Algae Volume 23(1): 83-90, 2008 Zostera marina L. Using a Digital Echosounder to Estimate Eelgrass (Zostera marina L.) Cover and Biomass in Kwangyang Bay Keunyong Kim, Ju-hyoung Kim and Kwang Young Kim* Department of Oceanography, Chonnam National University, Gwangju 500-757, Korea Eelgrass beds are very productive and provide nursery functions for a variety of fish and shellfish species. Management for the conservation of eelgrass beds along the Korean coasts is critical, and requires comprehensive strategies such as vegetation mapping. We suggest a mapping method to spatial distribution and quantify of eelgrass beds using a digital echosounder. Echosounding data were collected from the northeast part of Kwangyang Bay, on the south of Korea, in March, 2007. A transducer was attached to a boat equipped with a DGPS. The boat completed a transect survey scanning whole eelgrass beds of 11.7 km 2 with a speed of 1.5-2 m s -1 (3-4 knot). The acoustic reflectivity of eelgrass allowed for detection and explicit measurements of canopy cover and height. The results showed that eelgrass bed was distributed in depth from 1.19 to 3.6 m (below MSL) and total dry weight biomass of 4.1 ton with a vegetation area of 4.05 km 2. This technique was found to be an effective way to undertake the patch size and biomass of eelgrass over large areas as nondestructive sampling. Key Words: acoustic reflectivity, biomass, canopy cover, digital echosounder, eelgrass beds, Kwangyang Bay, vegetation mapping (Zostera marina L ) (Fortes 1986) (Duarte and Cebrián 1996) (Thayer et al 1984 Short and Neckles 1999) (Pergent-Martini et al 2006) 4% *Corresponding author (kykim@chonnam.ac.kr) 29% 11% (Duarte 1991 Dennison et al 1993) (Duarte 1995) (Livingston et al 1998) ( 2001 Kim and Choi 2004) 1970 1980 (Kim and Choi 2004) (submersed aquatic vegetation SAV)
84 Algae Vol. 23(1), 2008 (Dennis 1984 Madsen 1993) (Sabol et al 2002) (landscape) (Mumford et al 1995) 1 m s -1 (Norris et al 1997) (Sabol et al 2002 2006 Winfield et al 2007) GPS(global positioning system) (Sabol et al 2002 2006) (Stent and Hanley 1985 Carbo and Molero 1997 Guan et al 1999 Sabol et al 2002) (SAV) (Sabol et al 2002 Komatsu et al 2003 Erhan 2005 Haga et al 2007 Kevin et al 2007) (2006) (patch) (edge) Fig. 1. Map showing the study area and three eelgrass vegetations. Fig. 2. Processing and reporting for survey in hypothetical vegetated area. Circles indicate bottom area of ping, brown circle for bare and yellow for bare (Sabol et al. 2002). (fragmentation) (Kim and Choi 2004)
Kim et al.: Echosounder to Estimate Cover and Biomass 85 Fig. 3. Example of echogram obtained from Biosonics echosounder in an eelgrass bed. Pictures show bottom along track, ping by ping (left) and oscilloscope display showing voltage return for ping # 2400 only (right). Eelgrass shoots are green on soft bottom (yellow). 9 km 17 km 20-30 m (Zostera marina L ) 2007 3 25-26 ( 1) ( 2) ( 3) 3 (Fig 1) 1 2 1 3 (Kim and Choi 2004) DT-X Digital Echosounder (Biosonics Inc USA) 420 khz (Single-beam) 6 (pulse width) 0 1 ms (pings rate) 5 pings s -1 (transducer) (Biofin dead weight towing vehicle Biosonics USA) 0 5 m 1 5-2 m s -1 (3-4 knot) DGPS (Differential Global Positioning System) SAV (Mean Sea Level MSL) EcoSAV 1 0 (Biosonics USA) (canopy height) (cycle) 15 pings (Figs 2 3) (g DW m -2 ) 7 2 SCUBA 3 m 25 25 cm 2 m 3 6 m EcoSAV 15 pings 1 5 ping 2 m s -1 6 m 60 72 (g DW m -2 ) 3 11 7 km 2 1 (MSL) 1 19 2 42 m 60% 1 9 m (Fig 4A) 2 1 9 3 m 2 5 m (Fig 4B) 3 2 6 3 6 m 3 2 m (Fig 4C) 1 3 04 km 2 2 0 91 km 2 3 0 09 km 2
86 Algae Vol. 23(1), 2008 Fig. 4. Combined presentation of cover (below, in percent) and height (above, in meters) of eelgrass. Site 1 (A), Site 2(B) and Site 3(C). 1 ( ) ( ) 7 (Fig 4A) 2 3 (Fig 4B) 3 (Fig 4C) EcoSAV 1 60% 80 100% 3% 20 40% 1 37% 40-60% 23% 60 80% 6 4% (Fig 5A) 0 2 0 5 m (89%) 0 2 0 4 m 32 6% 0 6 0 7 m 1 6% (Fig 5B) 2 40% 20% 6% 20 40% 8% 40 60% 25% 60 80% 27% 80 100% 35% (Fig 5C) 2 0 3 0 7 m 90% 0 4 0 5 29 3% 0 7 0 8 m 0 9 1 0 m 1 7 % (Fig 5D) 3 20 40% 32 3% 20% 12 3% 40 60% 18 5% 60 80% 23 1% 80 100% 13 8% (Fig 5E) 3 0 3 0 8 m 87 1% 1 0 1 1 m 1 4% (Fig 5F) (db) -15 - -65 db EcoSAV 2 = 0 5072 + 38 054 (R 2 = 0 80) 5 (0 20 20 40 40 60 60 80 80 100%) 1 0 20% 30% 683 1 kg DW (Table 1) 20 40% 1064 8 kg DW 1
Kim et al.: Echosounder to Estimate Cover and Biomass 87 Fig. 5. The distributions of cover (left column) and height (right column) of eelgrass in Site 1(upper), Site 2 (mid) and Site 3 (lower). 1 2 9 (Table 1) 2 0 20% 920 8 m 2 39 7 kg DW (Table 1) 80 100% 5524 6 m 2 462 4 kg DW 2 1 1 3 20 40% 28 9 kg DW (Table 1) 0 20% 206 8 m 2 8 9 kg DW 3 0 1 (Pergent-Martini et al 2006)
88 Algae Vol. 23(1), 2008 Table 1. Area and biomass of eelgrass beds in each cover class in three sites of Kwangyang Bay derived from EcoSAV Site 1 Site 2 Site 3 Cover (%) class Area Biomass Area Biomass Area Biomass (m 2 ) (DW kg) (m 2 ) (DW kg) (m 2 ) (DW kg) 0-20 15840.0 683.1 920.8 39.7 206.8 8.9 20-40 19988.6 1064.8 1227.7 65.4 542.8 28.9 40-60 12068.6 765.3 3990.0 253.0 310.2 19.7 60-80 3394.3 249.7 4296.9 316.1 387.7 28.5 80-100 1508.6 126.3 5524.6 462.4 232.6 19.5 Sum of biomass (DW ton) 2.9 1.1 0.1 (DT-X Biosonics Inc USA) SAV(submerged aquatic vegetation) 60 g WW m -2 8 cm (Sabol et al 2002) 0 5 m (Sabol et al 2002) (Mohamed 2007) 0 1 m 153 7 g WW m -2 DT-X 4 05 km 2 4 1 1 2 60% 3 9 Kim and Choi(2004) 9 2002 1 12 (2004) 1 3 103 176 4 g DW m -2 2 1 60 0 838 5 g DW m -2 8 1 9 2000 ( 2000) 1 sand 1 4% silt 34 4% clay 64 2% 2 sand 2 4% silt 84 3% clay 13 3% 3 sand 7 7% silt 42 4% clay 49 9% 3 Kim and Choi (2004) (Lee et al 2005 Short et al 2007)
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