The relationship between seagrass (Posidonia oceanica) decline and sulfide porewater concentration in carbonate sediments

Estuarine, Coastal and Shelf Science 73 (2007) 583e588 www.elsevier.com/locate/ecss

The relationship between seagrass (Posidonia oceanica) decline and sulfide porewater concentration in carbonate sediments Maria Ll. Calleja*, Nu´ria Marba`, Carlos M. Duarte IMEDEA (CSIC-UIB), Grup d’Oceanografı´a Interdisciplinar, Institut Mediterrani d’Estudis Avanc¸ats, C/Miquel Marque`s, 21, 07190 Esporles (Illes Balears) Spain Received 16 August 2006; accepted 23 February 2007 Available online 11 April 2007

Abstract In this study we test the hypothesized negative relationship between seagrass status and porewater hydrogen sulfide (H2S) levels, through a comparative analysis within a range of seven Posidonia oceanica meadows growing over carbonate sediments in the NW Mediterranean Sea around Mallorca Island. The studied meadows range from meadows growing on sediments with very low sulfide porewater concentrations (4.6 mM) to those growing over higher sulfide conditions (33.5 mM). Organic matter content, sulfate reduction rates and sulfide porewater concentrations in the sediments were determined concurrently with the assessment of demographic plant dynamics (specific mortality and net population growth rates). Sulfide porewater concentration increased with increasing organic matter content in the sediment, while net population growth decreased significantly with low increases of sulfide concentrations. Our results confirm the previously suspected vulnerability of seagrass meadows growing on carbonate sediments to increased sulfide levels. An excess of 10 mmols H2S L1 porewater is identified to already conduce P. oceanica meadows to decline, which this study identifies, particularly, as strongly sensitive to sulfides. The results reported here suggest that even moderate increases in organic carbon inputs may lead to enhancement of dissolved sulfides and may be an important factor for seagrass status in these iron-depleted carbonate sediments from the Mediterranean Sea. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: carbonate sediments; demographic plant dynamics; hydrogen sulfide; seagrass; sulphate reduction rate Regional index terms: Spain; Western Mediterranean Sea; Mallorca and Cabrera Islands

1. Introduction Seagrass meadows are among the most productive of all marine ecosystems, contributing about 12% of the net production of organic carbon in the ocean (Duarte and Chiscano, 1999), and are valued for their high biodiversity and habitat services. In recent decades, extensive losses of seagrass habitats have been documented worldwide (Nienhuis, 1992; Hemminga, 1998; Duarte, 2002). Causes of decline have been commonly associated with increased eutrophication and consequent

* Corresponding author. E-mail address: [email protected] (M.Ll. Calleja). 0272-7714/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2007.02.016

degradation of underwater light environment (Orth and Moore, 1983; Cambridge and McComb, 1984; Larkum and West, 1990; Hemminga, 1998). However, light competition may not be the sole effect of eutrophication as it also implies an enhancement of organic matter fluxes to the vegetated sediments and therefore stimulating bacterial metabolism. Organic carbon inputs may induce sulfate reduction bacterial metabolism, which is the dominant biogeochemical process in coastal seagrass marine sediments (Jørgensen, 1982; Holmer et al., 2003) and is known to be particularly important for anaerobic organic carbon oxidation (Capone and Kiene, 1988; Howarth, 1993). The end product of sulfate reduction, sulfide, has been identified as extremely toxic to plants (Terrados et al., 1999;

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Duarte, 2002). Its accumulation has been directly related to episodic die-offs of seagrasses such as Thalassia testudinum (Robblee et al., 1991; Carlson et al., 1994). Sulfide intrusion from the sediments into the plants, experimentally documented in Zostera marina (Pedersen et al., 2004) and in T. testudinum (Borum et al., 2005), has been recently proposed as a possible mechanism that may control the growth and survival of rooted plants in sulfate-rich aquatic environments (Borum et al., 2005). A literature review suggested a threshold level of 100 mM (400e1000 mM) sulfide in sediment porewaters above which, seagrass growth would be impaired (Terrados et al., 1999). However, concentrations as high as 10 mM have been documented (Carlson et al., 1994, 2002), indicating that porewater sulfide concentrations vary widely among seagrass beds and that the resistance and tolerance to its accumulation varies among seagrass species and sediment biogeochemistry characteristics. The biogeochemical pathways of sulfide porewater removal are still to be understood. Two mechanisms exist to counteract sulfide toxicity under seagrass beds. Firstly, seagrass roots and rhizomes supply oxygen to the sediment through photosynthesis maintaining an oxygen microshield surrounding them. This oxygen can support both biotic and abiotic reoxidation of reduced compounds such as sulfides, therefore preventing the rhizosphere to sulfide invasion (Pedersen et al., 2004). Secondly, the negative impacts of free sulfides can be also attenuated by the presence of sedimentary labile iron pools (Smolders et al., 1995; Viaroli et al., 1997) which can remove sulfide from porewater by precipitation as pyrite (FeS2) and iron mono-sulfides (FeS), thereby acting as an alternative mechanism to sulfide detoxification and reducing its negative effects on seagrass growth. Therefore, low iron availability, which is characteristic of carbonate sediments (Berner, 1984; Duarte et al., 1995), limits the formation of iron-sulfide compounds and poorly buffer seagrass against dissolved sulfide toxicity (Chambers et al., 2001; Holmer et al., 2005). Accordingly, it has been suggested that the seagrasses growing on carbonate sediments may be particularly prone to sulfide toxicity (Hemminga and Duarte, 2000), and recent experimental studies concluded that sulfides impact seagrass (Posidonia oceanica) growing on carbonate sediments at very low sulfide levels (20e30 mM, Holmer et al., 2003). Despite the evidence that high sediment sulfide concentrations can have negative impacts on growth and survival of wetland plants and seagrasses (e.g. Havill et al., 1985; Bradley and Dunn, 1989; Koch and Mendelssohn, 1989), and that the sulfide removal by iron additions improve seagrass growth in impacted carbonate sediments (Chambers et al., 2001; Holmer et al., 2005), the direct links between growth conditions, invasion of sediment sulfide and subsequent shoot mortality have not yet been submitted to a comparable test. Several investigations have tried to establish clear relationships between sediment sulfide conditions and seagrass performance using different experimental approaches (e.g. Carlson et al., 1994; Goodman et al., 1995; Terrados et al., 1999; Erskine and Koch, 2000; Holmer and Bondgaard, 2001), but the experiments

have produced highly variable results. This inconsistency could be explained by differences in the tolerance of various seagrass species to sulfide exposure, but also because of the complex nature of the interactions among the controlling factors, that makes reproducible and controlled experimental treatments hard to obtain (Terrados et al., 1999). This study is addressed to test the hypothesized negative relationship between seagrass status and sulfide levels through a comparative analysis encompassing a range of seagrass meadows growing over carbonate sediments with variable sulfide porewater and organic carbon content levels. In order to be effective and to avoid confounding the results with species-specific responses or intrinsic differences in demographic rates across seagrass species, such comparative analysis has been referred to meadows of the same species. The relationship between the status of Posidonia oceanica meadows and sulfide levels has been tested by the use of a comparative analysis of seven meadows spanning a range of conditions around the Balearic Islands, in the West Mediterranean Sea. We assessed organic matter contents, sulfate reduction rates and sulfide porewater concentrations in the sediments, concurrently with the assessment of demographic dynamics (specific mortality and net population growth rates) using repeated census of tagged plants. 2. Methods The study was conducted at Mallorca Island and Cabrera Archipelago National Park (39 9’ N, 2 56’ E), a protected marine area, where a total of seven Posidonia oceanica meadows, selected on the basis of a broad survey of plant status, were sampled. Posidonia oceanica forms thick mats of rhizome and root material that occupy a substantial fraction of the sediment volume (Hemminga and Duarte, 2000). Study sites comprise a range of conditions from relatively pristine meadows (e.g. Magalluf and Sta Maria) to highly disturbed meadows (e.g. Sa Paret and Pollenc¸a, Table 1), growing on carbonate (>90% of dry weight, Holmer et al., 2003) sediments. Two sites in Mallorca (Magalluf and Cala Millor) were located in exposed areas, whereas two other sites (Porto Colom and Pollenc¸a) were in rather sheltered bays with relatively high nutrient loading. At Cabrera Island two sites (Es Castell, Sa Paret) were studied in a rather enclosed bay and one additional site (Sta Maria) in a more exposed bay, which is environmentally protected from human activities, with no public access. The meadows extend from near the surface (4 m depth, Pollenc¸a) down to 20 m (Es Castell), well above the 35 m depth limit of the plants (Table 1). Sulfate reduction rates and sulfide porewater concentrations had been investigated in some of these meadows in 2000e2001, as part of an earlier, exploratory study (Holmer et al., 2003) ranging from 0.7 to 12 mmol m2 d1 and from 0.1 to 29.5 mM, respectively. Impacts on the sediments derived from excess organic matter inputs from domestic sources, and the study sites did not receive industrial or substantial agricultural inputs. Seagrass status was characterized by shoot demographic dynamics, specifically the specific mortality and net population growth rates

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Table 1 Mean  standard error P. oceanica demography parameters (specific mortality and net population growth), and sediment characteristics (density, organic matter (OM), sulfate reduction rates (SRR) and sulfide porewater concentrations (H2S)) and depth (m) of the seven studied meadows. When the number of replicates was only two, the standard error of the mean is not provided. Coefficient of variation (CV) across sites is also provided for all variables Meadow

Depth (m)

Mortality (%, year1)

Net population growth (%, year1)

Sediment density (g cm3)

OM (g m2)

SRR 2 1 (mmol SO2 d ) 4 m

H2S (mM)

Es Castell Sa Paret Sta Maria Magalluf Pollenc¸a Cala Millor Porto Colom Cv (%)

20 17 13 6 4.1 6.5 6.5 6.0

2.9  3 8.6  6 12.9  8 17.5  3 22.2  14 8.5  2 9.7 55

2.9  3 6.3  8 9.9  5 7.6  4 14.7  16 4  4 2.9 92

1.82  0.03 1.44  0.01 1.83  0.06 1.57  0.06 1.34  0.02 1.71  0.04 1.70  0.05 12

2754 4675  224 2282 2220  152 5762  176 3396  132 3609  311 37

22.4  14.2 8.9  3.9 117.65  30.3 0.6  0.1 14.2  0.9 6.0  1.8 6.2  1.0 165

5.03  0.70 12.69  2.03 19.30  5.65 4.61  0.18 33.51  9.71 7.55  1.74 5.66  1.13 84

(NPG). Positive net population growth rates imply that the populations are expanding whereas negative ones imply population decline (Short and Duarte, 2001). The sampling was conducted during JulyeAugust 2004, the time of the year when microbial activity (Jørgensen, 1977; Hobbie and Cole, 1984) and seagrass productivity (e.g. Alcoverro et al., 1995) are the highest. 2.1. Plant demography parameters Seagrass shoot demographic parameters were quantified by direct shoot census in three permanent quadrats installed inside each studied meadow following the procedures described in Short and Duarte (2001), except at Porto Colom where there were only two quadrats. The area of the plots ranged between 0.09 m2 and 0.25 m2 as to include at least 100 shoots plot1. Shoot demography at all meadows was followed for more than 1-year period. The plots were installed in February 2003 in all meadows, except in those at Cala Millor (July 2003) and Porto Colom (September 2002), and then all shoots within the quadrats were tagged, with a plastic cable tie, and counted. In July 2004, the number of surviving shoots (i.e. shoots tagged with a cable tie) and the number of recruited shoots (i.e. young untagged shoots) in each permanent quadrat were counted. These measurements provided estimates of specific rates of shoot mortality, recruitment and population growth in between consecutive visits. The specific shoot mortality rate (M; % year1) was calculated as, M¼

ðlnðNT0 =NS1 ÞÞ100 365 t1  t0

where NT0 is the number of marked shoots at time t0 (days) at each plot and NS1 the number of marked shoots at time t0 that survived at t1 (days). The specific shoot recruitment rate (R; % year1) was estimated as,

R¼

½lnððNT0 þ NN1 Þ=NT0 Þ100 365 t1  t0

where NN1 was the number of recruited (i.e. young not marked) shoots between t1 and t0. The specific net population growth rate (NPG; % year1) was estimated as NPG ¼ R  M. Although the demographic parameters were measured over an extended period compared to that of sediment sulfur cycling measurements, Posidonia oceanica has been shown to exhibit little seasonality and reacts to sediment processes with a time lag (Alcoverro et al., 1995), rendering a longer observational time span to elucidate seagrass demographic responses to sediment perturbations necessary. 2.2. Sediment porewater and solid-phase characteristics Three sediment cores (i.d. 4.3 cm) from each station were collected by SCUBA divers inside seagrass meadows, avoiding cutting of roots and rhizomes. The cores were, within 2 h of collection, slized into one 0e8 cm interval, under N2 atmosphere to keep them anoxic. Porewaters were obtained under anaerobic conditions by centrifuging the sediment (3000 rpm for 10 min) for analysis of sulfate (SO2 4 ) and sulfides (H2S). Sulfate was determined using the turbidimetric assay described by Tabatabai (1974). Sulfides were kept in zinc acetate and determined spectrophotometrically according to Cline (1969). Sediment density was obtained by weighing a known volume, and the water content was obtained after drying it overnight at 105  C. Porosity was calculated from sediment density and water content. Organic matter content was obtained by ignition of the dried sediment overnight at 450  C. Number of replicates were three for each variable measured, but for Es Castell and Sta Maria only two replicate measurements were possible for the determination of organic matter content. 2.3. Sulfate reduction rate measurements Three sediment cores (i.d. 2.6 cm) from each station were collected, as the ones described before, to measure sulfate reduction rates (SRR) and total pools of reduced sulfides (TRS). The cores were within 1 h of collection injected with 2 ml of 35S-SO2 (70 kBq) at 1-cm intervals and incubated for 4

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2e3 h in darkness at in situ temperature to obtain the SRR (Jørgensen, 1978). Incubations were terminated by sectioning the cores into one 0e8 cm interval and fixing in 1 M zinc acetate (zinc acetate volume: sediment volume) and frozen immediately. The rates were obtained by the one-step distillation method (Fossing and Jørgensen, 1989). Radioactivity was counted on a Beckman LS3801 scintillation counter. Sulfate reduction rates (SRR, in 3 nmol SO2 day1) were calculated for each sediment 4 cm core following Fossing and Jørgensen (1989) as: SRR ¼

a  2  SO4 1:06 ða þ AÞt

where a is the total radioactivity in the traps, A is the total radioactivity of the sulfate pool after incubation, t is the incubation time (in days), [SO2 4 ] is the sulfate concentration in the sediment (nmol cm3) and 1.06 is the correction factor for microbial isotope fractionation between 32S and 35S. The concentrations of total reduced sulfides (TRS) from the traps were determined spectrophotometrically according to Cline (1969). 3. Results Sediment conditions ranged broadly across the studied meadows with sediment organic matter concentrations ranging more than twofold, from 2282 to 5762 g m2. Sulfate reduc2 1 tion rates ranged from 0.6 to 117.7 mmol SO2 d (see 4 m Table 1) and sulfide concentrations in the porewater were generally low (<34 mM), as in previous studies conducted in the same area (Holmer et al., 2003). There was a large variation between sites, with values ranging from 4.61 to 33.51 mM H2S, over one order of magnitude across the meadows (see Table 1). Sulfide porewater concentrations were not related with SRR (Pearson correlation, P > 0.05) and they tended to increase although not significantly with increasing sediment organic matter content (R ¼ 0.66, P ¼ 0.1, Fig. 1) when considering all the meadows. In Sta Maria meadow high sulfate reduction rates and sulfide porewater concentrations were found despite the low organic matter content of their sediments (Fig. 1). When this meadow is excluded from this analysis a strong exponential increase relationship of sulfide porewater concentration is found with sediment organic matter content (R ¼ 0.98, P < 0.01, Fig. 1). Considering sulfate reduction rates (in 2 1 mmol SO2 d ) and sulfide pools (in mmol m2), the 4 m calculated sulfide turnover rates among the meadows ranged from 0.006 to 0.23 day1. All seagrass meadows were in decline (i.e. negative specific net population growth rates) at rates ranging from 2.9 to 14.7% year1 (Table 1), except that at Porto Colom, which showed a positive net population growth at a rate of 2.9% year1 (Table 1). Mortality rate was very high at the meadow in Pollenc¸a, where shoot mortality was 22.2% year1 and presented the highest sulfide porewater concentration (33.51 mM) (Table 1). The average specific net population growth rate of 6.1% year1 in the meadows studied here (Table 1) is

40

30

H2S (µM)

586

20

10

0 2000

3000

4000

5000

6000

OM (g m-2) Fig. 1. Relationship between hydrogen sulfide concentrations in the sediment porewaters (H2S, mM) and the organic matter content in the sediment (OM, g m2). Empty circle represents Santa Maria meadow. Solid line shows the exponential function H2S ¼ 1.07(1.39)e[0.0006(0.00009)OM] (R ¼ 0.98, P < 0.01). Error bars represent 1 SE in both axis.

consistent with a trend towards a general decline of Posidonia oceanica meadows in the Mediterranean at an average decline rates of about 5% year1 (Marba` et al., 1996, 2005). Net population growth rate varied inter-annually when comparing the estimates provided here (year 2003e2004) with those provided by Marba` et al., 2005 (years 2000e2001). However, variability in net population growth rate between sites (74%) was much larger than that between years (26%), and no significant differences between years was observed when all meadows were considered (ANOVA, P > 0.05). The net specific growth rate declined significantly with increasing hydrogen sulfide concentration in the sediment (R ¼ 0.82, P < 0.05; Fig. 2). The intercept of the fitted regression equation (0.65  2.16% yr1, Fig. 2) is not significantly different from 0 (t-test, P ¼ 0.37) indicating that seagrass populations are balanced (i.e. net specific population growth ¼ 0; recruitment ¼ mortality) in the absence of sulfides. The threshold sulfide level for a significant Posidonia oceanica decline, considering a significant decline when NPG is 5% yr1, corresponded to the concentration of 10 mM H2S (Fig. 2). Mortality rates tended to decrease, although not significantly, with increasing water depth (R ¼ 0.72, P > 0.05), but no relationship was found between net population growth and depth. Neither sulfate reduction rates nor sulfide concentrations correlated with water depth. 4. Discussion The results presented suggest a close coupling between sulfur dynamics in carbonate sediments and the stability of Posidonia oceanica meadows. This study was conducted during summer, the time of the year when sediment microbial activity

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0

NPG (%, yr-1)

-5

-10

-15

-20

-25

-30

-35

0

10

20

30

40

H2S (µM) Fig. 2. Relationship between net specific population growth (m, % year1) of P. oceanica meadows and hydrogen sulfide concentrations in the sediment porewaters (H2S, mM). Solid line shows the fitted regression line m ¼ 0.65(2.16)  0.43(0.14)H2S (R ¼ 0.82, P < 0.05). Error bars represent 1 SE in both axis. Dashed line indicates the sulfide threshold for a significant seagrass decline.

and P. oceanica productivity are highest in response to high temperature (Jørgensen, 1977; Hobbie and Cole, 1984) and light (Alcoverro et al., 1995), respectively. Seasonal analyses of sediment sulfur dynamics in two of the studied meadows (Sa Paret and Sta Maria, Holmer et al., 2003), demonstrated seasonal SRR dynamics, with the highest SRR values observed during summer, as expected. Despite temporal variability SRR were consistently and significantly different between sites (Holmer et al., 2003). Comparison of the SRR variability across sites measured in the present study (coefficient of variation across sites 165%) and seasonal variability in Sta Maria (coefficient of variation 72%), and Sa Paret (coefficient of variation 44%), reported previously (Holmer et al., 2003), shows spatial variability to exceed seasonal variability. Hence, the sediment SRR and sulfide concentrations reported here are expected to represent annual maximum values for the meadows examined. SRR from this study exceeded by a factor of 10 those reported earlier, and indeed, the maximum rate (117 mmol 2 1 SO2 d , Table 1) reported at Sta. Maria (Cabrera Is4 m land) was much greater than the values measured in 2000e 2 1 2001 using the same methods (7.5 mmol SO2 d , 4 m Holmer et al., 2003). Sulfide pools at this meadow were also larger (19.3 mM, Table 1) than those measured in 2000e 2001 (11.1 mM, Holmer et al., 2003). In contrast, the low SRR rates in Magalluf observed in this study were comparable to those recorded in the 2000e2001 study (Holmer et al., 2003). Thus, the inter-annual differences observed suggests that sulfate reduction rates can exhibit large order-of-magnitude among years, which drivers are yet to be identified.

587

The fast sulfide turnover rates determined are consistent with results previously reported from carbonate sediments in the area (Holmer et al., 2003), and points to sulfide reoxidation to sulfate as the process likely responsible for the sulfide removal (Holmer et al., 2003), which should be a major sink for oxygen in the studied iron-poor sediments. This results in a feedback regime, with sulfides enhancing oxygen demand and anoxia, and thereby favoring microbial sulfate reduction activity and further production of sulfides. Accumulation of sulfides and consumption of oxygen penetrating into the sediment may lead to sulfide plant invasion and detrimental effects to seagrass. This study has identified the sulfide threshold level at which Posidonia oceanica experiences significant decline to be very low (10 mM H2S), compared to sulfide levels typically found in coastal sediments elsewhere (Carlson et al., 1994; Moeslund et al., 1994; Holmer and Nielsen, 1997; Azzoni et al., 2001), confirming the hypothesis that seagrass growing on ironpoor carbonate sediments are highly sensitive and vulnerable to sulfide. This vulnerability is attributable to the lack of pyrite formation in iron-poor carbonate sediments, one of the two pathways of sulfide removal from porewater. Sulfate reduction 2 1 rates in excess of about 8 mmol SO2 d will lead to ac4 m cumulation of sulfide pools above the threshold established here (Table 1), which is also a relatively low SRR value relative to values typically measured in other seagrass sediments (Marba` et al., 2006). Accordingly, even modest inputs of organic matter to the carbonate sediments studied may suffice to reach critical SRR and sulfide levels in carbonate sediments. There is evidence that Posidonia oceanica meadows are in decline throughout the Mediterranean, particularly in areas receiving additional organic inputs (Marba` et al., 1996, 2005). Previous descriptive and experimental results (Holmer et al., 2003, 2005) have provided evidence that increased sulfate reduction rates and sulfide pools may lead to seagrass decline. This study demonstrates that even modest porewater sulfide levels and organic matter content may lead to seagrass decline in carbonate sediments. We show a clear evidence of negative effects of organic enrichment, and consequent increase in porewater sulfide concentration, on the status of Mediterranean Posidonia oceanica meadows. These iron-depleted carbonate sediments lack on its sulfide buffering ability rendering the associated P. oceanica meadows strongly sensitive to sulfide. Sulfide porewater levels in excess of only 10 mM, resulting 2 1 from sulfate reduction rates in excess of 8 mmol SO2 d 4 m and moderate increases in organic carbon inputs, can lead to seagrass decline. Whether other seagrass species growing in carbonate sediments elsewhere are equally sensitive to sulfide, and the associated organic inputs, should be investigated further. Acknowledgements This study was funded by the Ministry of Environment of Spain (project 55/2002), and a grant of the Fundacio´n BBVA. We thank the officers and guards of Cabrera Archipelago National Park for providing access and facilities, Miguel

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Angel for his delicious cooking, and, Rocı´o Santiago, Regino Martı´nez and Natividad Cantero for field and laboratory assistance. Maria Ll. Calleja was supported by a Ph.D. grant from the Spanish Research Council.

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