Rapid recovery of the intertidal seagrass Zostera japonica following intense Manila clam (Ruditapes philippinarum) harvesting activity in Korea

Journal of Experimental Marine Biology and Ecology 407 (2011) 275–283

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Journal of Experimental Marine Biology and Ecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j e m b e

Rapid recovery of the intertidal seagrass Zostera japonica following intense Manila clam (Ruditapes philippinarum) harvesting activity in Korea Sang Rul Park a, 1, Young Kyun Kim a, Jong-Hyeob Kim a, Chang-Keun Kang b, Kun-Seop Lee a,⁎ a b

Department of Biological Sciences, Pusan National University, Pusan 609-735, Republic of Korea Ocean Science and Technology Institute, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea

a r t i c l e

i n f o

Article history: Received 28 January 2011 Received in revised form 27 May 2011 Accepted 24 June 2011 Available online 20 July 2011 Keywords: Disturbance Manila clam harvesting Population structure Recovery Reproduction Zostera japonica

a b s t r a c t Although the Manila clam (Ruditapes philippinarum) culture grounds are occasionally located in Zostera japonica beds along the coasts of Korea, plant responses to the clamming activity have not been reported for this seagrass species. Intense Manila clam harvesting activity took place in the intertidal Z. japonica bed during April 2004. The Z. japonica bed at the study site has been monitored since January 2003. Thus, this study provided a unique opportunity to compare the structure of the Z. japonica population before and after the clamming activity, which was conducted for approximately 1 week in April 2004. All Z. japonica shoots were removed and buried in the sediment immediately after the clamming activity. However, a few shoots were found at the disturbed area in July 2004, 3 months after the clamming activity. By September 2004, 5 months after the disturbance, shoot density and biomass were almost recovered to the levels reported before the clamming activity. No Z. japonica seedlings were observed when the shoot density rapidly increased in August and September 2004, 4–5 months after the disturbance, because revegetation of the disturbed seagrass bed has occurred before the seed germination time which is typically winter or early spring in this area. Thus, the initial rapid revegetation of the disturbed area occurred via asexual reproduction through new shoot formation from the buried below-ground tissues. The reproductive shoot density and reproductive efforts of Z. japonica were significantly higher after the disturbance relative to the levels recorded before the disturbance, and the duration of the fertile period was approximately three times longer following the clamming activity. The belowground biomass after the disturbance was also significantly higher than that before the disturbance. These results suggest that Z. japonica allocated more energy to sexual reproduction, as well as the maintenance of belowground tissues, to persist their population under unstable environmental conditions. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Seagrass meadows are an important component of estuarine and coastal ecosystems, providing food, shelter, and nursery areas for commercially and ecologically valuable marine organisms (Hemminga and Duarte, 2000). However, significant declines in these meadows, mainly caused by anthropogenic disturbances, have been reported from many parts of the world (Short and Wyllie-Echeverria, 1996). The main anthropogenic causes of seagrass declines are cultural eutrophication or toxic pollutants related to reductions in water quality and increased turbidity (Cambridge and McComb, 1984; Philippart et al., 1992). In addition, seagrass meadows can be damaged by mechanical disturbance related to dredging and reclamation (Park et al., 2009), boat traffic (Creed and Amado Filho, 1999; Dawes et al., 1997), and fishing activities ⁎ Corresponding author. Tel.: + 82 51 510 2255; fax: + 82 51 581 2962. E-mail address: [email protected] (K.-S. Lee). 1 Current address: University of Texas at Austin Marine Science Institute, 750 Channel View Drive, Port Aransas, TX 78373, United States. 0022-0981/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2011.06.023

such as clam harvesting and clam culture (Everett et al., 1995; Neckles et al., 2005; Orth et al., 2002). Clam harvesting has been identified as a major cause of seagrass declines in places where it is commercially important (Alexandre et al., 2005; Boese, 2002; Neckles et al., 2005). Clam harvesting usually encompasses digging and tilling the sediment, which damages the photosynthetic tissues of the seagrass or causes plant burial, resulting in reduced seagrass coverage (Short and Wyllie-Echeverria, 1996). Although some studies have examined seagrass responses to mechanical disturbance induced by clam harvesting (Alexandre et al., 2005; Boese, 2002; Cabaço et al., 2005), comparisons of seagrass conditions or population structure before and after clam harvesting and the recovery from damage caused by clam harvesting are rare (Neckles et al., 2005). The Manila clam Ruditapes philippinarum, widely distributed in the intertidal and upper subtidal flats along the coasts of Korea, is one of the most important commercial bivalves in Korea (Park and Choi, 2004). The Manila clam culture grounds are usually located in the higher [0.6–1.2 m mean lower low water (MLLW)] intertidal sand or

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mud flats (Dumbauld et al., 2009) and occasionally overlap with Zostera japonica beds or intertidal Zostera marina beds along the coasts of Korea as in western North America (Tsai et al., 2010; Authors pers. obs.). The Manila clams are usually harvested during spring when the tide is low. At a study site on the south coast of Korea, intensive clam harvesting activity took place for approximately 1 week in April 2004. Consequently, most Z. japonica shoots were uprooted or buried in the sediment. The seagrass bed as well as abiotic factors such as water temperature and nutrient conditions at the site had been monitored since January 2003. Therefore, this study site afforded a unique opportunity to compare seagrass population structures before and after intensive clam harvesting, as well as to examine the natural recovery process of Z. japonica populations that had been damaged during clam harvesting. Since Manila clam harvesting usually takes place once a year during spring before Z. japonica seed production occurs at the study site, we hypothesized that the initial revegetation of the disturbed bed occurred via asexual reproduction through the lateral shoot formation from the buried below-ground tissues in sediments. Z. japonica is mainly distributed in the intertidal and shallow subtidal zones in East Asia (den Hartog and Kuo, 2006). Since Z. japonica is usually distributed in intertidal areas, this species is probably more susceptible to human activities such as reclamation, seashore road construction, and clamming than seagrasses in subtidal areas. However, plant responses to the clamming activity have not been reported for this seagrass species. We also hypothesized that this intertidal seagrass species exhibits different responses in population structure to the clamming activity compared to the seagrass in subtidal areas. This study represents the first comprehensive examination of long-term patterns and environmental parameters within Z. japonica meadows and of the natural recovery processes following seagrass declines due to Manila clam harvesting. 2. Materials and methods 2.1. Study site The study site is located in Koje Bay (34°48′N, 128°35′E) on the southern coast of Korea. Four Zostera species (Z. japonica, Z. marina, Z. caespitosa, and Z. caulescens) are distributed along the different water depths. The tidal movements are semidiurnal with tidal amplitude of approximately 1.2 m at neap tides and 2.8 m at spring tides [Tide Tables (Coast of Korea), National Oceanographic Research Institute of Korea]. Z. japonica meadows, which are distributed in intertidal zones, are exposed to air during low tide for 1–4 h daily. Z. japonica is mixed with Z. marina in the lower intertidal zone. The Manila clam, R. philippinarum, was cultured in the intertidal flats of the area where Z. japonica formed dense meadows. This study was conducted on a monotypic meadow of Z. japonica in the middle intertidal zone, and the population structure and production of Z. japonica in relation to coincident measurements of environmental conditions were monitored from January 2003 to December 2005. Intensive Manila clam harvesting was conducted in the study area for approximately 1 week in April 2004, and the population structure and production of Z. japonica were documented before and after the clam harvesting activity.

from July 2004 to May 2006. Salinity was measured monthly at the water surface using YSI 85 (YSI Inc., Yellow Springs, OH, USA) throughout the experimental periods. To determine water column inorganic nutrient concentrations (NH4+, NO3− + NO2−, and PO43−), four replicate surface water samples were collected monthly from January 2003 to December 2005. The NO3− + NO2− concentration was determined after running the sample through a column containing copper-coated cadmium, which reduces NO3− to NO2−. Sediment pore water nutrients were measured monthly from six replicate sediment samples, which were collected randomly to a sediment depth of approximately 15 cm with a syringe corer and then frozen until laboratory analyses. Sediment pore water was obtained by centrifugation (5000 × g for 15 min) and then diluted with low-nutrient seawater (b0.1 μM) for determining pore-water nutrient concentrations. Water column and sediment pore-water nutrient concentrations were determined using standard colorimetric techniques following the methods of Parsons et al. (1984). 2.3. Biological measurements Shoot morphology, density, and biomass were measured monthly from January 2003 to December 2005. To measure shoot morphological characteristics, 10–15 mature terminal shoots at the study site were collected individually. Sheath length was measured from the meristem to the top of the sheath. Shoot height was measured from the meristem to the tip of the longest leaf to the nearest 1.0 mm, and the width of the longest leaf was measured to the nearest 0.1 mm. The number of leaves per shoot was counted. Diameter and internode length of the rhizome were measured from the first to sixth youngest nodes. To determine shoot density and biomass, six replicate core samples were collected with a 9-cm-diameter coring device. Z. japonica shoots were separated into seedling, vegetative and reproductive shoots, and then counted to estimate each shoot density. Seedlings were identified by the presence of a seed coat or a curved rhizome base. Shoot density was converted to per unit area estimates (shoots m − 2). Collected tissues were thoroughly cleaned of epiphytes and sediments, separated into above- (blade + sheath) and belowground tissues (root + rhizome), and dried at 60 °C to a constant weight. Samples were weighed and biomass was converted to per unit area estimates [g dry weight (DW) m − 2]. Leaf productivities were measured using a modified blade marking technique (Kowalski et al., 2001; Park et al., 2010; Zieman, 1974). Thirty randomly chosen shoots were marked through the sheath bundle approximately 3 cm above the meristem using a fine hypodermic needle (b0.2 mm diameter) and then harvested after a period of 3–5 weeks. Leaf materials were separated into leaf tissues produced before and after marking. Separated leaf tissues were dried at 60 °C to a constant weight. The leaf production rate per shoot (mg DW shoot − 1 d − 1) was determined by dividing the DW of new leaf tissues produced after marking by the number of days since marking. Areal leaf production rate (g DW m − 2 d − 1) was calculated by multiplying the leaf production rate per shoot by the average shoot density. The recovery rate (%) of biological parameters in the first year after the clamming activity was estimated by dividing the average values of biological parameters after the clamming activity in 2005 by the average values of biological parameters before the clamming activity in 2003.

2.2. Physical and chemical parameters 2.4. Statistical analyses Water temperature was measured at the study site every 15 min from January 2003 to December 2005 using HOBO data loggers (Onset Computer Corp., Bourne, MA, USA) encased in underwater housing. Continuous measurements of surface photon flux density (PFD) were conducted on the roof of a building at South Sea Fisheries Institute, approximately 70 km from the study site, using a LI-192 quantum sensor (2π) and an LI-1400 data-logger (LI-COR, Lincoln, NE, USA)

All values are presented as means ± 1SE. Data were tested for normality and homogeneity of variance to meet the assumptions of parametric statistics. If these assumptions were not satisfied, data were transformed by log(x + 1). Differences in water column and pore-water nutrient concentrations, plant morphological characteristics, shoot density, biomass, and leaf productivities of Z. japonica

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among sampling months were tested for significance using a one-way ANOVA. When a significant difference was observed among the sampling months, the means were estimated using the Student– Newman–Keuls test to determine where the significant differences occurred. The values of biological parameters (shoot density, biomass, shoot morphology, leaf productivity, and elongation) before (2003) and after (2005) the clamming activity were compared using a time series analysis to estimate the effects of the clamming activity on the growth dynamics of Z. japonica. Estimation of model parameters was accomplished using the Time Series Cross Section Regression (TSCSREG). This analysis is useful to evaluate the effect of an event on the pattern and extent of data measured before and after the event under continuous monitoring (Tabachnick and Fidell, 2007). The estimated proportional change in seagrass growth parameters due to the clamming activity was computed as E = 1 − exp (parameter estimate; Tabachnick and Fidell, 2007). Significant differences in maximum reproductive shoot density between 2003 and 2005 were analyzed using independent two samples t-tests. In all cases, the significance level was set at 5% probability. All statistical analyses were performed using SAS (version 9.1; SAS Institute, Cary, NC, USA). 3. Results 3.1. Physical and chemical parameters

Surface photon flux density (mol photons m-2 d-1)

Daily surface irradiance exhibited significant (p b 0.001) seasonal variation (Fig. 1A). Monthly average surface irradiance was highest in 70 60

A Daily values Monthly average

50 40 30 20 10 0

J F M AM J J A S O N D J F M AM J J A S O N D J F M AM J J A S O N D

Temperature (oC)

30

B

25 20 15 10 5 0 J F M AM J J A S O N D J F M AM J J A S O N D J F M AM J J A S O N D

35

C

34

Salinity

33 32 31 30 29 28 J F M AM J J A S O N D J F M AM J J A S O N D J F M AM J J A S O N D

2003

2004

2005

Fig. 1. Daily surface photon flux density (PFD; A), water temperature (B), and salinity (C) at the study site in Koje Bay on the southern coast of Korea from January 2003 to December 2005.

277

July 2004 (45.6 mol photons m − 2 d − 1) and lowest in December 2005 (17.6 mol photons m − 2 d − 1). Water temperature also exhibited a strong seasonal variation (p b 0.001), ranging from 5.0 °C in January 2004 to 27.9 °C in August 2005 (Fig. 1B). Salinity ranged from 30.4 to 33.5, with an average of 31.9 (Fig. 1C). The water column NH4+ concentrations varied significantly (p b 0.001) with sampling months, showing peaks during the summer period, but did not exhibit a clear seasonal trend (Fig. 2A). The NH4+ concentration in the water column was usually less than 4 μM except in summer. The NO3− + NO2− concentration in the water column also varied significantly (p b 0.001) with sampling months, but did not exhibit a clear seasonal trend (Fig. 2B). The average NO3− + NO2− concentration throughout the experimental period was 1.7 μM. The water column PO43− concentrations also did not exhibit a clear seasonal trend and were usually less than 2 μM (Fig. 2C). The sediment pore water NH4+ concentration exhibited a significant seasonal variation (p b 0.001), ranging from 47 μM in December 2005 to 352 μM in July 2005 (Fig. 2D). The sediment NO3− + NO2− concentrations varied significantly (p b 0.001) with sampling months, but did not show a clear seasonal trend (Fig. 2E). The PO43− concentration in the sediment pore water also varied significantly (p b 0.001) with sampling months, but did not exhibit a clear seasonal trend (Fig. 2F). The sediment NO3− + NO2− and PO43− concentrations were significantly (p b 0.001) higher in 2005 than in 2003 (Fig. 2E, F). 3.2. Shoot density and biomass Total shoot density exhibited a significant seasonal variation (p b 0.001) and showed maximum peaks during spring (11,083 shoots m − 2 in May 2003 and 9343 shoots m − 2 in March 2005; Fig. 3A). Prior to the clam harvesting, the total shoot density was 1965 shoots m − 2 in March 2004. However, all Z. japonica shoots were eliminated immediately in April 2004, the first sampling month after the clamming activity. Since plant samples were not collected for 2 months (May and June 2004) after the clamming activity, no shoot density data were collected during these months. Some shoots (approximately 20 shoots m − 2) were found at the study site in July 2004, 3 months after the clamming activity, and then shoot density increased rapidly in August and September 2004 (Fig. 3A). In September 2004, 5 months after the clamming activity, the total shoot density reached 4737 shoots m − 2, a density higher than that (2910 shoots m− 2) in September 2003, the year before the clamming activity. No Z. japonica seedlings were observed when the shoot density rapidly recovered in August and September 2004, because revegetation of the disturbed seagrass bed has occurred before the seed germination time which is typically winter or early spring in the study site. Vegetative shoot density also showed a significant seasonal variation (p b 0.001), but seasonal change patterns were distinctly different between before (2003) and after (2005) the clamming activity (Fig. 3B). In 2003, the year before the clamming activity, reproductive shoots appeared during May–June, whereas they appeared during March–August in 2005, the year after the clamming activity (Fig. 3C). The reproductive shoot density was highest in May (2293 shoots m− 2) and accounted for approximately 21% of the total shoot density in 2003, the year before clam harvesting. However, following the clamming activity, the reproductive shoot density was highest in March (5867 shoots m − 2) and accounted for approximately 63% of the total shoot density in 2005. Thus, the reproductive period in 2005, the first year after the clamming activity, was much longer than that in 2003, the year before the clamming, and reproductive efforts in 2005 were approximately three times higher than those before the clamming activity in 2003. The average total shoot density during the first year after the clamming activity (2005) nearly recovered to that before the clamming activity (2003), whereas the vegetative shoot density in 2005 reached 90% of the density in 2003 (Table 1). The results of the time series analysis showed that the total and vegetative shoot

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Water column

NH4+ (M)

10

Sediment pore water 500

A

8

400

6

300

4

200

2

100

0

0

J FMAMJ J ASOND J FMAMJ J ASOND J FMAMJ J ASOND 12

B

J FMAMJ J ASOND J FMAMJ J ASONDJ FMAMJ J ASOND 60

E

40 30

10

NO3- + NO2- (M)

D

8 20

6 4

10 2 0

0

J FMAMJ J ASOND J FMAMJ J ASOND J FMAMJ J ASOND 4

J FMAMJ J ASOND J FMAMJ J ASONDJ FMAMJ J ASOND 30

C

F

25

PO43- (M)

3 20 2

15 10

1 5 0

0

J FMAMJ J ASOND J FMAMJ J ASOND J FMAMJ J ASOND

2003 NH4+

2004 NO3− + NO2−

2005 PO43−

J FMAMJ J ASOND J FMAMJ J ASONDJ FMAMJ J ASOND

2003

2004

2005 NO3− + NO2−

Fig. 2. Seasonal changes in water column (A), (B), and (C) concentrations, and sediment pore water (D), (E), and PO43− (F) concentrations in Koje Bay on the southern coast of Korea from January 2003 to December 2005. Values represent means ± 1SE (n = 4–6). Vertical bars indicate the period of the clam harvesting activity.

densities were not significantly different before and after the clamming activity (Table 2). However, the maximum reproductive shoot density was significantly (p b 0.05) higher in 2005, the first year after the clamming activity (5867 shoots m − 2) than in 2003, the year before the clamming activity (2293 shoots m − 2; Fig. 3C, Table 1). The biomass of individual plant parts and total biomass varied significantly (p b 0.001) with season, increasing during spring and decreasing during fall (Fig. 4A). Total biomass was approximately 77 g DW m − 2 in March 2004, just before the clamming activity. Since all Z. japonica shoots were eliminated immediately at the study site due to the clamming activity, no aboveground tissues were found in April 2004, the first sampling month after the clamming activity. However, belowground tissues buried in the sediments included the apical meristematic parts, in which new lateral shoots can reproduce asexually. Seagrass biomass increased rapidly in August and September 2004, 4–5 months after the clamming activity (Fig. 4A). In September 2004, total biomass reached 123.1 g DW m − 2, which was higher than that in September 2003 (105.7 g DW m − 2), the year before the clamming activity. The contribution of belowground tissues

NH4+

to total biomass increased after the clamming activity (Fig. 4). Belowground tissues accounted for approximately 26% of the total biomass in 2003 and 48% of the biomass in 2005. Thus, the average below-/aboveground biomass ratios were significantly (p b 0.05) higher in 2005 (1.11) than in 2003 (0.43; Fig. 4B, Table 1). The average total biomass during the first year after the clamming activity (2005) was higher than that in 2003 (Table 1). The aboveground biomass in 2005 was 81% of the biomass in 2003, whereas the belowground biomass in 2005 was much higher than that in 2003 (Table 1). The time series analysis showed that total and aboveground biomass was not significantly different before and after the clamming activity (Table 2). However, the belowground biomass and below-/ aboveground biomass ratios increased significantly after the clamming activity (Table 2). 3.3. Shoot morphology and productivity Since no plant samples were collected for shoot morphology during April–June 2004, 1–2 months after the clamming activity, no

S.R. Park et al. / Journal of Experimental Marine Biology and Ecology 407 (2011) 275–283

Total shoot density (shoot m-2)

15000

Table 1 Zostera japonica. Average, ranges and of shoot densities, biomass, shoot morphology, leaf productivities, and leaf elongation rates before (2003) and after (2005) the clam harvesting activity and the recovery rates after the clamming activity in Koje Bay on the southern coast of Korea.

12000

9000

2003 6000

3000

0 12000

Vegetative shoot density

A

J F M AM J J A S O N D J F M AM J J A S O N D J F M AM J J A S O N D

B

10000 8000 6000 4000 2000 0 J F M AM J J A S O N D J F M AM J J A S O N D J F M AM J J A S O N D

10000

Reproductive shoot density

279

C

8000

6000

Shoot density (shoots m− 2) Total shoot Vegetative shoot Reproductive shoot Biomass (g DW m− 2) Total Above-ground Below-ground Below-/above-ground ratio Shoot morphology Shoot height (cm) Sheath length (cm) Blade width (mm) Number of leaves per shoot Rhizome internode length (mm) Rhizome diameter (mm) Leaf productivity Per shoot (mg DW shoot− 1 day− 1) Per unit area (g DW m− 2 day− 1) Leaf elongation Per shoot (cm leaf shoot− 1 day− 1) Per unit area (m leaf m− 2 day− 1)

2005

Recovery rate (%)

4914 (1258–11083) 4857 (2713–9343) 4490 (1258–8789) 4011 (2713–5780) 424 (0–2293) 846 (0–5867)

99 90 199

122.5 (58.5–201.8) 90.9 (46.9–177.3) 31.6 (13.3–49.4) 0.43 (0.14–0.74)

141.8 (98.2–220.3) 73.1 (42.8–126.3) 68.7 (47.5–94.0) 1.11 (0.63–1.74)

116 81 217 260

19.8 (14.0–27.3) 5.4 (3.4–7.2) 1.4 (0.9–1.9) 3.1 (2.8–3.4)

24.4(17.8–27.3) 5.6 (4.4–7.0) 1.5 (1.1–1.7) 2.8 (2.4–2.9)

123 105 106 89

14.5 (6.7–28.3)

27.7 (16.1–43.6)

191

1.5(1.2–1.8)

1.6 (1.4–1.8)

107

0.27 (0.12–0.42)

0.27 (0.16–0.37)

99

1.28 (0.37–3.22)

1.37 (0.54–2.47)

108

0.75 (0.41–1.12)

0.82 (0.56–1.12)

110

37.28 (9.32–82.23) 43.46 (15.81–78.97) 116

4000

2000

0 J F M AM J J A S O N D J F M AM J J A S O N D J F M AM J J A S O N D

2003

2004

2005

Fig. 3. Zostera japonica. Seasonal variations in densities of total shoot (A), vegetative shoot (B), and reproductive shoot (C) in Koje Bay on the southern coast of Korea from January 2003 to December 2005. Values represent means ± 1SE (n = 4–6). Vertical bars indicate the period of the clam harvesting activity. Seagrass samples were not collected for 2 months from May to June 2004.

shoot morphology results were available during these months. In August 2004, 4 months after the clamming activity, shoot morphological characteristics such as shoot height, sheath length, blade width, and the number of leaves per shoot were similar to those at the same season of 2003, the year before the clamming activity (Fig. 5). Thus, the newly emerged seagrass shoots during recolonization of Z. japonica after the clamming activity were not morphologically different from the preexisting shoots during the year before the clamming activity. The average shoot height in 2005 (24.4 cm) was greater than that in 2003 (19.8 cm), but these values were not significantly different before and after the clamming activity (Table 2). Shoot height and sheath length showed clear seasonal variation, increasing during late spring and decreasing in winter (Fig. 5A, B). Leaf blades were widest during summer and narrowest during winter (Fig. 5C). The number of leaves per shoot fluctuated with sampling time and was highest in June 2003 and lowest in February 2004 (Fig. 5D). Sheath length and blade width were almost the same before and after the clamming activity (Table 1). Although the number of leaves per shoot was slightly lower in 2005 (2.8 leaves shoot− 1) than in 2003 (3.1 leaves shoot− 1), they were not significantly different before and after

the clamming activity (Table 2). Rhizome internode length exhibited clear seasonal variation, increasing during early spring and decreasing during summer (Fig. 5E). The average rhizome internode length in 2005 was longer than that in 2003 (Table 1), and these values were significantly (p b 0.05) different before and after the clamming activity (Table 2). Rhizome diameter varied with sampling time, without clear seasonal trend (Fig. 5F). Although rhizome diameter was slightly lower

Table 2 Results of the time series analysis for Zostera japonica growth parameters before and after the clam harvesting activity. Dependent variable Shoot density Total shoot Vegetative shoot Biomass Total Above-ground Below-ground Below-/above-ground ratio Shoot morphology Shoot height Sheath length Blade width Number of leaves per shoot Rhizome internode length Rhizome diameter Leaf productivity Per shoot Per unit area Leaf elongation Per shoot Per unit area

Parameter estimate

Standard error

P value

0.423 − 0.022

0.213 0.181

0.067 0.904

0.188 0.050 0.423 0.280

0.207 0.265 0.201 0.079

0.380 0.854 0.050 0.003

− 0.040 − 0.072 − 0.056 − 0.039 0.384 0.046

0.073 0.073 0.041 0.028 0.176 0.032

0.588 0.338 0.191 0.175 0.047 0.168

0.024 0.149

0.031 0.117

0.441 0.219

− 0.010 0.137

0.044 0.218

0.824 0.537

280

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Total Aboveground Belowground

4.1. Seagrass recovery following the clamming activity

4.2. Changes in population structure following the clamming activity

Given that many Manila clam culture grounds are located in intertidal sand or mud flats, which are major distributional areas for the intertidal seagrass Zostera japonica on the coasts of Korea (Park and Choi, 2004), the population structure and growth dynamics of this seagrass species are likely affected by Manila clam culturing activities.

Remarkably increased reproductive efforts of seagrasses have been reported after a variety of disturbance types (Gallegos et al., 1992; Jacobs, 1982; Marbà and Duarte, 1995; Van Lent and Verschuure, 1994). The reproductive effort of Z. noltii was significantly higher in meadows affected by clam harvesting than in control meadows

Biomass (g DW m-2)

4. Discussion

Along the coasts of Korea, intensive Manila clam harvesting activities usually take place for 1–3 weeks in the spring during low tide. Although hand hoes are usually used to harvest Manila clams in this area, a clam harvesting vehicle was used in the 2004 harvesting event discussed here. Thus, nearly all Z. japonica shoots were uprooted or buried in sediment after the clamming activity at the study site. However, a few shoots were observed at the study site in July 2004, 3 months post-clamming activity, and the shoot density and biomass rapidly recovered within 4 months of the physical disturbance. In 2005, the year following the disturbance, Z. japonica had completely recovered to the levels recorded in 2003, the year before the disturbance. This result was similar to that of Boese (2002), who reported that no apparent differences in the biological parameters of Z. marina were observed between control and treatment areas a year after clam raking or digging treatments. Z. noltii, which is described as a fast-growing species, also rapidly recovered within 1 month of an experimental clam harvesting treatment (Cabaço et al., 2005). The recovery of seagrass habitats from disturbances is dependent on the intensity and frequency of disturbance as well as the seagrass species and biological characteristics (Curiel et al., 1996; Peterken and Conacher, 1997). Although the clamming activity was intensive, such disturbances do not often occur at this study site. Manila clam harvesting usually takes place once a year during spring. Additionally, like Z. noltii, Z japonica is a small and fast-growing seagrass species. Thus, the Z. japonica bed in the study site recovered rapidly within 1 year of disturbance. Sexual reproduction plays an important role in the revegetation of seagrass after destruction caused by large-scale disturbances (Lee et al., 2007; Peterken and Conacher, 1997). The contribution of Z. noltii shoots originating from seeds accounts for a high proportion of the total shoots in some meadows, suggesting the importance of sexual reproduction in space occupation for this species (Diekmann et al., 2005). However, the rapid revegetation of the disturbed bed has occurred before the seed germination time, which is typically winter and early spring in the study area. Consequently, no seedlings were observed during August and September 2004, when shoot density rapidly increased following the disturbance. Thus, the initial seagrass revegetation of this disturbed area probably did not occur through the seedling recruitment. Since asexual reproduction is usually a key component of space occupation for maintaining and expanding disturbed seagrass meadows (Duarte et al., 1997; Duarte and SandJensen, 1990), seagrass recolonization following large disturbance could occur via vegetative reproduction. The recovery of Z. noltii in an area disturbed by clam harvesting occurred through asexual reproduction of shoots with at least two modules including the apical parts, which were buried in sediment (Cabaço et al., 2005). In the present study, although nearly all aboveground tissues disappeared after the clamming activity, belowground tissues buried in sediments included the apical meristematic tissues, in which new lateral shoots can reproduce asexually. Since the amount of time required for vegetative reproduction is much shorter in small seagrass species than in large seagrass species (Hemminga and Duarte, 2000), recolonization of bare areas or disturbed areas through asexual reproduction is probably more important for small seagrass species such as Z. japonica than for large species. Thus, the rapid initial recovery at this study site probably occurred through asexual reproduction such as lateral shoot formation from belowground tissues buried in the sediments.

A

200

150

100

50

0

Below-/aboveground biomass ratio

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Fig. 4. Zostera japonica. Seasonal variations in biomass (A) and below-/aboveground biomass ratio (B) in Koje Bay on the southern coast of Korea from January 2003 to December 2005. Values represent means ± 1SE (n = 4–6). Vertical bars indicate the period of the clam harvesting activity. Seagrass samples were not collected for 2 months from May to June 2004.

in 2005 (1.6 mm) than in 2003 (1.5 mm), they were not significantly (p = 0.168) different before and after the clamming activity (Table 2). Recolonized Z. japonica shoots after the clamming activity showed similar leaf productivity and leaf elongation rates to the preexisting shoots in 2003 (Fig. 6). The average leaf productivity per shoot in 2005, the first year after the clamming activity, completely recovered to the level of the productivity before the clamming activity in 2003 (Table 1). Although areal leaf productivity and leaf elongation rates in 2005 were even higher than those in 2003 (Table 1), the time series analysis showed that leaf productivity and leaf elongation rates were not significantly different before and after the clamming activity (Table 2). Leaf productivities per shoot and per unit area showed clear seasonal variation, increasing during spring and summer and decreasing during late fall and winter (Fig. 6A). Leaf productivities per shoot were highest in summer or fall, but areal leaf productivities were highest in May/June. The seasonal pattern of leaf elongation rates was similar to that of leaf productivities (Fig. 6B). Leaf elongation rates per shoot ranged from 0.41 cm shoot − 1 d − 1 in January 2003 to 1.12 cm shoot − 1 d − 1 in June 2003 and 2005. Leaf elongation rates per unit area ranged between 7.9 m leaf m − 2 d − 1 in January 2004 and 82.2 m leaf m − 2 d − 1 in May 2003.

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Fig. 5. Zostera japonica. Seasonal variations in shoot height (A), sheath length (B), blade width (C), number of leaves per shoot (D), rhizome internode length (E), and rhizome diameter (F) in Koje Bay on the southern coast of Korea from January 2003 to December 2005. Values represent means ± 1SE (n = 10–15). Vertical bars indicate the period of the clam harvesting activity. Seagrass samples were not collected for 2 months from May to June 2004.

(Alexandre et al., 2005). The reproductive shoots of Z. japonica, which was invasive in Willapa Bay, Washington, USA, showed a 19-fold increase following a vegetation removal disturbance (Bando, 2006). In the present study, the reproductive shoot density and reproductive efforts conspicuously increased in 2005, the first year after the clam harvesting, compared to 2003 levels, the year before the clam harvesting. In 2003, reproductive shoot density showed a maximum peak (2293 shoots m − 2) in May and accounted for 21% of the total shoot density. This reproductive shoot density was slightly higher than those reported for other Z. japonica meadows (Huong et al., 2003; Kaldy, 2006; Lee et al., 2006), but was nearly identical to that reported by Lee et al. (2005). However, the reproductive efforts in the study site dramatically increased in 2005, the first year after the clamming activity. The maximum reproductive shoot density in 2005 was approximately 5867 shoots m − 2 in March, accounting for roughly 63% of the total shoot density. This result is similar to that reported by Plus et al. (2003), who showed that the biomass of reproductive shoots of Z. marina accounted for approximately 75% of the total aboveground biomass following a disturbance caused by an anoxic event in the French Mediterranean Sea.

Additionally, the duration of the fertile period of Z. japonica was approximately three times longer after the disturbance than before the disturbance. In 2003, the year before the clamming activity, reproductive shoots appeared for 2 months (from May to June), whereas they were present for approximately 6 months, from March to late August 2005, the year after the clamming activity. The duration of the fertile period for Z. japonica after the clam harvesting disturbance at this study site was also longer than that at an undisturbed Z. japonica meadow in Dadae Bay, approximately 8 km away (Lee et al., 2006). This result is similar to that reported by Alexandre et al. (2005), who reported an extended fertile season for Z. noltii following clam harvesting disturbance. The flowering of seagrasses is mainly controlled by water temperature, photoperiod, and salinity (Caye and Meinesz, 1985; DiazAlmela et al., 2006; Ramage and Schiel, 1999). Other environmental factors controlling seagrass flowering include sediment composition and heavy rainfalls (Alexandre et al., 2005). However, no significant differences in water temperature and salinity were observed between 2003 and 2005 at the study site. The sediment NO3− + NO2− and PO43− concentrations at the study site were significantly higher in 2005, when the reproductive efforts dramatically increased, than in 2003.

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Fig. 6. Zostera japonica. Seasonal variations in leaf productivity (A) and leaf elongation (B) in Koje Bay on the southern coast of Korea from January 2003 to December 2005. Values represent means ± 1SE (n = 10–20). Vertical bars indicate the period of the clam harvesting activity. Seagrass samples were not collected for 2 months from May to June 2004.

Addition of phosphate did not promote flowering and a high nitrogen status inhibited flowering in Syringodium filiforme (McMillan, 1980). Thus, the increased reproductive efforts after the clamming activity probably were not caused by the changes in sediment nutrient conditions at the study site. Based on the development model of Satake et al. (2001), annual plants adapted to an environment condition with unpredictable disturbances can evolve varied strategies for reproductive timing. Moreover, several researchers have suggested that seagrasses and wetland plants in disturbed environmental conditions invest in more reproductive effort for survival and sustenance of the populations (Crosslé and Brock, 2002; Gallegos et al., 1992; Jacobs, 1982; Plus et al., 2003; Van Lent and Verschuure, 1994). Increased reproductive efforts after the clamming activity may have been partially caused by interannual variability (Campey et al., 2002; Lee et al., 2005). However, main environmental factors controlling seagrass flowering were not significantly different before and after the clamming activity at the study site. Thus, the increased flowering shoot density and reproductive effort in the present study might have been induced by disturbances caused by the clamming activity. This suggests that Z. japonica tends to allocate more energy to produce seeds for survival and maintenance under unstable environmental conditions. A variety of responses of belowground tissue to burial and clamming activities have been reported for several seagrass species (Duarte et al., 1997; Marbà and Duarte, 1995). In the present study, the average belowground biomass in 2005, the first year after the disturbance, was 2.1 times higher than that in 2003, the year before the disturbance. Conversely, no difference was observed in aboveground biomass between the year before and that after the clamming activity. Thus, the contribution of belowground biomass to total biomass increased considerably after the clamming activity. The length of rhizome internodes of Z. noltii increased in response to disturbances caused by burial or erosion to relocate the leaf-producing meristems closer to the

sediment surface or to search for new sediment, thus avoiding the eroded area (Cabaço and Santos, 2007). Additionally, seagrasses produce more belowground tissues following disturbance to facilitate asexual reproduction from rhizomes remained in the sediment (Cabaço et al., 2005; Terrados et al., 1997). In this study, the rhizome internode length significantly increased after the clamming activity. The average rhizome internode length in 2005 was 1.9 times longer than that in 2003. Thus, the increased belowground biomass following the disturbance in this study also suggested that after the disturbance caused by clamming activities, the seagrasses enhanced the functioning of the belowground tissues for survival and sustenance under the unstable sediment conditions. In conclusion, nearly all Z. japonica shoots were removed and buried in sediments during Mania clam (R. philippinarum) harvesting activities, which occurred during low tide in the spring season of 2004. However, some shoots were observed at the disturbed site 3 months after the clamming activity. Then, the Z. japonica bed completely recovered to levels recorded before the disturbance via asexual reproduction within 1 year after the disturbance. Furthermore, the reproductive efforts and the fertile period of Z. japonica increased and were extended in response to the clamming activity. Additionally, the belowground biomass, below-/aboveground biomass ratio, and rhizome internode length increased considerably after the clamming activity. These results suggest that Z. japonica tends to allocate more energy to sexual reproduction and that the functions of the belowground tissues increased to promote survival and maintenance under unstable habitat conditions. Acknowledgments We thank YW Lee, W Li, J-I Park, SH Kim, and JW Kim for their countless hours of field and laboratory assistance. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST; NRF-2010-0026520). [SS] References Alexandre, A., Santos, R., Serrão, E., 2005. Effects of clam harvesting on sexual reproduction of the seagrass Zostera noltii. Mar. Ecol. Prog. Ser. 298, 115–122. Bando, K.J., 2006. The roles of competition and disturbance in a marine invasion. Biol. Invasions 8, 755–763. Boese, B.L., 2002. Effects of recreational clam harvesting on eelgrass (Zostera marina) and associated infaunal invertebrates: in situ manipulative experiments. Aquat. Bot. 73, 63–74. Cabaço, S., Santos, R., 2007. Effects of burial and erosion on the seagrass Zostera noltii. J. Exp. Mar. Biol. Ecol. 340, 204–212. Cabaço, S., Alexandre, A., Santos, R., 2005. Population-level effects of clam harvesting on the seagrass Zostera noltii. Mar. Ecol. Prog. Ser. 298, 123–129. Cambridge, M.L., McComb, A.J., 1984. The loss of seagrass from Cockburn Sound, Western Australia. I. The time course and magnitude of seagrass decline in relation to industrial development. Aquat. Bot. 20, 229–243. Campey, M.L., Kendrick, G.A., Walker, D.I., 2002. Interannual and small-scale spatial variability in sexual reproduction of the seagrasses Posidonia coriacea and Heterozoterea tasmanica, southwestern Australia. Aquat. Bot. 74, 287–297. Caye, G., Meinesz, A., 1985. Observations on the Vegetative Development, Flowering and Seeding of Cymodocea nodosa (Ucria) Ascherson on the Mediterranean Coasts of France. Creed, J.C., Amado Filho, G.M., 1999. Disturbance and recovery of the macroflora of a seagrass (Halodule wrightii Ascherson) meadow in the Abrolhos Marine National Park, Brazil: an experimental evaluation of anchor damage. J. Exp. Mar. Biol. Ecol. 235, 285–306. Crosslé, K., Brock, M.A., 2002. How do water regime and clipping influence wetland plant establishment from seed banks and subsequent reproduction? Aquat. Bot. 74, 43–56. Curiel, D., Bellato, A., Rismondo, A., Marzocchi, M., 1996. Sexual reproduction of Zostera noltii Hornemann in the lagoon of Venice (Italy, north Adriatic). Aquat. Bot. 52, 313–318. Dawes, C.J., Andorfer, J., Rose, C., Uranowski, C., Ehringer, N., 1997. Regrowth of the seagrass Thalassia testudinum into propeller scars. Aquat. Bot. 59, 139–155. den Hartog, C., Kuo, J., 2006. Taxonomy and biogeography in seagrasses. In: Larkum, A.W.D., Orth, R.R., Duarte, C.M. (Eds.), Seagrass: Biology, Ecology and Conservation. Springer, New York, pp. 1–23. Diaz-Almela, E., Marbà, N., Álvarez, E., Balestri, E., Fernández, Ruiz, Duarte, C.M., 2006. Patterns of seagrass (Posidnoia oceanica) flowering in the western Mediterranean. Mar. Biol. 148, 723–742.

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