Nutrient limitation of the tropical seagrass Enhalus acoroides (L.) Royle in Cape Bolinao, NW Philippines

Aquatic Botany 65 (1999) 123–139

Nutrient limitation of the tropical seagrass Enhalus acoroides (L.) Royle in Cape Bolinao, NW Philippines Jorge Terrados a,∗ , Nona S.R. Agawin b , Carlos M. Duarte a , Miguel D. Fortes c , Lars Kamp-Nielsen d , Jens Borum d a

Instituto Mediterráneo de Estudios Avanzados (CSIC-UIB) Edificio Mateu Orfila, Campus Universitario UIB Carretera de Valldemossa, Km. 7.5, 07071 Palma de Mallorca, Spain b Centro de Estudios Avanzados de Blanes, CSIC, Cam´ı de Santa Bárbara s/n, 17300 Blanes, Girona, Spain c Marine Science Institute, College of Science, University of the Philippines, Diliman, Quezon City 1101, Philippines d Freshwater Biological Laboratory, University of Copenhagen, Helsingørsgade 51, DK-3400 Hillerød, Denmark

Abstract Experimental additions of nutrients to the sediment of Enhalus acoroides stands were performed at four sites and three times along the year in Cape Bolinao, NW Philippines to test the hypothesis that seagrass growth in tropical environments is limited by the availability of nutrients. Both the nitrogen content (as % DW) and the nitrogen incorporation of E. acoroides leaves increased after the addition of nutrients. The size (g DW per shoot) and the leaf growth rates (g DW per shoot d−1 ) of E. acoroides shoots also increased after the addition of nutrients. Nitrogen rather than phosphorus was the nutrient limiting shoot size and leaf growth of E. acoroides in the area. The extent of nutrient limitation of E. acoroides showed high variability both in space and time which cannot be directly linked with differences in light or nutrient availability among the experimental sites. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Nutrient limitation; Seagrass growth; Enhalus acoroides; SE Asia

1. Introduction Seagrass meadows are among the most productive components of coastal ecosystems (Hillman et al., 1989). The availability of light (Dennison, 1987; Duarte, 1991), nutri∗ Corresponding author. Fax: +34-971-173248 E-mail address: [email protected] (J. Terrados)

0304-3770/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 7 7 0 ( 9 9 ) 0 0 0 3 6 - 4

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ents (Short, 1987), and water temperature (Bulthuis, 1987) are main environmental factors controlling the primary productivity of seagrass meadows. The typically clear, warm, and low-nutrient waters of tropical seas, together with the predominantly carbonate nature of the sediments where tropical seagrasses grow, led to consider seagrass growth to be nutrient-limited in these environments (Short, 1987). Early reports of low nutrient concentration in the tissues of tropical seagrasses (Short et al., 1985; Fourqurean et al., 1992a) were consistent with this hypothesis, which received experimental support from several in situ nutrient addition experiments in tropical and subtropical seagrass meadows (Powell et al., 1989; Short et al., 1990; Powell et al., 1991; Agawin et al., 1996). Tropical seagrasses, however, grow in a wide variety of sediment types (Nienhuis et al., 1989; Chansang and Poovachiranon, 1994; Kenyon et al., 1997), not only on carbonate sediments of marine origin (Brouns, 1985) but also on terrigenous sediments (Tomasko et al., 1993; Erftemeijer, 1994; Preen et al., 1995). Nutrient availability is low in fine-grained carbonate sediments (Short et al., 1985; Short, 1987; Fourqurean et al., 1992b) but increases in coarse-grained carbonate, and terrigenous sediments (Erftemeijer and Middelburg, 1993; Erftemeijer, 1994), which suggests that not all tropical seagrass meadows might be nutrient limited. Indeed, in situ nutrient addition experiments (Erftemeijer et al., 1994) and mass balances of nutrients (Erftemeijer and Middelburg, 1995) in some Indonesian seagrass meadows show that the growth of tropical seagrasses is not always nutrient-limited. Moreover, a decrease in the availability of light may reduce the nutrient requirements of seagrasses (Abal et al., 1994) and, therefore, alleviate the nutrient-limited status of some tropical seagrass meadows. Coastal ecosystems in southeast (SE) Asia are experiencing widespread deterioration, largely as a result of siltation (Fortes, 1988; Gómez, 1988). Siltation influences coastal systems through decreased light availability for benthic plants (Bach, 1997; Duarte et al., 1998), the smothering and burial of benthic organisms (Duarte et al., 1997a), and the increase of nutrient inputs to both the water column and the sediment (Malmer and Grip, 1994; Mitchell et al., 1997; Kamp-Nielsen et al., 1998). All these processes are particularly detrimental for seagrasses (Giesen et al., 1990; Duarte, 1991; Sand-Jensen and Borum, 1991; Duarte, 1995; Short and Burdick, 1996; Tomasko et al., 1996) and, therefore, siltation generates gradients of growth conditions for seagrasses along the tropical coasts as evidenced by a general decline of the species richness and community biomass in SE Asian seagrass meadows with increased siltation of the coastal zone (Bach, 1997; Terrados et al., 1998a). The goal of this study is to test experimentally if increased nutrient inputs promote the growth of the tropical seagrass Enhalus acoroides (L.) Royle. We test this hypothesis through a series of nutrient addition experiments to shallow stands of E. acoroides under different growth conditions driven by the siltation of the coastal zone in Cape Bolinao, NW Philippines. E. acoroides was chosen as the target species because (1) it is one of the most common species in SE Asian seagrass meadows (Johnstone, 1979; Nienhuis et al., 1989; Chansang and Poovachiranon, 1994; Erftemeijer, 1994), (2) it is the only species present in all of the seagrass-vegetated sites in the study area (Terrados et al., 1998a) and contributes significantly to total biomass of the seagrass meadows (Vermaat et al., 1995), and (3) it is strongly nutrient limited at one of the study sites (Agawin et al., 1996).

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Fig. 1. Map showing the study sites along the siltation gradient in Cape Bolinao, NW Philippines. Siltation is highest at Santa Barbara, and decreases in Pislatan and Binabalian Loob, with Silaqui being the least silted site (cf. Bach, 1997; Terrados et al., 1998a).

2. Materials and methods The Cape Bolinao area (Pangasinan, NW Luzon Island, The Philippines, Fig. 1) is characterised by a South to North siltation gradient, from the heavily silted mouth of the Alaminos river to the pristine reef lagoon north of Santiago Island (le Jeune, 1995; Bach, 1997), which determines a wide range of conditions for seagrass growth. Nutrient addition experiments were performed in shallow (≈1 m depth) E. acoroides stands at four sites (Fig. 1; Santa Barbara, Pislatan, Binabalian Loob, and Silaqui) encompassing the range of growth conditions (Table 1) present in the area. Light availability is lower in Santa Barbara and Pislatan than in Binabalian Loob and Silaqui. Gross sedimentation is lowest in Silaqui, while the sediment has higher content of total inorganic carbon, organic matter, total nitrogen and total phosphorus in Silaqui than in the other sites (Table 1). The species richness and leaf biomass of the seagrass communities present along this siltation gradient differ greatly from communities with 1–3 seagrass species and low leaf biomass (92 g DW m−2 ) at turbid-water sites (Santa Barbara), to communities with seven seagrass species and up to 550 g DW m−2 of leaf biomass at clear-water sites (Silaqui; cf.Terrados et al., 1998a). Three nutrient addition experiments were performed along the year at each site (from 18 March to 1 July 1996, from 14 June to 18 September 1996, and from 29 November to 6 March 1997) to account for temporal variability in the response of E. acoroides. Each nutrient addition experiment was initiated by setting randomly eight 50 cm × 50 cm plots at each E. acoroides stand. As shoots of this seagrass species show a patchy spatial distribution (Vermaat et al., 1995), each plot was established in a different E. acoroides patch. Plots were at least 2 m apart from each other. Four of the experimental plots received nutrient additions while the other four were used as controls. Nutrient additions

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Table 1 Selected features of the water column and the sediment at the four experimental sites in Cape Bolinao (NW Philippines) Features

Sites Santa Barbara Pislatan 16◦ 20.600 N

Location Total suspended solids in the water column (mg DW l−1 ) Vertical extinction coefficient for scalar irradiance (m−1 )

119◦ 55.580 E 12.54 ± 42a 0.91 ± 0.08a

Binabalian Loob Silaqui

16◦ 22.120

N 16◦ 23.090 N 119◦ 57.660 E 119◦ 54.780 E 8.30 ± 0.78a 8.26 ± 0.85a

16◦ 26.350 N 119◦ 55.460 E 6.11 ± 0.72a

1.31b 0.43 ± 0.06a

1.38b 0.31 ± 0.03a

0.61 ± 0.04a

0.57b Gross (mg 196.04 ± 65.70 92.58 ± 11.89 431.58 ± 114.46 0.90 3.56 3.16 Organic matter in the sedimentd (% DW) Total inorganic carbon in the sedimentd (% DW) 0.56 6.62 9.23 Total nitrogen in the sedimentd (% DW) 0.014 0.044 0.043 0.0088 0.0258 0.0218 Total phosphorus in the sedimentd (% DW) Coarse sand fraction d (>250 ␮m, % of DW) 29.68 61.98 42.04 68.95 35.27 56.24 Fine sand fractiond (63–250 ␮m, % of DW) Silt and clay fractiond (<63 ␮m, % of DW) 1.37 2.75 1.72 sedimentationc

DW m−2 d−1 )

0.08b 7.40 ± 0.69 5.24 9.32 0.053 0.0253 65.29 24.41 10.30

Mean ± SE of 16 measurements performed between June and August 1995 (Bach, 1997) April 1995 (Terrados et al., 1998a) c Mean ± SE (n = 4), October 1997 (E. Gacia, unpublished data) d April 1995 (L. Kamp-Nielsen, unpublished data). a

b

were based on previous experiments (Agawin et al., 1996) and consisted on the addition of a ‘tree-sized’ 245 g bar of the slow-release fertilizer (8% total nitrogen [0.4% organic, 3% nitrate, 4.6% ammonium], 8% P2 O5 ; 10% K2 O, 12% organic matter) ‘Le Clou Miracle’ (AMOSL, Granollers, Spain), which has been shown to promote seagrass growth in the area (Agawin et al., 1996). Each nutrient addition represented a load of 213 g N m−2 and 93 g P m−2 . Nutrient additions were repeated after 4, 8 and 12 weeks. The nutrient addition rate obtained was 55% higher than that applied by Agawin et al. (1996). The effect of nutrient additions on sediment nutrient concentrations was assessed by collecting porewater at the end of the last nutrient addition experiment, using a modification of the method of Bulthuis et al. (1992). Porewater was withdrawn from 10 cm below the sediment surface with 20 ml syringes. The collected porewater (100 ml) was preserved with 500 ␮l of 20% H2 SO4 until further analysis. NH4 + , NO3 − and PO4 3− were analyzed after neutralization with NaOH, on an Alpkem RAF autoanalyzer following standard methods (Parsons et al., 1984). At the time of the last nutrient addition of each experiment (after 12 weeks), all the E. acoroides shoots present in each plot (between 10 and 20 shoots) were marked by making two colinear holes with an hypodermic needle at the basal part of the shoot to estimate leaf growth rates (Agawin et al., 1996). After 2 weeks the marked shoots were retrieved, and the width, total length, and the ‘new length’ (the length corresponding to the portion of the leaf grown during the marking period) of all the leaves in each shoot were measured. A portion (about 5 cm in length) of the second youngest leaf of five shoots in each plot was collected, measured (length and width), dried (65◦ C, until constant weight, about 48 h), weighed to calculate the leaf specific weight (g DW cm−2 ), and subsequently used for

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analyses of nitrogen and phosphorus contents. Carbon and nitrogen in the leaves were determined using a Carlo-Erba NA-1500 CHN analyzer, and phosphorus was determined spectrophotometrically (Parsons et al., 1984) after wet oxidation with boiling H2 SO4 . Differences in the content (as % of DW) of nitrogen and phosphorus of E. acoroides leaves between control and nutrient-added plots were tested using a two-way ANOVA (Sokal and Rohlf, 1981) with ‘site’ and ‘nutrients’ as main effects. Differences in porewater nutrient concentrations between sites in control and nutrient-added stands were tested at the end of the last nutrient-addition experiment using a one-way ANOVA (Sokal and Rohlf, 1981). Differences in shoot size (g DW per shoot), leaf growth, (mg DW per shoot per day), and specific leaf growth (per day) of E. acoroides between the experimental sites were tested at the control plots using two-way ANOVA with ‘site’ and ‘time’ as main effects. The effect of nutrient addition on shoot size, leaf growth, and specific leaf growth was described by performing a two-way ANOVA analysis at each site with time and nutrients as main effects. Post-hoc comparisons of means were performed using a Tukey’s honestly significant difference method for unequal sample sizes (T-method, Sokal and Rohlf, 1981). Prior to the analysis, the homocedasticity of the data was tested, and the data transformed if necessary. The nutritional status of E. acoroides was assessed by calculating nutrient (N and P) incorporation rates in both control and nutrient-added plants as: Nutrientincorporationcontrol = growth ratecontrol × nutrient contentcontrol Nutrient incorporationnut. added = growth ratenut. added × nutrient contentnut. added The nutrient deficiency of E. acoroides at each site was calculated as the balance between the nutrient requirement and incorporation of the control plants (cf. Pedersen and Borum, 1993; Agawin et al., 1996; Alcoverro et al., 1997): Nutrient requirement = growth ratenut. added × nutrient contentcontrol Nutrient deficiency = nutrient requirement − nutrient incorporation 3. Results Porewater phosphate concentrations in control plots were lowest in Binabalian Loob (one-way ANOVA, F = 10.73, P < 0.01), while there were no significant differences in the concentration of ammonium and nitrate between sites (Fig. 2). Porewater nutrient concentrations increased by at least two orders of magnitude in response to the nutrient additions, reaching similar concentrations of ammonium at all the sites (one-way ANOVA, P > 0.05). After the nutrient additions nitrate and phosphate concentrations in porewater were, however, lower (one-way ANOVA, F = 7.45, P < 0.01 for nitrate; F = 0.53, P = 0.05 for phosphate) in Silaqui than at the other sites (Fig. 2). The addition of nutrients increased the nitrogen content of E. acoroides leaves (F = 7.32, P < 0.05, Table 2). However, the phosphorus content of the leaves was not affected. There were no differences in the content of nitrogen and phosphorus of the leaves between the different experimental sites (Table 2).

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Fig. 2. Mean (+ SE) porewater nutrient concentrations in control and nutrient-added plots of E. acoroides at the four experiment sites in Cape Bolinao in March 1997. Table 2 Average (±SE, n = 3) nitrogen and phosphorus content of Enhalus acoroides leaves in control and nutrient-added plots at each experimental site, and results of the two-way ANOVA analyses testing for differences in the content of leaf nutrients between sites and the effect of the nutrient additions Site Site

Leaf N (% DW) control plots

Leaf N (% DW) nutrient-added plots

Leaf P (% DW) control plots

Leaf P (% DW) nutrient-added plots

Santa Barbara Pislatan Binabalian Loob Silaqui

2.02 ± 0.08 1.84 ± 0.05 2.00 ± 0.12 1.98 ± 0.07

2.23 ± 0.05 2.10 ± 0.03 1.97 ± 0.05 2.17 ± 0.15

0.30 ± 0.03 0.36 ± 0.02 0.36 ± 0.02 0.34 ± 0.07

0.31 ± 0.05 0.33 ± 0.02 0.30 ± 0.04 0.34 ± 0.07

Two-way ANOVA Site Nutrient Site × nutrient

F = 1.54 n.s. F = 7.32∗ F = 1.20 n.s.

∗

P < 0.05; n.s., not significant

F = 0.32 n.s. F = 0.50 n.s. F = 0.25 n.s.

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Table 3 Results of two-way ANOVA analysis testing differences in shoot size, leaf growth, and specific leaf growth of E. acoroides in control plots between experiments and sites

Shoot size (g DW per shoot) Leaf growth (g DW per shoot per day) Specific leaf growth (per day) n.s., not significant; ∗ P < 0.05;

∗∗

Site

Time

Site × time

F = 47.02∗∗∗

F = 25.69∗∗∗

F = 6.63∗∗∗ F = 10.04∗∗∗ F = 4.62∗∗

F = 16.88∗∗∗ F = 12.84∗∗∗ P < 0.01;

∗∗∗

F = 11.50∗∗∗ F = 2.44 n.s.

P < 0.001.

Shoot size, leaf growth, and specific leaf growth of E. acoroides in control plots were different both between sites and experiments, and did not show any consistent temporal pattern in the Cape Bolinao area as evidenced by significant site × time interactions (Table 3, Fig. 3). Shoot size and leaf growth were smaller in Binabalian Loob than in the other sites (Figs. 3a, b), while specific leaf growth was lower in Santa Barbara and Silaqui than in Pislatan and Binabalian Loob (Fig. 3c). The addition of nutrients increased the size (g DW per shoot) of E. acoroides shoots in all the experimental sites (Fig. 4) as evidenced by a significant effect of the nutrient additions in all the sites (Table 4). The size of E. acoroides shoots was smaller in Binabalian Loob than in the other sites, and increased by 3-fold after the nutrient additions in June (Fig. 4). Significant increases (P < 0.05) in shoot size after nutrient additions were recorded in June in Binabalian Loob, September in Santa Barbara and Silaqui, and March in Pislatan (Fig. 4). Except for Pislatan, the leaf growth rates of E. acoroides generally increased in response to the nutrient additions (Table 4), but the significance of the effect varied with time (Table 4, Fig. 5). A 3-fold increase in the leaf growth rate of E. acoroides after the additions of nutrients ending in June was observed in Binabalian Loob (Fig. 5), while smaller but significant (P < 0.05) increases were observed in March in Santa Barbara, and September in Silaqui (Fig. 5). The specific leaf growth of E. acoroides increased (F = 12.36, P < 0.01) after the addition of nutrients in Santa Barbara, decreased (F = 5.22, P < 0.05) in Pislatan, and did not change in Binabalian Loob and Silaqui (Table 4). The significant effect of the addition of nutrients in Santa Barbara was driven by the 4-fold increase observed in March (Fig. 6). Nutrient incorporation of E. acoroides leaves did not differ between sites (P > 0.5, Table 5). The addition of nitrogen promoted an increase of nitrogen incorporation by E. acoroides leaves (P < 0.05, Table 5), while the addition of phosphorus did not promote a significant increase of phosphorus incorporation by the plants. E. acoroides in Binabalian Loob showed the highest requirements for N and P in June, resulting in high nutrient deficiency of the plants during this period (Fig. 7). However, there was no nutrient deficiency at this site in March. Enhalus acoroides showed nutrient deficiency during the three experiments in Silaqui and Santa Barbara, while in Pislatan the plants were not nutrient-deficient in June. There was no consistent temporal trend in nutrient deficiency between sites because the highest nutrient-deficiency was observed in June at Binabalian Loob, in March at Santa Barbara, and in September at Silaqui and Pislatan (Fig. 7). Overall, nitrogen and phosphorus deficiency were similar among sites (one-way ANOVA, P > 0.05).

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Fig. 3. Mean (+SE) shoot size (a), leaf growth (b), and specific leaf growth (c) of E. acoroides in control plots at the four experiment sites in Cape Bolinao in June and September (1996), and March (1997).

4. Discussion Shoot size and leaf growth of E. acoroides were limited by the availability of nutrients in Cape Bolinao, NW Philippines as evidenced by significant increases of shoot size (Table

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Fig. 4. Mean (+ SE) shoot size of E. acoroides in control and nutrient-added plots at the four experiment sites along the siltation gradient in Cape Bolinao in June and September (1996), and March (1997). Asterisk (*) denotes a significant difference (P < 0.05, Tukey post-hoc test) between control and nutrient-added plots at individual experiments.

4), leaf growth rate (except for Pislatan site, Table 4), and nitrogen content (Table 2) and incorporation (Table 5) by the leaves after the addition of nutrients in all the experimental sites. Our results confirm previous findings in the area (Agawin et al., 1996), and extend their generality because nutrient limitation of E. acoroides was detected at different times along the year and different sites in Cape Bolinao. However, our results also indicate that the nutrient limitation of this species was weak as evidenced, firstly, by significantly increased shoot sizes and growth rates were only detected in 4 and 3, respectively, of the 12 individual experiments conducted (Figs. 4, 5). Secondly, the nutrient additions did not affect the specific leaf growth rates in a consistent way (Table 4, Fig. 6). Thirdly, leaf nu-

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Table 4 Results of two-way ANOVA analysis testing differences between nutrient-added and control plots in shoot size, leaf growth, and specific leaf growth of E. acoroides at each experimental site Experimental site Santa Barbara Time Nutrients Time × nutrients Pislatan Time Nutrients Time × nutrients Binabalian loob Time Nutrients Time × nutrients Silaqui Time Nutrients Time × nutrients n.s., not significant;

∗

Shoot size (g DW per shoot)

Leaf growth (g DW per shoot per day)

Specific leaf growth (per day)

F = 10.86∗∗∗ F = 4.96∗ F = 5.59∗

F = 64.24∗∗∗ F = 20.85∗∗∗ F = 1.76 n.s.

F = 15.95∗∗∗ F = 12.36∗∗ F = 12.75∗∗∗

F = 0.78 n.s. F = 6.24∗ F = 0.40 n.s.

F = 12.54∗∗∗ F = 2.88 n.s. F = 1.15 n.s.

F = 12.49∗∗∗ F = 5.22∗ F = 0.01 n.s.

F = 33.41∗∗∗ F = 24.42∗∗∗ F = 11.05∗∗∗

F = 63.39∗∗∗ F = 38.26∗∗∗ F = 30.64∗∗∗

F = 13.59∗∗∗ F = 0.96 n.s. F = 3.62∗

F = 12.45∗∗∗ F = 5.85∗ F = 0.37 n.s.

F = 13.59∗∗∗ F = 8.47∗∗ F = 0.40 n.s.

F = 13.75∗∗∗ F = 1.07 n.s. F = 0.79 n.s.

P < 0.05;

∗∗

P < 0.01;

∗∗∗

P < 0.001

trient contents were high in all of the experimental sites (Table 2), above the thresholds typically separating nutrient-sufficient from nutrient-limited plants (Duarte, 1990), which suggests that E. acoroides is not strongly nutrient-limited in the stands studied. Indeed, the magnitude of the response of leaf growth to the addition of nutrients at Silaqui was smaller than previously reported (a maximum of 40% increase in growth in September in our study versus a 160% increase in January 1995, cf. Agawin et al., 1996). The content of phosphorus of E. acoroides leaves (between 0.30 and 0.36 % DW) was higher than the critical value for P-limited seagrass growth (0.20 % of DW; Duarte, 1990), while the nitrogen content (between 1.84 and 2.02 % DW) was similar to the critical value for N-limited seagrass growth (1.8 % of DW; Duarte, 1990), which suggests that E. acoroides growth is more likely to be limited by the availability of nitrogen than by phosphorus in Cape Bolinao. Indeed, an increase of the content (Table 2) and incorporation (Table 5) of nitrogen, but not phosphorus, by the leaves of E. acoroides after the addition of nutrients was observed. This result is consistent with previous studies in the area (Agawin et al., 1996) and indicates that either nitrogen or phosphorus can limit seagrass productivity in tropical environments, contrary to previous models (Short, 1987), which propose that phosphorus would be the main nutrient limiting seagrass growth in these environments. Primary productivity of seagrass meadows in tropical environments has been generally considered to be limited by the availability of nutrients (Powell et al., 1989; Short et al., 1990; Powell et al., 1991; Agawin et al., 1996), although this view has been falsified at certain places (Erftemeijer et al., 1994). Our results indicate that the tropical seagrass E. acoroides was only moderately nutrient-limited in Cape Bolinao, because the addition of

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Fig. 5. Mean (+ SE) leaf growth of E. acoroides in control and nutrient-added plots at the four experiment sites in Cape Bolinao in June and September (1996), and March (1997). Asterisk (*) denotes significant difference (P < 0.05, Tukey post-hoc test) between control and nutrient-added plots at individual experiments.

nutrients did not promote increases in shoot size and leaf growth rates in all the experiments performed at each site. The responses of E. acoroides shoot size and leaf growth to the addition of nutrients to the sediment were also heterogeneous in space, as evidenced by the different times at which the effects of the nutrient additions were significant at each site (Figs. 4, 5, 6). This indicates that the conditions for seagrass growth are different among sites. Background information in the area (Table 1) indicates that light availability is lower in Santa Barbara and Pislatan than in Binabalian Loob and Silaqui. A decrease of light availability can reduce the nutrient limitation of seagrasses through a reduction of seagrass growth

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Fig. 6. Mean (+ SE) specific leaf growth of E. acoroides in control and nutrient-added plots at the four experiment sites in Cape Bolinao in June and September (1996), and March (1997). Asterisk (*) denotes significant difference (P < 0.05, Tukey post-hoc test) between control and nutrient-added plots at individual experiments.

and, therefore, nutrient requirements (Abal et al., 1994), which might partially explain the heterogeneity of the responses of E. acoroides to the nutrient additions in Cape Bolinao. The leaf growth rates of nutrient-sufficient E. acoroides shoots (i.e., in nutrient-added stands) were different among sites (two-way ANOVA, site: F = 4.44, P < 0.01), which suggests that light availability might play role in the observed responses to the nutrient additions. The leaf growth rates of nutrient-sufficient E. acoroides shoots were higher (Tukey post-hoc tests, P < 0.05) in Silaqui (0.0792 g DW shoot per day) than in Pislatan (0.0656 g DW per shoot per day) and Binabalian Loob (0.0636 g DW per shoot per day), but similar to those in Santa Barbara (0.0756 g DW per shoot per day). These results are not consistent with the hypothesized effect of light availability on the growth responses because leaf growth

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Table 5 Average (±SE, n = 3) nutrient (N, and P) incorporation rates of E. acoroides during the three nutrient-addition experiments performed in each of the study sites at Cape Bolinao, NW Philippines, and results of the two-way ANOVA analyses testing for differences between sites and for the effect of the nutrient additions on nutrient incorporation Site

Nutrient-added plants Control plants Nutrient-added plants Control plants N-incorporation N-incorporation P-incorporation P-incorporation (g P (g N per shoot per day) (g N per shoot per day) (g P per shoot per day) per shoot per day)

Santa Barbara Pislatan Binabalian Loob Silaqui Two-way ANOVA Site Nutrients Site × Nut.

0.1679 (0.0266) 0.1391 (0.0253) 0.1223 (0.0502)

0.1279 (0.0232) 0.1056 (0.0114) 0.0741 (0.0124)

0.0236 (0.0069) 0.0212 (0.0024) 0.0183 (0.0069)

0.0189 (0.0034) 0.0207 (0.0022) 0.0137 (0.0028)

0.1747 (0.0318)

0.1221 (0.0186)

0.0201 (0.0019)

0.0201 (0.0038)

F = 1.52 n.s. F = 5.02∗ F = 0.05 n.s.

n.s., not significant;

∗

F = 1.02 n.s. F = 1.73 n.s. F = 0.15 n.s.

P < 0.05.

rates in Santa Barbara, the site with lower light availability (Table 1) were similar to those in Silaqui, the site with higher light availability. Similarly, the leaf growth of Thalassia hemprichii, Cymodocea rotundata and Cymodocea serrulata transplanted from Silaqui to the other experimental sites was not affected by the differences in light availability among sites (Bach, 1997). Overall, our results suggests that other environmental factors different from light availability should be responsible for the differences in the growth response of E. acoroides to the nutrient additions in Cape Bolinao. Differences in the content of organic matter, total nitrogen and total phosphorus in the sediment among the experimental sites (Table 1) suggest that nutrient availability might be lower in Santa Barbara than in the other sites. Phosphorus availability for seagrasses is lower in carbonate sediments than in siliceous sediments (Dennison, 1987), and also decreases with decreasing sizes of sediment particles (Erftemeijer and Middelburg, 1993). Sediment at Santa Barbara is finer and has higher content of total inorganic carbon than in the other sites (Table 1), which suggests that phosphorus availability might be lowest there. Phosphate concentrations in porewater were, however, lower in Binabalian Loob than in the other sites, and there were no differences among sites in the phosphorus concentration of E. acoroides leaves (Table 2), which indicates that phosphorus availability might be similar in all the experimental sites. On the other hand, ammonium and nitrate concentrations in porewater were similar in all the sites and, although, the nitrogen content of E. acoroides leaves increased after the nutrient additions, there were no differences among sites (Table 2), which suggests that nitrogen availability was also similar in the different E. acoroides stands studied. Indeed, both nitrogen and phosphorus deficiency were similar among sites. It is not likely, therefore, that the differences in the growth response of E. acoroides to the nutrient additions are due to differences in nutrient availability among sites. Our results show that the degree of nutrient limitation of E. acoroides in Cape Bolinao can be quite variable at the spatial scale of a few kilometers (among stands), and also along the year on the same stand. However, the differences in the growth response of E. acoroides

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to the nutrient additions cannot be directly linked with differences in either light or nutrient availability among the experimental sites. Which environmental factors might account for the among-site differences in the growth response of E. acoroides to the nutrient additions can not be elucidated here. In summary, our results provide evidence of a moderately nutrient (likely N) limited status of the tropical seagrass E. acoroides under contrasting growth conditions along a gradient of siltation in Cape Bolinao, NW Philippines. This species was nutrient-limited irrespective of the differences in light availability and nutrient levels in the sediment among the experiment sites, which indicates that the nutrient balance of this species is not affected by the increased siltation of the coastal zone. Wether the nutrient balance of other tropical seagrasses is influenced by siltation remains uncertain. E. acoroides experiences greater biomass and, therefore, nutrient losses than other tropical seagrasses to sexual reproduction, which accounts for 20.5% of above-ground production of the species (Duarte et al., 1997b). Consequently, E. acoroides might be more prone than other tropical seagrasses to nutrient limitation. The increased nutrient inputs (Malmer and Grip, 1994; Mitchell et al., 1997) associated with the siltation of the coastal zone do not affect the nutrient balance of E. acoroides but might alleviate the often nutrient-limited status of other tropical seagrasses with less nutrient losses.

Acknowledgements This study was funded by the STD-3 programme of the European Commission (Project TS3∗ -CT94-0301, Responses of Coastal Ecosystems to Deforestation-derived Siltation in Southeast Asia). N.S.R. Agawin was supported by the Spanish Agency for International Cooperation, and J. Terrados by the Spanish Ministry of Education and Culture. Our sincere gratitude to UP MSI-SEALAB staff (Jack Rengel, Iona Jalijali and Helen Dayao) for help. We also thank the reviewers’ comments, which greatly improved a former version of the manuscript. This is a contribution to the LOICZ program (Project No. 27), a core project of the IGBP program.

References Abal, E.G., Loneragan, N.R., Bowen, P., Perry, C.J., Udy, J.W., Dennison, W.C., 1994. Physiological and morphological responses of the seagrass Zostera capricorni Aschers. to light intensity. J. Exp. Mar. Biol. Ecol. 178, 113–129. Agawin, N.S.R., Duarte, C.M., Fortes, M.D., 1996. Nutrient limitation of Philippine seagrasses (Cape Bolinao, NW Philippines): in situ experimental evidence. Mar. Ecol. Prog. Ser. 138, 233–243. Alcoverro, T., Romero, J., Duarte, C.M., López, N.I., 1997. Spatial and temporal variations in nutrient limitation of seagrass Posidonia oceanica growth in the NW Mediterranean. Mar. Ecol. Prog. Ser. 146, 155–161. Bach, S., 1997. Havgræssers vækst og artssammensætning langs en siltgradient ved Kap Bolinao, Filippinerne. M.Sc. Thesis. Freshwater Biological Laboratory, University of Copenhagen Brouns, J.J.W.M., 1985. A comparison of the annual production and biomass in three monospecific stands of the seagrass Thalassia hemprichii (Ehrenb.) Aschers. Aquat. Bot. 23, 149–175. Bulthuis, D.A., 1987. Effects of temperature on photosynthesis and growth of seagrasses. Aquat. Bot. 27 (1), 27–40.

138

J. Terrados et al. / Aquatic Botany 65 (1999) 123–139

Bulthuis, D.A., Axelrad, D., Mickelson, M.J., 1992. Growth of the seagrass Heterozostera tasmanica limited by nitrogen in Port Phillip Bay, Australia. Mar. Ecol. Prog. Ser. 89, 269–275. Chansang, H., Poovachiranon, S., 1994. The distribution and species composition of seagrass beds along the Andaman Sea coast of Thailand. Phuket Mar. Biol. Cent. Res. Bull. 59, 43–52. Dennison, W.C., 1987. Effects of light on seagrass photosynthesis, growth, growth and depth distribution. Aquat. Bot. 27, 15–26. Duarte, C.M., 1990. Seagrass nutrient content. Mar. Ecol. Prog. Ser. 67, 207–210. Duarte, C.M., 1991. Seagrass depth limits. Aquat. Bot. 40, 363–377. Duarte, C.M., 1995. Submerged aquatic vegetation in relation to different nutrient regimes. Ophelia 41, 87–112. Duarte, C.M., Terrados, J., Agawin, N.S.R., Fortes, M.D., Bach, S., Kenworthy, W.J., 1997a. Response of a mixed Philippine seagrass meadow to experimental burial. Mar. Ecol. Prog. Ser. 147, 285–294. Duarte, C.M., Uri, J.S., Agawin, N.S.R., Fortes, M.D., Vermaat, J.E., Marbá, N., 1997b. Flowering frequency of Philippine seagrasses. Bot. Mar. 40, 497–500. Duarte, C.M., Agust´ı, S., Geertz-Hansen, O., 1998. Optical deterioration of silted SE Asian coastal ecosystems: implications for ecosystem autotrophs. In: Responses of Coastal Ecosystems to Deforestation-derived Siltation in Southeast Asia, Final Report. European Commision, DG XII, STD-3, Brussels Erftemeijer, P.L.A., 1994. Differences in nutrient concentrations and resources between seagrass communities on carbonate and terrigenous sediments in South Sulawesi, Indonesia. Bull. Marine. Sci. 54, 403–419. Erftemeijer, P.L.A., Middelburg, J.J., 1993. Sediment-nutrient interactions in tropical seagrass beds: a comparison between a terrigenous and a carbonate sedimentary environment in South Sulawesi (Indonesia). Mar. Ecol. Prog. Ser. 102, 187–198. Erftemeijer, P.L.A., Middelburg, J.J., 1995. Mass balance constraints on nutrient cycling in tropical seagrass beds. Aquat. Bot. 50, 21–36. Erftemeijer, P.L.A., Stapel, J., Smekens, M.J.E., Drossaert, W.M.E., 1994. The limited effect of in situ phosphorus and nitrogen additions to seagrass beds on carbonate and terrigenous sediments in South Sulawesi, Indonesia. J. Exp. Mar. Biol. Ecol. 182, 123–140. Fortes, M.D., 1988. Mangrove and seagrass beds of east Asia: habitats under stress. Ambio 17, 207–213. Fourqurean, J.W., Zieman, J.C., Powell, G.V.N., 1992a. Phosphorus limitation of primary production in Florida Bay: evidence from C : N : P ratios of the dominant seagrass Thalassia testudinum. Limnol. Oceanogr. 37, 162–171. Fourqurean, J.W., Zieman, J.C., Powell, G.V.N., 1992b. Relationships between porewater nutrients and seagrasses in a subtropical carbonate environment. Mar. Biol. 114, 57–65. Giesen, W.B.J.T., van Katwijk, M.M., den Hartog, C., 1990. Eelgrass condition and turbidity in the Dutch Wadden Sea. Aquat. Bot. 37, 71–85. Gómez, E.D., 1988. Overview of environmental problems in the east Asian seas region. Ambio 17, 166–169. Hillman, K., Walker, D.I., Larkum, A.W.D., McComb, A.J., 1989. Productivity and nutrient limitation. In: Larkum, A.W.D., McComb, A.J. and Shepherd, S.A. (Eds.). Biology of Seagrasses. Elsevier, Amsterdam, pp. 635−685. Johnstone, I.M., 1979. Papua New Guinea seagrasses and aspects of the biology and growth of Enhalus acoroides (L.f.) Royle. Aquat. Bot. 7, 197–208. Kamp-Nielsen, L., Vermaat, J.E., Wesseling, I., Borum, J., Geetz-Hansen, O., 1998. Sediment properties along siltation gradients in Southeast Asia. In: Responses of coastal ecosystems to deforestation-derived siltation in Southeast Asia, Final Report. European Commision, DG XII, STD-3, Brussels Kenyon, R.A., Conacher, C.A., Poiner, I.R., 1997. Seasonal growth and reproduction of Enhalus acoroides (L.f.) Royle in a shallow bay in the western Gulf of Carpentaria. Mar. Freshwater Res. 48, 335–342. le Jeune, E.L., 1995. Causes of siltation in the Santiago Island Reef System, Philippines. Water Quality Management and Aquatic Ecology, Report No. 030/95, Wageningen Agricultural University Malmer, A., Grip, H., 1994. Converting tropical rainforest to forest plantation in Sabah, Malaysia Part II. Effects on nutrient dynamics and net losses in streamwater.. Hydrological Processes 8, 195–209. Mitchell, A.W., Bramley, R.G.V., Johnson, A.K.L., 1997. Export of nutrients and suspended sediment during a cyclone-mediated flood event in the Herbert river catchment, Australia. Mar. Freshwater Res. 48, 79–88. Nienhuis, P.H., Coosen, J., Kiswara, W., 1989. Community structure and biomass distribution of seagrasses and macrofauna in the Flores Sea, Indonesia. Neth. J. Sea Res. 23, 197–214. Parsons, T.R., Maita, Y., Lalli, C.M., 1984. A Manual of Chemical and Biological Methods for Seawater Analysis. Pergamon Press, Oxford

J. Terrados et al. / Aquatic Botany 65 (1999) 123–139

139

Pedersen, M.F., Borum, J., 1993. An annual nitrogen budget for a seagrass Zostera marina population. Mar. Ecol. Prog. Ser. 101, 169–177. Powell, G.V.N., Fourqurean, J.W., Kenworthy, W.J., Zieman, J.C., 1991. Bird colonies cause seagrass enrichment in a subtropical estuary: observational and experimental evidence. Estuarine Coastal Shelf Sci. 32, 567–579. Powell, G.V.N., Kenworthy, W.J., Fourqurean, J.W., 1989. Experimental evidence for nutrient limitation of seagrass growth in a tropical estuary with restricted circulation. Bull. Mar. Sci. 44 (1), 324–340. Preen, A.R., Lee Long, W.J., Coles, R.G., 1995. Flood and cyclone related loss, and partial recovery, of more than 1000 km2 of seagrass in Hervey Bay, Queensland, Australia. Aquat. Bot. 52, 3–17. Sand-Jensen, K., Borum, J., 1991. Interactions among phytoplankton, periphyton, periphyton and macrophytes in temperate freshwaters and estuaries. Aquat. Bot. 41, 137–175. Short, F.T., 1987. Effects of sediment nutrients on seagrasses: literature review and mesocosm experiment. Aquat. Bot. 27, 41–57. Short, F.T., Burdick, D.M., 1996. Quantifying elgrass habitat loss in relation to housing development and nitrogen loading in Waquoit Bay, Massachusetts. Estuaries 19, 730–739. Short, F.T., Davis, M.W., Gibson, R.A., Zimmermann, C.F., 1985. Evidence for phosphorus limitation in carbonate sediments of the seagrass Syringodium filiforme. Estuarine Coastal Shelf Sci. 20, 419–430. Short, F.T., Dennison, W.C., Capone, D.G., 1990. Phosphorus-limited growth of the tropical seagrass Syringodium filiforme in carbonate sediments. Mar. Ecol. Prog. Ser. 62, 169–174. Sokal, R.R., Rohlf, F.J., 1981. Biometry. The Principles and Practice of Statistics in Biological Research. Freeman, New York Terrados, J., Duarte, C.M., Fortes, M.D., Borum, J., Agawin, N.S.R., Bach, S., Thampanya, U., Kamp-Nielsen, L., Kenworthy, W.J., Geertz-Hansen, O., Vermaat, J.E., 1998a. Changes in community structure and biomass of seagrass communities along gradients of siltation in SE Asia.. Estuarine Coastal Shelf Sci. 46, 757–768. Tomasko, D.A., Dawes, C.J., Fortes, M.D., Largo, D.B., Alava, M.N.R., 1993. Observations on a multi-species seagrass meadow offshore of Negros Oriental, Republic of the Philippines. Bot. Mar. 36, 303–311. Tomasko, D.A., Dawes, C.J., Hall, M.O., 1996. The effects of anthropogenic nutrient enrichment on turtle grass (Thalassia testudinum) in Sarasota Bay, Florida. Estuaries 19, 448–456. Vermaat, J.E., Agawin, N.S.R., Duarte, C.M., Fortes, M.D., Marbá, N., Uri, J.S., 1995. Meadow maintenance, growth, growth and productivity of a mixed Philippine seagrass bed. Mar. Ecol. Prog. Ser. 124, 215–225.