Marine Environmental Research 106 (2015) 92e102
Contents lists available at ScienceDirect
Marine Environmental Research journal homepage: www.elsevier.com/locate/marenvrev
Carbon budgets of multispecies seagrass beds at Dongsha Island in the South China Sea Yen-Hsun Huang a, Chen-Lu Lee a, Chia-Yun Chung a, Shu-Chuan Hsiao a, Hsing-Juh Lin a, b, * a b
Department of Life Sciences and Research Center for Global Change Biology, National Chung Hsing University, Taichung 402, Taiwan Biodiversity Research Center, Academia Sinica, Taipei 115, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2015 Received in revised form 5 March 2015 Accepted 8 March 2015 Available online 10 March 2015
Biomass, production, consumption, and detrital export and decomposition of four dominant seagrass species were determined in tropical multispecies beds as a means of constructing carbon budgets. These processes varied among seagrass species. The living biomass held a high carbon stock. The leaf production of multispecies beds was also higher than that of monospecific beds. However, the sediment organic carbon stock was much lower than the global median stock, which was likely due to decomposition of most of the detritus and export to nearshore waters. Reliable measurements of decomposition and export are particularly needed to estimate the organic carbon storage rate. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Biomass Carbon sink Decomposition Export Herbivory Production Tropical
1. Introduction Seagrass beds are considered to be significant sinks of Blue Carbon (Nellemann et al., 2009). On an areal basis, seagrass beds are not only highly productive (Duarte and Chiscano, 1999) but also trap allochthonous suspended organic carbon (C) in the sediments (Fourqurean et al., 2012). Despite the small amount of living biomass in a seagrass bed, the bed may hold as much C as the same surface area of a tropical forest (Pendleton et al., 2012). On a global scale, seagrass beds can store 19.9 Pg of organic C in living seagrass biomass and the underlying sediments (Fourqurean et al., 2012). While the accumulation rate of organic C in the sediments of seagrass beds has been estimated (e.g. Lavery et al., 2013), little is actually known about the fate of production by seagrasses. Although the average short-term (years) and long-term (decades) storage rates of organic C from seagrass leaves in sediments n, 2002) and 83 g m2 yr1 have been estimated to be 133 (Cebria (Duarte et al., 2005), respectively, few studies have tried to quantify
* Corresponding author. Department of Life Sciences and Research Center for Global Change Biology, National Chung Hsing University, Taichung 402, Taiwan. E-mail address: [email protected] (H.-J. Lin). http://dx.doi.org/10.1016/j.marenvres.2015.03.004 0141-1136/© 2015 Elsevier Ltd. All rights reserved.
and integrate the processes constructing seagrass C budgets. Based n on a compilation of globally available data, Duarte and Cebria (1996) estimated that the majority of seagrass net production is decomposed (50%), with export and herbivory accounting for 24% and 19%, respectively, and the remaining 16% being stored. However, these estimates of processes of C budgets are primarily limited to two seagrass species (Thalassia testudinum and Posidonia oceanica), which have been studied only in the Caribbean and Mediterranean. Although there is a wealth of available data on seagrass leaf production, there are still few field measurements of herbivory, export, decomposition, and storage or of belowground processes € rk, 2009). To obtain a more accurate global stor(Kennedy and Bjo age rate estimate for seagrass Blue Carbon, reliable estimates of these processes in dominant seagrass species across a broad geographic range and of the C storage potential of each species are needed. The tropics may include the most biogeochemically active coastal regions and represent potentially important C sinks in the biosphere (Twilley et al., 1992). However, seagrass biomass and production were lower at low latitudes (Duarte and Chiscano, 1999). Tropical seagrass beds are also generally dominated by a variety of smaller-sized seagrasses (Short et al., 2007), which
Y.-H. Huang et al. / Marine Environmental Research 106 (2015) 92e102
typically have a lower organic C stock in living biomass than the temperate species. The observed lower seagrass biomass and production at low latitudes may be partly attributed to the higher level of grazing on seagrasses (Heck and Valentine, 2006). It is also thought that the higher temperatures in the tropics promote more efficient decomposition of seagrass detritus in the sediment. It appears that the remaining amount of organic C in tropical seagrass beds would be lower than in temperate seagrass beds. The tropical Indo-Pacific has the highest seagrass biodiversity in the world, with as many as 14 species growing together in mixed meadows (Short et al., 2007). A multispecies seagrass bed may exhibit a higher production rate than a monospecific seagrass bed (Erftemeijer and Stapel, 1999). Empirical work on the processes occurring in tropical multispecies seagrass beds is needed to improve our understanding of the contribution of seagrass ecosystems to Blue Carbon. Dongsha Island (20 420 N, 116 430 E), also known as Pratas Island, is a pristine island that is little affected by human activities and is located in the South China Sea (Fig. 1). There is a lagoon on the western side of Dongsha Island with only one inlet (80 m wide) connecting it to the sea. The shoot density and coverage of seagrasses, as well as their species diversity, are high on this remote island (Lin et al., 2005). In this study, the C budgets of the leaf and belowground production of dominant seagrass species at Dongsha Island were further examined. The objectives of this study were 1) to quantify the biomass, production, consumption, and detrital export and decomposition of dominant seagrass species in the multispecies seagrass beds, 2) to determine whether there is variation in these processes among the dominant species, 3) to determine whether there is seasonal variation in these processes, 4) to examine whether there is spatial variation (in the semi-enclosed lagoon vs. on the open coast) in these processes, and 5) to estimate the organic C storage rates in the multispecies seagrass beds at Dongsha Island by integrating the processes described above. 2. Materials and methods 2.1. Study sites Dongsha Island is 2.80 km long and 0.87 km wide, covering an area of 1.74 km2. The lagoon exhibits a surface area of 0.64 km2, a mean depth of approximately 1 m at low tide, and a small tidal amplitude of approximately 80 cm. Despite the small area of the island, the total coverage area of seagrass beds is 11.85 km2. These beds grow on carbonate sediment in the lagoon and around Dongsha Island. A total of seven seagrass species from six genera and two families have been identified around the island (Lin et al., 2005): Cymodocea serrulata (CS), Cymodocea rotundata (CR),
93
Table 1 Environmental factors and seagrass characteristics in the lagoon and on the northern (N) coast of Dongsha Island.
Location Habitat Vegetation Mean water temperature ( C) Mean water salinity Mean sediment silt/clay content (%) Median sediment grain size (mm) Sediment sorting coefficient Mean sediment organic content (%) Mean seagrass cover (%) Mean shoot density (shoots m2) Mean seagrass total biomass (g DW m2)
Lagoon
N coast
20 420 N, 116 420 E subtidal mixed 19.4e36.3 22.3e40.7 0.64e3.93 0.21e0.32 0.57e2.02 2.45e4.48 85 2200e5321 775e1359
20 420 N, 116 430 E subtidal mixed 25.3e31.9 31.7e38.5 1.66e2.62 0.35e0.43 1.35e1.84 2.22e2.40 81 455e2522 713e1147
Syringodium isoetifolium (SI), Halodule uninervis (HU), Thalassodendron ciliatum (TC), Halophila ovalis (HO), and Thalassia hemprichii (TH). This study was conducted in the seagrass beds in the lagoon and on the northern (N) coast of Dongsha Island (Fig. 1). Although Dongsha Island is dominated by a tropical climate, the intense northeast monsoons from November to April can markedly decrease water temperature. Therefore, our samples were collected in February (winter) and April (spring) for the cool season and June (summer) and October (fall) 2011 for the warm season. The water temperature and salinity were more variable in the lagoon than on the N coast due to the longer residence times (Table 1). The mean water temperature in the lagoon ranged from 19.4 C in winter to 36.3 C in summer. The mean water salinity in the lagoon ranged from 22.3 in summer to 40.7 in winter. The mean silt/clay content of the sediment was lower in the lagoon (1.43%) than on the N coast (2.42%). The median sediment grain size was smaller in the lagoon (0.26 mm) than on the N coast (0.37 mm). Although the mean sorting coefficient of the sediment was also lower in the lagoon (1.08) relative to the N coast (1.57), indicating that the sediment in the lagoon was relatively well sorted, the sorting coefficients were more variable in the lagoon than on the N coast. Conversely, the sediment mean organic content was greater in the lagoon (3.14%) than on the N coast (2.37%).
2.2. Seagrass biomass and production To collect data that were representative of the seagrass beds at Dongsha Island, five and three 50-m transects were surveyed in mixed meadows in the lagoon and on the N coast, respectively (Fig. 1). The coverage and shoot density of the seagrasses at each
Fig. 1. (a) The location of Dongsha Island in the South China Sea and (b) transects where seagrasses were investigated at Dongsha Island.
94
Y.-H. Huang et al. / Marine Environmental Research 106 (2015) 92e102
site in each season were determined at 5-m intervals along each transect by placing a quadrat (50 50 cm) divided into 25 squares (10 10 cm) on the substratum following Lin and Shao (1998). The coverage of each species in each quadrat was estimated according to the frequency of the mid-point percentage of 6 cover classes of the 25 squares for a given species following the method of Saito and Atobe (1970). This procedure was repeated for each species present in the quadrat. The shoot density of each species was estimated from five counts performed within five squares along a diagonal line in each quadrat along each transect. Based on the coverage data, the seagrass bed in the lagoon consists of a Thalassia-Halodule-Cymodocea community, but the seagrass bed on the N coast is a Cymodocea-Thalassia community. The biomass, production, consumption, and detrital export and decomposition of the four dominant species (TH, CR, and HU in the lagoon and TH, CR, and CS on the N coast) were further determined to construct the C budgets of leaf and belowground productions in the multispecies seagrass beds at Dongsha Island. In each season, five biomass samples consisting of approximately 20 seagrass shoots each, together with the associated belowground portion, were randomly collected near each transect but not within each transect. In the laboratory, these biomass samples (on average, 634 shoots in the lagoon and 216 shoots on the N coast) were sorted, rinsed with fresh water, and weighed (wet weight, WW). Dry weight (DW) was then determined by drying the seagrass samples at 60 C to a constant weight. Epiphytes were excluded by gently scraping them off the leaves with the edge of a glass slide. The total area of each leaf was calculated using ImagePro Plus software (Media Cybernetics, Silver Springs, MD, USA). For each seagrass species, conversion factors between leaf area and WW and between WW and DW were determined by linear regression. The leaf marking method described by Short and Coles (2001) was used to estimate the leaf production of each seagrass species in three random plots (10 10 cm each) along each transect in each season. In each plot, 15e29 shoots of TH, 17e42 shoots of HU, 11e24 shoots of CR, or 10e17 shoots of CS were marked in each season depending on the shoot density of each seagrass species. A small hole was punched through each leaf at the base of the live shoot to provide a reference level. Five days after the initial marking, the shoot was cut at the base, and the new growth increments of the leaves were cut off and dried at 60 C to a constant weight. Using these increments, leaf production was expressed as the DW per shoot per day (g DW shoot1 d1) and then transformed into leaf production per unit area per day (g DW m2 d1) by multiplying by the shoot density (shoot m2). The belowground production was not quantified due to the constraints of survey time on the remote island. Instead, the belowground production for each species at each site in each season for the C budget was estimated using the regression equation between belowground biomass and production derived from Duarte and Chiscano (1999). 2.3. Herbivory by fish and invertebrates We modified the tethering method of Kirsch et al. (2002) to quantify grazing on leaves of each seagrass species. Fifteen replicate quadrats (10 10 cm each) of ungrazed seagrass shoots were randomly deployed within the multispecies seagrass beds in each season. Each quadrat was at least 5 m away from the previously deployed one. Within each quadrat, seagrass shoots of mixed dominant species were deployed, and the shoot densities of each seagrass species approximated the natural densities at each site described above. Before deployment, ungrazed seagrass shoots were collected from each site, and their leaf areas were recorded with a digital camera and analyzed using Image-Pro Plus software
Table 2 Summary statistics obtained with ANOVAs. A two-way ANOVA was applied to determine whether aboveground and belowground biomass (g DW m2), leaf production rate (g DW m2 d1), or leaf consumption rate (g DW m2 d1) differed statistically among the different species and the four seasons. A one-way ANOVA was applied to examine whether detrital export rate (g DW m2 d1) differed seasonally. If significant differences were detected, a Duncan's new multiple range test (MRT) test was used to determine which means differed. df: degrees of freedom, CS: Cymodocea serrulata, CR: Cymodocea rotundata, HU: Halodule uninervis, HO: Halophila ovalis, TH: Thalassia hemprichii, spr: spring, sum: summer, fal: fall, win: winter. Variable
Factor
df
F-value
p value
Duncan's test
2
Aboveground biomass (g DW m ) Lagoon species 2 39.692 season 3 17.027 species season 6 7.392 N coast species 2 2.043 season 3 1.556 species season 6 1.092 Belowground biomass (g DW m2) Lagoon species 2 13.872 season 3 7.515 species season 6 2.29 N coast species 2 5.49 season 3 1.564 species season 6 2.063 Leaf production rate (g DW m2 d1) Lagoon species 2 6.547 season 3 15.261 species season 6 1.841 N coast species 2 8.797 season 3 21.883 species season 6 4.824 Leaf consumption rate (g DW m2 d1) Lagoon species 1 2.173 season 3 0.929 species season 3 9.003 N coast species 2 85.974 season 3 71.928 species season 6 10.576 Detrital export rate (g DW m2 d1) Lagoon season 3 9.463 N coast season 3 9.257
Contents lists available at ScienceDirect
Marine Environmental Research journal homepage: www.elsevier.com/locate/marenvrev
Carbon budgets of multispecies seagrass beds at Dongsha Island in the South China Sea Yen-Hsun Huang a, Chen-Lu Lee a, Chia-Yun Chung a, Shu-Chuan Hsiao a, Hsing-Juh Lin a, b, * a b
Department of Life Sciences and Research Center for Global Change Biology, National Chung Hsing University, Taichung 402, Taiwan Biodiversity Research Center, Academia Sinica, Taipei 115, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2015 Received in revised form 5 March 2015 Accepted 8 March 2015 Available online 10 March 2015
Biomass, production, consumption, and detrital export and decomposition of four dominant seagrass species were determined in tropical multispecies beds as a means of constructing carbon budgets. These processes varied among seagrass species. The living biomass held a high carbon stock. The leaf production of multispecies beds was also higher than that of monospecific beds. However, the sediment organic carbon stock was much lower than the global median stock, which was likely due to decomposition of most of the detritus and export to nearshore waters. Reliable measurements of decomposition and export are particularly needed to estimate the organic carbon storage rate. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Biomass Carbon sink Decomposition Export Herbivory Production Tropical
1. Introduction Seagrass beds are considered to be significant sinks of Blue Carbon (Nellemann et al., 2009). On an areal basis, seagrass beds are not only highly productive (Duarte and Chiscano, 1999) but also trap allochthonous suspended organic carbon (C) in the sediments (Fourqurean et al., 2012). Despite the small amount of living biomass in a seagrass bed, the bed may hold as much C as the same surface area of a tropical forest (Pendleton et al., 2012). On a global scale, seagrass beds can store 19.9 Pg of organic C in living seagrass biomass and the underlying sediments (Fourqurean et al., 2012). While the accumulation rate of organic C in the sediments of seagrass beds has been estimated (e.g. Lavery et al., 2013), little is actually known about the fate of production by seagrasses. Although the average short-term (years) and long-term (decades) storage rates of organic C from seagrass leaves in sediments n, 2002) and 83 g m2 yr1 have been estimated to be 133 (Cebria (Duarte et al., 2005), respectively, few studies have tried to quantify
* Corresponding author. Department of Life Sciences and Research Center for Global Change Biology, National Chung Hsing University, Taichung 402, Taiwan. E-mail address: [email protected] (H.-J. Lin). http://dx.doi.org/10.1016/j.marenvres.2015.03.004 0141-1136/© 2015 Elsevier Ltd. All rights reserved.
and integrate the processes constructing seagrass C budgets. Based n on a compilation of globally available data, Duarte and Cebria (1996) estimated that the majority of seagrass net production is decomposed (50%), with export and herbivory accounting for 24% and 19%, respectively, and the remaining 16% being stored. However, these estimates of processes of C budgets are primarily limited to two seagrass species (Thalassia testudinum and Posidonia oceanica), which have been studied only in the Caribbean and Mediterranean. Although there is a wealth of available data on seagrass leaf production, there are still few field measurements of herbivory, export, decomposition, and storage or of belowground processes € rk, 2009). To obtain a more accurate global stor(Kennedy and Bjo age rate estimate for seagrass Blue Carbon, reliable estimates of these processes in dominant seagrass species across a broad geographic range and of the C storage potential of each species are needed. The tropics may include the most biogeochemically active coastal regions and represent potentially important C sinks in the biosphere (Twilley et al., 1992). However, seagrass biomass and production were lower at low latitudes (Duarte and Chiscano, 1999). Tropical seagrass beds are also generally dominated by a variety of smaller-sized seagrasses (Short et al., 2007), which
Y.-H. Huang et al. / Marine Environmental Research 106 (2015) 92e102
typically have a lower organic C stock in living biomass than the temperate species. The observed lower seagrass biomass and production at low latitudes may be partly attributed to the higher level of grazing on seagrasses (Heck and Valentine, 2006). It is also thought that the higher temperatures in the tropics promote more efficient decomposition of seagrass detritus in the sediment. It appears that the remaining amount of organic C in tropical seagrass beds would be lower than in temperate seagrass beds. The tropical Indo-Pacific has the highest seagrass biodiversity in the world, with as many as 14 species growing together in mixed meadows (Short et al., 2007). A multispecies seagrass bed may exhibit a higher production rate than a monospecific seagrass bed (Erftemeijer and Stapel, 1999). Empirical work on the processes occurring in tropical multispecies seagrass beds is needed to improve our understanding of the contribution of seagrass ecosystems to Blue Carbon. Dongsha Island (20 420 N, 116 430 E), also known as Pratas Island, is a pristine island that is little affected by human activities and is located in the South China Sea (Fig. 1). There is a lagoon on the western side of Dongsha Island with only one inlet (80 m wide) connecting it to the sea. The shoot density and coverage of seagrasses, as well as their species diversity, are high on this remote island (Lin et al., 2005). In this study, the C budgets of the leaf and belowground production of dominant seagrass species at Dongsha Island were further examined. The objectives of this study were 1) to quantify the biomass, production, consumption, and detrital export and decomposition of dominant seagrass species in the multispecies seagrass beds, 2) to determine whether there is variation in these processes among the dominant species, 3) to determine whether there is seasonal variation in these processes, 4) to examine whether there is spatial variation (in the semi-enclosed lagoon vs. on the open coast) in these processes, and 5) to estimate the organic C storage rates in the multispecies seagrass beds at Dongsha Island by integrating the processes described above. 2. Materials and methods 2.1. Study sites Dongsha Island is 2.80 km long and 0.87 km wide, covering an area of 1.74 km2. The lagoon exhibits a surface area of 0.64 km2, a mean depth of approximately 1 m at low tide, and a small tidal amplitude of approximately 80 cm. Despite the small area of the island, the total coverage area of seagrass beds is 11.85 km2. These beds grow on carbonate sediment in the lagoon and around Dongsha Island. A total of seven seagrass species from six genera and two families have been identified around the island (Lin et al., 2005): Cymodocea serrulata (CS), Cymodocea rotundata (CR),
93
Table 1 Environmental factors and seagrass characteristics in the lagoon and on the northern (N) coast of Dongsha Island.
Location Habitat Vegetation Mean water temperature ( C) Mean water salinity Mean sediment silt/clay content (%) Median sediment grain size (mm) Sediment sorting coefficient Mean sediment organic content (%) Mean seagrass cover (%) Mean shoot density (shoots m2) Mean seagrass total biomass (g DW m2)
Lagoon
N coast
20 420 N, 116 420 E subtidal mixed 19.4e36.3 22.3e40.7 0.64e3.93 0.21e0.32 0.57e2.02 2.45e4.48 85 2200e5321 775e1359
20 420 N, 116 430 E subtidal mixed 25.3e31.9 31.7e38.5 1.66e2.62 0.35e0.43 1.35e1.84 2.22e2.40 81 455e2522 713e1147
Syringodium isoetifolium (SI), Halodule uninervis (HU), Thalassodendron ciliatum (TC), Halophila ovalis (HO), and Thalassia hemprichii (TH). This study was conducted in the seagrass beds in the lagoon and on the northern (N) coast of Dongsha Island (Fig. 1). Although Dongsha Island is dominated by a tropical climate, the intense northeast monsoons from November to April can markedly decrease water temperature. Therefore, our samples were collected in February (winter) and April (spring) for the cool season and June (summer) and October (fall) 2011 for the warm season. The water temperature and salinity were more variable in the lagoon than on the N coast due to the longer residence times (Table 1). The mean water temperature in the lagoon ranged from 19.4 C in winter to 36.3 C in summer. The mean water salinity in the lagoon ranged from 22.3 in summer to 40.7 in winter. The mean silt/clay content of the sediment was lower in the lagoon (1.43%) than on the N coast (2.42%). The median sediment grain size was smaller in the lagoon (0.26 mm) than on the N coast (0.37 mm). Although the mean sorting coefficient of the sediment was also lower in the lagoon (1.08) relative to the N coast (1.57), indicating that the sediment in the lagoon was relatively well sorted, the sorting coefficients were more variable in the lagoon than on the N coast. Conversely, the sediment mean organic content was greater in the lagoon (3.14%) than on the N coast (2.37%).
2.2. Seagrass biomass and production To collect data that were representative of the seagrass beds at Dongsha Island, five and three 50-m transects were surveyed in mixed meadows in the lagoon and on the N coast, respectively (Fig. 1). The coverage and shoot density of the seagrasses at each
Fig. 1. (a) The location of Dongsha Island in the South China Sea and (b) transects where seagrasses were investigated at Dongsha Island.
94
Y.-H. Huang et al. / Marine Environmental Research 106 (2015) 92e102
site in each season were determined at 5-m intervals along each transect by placing a quadrat (50 50 cm) divided into 25 squares (10 10 cm) on the substratum following Lin and Shao (1998). The coverage of each species in each quadrat was estimated according to the frequency of the mid-point percentage of 6 cover classes of the 25 squares for a given species following the method of Saito and Atobe (1970). This procedure was repeated for each species present in the quadrat. The shoot density of each species was estimated from five counts performed within five squares along a diagonal line in each quadrat along each transect. Based on the coverage data, the seagrass bed in the lagoon consists of a Thalassia-Halodule-Cymodocea community, but the seagrass bed on the N coast is a Cymodocea-Thalassia community. The biomass, production, consumption, and detrital export and decomposition of the four dominant species (TH, CR, and HU in the lagoon and TH, CR, and CS on the N coast) were further determined to construct the C budgets of leaf and belowground productions in the multispecies seagrass beds at Dongsha Island. In each season, five biomass samples consisting of approximately 20 seagrass shoots each, together with the associated belowground portion, were randomly collected near each transect but not within each transect. In the laboratory, these biomass samples (on average, 634 shoots in the lagoon and 216 shoots on the N coast) were sorted, rinsed with fresh water, and weighed (wet weight, WW). Dry weight (DW) was then determined by drying the seagrass samples at 60 C to a constant weight. Epiphytes were excluded by gently scraping them off the leaves with the edge of a glass slide. The total area of each leaf was calculated using ImagePro Plus software (Media Cybernetics, Silver Springs, MD, USA). For each seagrass species, conversion factors between leaf area and WW and between WW and DW were determined by linear regression. The leaf marking method described by Short and Coles (2001) was used to estimate the leaf production of each seagrass species in three random plots (10 10 cm each) along each transect in each season. In each plot, 15e29 shoots of TH, 17e42 shoots of HU, 11e24 shoots of CR, or 10e17 shoots of CS were marked in each season depending on the shoot density of each seagrass species. A small hole was punched through each leaf at the base of the live shoot to provide a reference level. Five days after the initial marking, the shoot was cut at the base, and the new growth increments of the leaves were cut off and dried at 60 C to a constant weight. Using these increments, leaf production was expressed as the DW per shoot per day (g DW shoot1 d1) and then transformed into leaf production per unit area per day (g DW m2 d1) by multiplying by the shoot density (shoot m2). The belowground production was not quantified due to the constraints of survey time on the remote island. Instead, the belowground production for each species at each site in each season for the C budget was estimated using the regression equation between belowground biomass and production derived from Duarte and Chiscano (1999). 2.3. Herbivory by fish and invertebrates We modified the tethering method of Kirsch et al. (2002) to quantify grazing on leaves of each seagrass species. Fifteen replicate quadrats (10 10 cm each) of ungrazed seagrass shoots were randomly deployed within the multispecies seagrass beds in each season. Each quadrat was at least 5 m away from the previously deployed one. Within each quadrat, seagrass shoots of mixed dominant species were deployed, and the shoot densities of each seagrass species approximated the natural densities at each site described above. Before deployment, ungrazed seagrass shoots were collected from each site, and their leaf areas were recorded with a digital camera and analyzed using Image-Pro Plus software
Table 2 Summary statistics obtained with ANOVAs. A two-way ANOVA was applied to determine whether aboveground and belowground biomass (g DW m2), leaf production rate (g DW m2 d1), or leaf consumption rate (g DW m2 d1) differed statistically among the different species and the four seasons. A one-way ANOVA was applied to examine whether detrital export rate (g DW m2 d1) differed seasonally. If significant differences were detected, a Duncan's new multiple range test (MRT) test was used to determine which means differed. df: degrees of freedom, CS: Cymodocea serrulata, CR: Cymodocea rotundata, HU: Halodule uninervis, HO: Halophila ovalis, TH: Thalassia hemprichii, spr: spring, sum: summer, fal: fall, win: winter. Variable
Factor
df
F-value
p value
Duncan's test
2
Aboveground biomass (g DW m ) Lagoon species 2 39.692 season 3 17.027 species season 6 7.392 N coast species 2 2.043 season 3 1.556 species season 6 1.092 Belowground biomass (g DW m2) Lagoon species 2 13.872 season 3 7.515 species season 6 2.29 N coast species 2 5.49 season 3 1.564 species season 6 2.063 Leaf production rate (g DW m2 d1) Lagoon species 2 6.547 season 3 15.261 species season 6 1.841 N coast species 2 8.797 season 3 21.883 species season 6 4.824 Leaf consumption rate (g DW m2 d1) Lagoon species 1 2.173 season 3 0.929 species season 3 9.003 N coast species 2 85.974 season 3 71.928 species season 6 10.576 Detrital export rate (g DW m2 d1) Lagoon season 3 9.463 N coast season 3 9.257
