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Deterioration of eelgrass, Zostera marina L., meadows by water pollution in Seto Inland Sea, Japan
Marine Pollution Bulletin 44 (2002) 1253–1258 www.elsevier.com/locate/marpolbul
Deterioration of eelgrass, Zostera marina L., meadows by water pollution in Seto Inland Sea, Japan Hitoshi Tamaki
a,*
, Makoto Tokuoka b, Wataru Nishijima a, Toshinobu Terawaki c, Mitsumasa Okada a
a
c
Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higahi-Hiroshima 739-8527, Japan b Mikuniya Corporation, 4-4-7 Miyuki Ujina, Minami, Hiroshima, Japan Fisheries Research Agency, National Research Institute of Fisheries and Environment of Inland Sea, 2-17-5 Maruishi, Ohno, Saeki, Hiroshima 739-0452, Japan
Abstract Survival of transplanted Zostera marina L. (eelgrass) and environmental conditions (water quality, bottom sediments, sedimentation on leaves and flow regime) were studied concurrently in the center, edge, and at the outside of a eelgrass meadow located in a eutrophic coastal zone in northern Hiroshima Bay, Seto Inland Sea, Japan. Eelgrass transplants at the outside of the meadow declined significantly, whereas those at the center were consistently well established. Silt content in the bottom sediments at the outside was higher than that at the center. The sediment was oxic from the surface to 2 cm deep at the center, whereas those at the edge and the outside were reductive almost from the surface. The sediment characteristics typical in eutrophic water seemed to be a factor responsible for the deterioration of eelgrass meadows. Although suspended solid concentrations in the water columns were almost the same, the amount of sediments deposited on leaves of eelgrass at the outside was higher than that at the center of the meadow. The amount of the deposition at the outside seems to be enough to inhibit photosynthesis; i.e. photosynthetic photon flux density (PPFD) available for eelgrass was only 36% of that without any deposition. The deposition in the center, however, was small enough to allow 84% of the original PPFD. Flow rates, determined at 30 cm above the bottom, a half height of average eelgrass, suggested that the rate at the outside was not enough to remove deposited sediments from the surface of eelgrass leaves. Thus, the large amount of sediment deposition caused by water pollution and/or eutrophication seemed to be another factor to inhibit the survival of eelgrass at the outside edge of the meadow. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Sediment deposition on eelgrass leaves; Flow velocity; Transplant; Survival; Eelgrass; Zostera marina L.
1. Introduction Eelgrass (Zostera marina L.) is a prolific temperate seagrass that grows in shallow coastal areas. Eelgrass meadows are highly productive components of estuaries and coastal ecosystems and support diverse faunal assemblages (Orth et al., 1993; Thayer et al., 1984; Heck et al., 1995). They are excellent habitats for many commercial fishes and are nurseries for juvenile fishes (Ohno et al., 1996). Eelgrass filters and retains nutrients from water column (Short et al., 1984) and provides a
*
Corresponding author. Tel./fax: +81-824-24-7626. E-mail address: thitoshi@affrc.go.jp (H. Tamaki).
major component of biomass for the detrital food chain (Ward et al., 1984). Eelgrass around the world is declining as a result of human impacts in coastal and estuarine environments (Tokuda et al., 1987; Short and Wyllie-Echeverria, 1996). Losses of eelgrass habitat in Chesapeake Bay, USA, have resulted primarily from the deterioration of water quality linked to upland development, agriculture, and shoreline constructions (Orth and Moore, 1983; Dennison et al., 1993). In Seto Inland Sea, Japan, there were more than 23,600 ha of eelgrass meadows in 1960, whereas it decreased down to 6409 ha in 1981 due to water pollution and/or coastal developments (Okaichi et al., 1996). Thus, many transplanting projects have been attempted in order to restore eelgrass meadows either as
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mitigation for loss from development or as enhancement of fishery production (Bosworth and Short, 1993; Terawaki et al., 2001). However, many of them failed to maintain sustainable habitats, i.e. transplanted habitats disappeared or declined within a few years (Moore et al., 1996; Dan et al., 1998). It is likely that the environmental conditions in the transplanted sites were not favorable for the survival of eelgrass. Tidal and wave actions and the availability of photosynthetically active radiation (PAR) are known to regulate distribution and survival of eelgrass meadows (Maruyama et al., 1987; Duarte, 1991; Zimmerman et al., 1995). However, there are many areas where eelgrass declined even with enough light underwater and little disturbances by waves in Seto Inland Sea. The objective of this study is to elucidate factors responsible for the deterioration of eelgrass meadows in these areas. We carried out a comparative study on water quality, bottom sediments, sedimentation on eelgrass leaves, flow regime, and survival of transplanted eelgrass, in the center, edge, and at the outside of a eelgrass meadow located in a eutrophic coastal zone in northern Hiroshima Bay, Seto Inland Sea, Japan.
2. Methods 2.1. Study site Study sites are located in a eutrophic and shallow zone near Ohno in northern Hiroshima Bay (34°16.20 N; 132°15.60 E) (Fig. 1). There is a healthy eelgrass meadow and also an adjacent unvegetated area off the meadow in spite of enough PAR and little disturbance by waves and tidal currents. We selected stations in the center (St. 1 and St. 2), at the edge (St. 3) and the outside of the eelgrass meadow (St. 4). Stations 2, 3 and 4 are located
with intervals of 30, 60 and 90 m away from St. 1 to the east. Water depths in all the stations were the same, i.e. approximately 1.0 m relative to mean low water. 2.2. Transplanting experiments Transplanting was carried out in December 1997 to confirm the environmental condition to limit the distribution of eelgrass in St. 3 and St. 4. Eelgrass was harvested from St. 1, and was washed to remove sediments and epiphytes on leaves and trimmed to leave only three nodes of rhizome. Five shoots were transferred into a pot filled with sediments at St. 1 within 24 h. All pots were cultivated in an aquarium for one month to stabilize rhizome in the soil. Two pots (approximately 10 shoots) were deployed to St. 1, St. 3 and St. 4 and were monitored for shoot density almost every month. 2.3. Deposition of suspended sediments on leaves The amount of suspended solids deposited on eelgrass leaves was determined by using plants collected at St. 1 in October 1997 and September 1998. The plants were washed to remove attached sediments and epiphytes on leaves before the experiment and three each of them were deployed to St. 1, St. 3 and St. 4. Deposition was defined as the amount of accumulated sediments on eelgrass leaves in five days. Deposited sediments were collected with shoots using plastic bags. In the laboratory, sediments were washed away from the shoots in the bag, and the suspensions of the accumulated sediments were filtered through a glass fiber filter (pore size ¼ 1:2 lm, Whatman GF/C) to determine suspended solid concentration. The area of leaves for each sample was calculated by measuring the leaf length and width. After subtracting suspended solids concentration in
Fig. 1. Location of the study sites on the coast of Ohno in northern Hiroshima Bay, Seto Inland Sea, Japan.
H. Tamaki et al. / Marine Pollution Bulletin 44 (2002) 1253–1258
water column, the amount of deposition per surface area of leaves was calculated. 2.4. Shading by deposited sediments The effects of shading by deposited sediments on photosynthetic activity were estimated using suspended solids around the meadow (Kawasaki and Yamada, 1991). Suspended solids for the shading experiment were collected using a sediment trap (length ¼ 13 cm diameter ¼ 3 cm) at each station in February 1998, and sieved to capture particles less than 0.075 mm (Nakano et al., 1995). The shading of solar irradiation by sediments was determined by the decrease in photosynthetic photon flux density (PPFD) using an underwater 2p, cosinecorrected sensor (LI-192SB, LI-COR, Inc.) placed at the bottom of a cylinder (length ¼ 13 cm diameter ¼ 25 cm). Suspended solids were placed into a cylinder and allowed for deposition for 24 h, and the decrease in the photon flux was determined.
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wide and 1.0 m high. The flow velocity can be controlled from 0 to 55 cm s1 . Eelgrass was transplanted into the simulator. Sediment was collected at St. 4 and sieved to capture particles less than 0.075 mm and placed on the leaves. The amount of suspended solids on leaves per shoot was approximately 80 mg. The minimum velocity to remove the deposition was observed by changing flow rates. 2.7. Flow regime inside and outside of the eelgrass meadow Field survey on the flow rates at St. 1 and St. 4 were conducted at neap tide (11 November, 1998) and spring tide (27 November, 1997 and 16 December, 1998). Flow rates were determined at 30 cm above the bottom, a half height of average eelgrass, using an acoustic Doppler velocity meter (ADV––filed, Nortek, Inc., Norway). 3. Result and discussion
2.5. Characterization of bottom sediments
3.1. Survival of transplanted eelgrass
Bottom sediments were characterized by particle size distribution and oxidation–reduction potential (ORP). Three samples were collected at each station with an interval of 1 m. In order to determine particle size distribution of the bottom sediments, sediment sample collected from the surface (0–2 cm depth) was washed and desalinated by distilled water. Organic matter in the sample was decomposed by 30% hydrogen peroxide (H2 O2 ) (Lee et al., 1997). After drying at 100 °C, the soil sample was separated into pebble (>2.0 mm), coarse sand (0.425–2.0 mm), fine sand (0.075–0.425 mm) and silt (<0.075 mm) using sieves (Nakano et al., 1995). The particle size distribution was expressed by the / unit following to the Wentworth size classification (Sorensen, 1997):
Fig. 2 shows percent survival of the transplants from December 1997 to August 1998. The percent survival at St.1 increased to 210% in April. After some losses during the summer, the percent survival at St.1 was 100% in August. However, the percent survival at St. 3 and St. 4 decreased down 0% and 70%, respectively in February. Although the percent survival at St. 4 increased to 160% in April as well as at St. 1, it decreased again down to 20% by August. The distinct difference in the survival among stations suggests significant differences in environmental conditions.
/ ¼ log2 d where d ¼ particle diameter (mm). Intact sediment samples for the determination of vertical distribution of ORP were collected using a polycarbonate core with 3.0 cm in diameter and 10 cm in length. Vertical distribution of ORP was determined immediately after the sampling by ORP electrode (Yokogawa Model pH82).
3.2. Shading by deposited sediments Fig. 3 shows the amount of sediment deposited on leaves and suspended solid concentration in water columns. Although suspended solid concentrations in the water columns were almost the same, the amount of the deposition on leaves were significantly different among St. 1, St. 3 and St. 4; i.e. 0.41 mg cm2 , 1.20 mg cm2 and 2.27 mg cm2 , respectively in five days. The
2.6. Effect of flow rates on the removal of suspended solids from leaves A coastal flow simulator (JSES-500 Japan Aquatic) with flow control was used to study the effect of flow rates on the removal of suspended solids from eelgrass leaves. The length of the simulator was 1.0 m with 0.5 m
Fig. 2. Percent survival of Z. marina L. at transplant stations.
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Fig. 3. The amount of deposition on eelgrass leaves in the center (St. 1), at the edge (St. 3), and the outside of the eelgrass meadow (St. 4). Bars are maximum and minimum values.
Fig. 5. Cumulative distributions of particle size of the bottom sediments in the center (St. 1 and St. 2), at the edge (St. 3), and outside of the eelgrass meadow (St. 4). / ¼ log2 d (d: particle size, mm).
Fig. 4. Relationship between the amount of sediment deposition and percentages of PPFD reduction for suspended solids from Hiroshima Bay.
deposition on leaves at St. 4 was five times higher than that at St. 1. Fig. 4 shows the decrease in PPFD with the increase in the amount of deposited suspended solids. The deposition of suspended solids equivalent with those on leaves at St. 1 and St. 4 led to reduce PPFD to 84% and 36% of that without any deposition, respectively. PPFD at the bottom in our study stations was around 500 lE s1 m2 . PPFD for eelgrass leaves in St. 1 and St. 4 were estimated to reduce down to 420 lE s1 m2 and 180 lE s1 m2 , respectively by the deposition. PPFD required to saturate photosynthesis of eelgrass ranges from 100 lE s1 m2 to 300 lE s1 m2 (Dennison and Alberte, 1985; Zimmerman et al., 1995; Koch and Beer, 1996). The deposition at St. 4 seems to be enough to inhibit photosynthesis. 3.3. Characterization of bottom sediments The particle size distribution and vertical distributions of ORP of the bottom sediments are shown in Figs. 5 and 6, respectively. Fine sand and silt contents (/ ¼ 3–4) at St. 4 were higher than those at St. 1 and St. 2 (Fig. 5). The sediment at St. 1 was oxic from the surface to 2 cm deep, whereas those at St. 3 and St. 4
Fig. 6. Vertical distributions of ORP in the center (St. 1), at the edge (St. 3), and outside of the eelgrass meadow (St. 4). Bars are maximum and minimum values.
were reductive almost from the surface (Fig. 6). It is clear that there are significant differences in the particle size distribution of the sediment and vertical profiles of ORP between the center and the outside of eelgrass meadow. These facts suggest that growth and survival of eelgrass was negatively affected by the reductive conditions (Goodman et al., 1995; Holmer and Bondgaard, 2001) and the presence of high silt condition (Kawabata, 1993). High silt content in the bottom sediments led to a decrease in the pore water exchange with the overlying water column (Huettel and Rusch, 2000). This may result in the increase in the phytotoxins such as sulfide in the sediments (Koch, 2001). Thus, these sediment characteristics at the outside of the meadow seemed to be factors responsible for the deterionation of eelgrass meadows. 3.4. Effect of the flow regime on suspended solid deposition The removal of deposited sediments was observed at the flow rates more than 8 cm s1 in the simulator.
H. Tamaki et al. / Marine Pollution Bulletin 44 (2002) 1253–1258
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trophic coastal zone with small disturbance by wave and tidal currents in Hiroshima Bay. Specific conclusions derived from this study are as follows:
Fig. 7. Percent occurrences of flow rates above 8 cm s1 at a neap tide in St. 1 and St. 4. ( ) water level at flood and ebb tides.
(1) The sediment characteristics seemed to be a factor responsible for the deterioration of eelgrass meadows. (2) Although suspended solid concentrations in the water columns were almost the same, the amount of sediment deposited on eelgrass leaves at the outside was higher than that in the center of the meadow. The deposition at the outside seems to be enough to inhibit photosynthesis; i.e. PPFD available for eelgrass was only 36% of that without any deposition. The deposition in the center, however, was small enough to allow 84% of the original PPFD. (3) Flow rates determined at 30 cm above the bottom suggested that the rate at the outside was not enough to remove deposited sediments from the surface of eelgrass leaves. Thus, the large amount of sediment deposition caused by water pollution and/or eutrophication seemed to be another limiting factor for the growth of eelgrass.
Acknowledgements Fig. 8. Percent occurrences of flow rates above 8 cm s1 at a spring tide in St. 1 and St. 4. ( ) water level at flood and ebb tides, and left value was measured in November, 1997 and right value in December, 1998.
Therefore, the minimum velocity needed to remove the sediment deposited on leaves was estimated to be around 8 cm s1 . In order to determine the effect of flow rates on the removal of suspended solids from leaves, percent occurrences of flow rates above 8 cm s1 were estimated at St. 1 and St. 4 as shown in Figs. 7 and 8, respectively. At a neap tide, flow rates never exceed 8 cm s1 both at St. 1 and St. 4 (Fig. 7). At a spring tide, however, flow rates above 8 cm s1 were noted more frequently at St. 1 than at St. 4 (Fig. 8). Percent occurrences of flow rates above 8 cm s1 at St. 4 were ca. 50% or less of those at St. 1. It is most probable, therefore, that the flow rates at the outside of the eelgrass meadow were not high enough to remove deposited sediments from the surface of eelgrass leaves. St. 4 is located in the bay, while St. 1 is located on the coast directly facing with Itsukusima Island (Fig. 1). The difference in topography at St. 4 may be responsible for the low flow rates.
4. Conclusions The purpose of this study was to clarify the factors responsible for the loss of eelgrass meadows in a eu-
We especially thank J.G. Lee, Yoshihisa Mae and Tomohiro Kose for the field assistance. The work would not have been possible without the assistance of Syogo Arai. We also thank the Fisheries Research Agency, National Research Institute of Fisheries and Environment of Inland Sea for their support and opportunities to do research at the facilities. This research was supported by the fisherman’s association at Ohno in Hiroshima Prefecture.
References Bosworth, W., Short, F.T., 1993. Port of New Hampshire Mitigation Plan. For the New Hampshire Commercial Marine Terminal Development Project in Portsmouth, New Hampshire, p. 98. Dan, A., Izumi, Y., Mori, K., Hirosawa, A., Terawaki, T., 1998. Flourishing of Zostera marina L. by artificial seeding on sediment stabilizer mat. Fish Eng. 35 (1), 37–42. Dennison, W.C., Alberte, R.S., 1985. Role of daily light period in the depth distribution of Zostera marina (eelgrass). Mar. Eco. Pro. Ser. 25, 51–61. Dennison, W.C., Orth, R.J., Moore, K.A., Stevenson, J.C., Carter, V., Kollar, S., Bergstorm, P.W., Batiuk, R.A., 1993. Assessing water quality with submerged aquatic vegetation. Bio. Sci. 43, 86–94. Duarte, C.M., 1991. Seagrass depth limits. Aquat. Bot. 40, 363–377. Goodman, J.L., Moore, K.A., Dennison, W.C., 1995. Photosynthetic responses of eelgrass (Zostera marina L.) to light and sediment sulfide in a shallow barrier island lagoon. Aquat. Bot. 50, 37–47. Heck Jr., K.L., Able, K.W., Roman, C.T., Fahay, M.P., 1995. Composition, abandance, biomass and production of macrofauna
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in a New England estuary: comparisons among eelgrass meadows and other nursery habitats. Estuaries 18 (2), 379–389. Holmer, M., Bondgaard, E.J., 2001. Photosynthetic and growth response of eelgrass to low oxygen and high sulfide concentrations during hypoxic events. Aquat. Bot. 70, 29–38. Huettel, M., Rusch, A., 2000. Transport of phytoplankton in permeable sediment. Limnol. Oceanogr. 45, 534–549. Kawabata, Y., 1993. The study on restoration eelgrass (Zostera marina L.) habitat. Ph.D. Dissertation. University of Nagasaki, Japan, p. 181. Kawasaki, Y., Yamada, S., 1991. Development of marine macrophyte bed creation on sandy sea beds. V. Critical values of irradiance and mud density for the growth of gametophytes and juvenile sporophytes of Eisenia bicyclis, Ecklonia cava, and Ec. Kurome. J. Cent. Res. Inst. Electr. Ind., 12–22. Koch, E.W., Beer, S., 1996. Tides, light and the distribution of Zostera marina in Long Island Sound, USA. Aquat. Bot. 53, 97–107. Koch, E.W., 2001. Beyond light: physical, geological, and geochemical parameters as possible submersed aquatic vegetation habitat requirements. Estuaries 24, 1–17. Lee, J.G., Nishijima, W., Mukai, T., Takimoto, K., Seiki, T., Hiraoka, K., Okada, M., 1997. Comparison for structure and functions of organic matter degradation at natural and constructed tidal flats–– a case study in Hiroshima Bay. J. Soc. Water Environ. Jpn. 20 (3), 175–184. Maruyama, T., Igarashi, Y., Ishikawa, Y., 1987. The suitable eelgrass meadow creation: the limit of change in the sea bottom level. Proc. Coast. Eng. 34, 227–231. Moore, K.A., Neckles, H.A., Orth, R.J., 1996. Zostera marina (eelgrass) growth and survival along a gradient of nutrients and turbidity in the lower Chesapeak Bay. Mar. Ecol. Prog. Ser. 142, 247–259. Nakano, M., Miyazaki, T., Shinozawa, S., Nishimura, T., 1995. In: Physical and Environmental Analysis of Soil. University of Tokyo Press, Tokyo, pp. 64–74.
Ohno, M. et al., 1996. In: The resource of Seaweeds on 21Century. Midori Press, Tokyo, pp. 17–30. Okaichi, T. et al., 1996. In: The Resource of Biomass and Environment in Seto Inland Sea. Kouseisya-Kouseikaku, Tokyo, p. 75. Orth, R.J., Moore, K.A., 1983. Chesapeake Bay: an unprecedented decline in submerged aquatic vegetation. Science 222, 51–53. Orth, R.J., Heck Jr., K.L., vans Montfrans, J., 1993. Faunal communities in seagrass beds: a review of the influence of plant structure and prey characteristics on predator–prey relationships. Estuaries 7 (4A), 339–350. Short, F.T., Mathieson, A.C., Nelson, J.I., 1984. The seagrass filter: purification of estuarine and coastal waters. In: Kennedy, V.S. (Ed.), The Estuarine as a Filter. Academic Press, London, pp. 395– 413. Short, F.T., Wyllie-Echeverria, S., 1996. Natural and human induced disturbance seagrass. Environ. Conserv. 23. Sorensen, R.M., 1997. In: Basic Coastal Engineering. Chapman & Hall, USA, pp. 234–238. Terawaki, T., Hasegawa, H., Arai, S., Ohno, M., 2001. Managementfree techniques for restoration of Eisenia and Ecklonia beds along the central Pacific coast of Japan. J. Appl. Phycol. 13, 13– 17. Thayer, G.W., Kenworth, W.J., Fonseca, M.S., 1984. The ecology of eelgrass meadow of Atlantic coast: a community profile. US Fish and Wildlife Service, FWS/OBS-84/24, p. 85. Tokuda, H. et al., 1987. The lecture of aquaculture: 10, The study of resource of seaweed and aquaculture. Midori Press, Tokyo. Ward, L.G., Kemp, W.M., Boyton, W.R., 1984. The influence of waves and seagrass communities on suspended particulates in an estuarine embayment. Mar. Geol. 59, 85–103. Zimmerman, R.C., Reguzzoni, J.L., Alberte, R.S., 1995. Eelgrass (Zostera marina L.) transplants in San Francisco Bay: role of light availability on metabolism, growth and survival. Aquat. Bot. 51, 67–86.
Deterioration of eelgrass, Zostera marina L., meadows by water pollution in Seto Inland Sea, Japan Hitoshi Tamaki
a,*
, Makoto Tokuoka b, Wataru Nishijima a, Toshinobu Terawaki c, Mitsumasa Okada a
a
c
Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higahi-Hiroshima 739-8527, Japan b Mikuniya Corporation, 4-4-7 Miyuki Ujina, Minami, Hiroshima, Japan Fisheries Research Agency, National Research Institute of Fisheries and Environment of Inland Sea, 2-17-5 Maruishi, Ohno, Saeki, Hiroshima 739-0452, Japan
Abstract Survival of transplanted Zostera marina L. (eelgrass) and environmental conditions (water quality, bottom sediments, sedimentation on leaves and flow regime) were studied concurrently in the center, edge, and at the outside of a eelgrass meadow located in a eutrophic coastal zone in northern Hiroshima Bay, Seto Inland Sea, Japan. Eelgrass transplants at the outside of the meadow declined significantly, whereas those at the center were consistently well established. Silt content in the bottom sediments at the outside was higher than that at the center. The sediment was oxic from the surface to 2 cm deep at the center, whereas those at the edge and the outside were reductive almost from the surface. The sediment characteristics typical in eutrophic water seemed to be a factor responsible for the deterioration of eelgrass meadows. Although suspended solid concentrations in the water columns were almost the same, the amount of sediments deposited on leaves of eelgrass at the outside was higher than that at the center of the meadow. The amount of the deposition at the outside seems to be enough to inhibit photosynthesis; i.e. photosynthetic photon flux density (PPFD) available for eelgrass was only 36% of that without any deposition. The deposition in the center, however, was small enough to allow 84% of the original PPFD. Flow rates, determined at 30 cm above the bottom, a half height of average eelgrass, suggested that the rate at the outside was not enough to remove deposited sediments from the surface of eelgrass leaves. Thus, the large amount of sediment deposition caused by water pollution and/or eutrophication seemed to be another factor to inhibit the survival of eelgrass at the outside edge of the meadow. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Sediment deposition on eelgrass leaves; Flow velocity; Transplant; Survival; Eelgrass; Zostera marina L.
1. Introduction Eelgrass (Zostera marina L.) is a prolific temperate seagrass that grows in shallow coastal areas. Eelgrass meadows are highly productive components of estuaries and coastal ecosystems and support diverse faunal assemblages (Orth et al., 1993; Thayer et al., 1984; Heck et al., 1995). They are excellent habitats for many commercial fishes and are nurseries for juvenile fishes (Ohno et al., 1996). Eelgrass filters and retains nutrients from water column (Short et al., 1984) and provides a
*
Corresponding author. Tel./fax: +81-824-24-7626. E-mail address: thitoshi@affrc.go.jp (H. Tamaki).
major component of biomass for the detrital food chain (Ward et al., 1984). Eelgrass around the world is declining as a result of human impacts in coastal and estuarine environments (Tokuda et al., 1987; Short and Wyllie-Echeverria, 1996). Losses of eelgrass habitat in Chesapeake Bay, USA, have resulted primarily from the deterioration of water quality linked to upland development, agriculture, and shoreline constructions (Orth and Moore, 1983; Dennison et al., 1993). In Seto Inland Sea, Japan, there were more than 23,600 ha of eelgrass meadows in 1960, whereas it decreased down to 6409 ha in 1981 due to water pollution and/or coastal developments (Okaichi et al., 1996). Thus, many transplanting projects have been attempted in order to restore eelgrass meadows either as
0025-326X/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 3 2 6 X ( 0 2 ) 0 0 2 1 8 - 7
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H. Tamaki et al. / Marine Pollution Bulletin 44 (2002) 1253–1258
mitigation for loss from development or as enhancement of fishery production (Bosworth and Short, 1993; Terawaki et al., 2001). However, many of them failed to maintain sustainable habitats, i.e. transplanted habitats disappeared or declined within a few years (Moore et al., 1996; Dan et al., 1998). It is likely that the environmental conditions in the transplanted sites were not favorable for the survival of eelgrass. Tidal and wave actions and the availability of photosynthetically active radiation (PAR) are known to regulate distribution and survival of eelgrass meadows (Maruyama et al., 1987; Duarte, 1991; Zimmerman et al., 1995). However, there are many areas where eelgrass declined even with enough light underwater and little disturbances by waves in Seto Inland Sea. The objective of this study is to elucidate factors responsible for the deterioration of eelgrass meadows in these areas. We carried out a comparative study on water quality, bottom sediments, sedimentation on eelgrass leaves, flow regime, and survival of transplanted eelgrass, in the center, edge, and at the outside of a eelgrass meadow located in a eutrophic coastal zone in northern Hiroshima Bay, Seto Inland Sea, Japan.
2. Methods 2.1. Study site Study sites are located in a eutrophic and shallow zone near Ohno in northern Hiroshima Bay (34°16.20 N; 132°15.60 E) (Fig. 1). There is a healthy eelgrass meadow and also an adjacent unvegetated area off the meadow in spite of enough PAR and little disturbance by waves and tidal currents. We selected stations in the center (St. 1 and St. 2), at the edge (St. 3) and the outside of the eelgrass meadow (St. 4). Stations 2, 3 and 4 are located
with intervals of 30, 60 and 90 m away from St. 1 to the east. Water depths in all the stations were the same, i.e. approximately 1.0 m relative to mean low water. 2.2. Transplanting experiments Transplanting was carried out in December 1997 to confirm the environmental condition to limit the distribution of eelgrass in St. 3 and St. 4. Eelgrass was harvested from St. 1, and was washed to remove sediments and epiphytes on leaves and trimmed to leave only three nodes of rhizome. Five shoots were transferred into a pot filled with sediments at St. 1 within 24 h. All pots were cultivated in an aquarium for one month to stabilize rhizome in the soil. Two pots (approximately 10 shoots) were deployed to St. 1, St. 3 and St. 4 and were monitored for shoot density almost every month. 2.3. Deposition of suspended sediments on leaves The amount of suspended solids deposited on eelgrass leaves was determined by using plants collected at St. 1 in October 1997 and September 1998. The plants were washed to remove attached sediments and epiphytes on leaves before the experiment and three each of them were deployed to St. 1, St. 3 and St. 4. Deposition was defined as the amount of accumulated sediments on eelgrass leaves in five days. Deposited sediments were collected with shoots using plastic bags. In the laboratory, sediments were washed away from the shoots in the bag, and the suspensions of the accumulated sediments were filtered through a glass fiber filter (pore size ¼ 1:2 lm, Whatman GF/C) to determine suspended solid concentration. The area of leaves for each sample was calculated by measuring the leaf length and width. After subtracting suspended solids concentration in
Fig. 1. Location of the study sites on the coast of Ohno in northern Hiroshima Bay, Seto Inland Sea, Japan.
H. Tamaki et al. / Marine Pollution Bulletin 44 (2002) 1253–1258
water column, the amount of deposition per surface area of leaves was calculated. 2.4. Shading by deposited sediments The effects of shading by deposited sediments on photosynthetic activity were estimated using suspended solids around the meadow (Kawasaki and Yamada, 1991). Suspended solids for the shading experiment were collected using a sediment trap (length ¼ 13 cm diameter ¼ 3 cm) at each station in February 1998, and sieved to capture particles less than 0.075 mm (Nakano et al., 1995). The shading of solar irradiation by sediments was determined by the decrease in photosynthetic photon flux density (PPFD) using an underwater 2p, cosinecorrected sensor (LI-192SB, LI-COR, Inc.) placed at the bottom of a cylinder (length ¼ 13 cm diameter ¼ 25 cm). Suspended solids were placed into a cylinder and allowed for deposition for 24 h, and the decrease in the photon flux was determined.
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wide and 1.0 m high. The flow velocity can be controlled from 0 to 55 cm s1 . Eelgrass was transplanted into the simulator. Sediment was collected at St. 4 and sieved to capture particles less than 0.075 mm and placed on the leaves. The amount of suspended solids on leaves per shoot was approximately 80 mg. The minimum velocity to remove the deposition was observed by changing flow rates. 2.7. Flow regime inside and outside of the eelgrass meadow Field survey on the flow rates at St. 1 and St. 4 were conducted at neap tide (11 November, 1998) and spring tide (27 November, 1997 and 16 December, 1998). Flow rates were determined at 30 cm above the bottom, a half height of average eelgrass, using an acoustic Doppler velocity meter (ADV––filed, Nortek, Inc., Norway). 3. Result and discussion
2.5. Characterization of bottom sediments
3.1. Survival of transplanted eelgrass
Bottom sediments were characterized by particle size distribution and oxidation–reduction potential (ORP). Three samples were collected at each station with an interval of 1 m. In order to determine particle size distribution of the bottom sediments, sediment sample collected from the surface (0–2 cm depth) was washed and desalinated by distilled water. Organic matter in the sample was decomposed by 30% hydrogen peroxide (H2 O2 ) (Lee et al., 1997). After drying at 100 °C, the soil sample was separated into pebble (>2.0 mm), coarse sand (0.425–2.0 mm), fine sand (0.075–0.425 mm) and silt (<0.075 mm) using sieves (Nakano et al., 1995). The particle size distribution was expressed by the / unit following to the Wentworth size classification (Sorensen, 1997):
Fig. 2 shows percent survival of the transplants from December 1997 to August 1998. The percent survival at St.1 increased to 210% in April. After some losses during the summer, the percent survival at St.1 was 100% in August. However, the percent survival at St. 3 and St. 4 decreased down 0% and 70%, respectively in February. Although the percent survival at St. 4 increased to 160% in April as well as at St. 1, it decreased again down to 20% by August. The distinct difference in the survival among stations suggests significant differences in environmental conditions.
/ ¼ log2 d where d ¼ particle diameter (mm). Intact sediment samples for the determination of vertical distribution of ORP were collected using a polycarbonate core with 3.0 cm in diameter and 10 cm in length. Vertical distribution of ORP was determined immediately after the sampling by ORP electrode (Yokogawa Model pH82).
3.2. Shading by deposited sediments Fig. 3 shows the amount of sediment deposited on leaves and suspended solid concentration in water columns. Although suspended solid concentrations in the water columns were almost the same, the amount of the deposition on leaves were significantly different among St. 1, St. 3 and St. 4; i.e. 0.41 mg cm2 , 1.20 mg cm2 and 2.27 mg cm2 , respectively in five days. The
2.6. Effect of flow rates on the removal of suspended solids from leaves A coastal flow simulator (JSES-500 Japan Aquatic) with flow control was used to study the effect of flow rates on the removal of suspended solids from eelgrass leaves. The length of the simulator was 1.0 m with 0.5 m
Fig. 2. Percent survival of Z. marina L. at transplant stations.
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Fig. 3. The amount of deposition on eelgrass leaves in the center (St. 1), at the edge (St. 3), and the outside of the eelgrass meadow (St. 4). Bars are maximum and minimum values.
Fig. 5. Cumulative distributions of particle size of the bottom sediments in the center (St. 1 and St. 2), at the edge (St. 3), and outside of the eelgrass meadow (St. 4). / ¼ log2 d (d: particle size, mm).
Fig. 4. Relationship between the amount of sediment deposition and percentages of PPFD reduction for suspended solids from Hiroshima Bay.
deposition on leaves at St. 4 was five times higher than that at St. 1. Fig. 4 shows the decrease in PPFD with the increase in the amount of deposited suspended solids. The deposition of suspended solids equivalent with those on leaves at St. 1 and St. 4 led to reduce PPFD to 84% and 36% of that without any deposition, respectively. PPFD at the bottom in our study stations was around 500 lE s1 m2 . PPFD for eelgrass leaves in St. 1 and St. 4 were estimated to reduce down to 420 lE s1 m2 and 180 lE s1 m2 , respectively by the deposition. PPFD required to saturate photosynthesis of eelgrass ranges from 100 lE s1 m2 to 300 lE s1 m2 (Dennison and Alberte, 1985; Zimmerman et al., 1995; Koch and Beer, 1996). The deposition at St. 4 seems to be enough to inhibit photosynthesis. 3.3. Characterization of bottom sediments The particle size distribution and vertical distributions of ORP of the bottom sediments are shown in Figs. 5 and 6, respectively. Fine sand and silt contents (/ ¼ 3–4) at St. 4 were higher than those at St. 1 and St. 2 (Fig. 5). The sediment at St. 1 was oxic from the surface to 2 cm deep, whereas those at St. 3 and St. 4
Fig. 6. Vertical distributions of ORP in the center (St. 1), at the edge (St. 3), and outside of the eelgrass meadow (St. 4). Bars are maximum and minimum values.
were reductive almost from the surface (Fig. 6). It is clear that there are significant differences in the particle size distribution of the sediment and vertical profiles of ORP between the center and the outside of eelgrass meadow. These facts suggest that growth and survival of eelgrass was negatively affected by the reductive conditions (Goodman et al., 1995; Holmer and Bondgaard, 2001) and the presence of high silt condition (Kawabata, 1993). High silt content in the bottom sediments led to a decrease in the pore water exchange with the overlying water column (Huettel and Rusch, 2000). This may result in the increase in the phytotoxins such as sulfide in the sediments (Koch, 2001). Thus, these sediment characteristics at the outside of the meadow seemed to be factors responsible for the deterionation of eelgrass meadows. 3.4. Effect of the flow regime on suspended solid deposition The removal of deposited sediments was observed at the flow rates more than 8 cm s1 in the simulator.
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trophic coastal zone with small disturbance by wave and tidal currents in Hiroshima Bay. Specific conclusions derived from this study are as follows:
Fig. 7. Percent occurrences of flow rates above 8 cm s1 at a neap tide in St. 1 and St. 4. ( ) water level at flood and ebb tides.
(1) The sediment characteristics seemed to be a factor responsible for the deterioration of eelgrass meadows. (2) Although suspended solid concentrations in the water columns were almost the same, the amount of sediment deposited on eelgrass leaves at the outside was higher than that in the center of the meadow. The deposition at the outside seems to be enough to inhibit photosynthesis; i.e. PPFD available for eelgrass was only 36% of that without any deposition. The deposition in the center, however, was small enough to allow 84% of the original PPFD. (3) Flow rates determined at 30 cm above the bottom suggested that the rate at the outside was not enough to remove deposited sediments from the surface of eelgrass leaves. Thus, the large amount of sediment deposition caused by water pollution and/or eutrophication seemed to be another limiting factor for the growth of eelgrass.
Acknowledgements Fig. 8. Percent occurrences of flow rates above 8 cm s1 at a spring tide in St. 1 and St. 4. ( ) water level at flood and ebb tides, and left value was measured in November, 1997 and right value in December, 1998.
Therefore, the minimum velocity needed to remove the sediment deposited on leaves was estimated to be around 8 cm s1 . In order to determine the effect of flow rates on the removal of suspended solids from leaves, percent occurrences of flow rates above 8 cm s1 were estimated at St. 1 and St. 4 as shown in Figs. 7 and 8, respectively. At a neap tide, flow rates never exceed 8 cm s1 both at St. 1 and St. 4 (Fig. 7). At a spring tide, however, flow rates above 8 cm s1 were noted more frequently at St. 1 than at St. 4 (Fig. 8). Percent occurrences of flow rates above 8 cm s1 at St. 4 were ca. 50% or less of those at St. 1. It is most probable, therefore, that the flow rates at the outside of the eelgrass meadow were not high enough to remove deposited sediments from the surface of eelgrass leaves. St. 4 is located in the bay, while St. 1 is located on the coast directly facing with Itsukusima Island (Fig. 1). The difference in topography at St. 4 may be responsible for the low flow rates.
4. Conclusions The purpose of this study was to clarify the factors responsible for the loss of eelgrass meadows in a eu-
We especially thank J.G. Lee, Yoshihisa Mae and Tomohiro Kose for the field assistance. The work would not have been possible without the assistance of Syogo Arai. We also thank the Fisheries Research Agency, National Research Institute of Fisheries and Environment of Inland Sea for their support and opportunities to do research at the facilities. This research was supported by the fisherman’s association at Ohno in Hiroshima Prefecture.
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