium for the whole period of experiments. Values in meq m2. Numbers in brackets were added in order to match the balance.
carbonate precipitation in brackish waters cannot be detected in daily measurements. In the marine incubations, the total flux of DIC to water was 850 and 750 mmol m2 (S1 and S6, respectively). DIC content in porewater did not change at S6 and slightly decreased at S1 (Table 4). The production of DIC in water (150 mmol m2 in both stations) was the net balance between water respiration and carbon removal from precipitation. From the removal of Ca2+ (700 and 500 mmol m2 in S1 and S6, respectively), the predicted loss of DIC was about 1400 and 1000 mmol m2, respectively. Half of this amount precipitated as CaCO3, while the other half diffused to the atmosphere as CO2 (Fig. 10). To calculate the whole balance of DIC, we assumed that diffusion of CO2 to the atmosphere took place in periods between measurements, therefore the stoichiometry for the blank flux (before CO2 diffuses to the atmosphere) is the loss of 1 mmol DIC/1 mmol Ca. As a consequence, the estimated water respiration should be 850 mmol m2 (i.e. 700 mmol m2 precipitated as CaCO3+150 mmol m2 observed as net balance) at S1 and 600 mmol m2 at S6 (Fig. 10). This agrees with the increase in DIC observed in water (around 350 mmol m2), revealing a net loss of DIC from the system of 700 and 500 mmol m2 (S1 and S6, respectively). This loss should be consistent with the diffusion
of the CO2 formed by calcification and released to the atmosphere during the periods between daily flux measurements.
4. Discussion 4.1. Control incubations The results observed in control incubations at moderate salinity may be related to two main processes that control the diffusive fluxes of DIC, Ca2+ and NH+ 4 : consumption in water and decomposition of organic matter in sediments. Consumption in water was especially significant for NH+ 4 dynamics, since it caused removal of all the efflux from sediments in 17 days. As water mass was saturated in oxygen, NH+ removal may be associated with 4 nitrification. This has been widely reported elsewhere (e.g. Joye, Connell, Miller, Oremland, & Jellison, 1999; Spagnoli & Bergamini, 1997), with rates in estuarine and freshwater environments ranging between 0.05 and 32 lM d1 (Berounsky & Nixon, 1993; Enoksson, 1986). Moreover, for superficial sediments, it has been established that nitrification rates depend on NH+ 4 concentration, when oxygen is not a limiting factor
P. Lo´pez / Estuarine, Coastal and Shelf Science 56 (2003) 943–956
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Fig. 10. Mass balance of dissolved inorganic carbon for the whole period of experiments. Values in mmol m2. Numbers underlined were calculated in order to match calcium balance. Number in brackets were added in order to adjust DIC balance.
(Rysgaard & Berg, 1996). The rates of NH+ 4 consumption observed in the Albufera system (from 0.85 to 5 lM d1) are thus in the range previously reported, and the correlation observed between NH+ 4 consumption and NH+ concentration suggests that nitrification was a 4 significant process which controlled nitrogen water dynamics in the Albufera system. DIC and Ca2+ dynamics were only slightly affected by water processes. In the incubations without sediment (i.e. blank fluxes), Ca2+ concentrations did not change and, as in many coastal environments (Cai & Wang, 1998), DIC production, which can be related to water respiration, was very low. The exponential decay observed for the blank flux of DIC could be associated with the decrease in bacterial activity caused by the exhaustion of organic carbon under experimental conditions (without new synthesis of organic matter or sedimentation of allocthonous material). Although there was no significant difference between the two sites regarding the saturation state with respect to CaCO3, removal of Ca2+ and DIC by CaCO3 precipitation in water was observed only at the deepest station (S6). This may be due to the presence of a higher amount of fine particulate matter at this site (Table 1B).
Smaller sediment particles may be resuspended during incubation in S6 samples and behave as nucleation agents (Stabel, 1986), thereby improving CaCO3 precipitation. The removal of DIC from water, however, only accounted for a minor part of the efflux of carbon from sediments. Since no dissolution of CaCO3 was observed in sediments in brackish incubations, it is concluded that mineralization of organic matter is the main source of DIC to the water in the Albufera system. The good agreement between the temporal variation of NH+ 4 fluxes and the NH+ mass balance with the diffusion4 reaction model assuming a zero-order reaction for NH+ 4 production in sediments also indicates that organic matter decomposition is the main source of NH+ 4 to the water. The total efflux of DIC was higher than that of NH+ 4 , with a molar ratio of 55 and 75 in S1 and S6, respectively. Even if accumulation of DIC and NH+ 4 in sediments is taken into account, this ratio was notably higher than in other coastal areas (e.g. Lo´pez, Vidal, Lluch, & Morguı´ , 1995; Thamdrup, Canfield, Ferdelman, Glud, & Gundersen, 1996) and higher than the average C/N ratio in sediments of the Albufera (18). This observation points to slow mineralization of NH+ 4 ,
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especially at S6. On the other hand, the changes observed in porewater profiles of DIC and NH+ 4 indicated that mineralization of organic carbon was inversely proportional to depth, as reported elsewhere (e.g. Cermelj et al., 1997; Ullman & Aller, 1989), whereas mineralization of NH+ 4 was similar across the 6 cm studied. Two additional factors should be taken into account in order to interpret these data. First, the adsorption of NH+ 4 into clay particles (Rosenfeld, 1979) may lead to an underestimation of the NH+ 4 produced from measured fluxes and accumulation in porewater (Thamdrup et al., 1996). At low salinity, clay minerals, like illites, which are a main component of the Albufera sediments, have a strong capacity to adsorb NH+ 4 (Drever, 1982), which may explain the small flux of NH+ 4 observed, as well as its regular profile with depth. Mineralization of organic matter shows seasonal variations in many shallow systems, for example, in the Mediterranean area, Bertuzzi et al. (1997) and Bartoli et al. (1996) reported low NH+ 4 fluxes in early summer followed by higher fluxes in early autumn, in response to changes in organic matter sedimentation. In the Albufera, the organic matter accumulated in sediments is not only from algal origin, but mostly from macrophyte debris. Decomposition of this material takes place in autumn, and therefore the organic matter present in sediment in early July, when the study was carried out, was refractory material with a low N content. Finally, solubilization of CaCO3 in sediments by metabolically produced CO2 is a common process in many systems (e.g. Green, Aller, & Aller, 1993; Jahnke & Jahnke, 2000; Ullman & Aller, 1989), but in temperate nearshore regions carbonate dynamics is controlled by alternating periods of net dissolution and precipitation on a year scale (Green & Aller, 1998). In the Albufera lagoon, a small amount of Ca2+ and DIC was removed from porewater throughout precipitation because it remained supersaturated throughout the experiment with respect to calcite and aragonite. Saturation of CaCO3 may be due to the high amount of F. enigmaticus (Fauvel) debris, whose tubes contain 80% of high-Mg calcite (Fornos, Forteza, & Martinez Taberner, 1997). Increased supersaturation in porewater during the experiment indicates the presence of some factor that hinders calcification. The increase in porewater phosphate and magnesium (DCpw PO4 81– 85 lmol m2; DCpw Mg 100–500 mmol m2) may inhibit calcite precipitation, as reported elsewhere (Burton & Walter, 1990; Dove & Hochella, 1993). 4.2. Seawater incubations The sudden change in salinity from brackish to seawater levels significantly altered the DIC, Ca2+ and NH+ dynamics at the sediment/water interface at 4 different time scales. One of the most drastic changes,
the high efflux of NH+ 4 from sediment, was observed 24 h after seawater input. This high flux could be related to a rapid bacterial response to the increase of organic carbon caused by the mortality or physiological stress of organisms that cannot withstand the new salinity level. This process might have strongly and rapidly enhanced benthic NH+ 4 -flux. Although this process should have produced a significant input of DIC to the sediments, the mass balance of DIC in seawater incubations did not show a significantly higher input of DIC to sediments when compared with brackish water incubations. Thus, the main cause of the NH+ 4 release appears to be the decrease in the NH+ 4 -adsorption capacity of clays which is induced by high salinity (Rysgaard et al., 1999). Another major initial change was the removal of Ca2+ and DIC from water by CaCO3 precipitation and the associated CO2 release to the atmosphere. Abiotic calcification is expected to occur when brackish alkalinity-rich water mixes with Ca-rich seawater (Stumm and Morgan, 1981), which, like biogenic calcification, is a source of CO2 to the surrounding water and to the atmosphere (Frankignoulle et al., 1998; Frankignoulle, Canon, & Gattuso, 1994; Purdie & Finch, 1994). The last initial change was a shift from NH+ 4 consumption in water in brackish system to NH+ 4 production in saline incubations. NH+ 4 production in water exponentially decreased to negative values (i.e. NH+ 4 consumption) after 6 days. Physiological stress of organisms, as well as desorption of NH+ from 4 resuspended sediment particles, is the most probable cause of initial release in water. In any case, NH+ 4 production from suspended matter accounted for only a minor source of this compound to the water. After the initial modifications following seawater input, the Albufera sediment system evolved to a new state of equilibrium, which differed in several aspects from that observed in brackish conditions. First, the rate of NH+ 4 consumption in water after the initial production was much lower than that measured in brackish incubations. Reduction of nitrifying activity at high salinity has not only been linked to limitation of NH+ 4 availability but also to a physiological effect of salinity on nitrifying organisms (Rysgaard et al., 1999). In spite of the higher levels of NH+ 4 observed in seawater incubations, reduction of nitrifying activity may explain the decreased consumption rates in water. Moreover, several authors have reported that the rates of NH+ 4 excretion of macrofauna decrease with salinity (Cheung, 1997; Rosas et al., 1999). The diffusion-reaction model and the mass-balance calculation showed that the rates of DIC and NH+ 4 production that could be attributed to the organic matter mineralization in sediments were lower than in brackish incubations. This is an unexpected result, because marine sediments do not present lower rates of mineralization than freshwater. Nevertheless, changes
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in the burrowing activity of benthic macrofauna could provide a possible explanation. The apparent diffusion coefficient deduced from water/sediment fluxes and porewater concentrations were higher in brackish waters and agreed with the density of living Nereis diversicolor observed at the end of experiment (Aller, 1980; Aller & Yingst, 1985). In cores incubated with seawater, the number of living N. diversicolor at the end of experiment was similar to that observed in control cores, but the Ds value was lower and approached molecular values. Reduction of the physiological activity of interstitial organisms as a result of sudden changes in salinity has been reported elsewhere (Cheung, 1997). The lower values of Ds observed in seawater were thus in agreement with decreased burrowing activity because of increased salinity. Moreover, benthic fauna not only directly release CO2 and ammonia (Kristensen, Jensen, & Aller, 1991), but also promote the mineralization of organic matter (Aller, 1994; Aller & Aller, 1998). The reduction of Nereis activity in sediments of Albufera caused by the drastic change in salinity may therefore explain the decrease observed in the rates of NH+ 4 and DIC production. Reduction of bioturbation was more severe at S1, leading to a higher increase in porewater accumulation of NH+ 4 . Total DIC flux was less modified than NH+ 4 flux, because the increase in porewater salinity altered equilibrium concentrations with respect to CaCO3. Dissolution of CaCO3 solid phase by metabolically produced CO2 compensated for the decrease of DIC production by mineralization.
5. Conclusions The sudden change in salinity from 10 to 37 2+ significantly altered the benthic fluxes of NH+ 4 , Ca and DIC. 1. A substantial amount of NH+ 4 was released from sediments to the water 24 h after the salinity change. This may increase nitrogen availability for the phytoplanktonic community and consequently stimulate planktonic productivity. The initial release to the water was then followed by a reduction of sediment-flux and a higher retention in porewater, therefore the nitrogen-stimulated production, if done, should be short. 2. The most significant change in the carbon cycle was the intense precipitation of calcium carbonate in water, which was not compensated with calcium carbonate dissolution in sediments. This represented a significant export of carbon from the system to the atmosphere. Because the rise in sea level (and consequently, the increase in salinity in shallow coastal waters) may be a result of atmospheric CO2 increase, a process like that observed in the Albufera
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system may involve a positive feed-back, thereby causing a further increase in atmospheric CO2. Acknowledgements I thank, J.L. Pretus, X. Lluch and J. Gonzalez for helping with sample collection, S. Joye for helpful comments and R. Rycroft for improving the manuscript. I also thank two anonymous reviewers for their valuable comments. This study was supported by a CYCIT grant (PB97-0953).
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Lo´pez, P., Vidal, M., Lluch, X., & Morguı´ , J. A. (1995). Sediment metabolism in a transitional continental/marine area: the Albufera of Majorca Balearic Islands Spain. Marine and Freshwater Research 46, 45–53. Millero, F. (1995). Thermodynamics of the carbon dioxide system in the oceans. Geochimica et Cosmochimica Acta 59, 661–677. Pretus, J. L. (1989). Limnologı´a de la Albufera de Menorca Menorca, Espana. Limnetica 5, 69–81. Purdie, D. A., & Finch, M. S. (1994). Impact of a coccolithophorid bloom on dissolved carbon dioxide in sea water enclosures in a Norwegian fjord. Sarsia 79, 379–387. Rosas, C., Martinez, E., Gaxiola, G., Brito, R., Sanchez, A., & Soto, L. A. (1999). The effect of dissolved oxygen and salinity on oxygen consumption, ammonia excretion and osmotic pressure of Penaeus setiferus Linnaeus juveniles. Journal of Experimental Marine Biology and Ecology 234, 41–57. Rosenfeld, J. K. (1979). Ammonium adsorption in nearshore anoxic sediments. Limnology and Oceanography 24, 356–364. Rysgaard, S., & Berg, P. (1996). Mineralization in a northeastern Greenland sediment, mathematical modelling, measured sediment porewater profiles and actual activities. Aquatic Microbial Ecology 11, 297–305. Rysgaard, S., Thastum, P., Dalsgaard, T., Christensen, P. B., & Sloth, N. P. (1999). Effects of salinity on NH+ 4 adsorption capacity, nitrification, and denitrification in Danish estuarine sediments. Estuaries 22, 21–30. Sondergaard, M., Kristensen, P., & Jeppesen, E. (1992). Phosphorus release from resuspended sediment in the shallow and windexposed lake Arreso, Denmark. Hydrobiologia 228, 91–99. Spagnoli, F., & Bergamini, M. C. (1997). Water–sediment exchange of nutrients during early diagenesis and resuspension of anoxic sediments from the Northern Adriatic Sea shelf. Water, Air and Soil Pollution 99, 541–556. Stabel, H. H. (1986). Calcite precipitation in lake Constance, chemical equilibrium, sedimentation and nucleation by algae. Limnology and Oceanography 31, 1081–1093. Stumm, W., & Morgan, J. (1981). Aquatic chemistry, (2nd ed., 780 pp). New York: Wiley. Talling, J. F. (1973). The application of some electrochemical methods to the measurement of photosynthesis and respiration in fresh waters. Freshwater Biology 3, 335–363. Thamdrup, B., Canfield, D. E., Ferdelman, T. G., Glud, R. N., & Gundersen, J. K. (1996). A biogeochemical survey of the anoxic basin Golfo Dulce, Costa Rica. Revista de Biologia Tropical 44, 19–33. Ullman, W. J., & Aller, R. C. (1989). Nutrient release rates from the sediments of Saginaw Bay, Lake Huron. Hydrobiologia 171, 127– 140. Viaroli, P., Bartoli, M., Bondavalli, C., Christian, R. R., Giordani, G., & Naldi, M. (1996). Macrophyte communities and their impact on benthic fluxes of oxygen, sulphide and nutrients in shallow eutrophic environments. Hydrobiologia 329, 105–119. Vidal, M., & Morguı´ , J. A. (2000). Close and delayed benthic–pelagic coupling in coastal ecosystems, the role of physical constraints. Hydrobiologia 429, 105–113.
Effect of changes in water salinity on ammonium, calcium, dissolved inorganic carbon and influence on water/sediment dynamics P. Lo´pez Department of Ecology, University of Barcelona, Avda Diagonal 645, E-08028 Barcelona, Spain Received 14 May 2001; received in revised form 22 April 2002; accepted 25 April 2002
Abstract The effect of a sudden increase in salinity from 10 to 37 in porewater concentration and the benthic fluxes of ammonium, calcium and dissolved inorganic carbon were studied in sediments of a small coastal lagoon, the Albufera d’Es Grau (Minorca Island, Spain). The temporal effects of the changes in salinity were examined over 17 days using a single diffusion-reaction model and a + mass-balance approach. After the salinity change, NH+ 4 -flux to the water and Ca-flux toward sediments increased (NH4 -flux: 5000– 2 1 2 1 2 1 3000 lmol m d in seawater and 600/250 lmol m d in brackish water; Ca-flux: 40/76 meq m d at S ¼ 37 and 13/ 10 meq m2 d1 at S ¼ 10); however, later NH+ 4 -flux decreased in seawater, reaching values lower than in brackish water. In contrast, Ca-flux presented similar values in both conditions. The fluxes of dissolved inorganic carbon, which were constant at S ¼ 10 (55/45 mmol m2 d1), increased during the experiment at S ¼ 37 (from 30 mmol m2 d1 immediately after salinity increase to 60 mmol m2 d1 after 17 days). 2+ In brackish conditions, NH+ fluxes were consistent with a single diffusion-reaction model that assumes a zero-order 4 and Ca + reaction for NH4 production and a first-order reaction for Ca2+ production. In seawater, this model explained the Ca-flux observed, but did not account for the high initial flux of NH+ 4. The mass balance for 17 days indicated a higher retention of NH+ 4 in porewater in the littoral station in seawater conditions (9.5 mmol m2 at S ¼ 37 and 1.6 mmol m2 at S ¼ 10) and a significant reduction in the water consumption at both sites (5 mmol m2 at S ¼ 37; 35/23 mmol m2 at S ¼ 10). In contrast, accumulation of dissolved inorganic carbon in porewater was lower in seawater incubations (10/1 meq m2 at S ¼ 37; 50/90 meq m2 at S ¼ 10) and was linked to a higher efflux of CO2 to the atmosphere, because of calcium carbonate precipitation in water (675/500 meq m2). These results indicate that in‘reased salinity in shallow coastal waters could play a major role in the global carbon cycle. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: benthic fluxes; coastal lagoons; porewater; diffusion-reaction model; carbon balance; ammonium balance
1. Introduction In many coastal lagoons sudden changes in salinity can occur because of seawater input from tidal exchange or through man-regulated channels. Moreover, the rise in seawater level caused by climatic changes may increase the salinity of such coastal water bodies. The effect of these changes on the benthic flux of nutrients from sediments, however, remains unclear. The role of sediments as a source of nutrients to the overlying water is well documented in shallow coastal
E-mail address: [email protected] (P. Lo´pez).
areas, benthic fluxes being determined by multiple processes. Diffusion-controlled transport of the products of organic matter decomposition is the key mechanism that determines porewater concentration and vertical distribution of ammonium and dissolved inorganic carbon (e.g. Cermelj, Bertuzzi, & Faganeli, 1997; Ullman & Aller, 1989) and thus their benthic fluxes (Aller & Mackin, 1989; Berner, 1977). Other factors like bioturbation (Aller, 1994; Aller & Aller, 1998; Barbanti, Ceccherelli, Frascari, Reggiani, & Rosso, 1992; Hammond et al., 1985), sediment structure (Vidal & Morguı´ , 2000), macrophyte community (Bartoli, Cattadori, Giordani, & Viaroli, 1996; Viaroli et al., 1996), benthic microalgae (Bertuzzi, Faganeli, Welker,
0272-7714/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0272-7714(02)00299-8
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& Brambati, 1997) and resuspension of sediments (Sondergaard, Kristensen, & Jeppesen, 1992; Spagnoli & Bergamini, 1997) also affect the extent of the transfer of chemicals across the water–sediment interface. Salinity affects not only the chemical reactions that occur in sediments, like precipitation/dissolution of CaCO3 (Stumm & Morgan, 1981) and the NH+ 4 adsorption capacity of clays (Rysgaard, Thastum, Dalsgaard, Christensen, & Sloth, 1999), but also the sediment structure—through the formation of a viscous layer by flocculation of colloids (Ayukai & Wolanski, 1997). In addition, drastic changes in water salinity may induce physiological stress on living organisms and alter the biotic activity of the macrofauna (Cheung, 1997; Rosas et al., 1999) and microbial community (Rysgaard et al., 1999). Consequently, changes in benthic fluxes that occur after a sudden increase in salinity may be due to a complex web of interactions, thereby making it difficult to predict these changes from a theoretical basis. The best approach to the question is from an experimental basis. Because of the complexity of the processes that determine nutrient release from sediments, comparison between porewater profiles and direct measurement of benthic fluxes is preferable to a single approach (e.g. Barbanti et al., 1992; Gomez-Parra & Forja, 1993). The ÔAlbufera d’Es GrauÕ is a small mesohaline coastal lagoon connected to the sea by a narrow channel with a floodgate that regulates the flux between seawater and lagoon water. The system prevents seawater from flowing into the lagoon, thereby avoiding summer stratification, which caused a serious dystrophic crisis in 1983 (Pretus, 1989). In recent years, controlled
seawater supply to the lagoon has been proposed as a mechanism of preventing the desiccation of shallow areas caused by the high loss of water by evaporation during severely dry summers. To evaluate the impact of this measure it is necessary to establish the effect of salinity on nutrient release from sediments and consequently on the trophic dynamics of the lagoon. The results of a preliminary experiment, which studied the changes in sediment/water fluxes of ammonium and dissolved inorganic carbon caused by the replacement of the natural lagoon water (salinity: 10) by seawater (salinity: 37) are presented here. The aim of this study was not only to draw up guidelines for the adequate management of the Albufera system, but also to improve knowledge of the effect of salinity changes on nutrient fluxes in such coastal areas.
2. Material and methods 2.1. Study area The Albufera d’Es Grau is a small lagoon located on the northeastern coast of the island of Minorca (Fig. 1). The watershed is mainly formed of carboniferous sandstones and has a maximum length of 1700 m, maximum width of 880 m and maximum depth of 3 m, with a mean depth of 1.37 m and a volume of 1 hm3. Since the lagoon is connected to the sea through a narrow, sinuous channel about 300 m long, the role of seawater exchange in the turnover of water mass is quantitatively of minor importance. The lagoon also has low anthropogenic inputs, and the cycle of nutrients is
Fig. 1. Map of Albufera d’Es Grau showing locations at which sediment samples were collected for experiments.
P. Lo´pez / Estuarine, Coastal and Shelf Science 56 (2003) 943–956 Table 1 Annual trends in water characteristics in the Albufera d’Es Grau Station 1
Station 6
(A) Mean annual values of selected water variables (standard deviation in brackets) 23.2 (6.4) 23.0 (5.6) Temperature ( C) pH 8.95 (0.41) 8.41 (0.07) O2 (mg l1) 10.16 (2.27) 7.85 (1.66) O2 (%Saturation) 125 (33) 96 (12) Salinity 11.9 (2.1) 11.6 (2.1) Alkalinity (meq l1) 3.55 (1.37) 3.50 (1.36) Ca2+ (meq l1) 7.94 (0.57) 7.64 (0.65) SRP (PO43) (lM) 0.136 (0.069) 0.121 (0.063) 13.140 (11.15) 13.066 (9.713) NH+ 4 (lM) NO3+NO2 (lM) 1.913 (0.950) 2.611 (1.915) (B) Characteristics of superficial sediments Porosity 0.670–0.762 Mean ; (lm) 49.90 LOI (%) 5.70–12.71 1.78 Al (mmol g1) 0.467 Fe (mmol g1) Ca (mmol g1) 4.43 C/N atomsa 13–18 b Abundance of Nereis ind. m2 1000–1500
0.604–0.777 25.03 5.26–13.97 2.22 0.498 3.74 9–17 350–650
a
Values from Lo´pez, Lluch, Vidal, & Morguı´ , 1996. Abundance of Nereis is a rough estimate from the number of living Nereis observed at the end of the experiment. b
strongly associated with the annual cycle of phytoplankton and macrophytic population (Pretus, 1989). Some annual trends in water characteristics are given in Table 1A. Sediments are fine-grained and have a high content of aluminum (>1.75 mmol g1 dw) and iron (>0.45 mmol g1 dw) and a high C/N ratio (18) (Table 1B). The lagoon is inhabited by a dense population of Ficopomatus enigmaticus (Fauvel), whose debris are abundant in sediments. Nereis diversicolor is the main polychaete of bottom macrofauna. Macrophytic vegetation was present in the littoral area (<1.5 m), with Potamogeton pectinatus and Ruppia cirrosa as the main species at the time of the experiment. 2.2. Experimental procedure Experiments were performed in July (month proposed for seawater input). Sediment cores with a diameter of 6 cm were collected from two sites free of macrophytes: one (S1) representative of the littoral area (1 m in depth) and the other (S6) representative of the central basin (3 m in depth) (Fig. 1). Undisturbed cores were taken to the laboratory within 24 h of collection and analyzed as follows. (A) One core from each station was sliced into 1-cm thick sections under N2 atmosphere, pH was measured and porewater extracted by centrifugation (15 min at 3500 rpm). The water obtained was immediately analyzed for alkalinity and nutrients,
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and samples for metal determinations were acidified with HNO3 and kept at 4 C until analysis. Previous results indicated that variability of porewater analysis in Albufera was lower than 5%. (B) Six cores from each station were incubated in the dark in a continuous flow system, as described by Andersen and Jensen (1992) and Jensen, Mortensen, Andersen, Rasmusen, and Jensen (1995). Filtered seawater in experimental conditions (three cores) and filtered deep-lagoon water in control conditions (three cores) were circulated between each core (headspace ca. 12 cm) and a reservoir, the surface sediment/water volume ratio being 10 cm2 l1. Reservoir water was aerated to maintain oxygen saturation as in natural conditions (see Table 1A). Continuous circulation of water ensured oxic conditions in the overlying water and prevented gradient attenuation throughout the experiment. Sediment–water exchange of nutrients was measured at intervals (on days 1, 9 and 17) by stopping the water flow for 20 h and gently aerating headspace water to mix the water and to maintain air saturation. Water samples for analysis were taken at the beginning and end of 20-h incubations, and flux rates were calculated from the change in concentrations. Blank incubations (i.e. changes in water without contact with sediment) were performed simultaneously. (C) After the last incubation (day 17), cores incubated with seawater and deep lagoon water were treated as indicated earlier in (A). Dissolved compounds were analyzed after filtration through GF/F glass fiber filters. Alkalinity was analyzed by Gran titration (Talling, 1973), NH+ 4 by the standard method described by Grasshoff, Ehrhardt, and Kremling (1983) and Ca2+ by inductively coupled plasma (OES), the detection limits being 0.5 lM for NH+ 4 and 1 mg l1 for Ca. The concentrations of dissolved inorganic carbon and carbonate were calculated from alkalinity and pH values following Millero (1995). The saturation index with respect to calcium carbonate 0 was calculated as: log X ¼ logð½CO3 ½Ca=Kara Þ where [CO3] and [Ca] are the concentrations of carbonate and 0 calcium, respectively, and Kara is the apparent solubility constant of aragonite for the salinity and temperature observed. 3. Results In control incubations, water concentrations of NH+ 4 decreased to values near zero at the end of the experiments, whereas DIC slightly increased. No significant changes were observed in the concentration of Ca2+. In marine experiments, NH+ 4 and DIC increased with time and Ca2+ showed a significant decrease (Table 2).
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Table 2 2+ and DIC and pH in the incubation Concentrations of NH+ 4 , Ca water at the initial conditions and on days 9 and 17 Control
Seawater
S1 Mean
S6 SD
Mean
S1 SD
Mean
S6 SD
Mean
SD
NH+ 4
(lM) Initial 24.260 1.630 22.500 1.610 6.840 0.460 6.840 0.460 9-day 13.310 2.060 20.100 0.660 52.000 1.233 49.000 2.000 17-day 6.350 2.380 1.600 0.200 20.000 1.000 10.000 2.000
Ca (meq l1) Initial 8.980 0.210 9-day 8.570 0.100 17-day 8.920 0.130
8.570 0.010 23.220 0.940 23.220 0.940 8.320 0.090 21.740 0.380 21.900 0.900 8.330 0.100 20.500 0.220 20.900 1.000
DIC (mM) Initial 4.673 0.132 9-day 5.343 0.107 17-day 5.701 0.066
4.534 0.069 5.219 0.059 5.562 0.053
2.931 0.065 3.177 0.160 3.534 0.039
2.931 0.065 3.426 0.007 3.560 0.021
pH Initial 9-day 17-day
8.643 0.009 8.520 0.002 8.541 0.002
8.150 0.042 8.055 0.006 8.191 0.006
8.150 0.042 8.055 0.001 8.189 0.001
8.596 0.039 8.513 0.002 8.528 0.029
In porewater (Figs. 2 and 3), the concentration of Ca2+ increased at the end of the experiment, the highest increases occurring in marine incubations. The pattern of variation for DIC concentrations differed: in control sediments a significant increase with time was observed, while in seawater cores concentrations were nearly constant. Porewater NH+ 4 showed slight differences between the two study sites: at S1, the increase in marine incubations was much higher than in controls, whereas at S6 the magnitude of the NH+ 4 increase was similar in the two experimental conditions. The main mechanism that controls the concentration of calcium in porewater is the equilibrium with respect to calcium carbonate (aragonite or calcite). Water and porewater were always supersaturated with respect to calcite and aragonite (Fig. 4). In control sediments, supersaturation increased in porewater at the end of the experiment, indicating the presence of factors that prevent calcium carbonate precipitation. In contrast,
Fig. 2. Porewater profiles of pH, dissolved inorganic carbon, calcium, magnesium, ammonium and phosphate in station S1. Circles: initial concentrations; squares: final concentrations in control incubations; triangles: final concentrations in seawater incubations.
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Fig. 3. As Fig. 2, but in station S6.
in marine sediments, supersaturation decreased in superficial porewater (top 3 cm). The evolution of sediment/water fluxes is shown in Figs. 5–7. In control incubations, the NH+ 4 -flux rose from 600/250 lmol m2 d1 (S1 and S6, respectively) on the first day to 1100/400 lmol m2 d1 on days 9 and 17 (Fig. 5A). Blank flux indicated a net consumption in water that was linearly correlated with water NH+ 4 1 concentration (NH+ ¼ [NH+ 4 consumption lM d 4 ] 0.29+1.08, p < 0:009). In marine incubations, the NH+ 4 flux was very high on the first day (5000/3000 lmol m2 d1) and then decreased to values lower than those observed in control experiments on days 9 and 17 (Fig. 5B). Blank flux indicated a net production in water on the first days, which then became a net consumption on the final days of the experiment (NHþ 4 consumption lM d1 ¼ 2:91 lnðTIMEÞ þ 6:76, p < 0:003).
In the control experiment, the Ca-flux varied from 10 meq m2 d1 on the first day to values near zero on days 9 and 17 (Fig. 6A), whereas in seawater this flux varied from 40 to 2 meq m2 d1 at S1 and from 70 to 10 meq m2 d1 at S6 (Fig. 6B). Blank flux was undetectable in control incubations and showed an exponential decay with time in marine conditions (S1: y ¼ 0:35 expð0:1299 TIME) p < 0:0001; S6: y ¼ 0:43 expð0:1919 TIME) p < 0:00001). The sediment–water flux of DIC remained close to 45–55 mmol m2 d1 throughout the control experiment (Fig. 7A) and increased almost linearly with time in marine incubations (Fig. 7B). Blank fluxes showed an exponential decay with time in both conditions (Control—S1: y ¼ 0:119 expð0:277 TIME) p < 0:0001; S6: y ¼ 0:065 expð0:210 TIME) p < 0:0001; Seawater— S1 and S6: y ¼ 0:033 expð0:117 TIME) p < 0:009).
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Fig. 4. Vertical profiles of saturation index with respect to aragonite. Black symbols: initial conditions; white symbols: final conditions.
3.1. Diffusion-reaction model The sediment–water fluxes of Ca2+ and NH+ 4 , as well as the changes in water and porewater concentrations, were first analyzed using a single diffusion model. The values observed were compared with time evolution of diffusive fluxes and final porewater and water concentrations estimated from the initial concentrations, assuming the following diffusion-reaction model: Js ¼ Ds /ðdC=dzÞ
(cm2 d1), dCT/dt, the change in water concentration with time, /, porosity (cm3 cm3), S/V, the sediment surface/water volume ratio (m2 l1), z, the depth (cm) and R (lM d1 or meq l1 d1) is the production rate in sediments. The parameter R is constant if the substance is produced by a zero-order reaction (for example, NH+ 4 ) and R ¼ KðC Ceq Þ (where K is a constant and Ceq (meq 11) is the apparent equilibrium constant of the reaction) if it is produced by a first-order reaction. Values of Ds, R, K and Ceq were tested to obtain the same final concentrations (after 17 days) in water and porewater as those observed in the experiments (Table 3). Finally, C, a function that describes the removal of the substance from water, was deduced from blank incubations. The values predicted are shown as solid lines in Figs. 5 and 6. In control sediments, the temporal variation of NH+ 4 flux and the final concentrations observed in water and porewater were consistent with a diffusion-reaction model that assumes a zero-order reaction: dC=dt ¼ dJ=dz þ R and includes water consumption (Fig. 5A; Table 3). The values of Ds used in the model (6 and 4 cm2 d1) were three or fivefold the molecular diffusion 2 1 coefficient of NH+ at 25 C for U ¼ 0:77 4 (1.014 cm d (Li & Gregory, 1974)), which is consistent with the density of Nereis diversicolor observed in the cores (Table 1B). The same model was applied to the seawater incubation by assuming that the final flux was the steady-state flux (i.e. that the total production rate of 1 NH+ in S1 and 4 in sediments, R, was 28 and 10 lM d S6, respectively). When these R values and the water production were included in the diffusion-reaction model, a Ds value around 2 cm2 d1 was needed to adjust the final flux predictions to those observed, however, the model could not explain the high flux observed in the first incubation nor the final concentrations (Fig. 5B; Table 3). The temporal variation of Ca-flux and the final water and porewater concentrations were consistent with a diffusion-reaction model that assumes a first-order reaction: dC=dt ¼ dJ=dz þ KðC Ceq Þ. In control conditions, the model was a reliable predictor of the observed values when Ceq was considered not to be constant with time but to linearly vary from 4 to around 10 meq l1 (Fig. 6A; Table 3). In seawater, an apparent equilibrium concentration of 20 meq l1 and water removal depending on time allowed good agreement between the observed and predicted values (Fig. 6B; Table 3).
dCs =dt ¼ Ds ðd2 C=dz2 Þ þ R 3.2. Mass balance dCT =dt ¼ /Ds ðdC=dzÞðS=VÞ C ðAller & Mackin,1989Þ where Js is the water/sediment flux (lmol m2 d1 or meq m2 d1), dCs/dt, the change in porewater concentration with time, Ds, the apparent diffusion coefficient
The sediment–water fluxes and changes in water and porewater concentrations were also analyzed from a mass-balance approach, which was calculated as follows for the whole experimental period.
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Fig. 5. Mean and standard error of measured benthic fluxes of NH4+. (A) Control incubations; (B) seawater incubations. White symbols: S1; black symbols: S6. Solid lines indicate the temporal variation predicted from a diffusion-reaction model (see text for more explanation).
1. DCw: change observed in the water mass, calculated as the difference between initial and final concentrations in water incubation. 2. DCpw: change observed in the porewater, calculated as the difference between initial and final concentrations in porewater. P 3. Js: total amount of the substance released (or incorporated) by sediments, calculated by adjusting
the data of observed fluxes as a function of time by the polynomic equation that best fits the data ðp < 0:005Þ. This equation was then integrated for 17 P days to obtain the total flux. 4. Jb: total amount of the substance released (or incorporated) in water P by organisms or suspended matter, calculated as Js, but using the blank fluxes instead of the sediment/water fluxes.
Fig. 6. Mean and standard error of measured benthic fluxes of calcium. (A) Control incubations; (B) seawater incubations. White symbols: S1; black symbols: S6. Solid lines indicate the temporal variation predicted from a diffusion-reaction model (see text for more explanation).
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Fig. 7. Mean and standard error of measured benthic fluxes of dissolved inorganic carbon. (A) Control incubations; (B) seawater incubations. White symbols: S1; black symbols: S6.
All these values were expressed in mmol m2 by using the porosity and the S/V ratio for comparison purposes (Table 4). P P In a closed P system, DCw should be J s J b, whereas DCpw+ Js will be the total production in sediments by organic matter decomposition or solid phase dissolution (or the total amount precipitated as solid phase when negative). The whole balance of NH+ 4 indicates that consumption in water throughout the control experiment exceeded the input from sediments by diffusion in both stations, leading to the observed decrease in water concentration (Fig. 8; Table 4). This acted as a driving
force in maintaining a high flux from sediments and only a small fraction (10% in station 1, 26% in station 6) of the NH+ 4 produced in sediments remained in porewater. In contrast, in seawater, the net consumption in water was lower than the input from sediment, which determined the increase in water at the end of the experiment (Table 4). Moreover, the production rate in sediments did not reach the total amount of NH+ 4 accumulated in porewater and released to the water,
Table 3 Parameters in the equations that describe the time evolution of benthic fluxes and final concentrations predicted and observed in water and porewater (see text for explanation)
70 200 1540 1314
7 20 250 300
Ds
R K
Ceq
Cw obs Cw pre Cpw obs Cpw pre
Ca2+ S1 control S6 control S1 seawater S6 seawater
6 4 2 1.75
37 19 28 10
— — — —
6.4 1.6 20.2 10.1
NH4+ S1 control S6 control S1 seawater S6 seawater
20.90 17.42 7.46 1.79
2.78 1.34 0.57 0.26
1.2 1.2 1.2 1.2
— 0.75 3.5–11 8.92 0.75 4.0–10 8.33 0.75 21.5 20.5 0.75 19.5 20.9
DIC S1 control S6 control S1 seawater S6 seawater
1299 857 342 372
77 57 22 37
NH+ 4 S1 S6 S1 S6
control control seawater seawater
Ca2+ S1 control S6 control S1 seawater S6 seawater
Table 4 Components of the mass balance (see text for explanation) for the total duration of experiment (see text for explanation) P P DCw DCpw Jb Js
— — — —
6.7 5.9 12.1 2.7 8.91 8.48 20.93 20.89
71 76 241 82 10.09 9.69 21.61 19.84
82 64 180 77 10.08 9.11 21.57 19.91
Cw obs, final concentration observed in water; Cw pre, final concentration in water predicted from the model; Cpw obs, final concentration observed in porewater; Cpw pre, final concentration in porewater predicted from the model.
66 90 599 576 1.6 2.7 9.5 2.9 46 89 10 1
n.d. n.d. 1270 80 1082 50
73 85 142 333
35.14 22.46 5.24 5.24
2.17 1.95 0.68 0.68
15.72 1.98 7.45 2.10 8.95 3.04
431 226 135 135
50 98 25 25
935 765 834 739
4 15 18 60
100 85 70 57
and DIC and in Quantities are expressed in mmol m2 for NH4 P meq m2 for dissolved calcium. Negative values of Js indicate flux P from water to sediment. Negative values of Jb indicate consumption in water.
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Fig. 8. Mass balance of NH4 for the whole period of experiments. Values in mmol m2. Numbers in brackets were added in order to match the balance.
with a difference of 5 and 3 mmol m2 in S1 and S6, respectively. These differences were close to the fluxes observed on the first day, which could not be explained by the single diffusion-reaction model. The integrated flux of calcium during the control experiment accounted for 70 to 85 meq m2 (in S1 and S6, respectively), which was slightly higher than the increase in porewater content (Fig. 9; Table 4). Thus, only a small fraction of calcium was removed from solution to the solid phase, a finding that is consistent with the increase of the apparent equilibrium concentration used in the diffusion-reaction model. This supports the hypothesis that some factors inhibited the precipitation of calcium carbonate in the sediments. The integrated flux was also consistent with the diminution observed in water at S1. In contrast, the decrease of Ca2+ in water was twice the integrated flux at S6 (Fig. 9; Table 4), although no Ca2+ removal was observed in blank incubations. This loss of about 100 meq m2 in water will be discussed on the basis of dissolved inorganic carbon changes. In seawater, the estimated total flux toward sediments accounted for 142 and 330 meq m2 (in S1 and S6, respectively), values which were notably lower than the increase observed in porewater content (Fig. 9; Table 4). This was in agreement with the dissolution of calcium carbonate
from the solid phase, which could be expected from the change in the apparent equilibrium constant caused by the increase in salinity. Removal of Ca2+ from water by diffusion toward sediment was much lower at both stations than removal through precipitation (estimated from blank incubations), the sum of both mechanisms being close to the decrease observed in water (Fig. 9; Table 4). Significant differences were observed in the mass balance of DIC between the two study sites in the two experimental conditions. In control incubations, the total amount of DIC released to the water at S1 (around 1000 mmol m2) accounted for more than 80% of the increase observed in DIC in water (Fig. 10; Table 4). The remaining 400 mmol m2 were provided by respiration in water (estimated from the blank flux), which followed an exponential decay. However, the increase observed in water at S6 was lower than the sum of the flux from sediments and the water respiration (Fig. 10; Table 4). The lack of about 100 mmol m2 of DIC in water agrees with the lack of Ca2+ (around 100 meq m2=50 mmol m2) observed at this station, and indicates the precipitation of 50 mmol m2 of calcium carbonate by the reaction: Ca2þ þ 2HCO 3 $ CaCO3 þ CO2 . Precipitation was not observed in the blank flux, probably because calcium
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Fig. 9. Mass balance of calcium for the whole period of experiments. Values in meq m2. Numbers in brackets were added in order to match the balance.
carbonate precipitation in brackish waters cannot be detected in daily measurements. In the marine incubations, the total flux of DIC to water was 850 and 750 mmol m2 (S1 and S6, respectively). DIC content in porewater did not change at S6 and slightly decreased at S1 (Table 4). The production of DIC in water (150 mmol m2 in both stations) was the net balance between water respiration and carbon removal from precipitation. From the removal of Ca2+ (700 and 500 mmol m2 in S1 and S6, respectively), the predicted loss of DIC was about 1400 and 1000 mmol m2, respectively. Half of this amount precipitated as CaCO3, while the other half diffused to the atmosphere as CO2 (Fig. 10). To calculate the whole balance of DIC, we assumed that diffusion of CO2 to the atmosphere took place in periods between measurements, therefore the stoichiometry for the blank flux (before CO2 diffuses to the atmosphere) is the loss of 1 mmol DIC/1 mmol Ca. As a consequence, the estimated water respiration should be 850 mmol m2 (i.e. 700 mmol m2 precipitated as CaCO3+150 mmol m2 observed as net balance) at S1 and 600 mmol m2 at S6 (Fig. 10). This agrees with the increase in DIC observed in water (around 350 mmol m2), revealing a net loss of DIC from the system of 700 and 500 mmol m2 (S1 and S6, respectively). This loss should be consistent with the diffusion
of the CO2 formed by calcification and released to the atmosphere during the periods between daily flux measurements.
4. Discussion 4.1. Control incubations The results observed in control incubations at moderate salinity may be related to two main processes that control the diffusive fluxes of DIC, Ca2+ and NH+ 4 : consumption in water and decomposition of organic matter in sediments. Consumption in water was especially significant for NH+ 4 dynamics, since it caused removal of all the efflux from sediments in 17 days. As water mass was saturated in oxygen, NH+ removal may be associated with 4 nitrification. This has been widely reported elsewhere (e.g. Joye, Connell, Miller, Oremland, & Jellison, 1999; Spagnoli & Bergamini, 1997), with rates in estuarine and freshwater environments ranging between 0.05 and 32 lM d1 (Berounsky & Nixon, 1993; Enoksson, 1986). Moreover, for superficial sediments, it has been established that nitrification rates depend on NH+ 4 concentration, when oxygen is not a limiting factor
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Fig. 10. Mass balance of dissolved inorganic carbon for the whole period of experiments. Values in mmol m2. Numbers underlined were calculated in order to match calcium balance. Number in brackets were added in order to adjust DIC balance.
(Rysgaard & Berg, 1996). The rates of NH+ 4 consumption observed in the Albufera system (from 0.85 to 5 lM d1) are thus in the range previously reported, and the correlation observed between NH+ 4 consumption and NH+ concentration suggests that nitrification was a 4 significant process which controlled nitrogen water dynamics in the Albufera system. DIC and Ca2+ dynamics were only slightly affected by water processes. In the incubations without sediment (i.e. blank fluxes), Ca2+ concentrations did not change and, as in many coastal environments (Cai & Wang, 1998), DIC production, which can be related to water respiration, was very low. The exponential decay observed for the blank flux of DIC could be associated with the decrease in bacterial activity caused by the exhaustion of organic carbon under experimental conditions (without new synthesis of organic matter or sedimentation of allocthonous material). Although there was no significant difference between the two sites regarding the saturation state with respect to CaCO3, removal of Ca2+ and DIC by CaCO3 precipitation in water was observed only at the deepest station (S6). This may be due to the presence of a higher amount of fine particulate matter at this site (Table 1B).
Smaller sediment particles may be resuspended during incubation in S6 samples and behave as nucleation agents (Stabel, 1986), thereby improving CaCO3 precipitation. The removal of DIC from water, however, only accounted for a minor part of the efflux of carbon from sediments. Since no dissolution of CaCO3 was observed in sediments in brackish incubations, it is concluded that mineralization of organic matter is the main source of DIC to the water in the Albufera system. The good agreement between the temporal variation of NH+ 4 fluxes and the NH+ mass balance with the diffusion4 reaction model assuming a zero-order reaction for NH+ 4 production in sediments also indicates that organic matter decomposition is the main source of NH+ 4 to the water. The total efflux of DIC was higher than that of NH+ 4 , with a molar ratio of 55 and 75 in S1 and S6, respectively. Even if accumulation of DIC and NH+ 4 in sediments is taken into account, this ratio was notably higher than in other coastal areas (e.g. Lo´pez, Vidal, Lluch, & Morguı´ , 1995; Thamdrup, Canfield, Ferdelman, Glud, & Gundersen, 1996) and higher than the average C/N ratio in sediments of the Albufera (18). This observation points to slow mineralization of NH+ 4 ,
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especially at S6. On the other hand, the changes observed in porewater profiles of DIC and NH+ 4 indicated that mineralization of organic carbon was inversely proportional to depth, as reported elsewhere (e.g. Cermelj et al., 1997; Ullman & Aller, 1989), whereas mineralization of NH+ 4 was similar across the 6 cm studied. Two additional factors should be taken into account in order to interpret these data. First, the adsorption of NH+ 4 into clay particles (Rosenfeld, 1979) may lead to an underestimation of the NH+ 4 produced from measured fluxes and accumulation in porewater (Thamdrup et al., 1996). At low salinity, clay minerals, like illites, which are a main component of the Albufera sediments, have a strong capacity to adsorb NH+ 4 (Drever, 1982), which may explain the small flux of NH+ 4 observed, as well as its regular profile with depth. Mineralization of organic matter shows seasonal variations in many shallow systems, for example, in the Mediterranean area, Bertuzzi et al. (1997) and Bartoli et al. (1996) reported low NH+ 4 fluxes in early summer followed by higher fluxes in early autumn, in response to changes in organic matter sedimentation. In the Albufera, the organic matter accumulated in sediments is not only from algal origin, but mostly from macrophyte debris. Decomposition of this material takes place in autumn, and therefore the organic matter present in sediment in early July, when the study was carried out, was refractory material with a low N content. Finally, solubilization of CaCO3 in sediments by metabolically produced CO2 is a common process in many systems (e.g. Green, Aller, & Aller, 1993; Jahnke & Jahnke, 2000; Ullman & Aller, 1989), but in temperate nearshore regions carbonate dynamics is controlled by alternating periods of net dissolution and precipitation on a year scale (Green & Aller, 1998). In the Albufera lagoon, a small amount of Ca2+ and DIC was removed from porewater throughout precipitation because it remained supersaturated throughout the experiment with respect to calcite and aragonite. Saturation of CaCO3 may be due to the high amount of F. enigmaticus (Fauvel) debris, whose tubes contain 80% of high-Mg calcite (Fornos, Forteza, & Martinez Taberner, 1997). Increased supersaturation in porewater during the experiment indicates the presence of some factor that hinders calcification. The increase in porewater phosphate and magnesium (DCpw PO4 81– 85 lmol m2; DCpw Mg 100–500 mmol m2) may inhibit calcite precipitation, as reported elsewhere (Burton & Walter, 1990; Dove & Hochella, 1993). 4.2. Seawater incubations The sudden change in salinity from brackish to seawater levels significantly altered the DIC, Ca2+ and NH+ dynamics at the sediment/water interface at 4 different time scales. One of the most drastic changes,
the high efflux of NH+ 4 from sediment, was observed 24 h after seawater input. This high flux could be related to a rapid bacterial response to the increase of organic carbon caused by the mortality or physiological stress of organisms that cannot withstand the new salinity level. This process might have strongly and rapidly enhanced benthic NH+ 4 -flux. Although this process should have produced a significant input of DIC to the sediments, the mass balance of DIC in seawater incubations did not show a significantly higher input of DIC to sediments when compared with brackish water incubations. Thus, the main cause of the NH+ 4 release appears to be the decrease in the NH+ 4 -adsorption capacity of clays which is induced by high salinity (Rysgaard et al., 1999). Another major initial change was the removal of Ca2+ and DIC from water by CaCO3 precipitation and the associated CO2 release to the atmosphere. Abiotic calcification is expected to occur when brackish alkalinity-rich water mixes with Ca-rich seawater (Stumm and Morgan, 1981), which, like biogenic calcification, is a source of CO2 to the surrounding water and to the atmosphere (Frankignoulle et al., 1998; Frankignoulle, Canon, & Gattuso, 1994; Purdie & Finch, 1994). The last initial change was a shift from NH+ 4 consumption in water in brackish system to NH+ 4 production in saline incubations. NH+ 4 production in water exponentially decreased to negative values (i.e. NH+ 4 consumption) after 6 days. Physiological stress of organisms, as well as desorption of NH+ from 4 resuspended sediment particles, is the most probable cause of initial release in water. In any case, NH+ 4 production from suspended matter accounted for only a minor source of this compound to the water. After the initial modifications following seawater input, the Albufera sediment system evolved to a new state of equilibrium, which differed in several aspects from that observed in brackish conditions. First, the rate of NH+ 4 consumption in water after the initial production was much lower than that measured in brackish incubations. Reduction of nitrifying activity at high salinity has not only been linked to limitation of NH+ 4 availability but also to a physiological effect of salinity on nitrifying organisms (Rysgaard et al., 1999). In spite of the higher levels of NH+ 4 observed in seawater incubations, reduction of nitrifying activity may explain the decreased consumption rates in water. Moreover, several authors have reported that the rates of NH+ 4 excretion of macrofauna decrease with salinity (Cheung, 1997; Rosas et al., 1999). The diffusion-reaction model and the mass-balance calculation showed that the rates of DIC and NH+ 4 production that could be attributed to the organic matter mineralization in sediments were lower than in brackish incubations. This is an unexpected result, because marine sediments do not present lower rates of mineralization than freshwater. Nevertheless, changes
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in the burrowing activity of benthic macrofauna could provide a possible explanation. The apparent diffusion coefficient deduced from water/sediment fluxes and porewater concentrations were higher in brackish waters and agreed with the density of living Nereis diversicolor observed at the end of experiment (Aller, 1980; Aller & Yingst, 1985). In cores incubated with seawater, the number of living N. diversicolor at the end of experiment was similar to that observed in control cores, but the Ds value was lower and approached molecular values. Reduction of the physiological activity of interstitial organisms as a result of sudden changes in salinity has been reported elsewhere (Cheung, 1997). The lower values of Ds observed in seawater were thus in agreement with decreased burrowing activity because of increased salinity. Moreover, benthic fauna not only directly release CO2 and ammonia (Kristensen, Jensen, & Aller, 1991), but also promote the mineralization of organic matter (Aller, 1994; Aller & Aller, 1998). The reduction of Nereis activity in sediments of Albufera caused by the drastic change in salinity may therefore explain the decrease observed in the rates of NH+ 4 and DIC production. Reduction of bioturbation was more severe at S1, leading to a higher increase in porewater accumulation of NH+ 4 . Total DIC flux was less modified than NH+ 4 flux, because the increase in porewater salinity altered equilibrium concentrations with respect to CaCO3. Dissolution of CaCO3 solid phase by metabolically produced CO2 compensated for the decrease of DIC production by mineralization.
5. Conclusions The sudden change in salinity from 10 to 37 2+ significantly altered the benthic fluxes of NH+ 4 , Ca and DIC. 1. A substantial amount of NH+ 4 was released from sediments to the water 24 h after the salinity change. This may increase nitrogen availability for the phytoplanktonic community and consequently stimulate planktonic productivity. The initial release to the water was then followed by a reduction of sediment-flux and a higher retention in porewater, therefore the nitrogen-stimulated production, if done, should be short. 2. The most significant change in the carbon cycle was the intense precipitation of calcium carbonate in water, which was not compensated with calcium carbonate dissolution in sediments. This represented a significant export of carbon from the system to the atmosphere. Because the rise in sea level (and consequently, the increase in salinity in shallow coastal waters) may be a result of atmospheric CO2 increase, a process like that observed in the Albufera
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system may involve a positive feed-back, thereby causing a further increase in atmospheric CO2. Acknowledgements I thank, J.L. Pretus, X. Lluch and J. Gonzalez for helping with sample collection, S. Joye for helpful comments and R. Rycroft for improving the manuscript. I also thank two anonymous reviewers for their valuable comments. This study was supported by a CYCIT grant (PB97-0953).
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