Current status and ecological roles of Zostera marina after recovery from large-scale reclamation in the Nakdong River estuary, Korea
Introduction
Estuaries have high biodiversity and primary production due to their highly variable habitat types and environmental conditions (Day et al., 1989). Estuaries also play a pivotal role as nursery habitats for various commercial fishes and invertebrates, and act as nutrient and sediment sinks (Lirman et al., 2008). However, many estuarine ecosystems have been destroyed by coastal development; consequently, submerged aquatic vegetation has seriously declined (Lotze et al., 2006). The Nakdong River estuary, which has been protected by law since the 1960s, is a representative estuary in Korea, with 32 species of vascular hydrophytes, including four seagrass species (Kim et al., 2005). This estuary is of great value as seaweed and shellfish grounds and is a critical stopover site for migratory birds because of its location in the center of the East Asia–Australia flyway (Kim et al., 2005).
Before disturbance, meadows of Zostera marina, which is a dominant primary producer in this estuary, occupied about 13.6 km2 (Chung and Choi, 1985). However, large-scale reclamation for an industrial complex, dam construction, and port development during the late 1980s resulted in the disappearance of these large Z. marina meadows from this estuary (Kim et al., 2005). Worldwide, disturbance of seagrass habitats because of dredging or filling has increased since the 1980s (Short and Wyllie-Echeverria, 1996). Dredging decreases water clarity by increasing water turbidity, levels of total suspended solids, and algal blooms, and consequently leads to seagrass declines (Cambridge et al., 1986, Goldsborough and Kemp, 1988, Onuf, 1994).
Recently, Z. marina meadows have partially recovered and small meadows have been observed in the Nakdong River estuary. Although the biomass and coverage of currently existing Z. marina beds is dramatically lower than those in 1985 (Fig. 1; Table 1), the existing meadows seem persistent and have adjusted to the changed environmental conditions. The health of seagrasses is evaluated based on shoot morphology, productivity, density, and biomass (Pergent et al., 1995, Buia et al., 2004, Terrados et al., 2008). Gaeckle et al. (2006) estimated eelgrass leaf productivity by measuring sheath length and found a significant relationship between shoot morphology and plant metabolic status. Shoot density, biomass, and productivity of seagrass meadows have been compared with those of persistent meadows in similar geographical areas to assess the status of seagrass meadows (Agostini et al., 2003, Terrados et al., 2008).
Seagrass meadows are among the most productive plant communities, providing food and habitats for commercially and ecologically valuable marine organisms (Holmquist et al., 1989, Montague and Ley, 1993). Because of low direct grazing pressure from herbivores, a substantial fraction of seagrass carbon enters coastal and estuarine food webs through microbial transformation of litter and particulate detritus (Duarte and Cebrián, 1996, Cebrían and Duarte, 2001). Thus, organic matter produced by seagrasses is important for supporting commercial fishery production in coastal and estuarine ecosystems (Melville and Connolly, 2005).
Because seagrasses have high productivity, they require high nutrient incorporation and play an important role in nutrient cycling in coastal and estuarine ecosystems (Hemminga et al., 1991, Blackburn et al., 1994). In addition, seagrasses can take up inorganic nutrients through both leaf and root tissues (Stapel et al., 1996, Pedersen et al., 1997, Lee and Dunton, 1999). Nutrient uptake through leaf tissues from the water column significantly contributes to total nutrient acquisition (Lee and Dunton, 1999). Thus, seagrasses in estuarine areas might reduce levels of over-enriched nutrients caused by anthropogenic loading and consequently reduce cultural eutrophication in estuarine ecosystems. In the present study, we hypothesized that the existing Z. marina beds, which partially recovered from large-scale reclamation, are persisting and play significant ecological roles in this estuary. The current status and the ecological roles of the Z. marina meadows in this estuary were evaluated based on the measurements of plant growths and nutrient uptakes by Z. marina shoots.
Section snippets
Study area
The study site was located in the Nakdong River estuary on the southern coast of Korea (Fig. 1). This study was conducted in a typical Z. marina meadow of the estuary at an average water depth of about 1.5 m relative to the mean sea level. This estuary has a well-developed delta and is protected by sand dunes that parallel the coastline. Commercial shellfish (mainly oysters) are cultured intensively in the vicinity of seagrass beds. The estuary has undergone significant changes in environmental
Physical and chemical parameters
Water temperature showed a strong seasonal pattern, ranging from 5.1 °C in February to 26.9 °C in August (Fig. 2A). Average daily underwater irradiance exhibited significant (p < 0.001) temporal variation (Fig. 2B). Underwater irradiance was highest in February 2005 (25.0 mol photons m−2 d−1) and lowest in July 2005 (1.7 mol photons m−2 d−1). Salinity was relatively constant (ca. 30) throughout the experimental period, except during late summer when it decreased to 20 as a result of increased precipitation
The current status of Z. marina meadows
Zostera marina beds in the Nakdong River estuary were almost completely eliminated due to anthropogenic activities, mainly reclamation and dam construction, during the last 20 years (Kim et al., 2005). However, many small Z. marina beds have been observed in the estuary recently. Recovery of seagrass beds following destruction by anthropogenic disturbances has been reported in many geographical areas (Frederiksen et al., 2004, Orth et al., 2006). Shoot density of existing beds in this estuary
Acknowledgments
We thank J.I. Park, Y.K. Kim, T.H. Kim, W. Li, and S.H. Kim for field assistance and laboratory support. Two anonymous reviewers and an editor provided useful comments on earlier version of the manuscript. This work was supported by the Ministry of Environment (Eco-technopia 21 project #050010013) and the Ministry of Land, Transport and Maritime Affairs of Korea (Program of Greenhouse Gas Emissions Reduction Using Seaweeds).
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