Changes in dominant species of seagrass bed off Iwakuni, Seto Inland Sea, Japan Kenji SUGIMOTO, Yorihide ASAOKA Toshinobu TERAWAKI and Mitsumasa OKADA
Background 1 Seagrass Highly productive components of estuarine and coastal ecosystems. Support large and diverse faunal assemblages. Excellent habitats for many commercial fishes. Japanese rockfish Seagrass grows in estuaries and shallow coastal areas.
Background 2 Zostera marina L. and Zostera japonica distributed off Iwakuni Zostera japonica Shallow From middle intertidal zone to subtidal zone Shoot height < Zostera marina L. Distribution From low intertidal zone to subtidal zone Deep
Dominant species of seagrass bed off Iwakuni Z. marina L. distributed before typhoon attacked in 24. Z. marina L. was disappeared by typhoon attacked in 24. It is appropriate reasonable in Z. marina L. not growing. Even if Z. marina L. grow temporarily, it disappears again after typhoon attacked. Z. japonica distributed in 27. Seagrass bed changed in Z. marina L. into Z japonica off Iwakuni.
What is seagrass bed recovered automatically? Seed sources of supply seed Bottom Level Underwater light quantity Sand movement Flow velocity Seed supply hydraulic conditions Seagrass bed Habitat suitability Sediment quality Water temperature Competition Salinity Particle distribution Nutrient Sediment hardness grazer Alga Benthos
Objective of this study To clarify the changes in dominant species of seagrass bed off Iwakuni, Seto Inland Sea, Japan. We monitored Z. marina L. and Z. japonica distribution range, shoot density, shoot height off two sites. We investigated water temperature, water irradiate off two sites. Sediment in the eelgrass bed was collected and particle diameter, silt and clay contents, chemical oxygen demand, total sulfide was analyzed. Z. marina L. and Z. japonica leaves was collected and chlorophyll a contents in the leaves was analyzed
Study site 1 Seto Inland Sea Japan
Study site 2 Seagrass bed(1997) Itsukushima Is. Nomijima Is. Iwakuni Hiroshima Bay Iwakuni Yashiro Is. Ozu
Study site 3 Ozu 45m D.L.+.5~-2.5m Water temperature Sediment quality Water irradiate Seagrass distribution Shoot density Shoot height We investigated from June 1997 to August 211.
Typhoon feature off Iwakuni (24-27) Typhoon name Songda Nabi Shanshan Usagi Date 24/9/8 25/9/5 26/9/18 27/8/2 Category 4 5 4 4 Max wind velocity (m s -1 ) (wind direction) 18.9 (S) 13.6 (E) 13.4 (S) 12.3 (E) Tidel level(cm) 385 22 334 42 Max river flow rate (m 3 s -1 ) 14 5398 455 29 SS after 3 days (mg L -1 ) 3.3 1.1 3.3 -
Vegetated area (m) Result1 Changes in Z. marina L. and Z. japonica vegetated area off Ozu 45 4 35 3 25 Nabi Shanshan 2 15 1 5 Zostera marina L. Zostera japonica Songda Usagi AJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJ 1997 1998 1999 2 21 22 23 24 25 26 27 28 29 21 211 Year Z. marina L. The vegetated area was narrow after 26. Z. japonica The vegetated area expanded after 27.
Z. marina L. shoot density(shoots m -2 ) Result2 Changes in Z. marina L. shoot density and shoot height off Ozu Z. marina L. shoot height (cm) 1 Shoot density Shoot height 2 8 Nabi 16 6 Songda Shanshan 12 4 Usagi 8 2 4 AJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJ 1997 1998 1999 2 21 22 23 24 25 26 27 28 29 21 211 Year Shoot density Before 24 > After 27 Shoot height Before 24 > After 24
Z. japonica shoot density (shoots m -2 ) Z. japonica shoot height (cm) Result3 Changes in Z. japonica. shoot density and shoot height off Ozu 4 3 Shoot density Shoot height 4 3 2 2 1 1 A J A O D F A J A 21 211 Year Shoot density 21 < 211 Shoot height 21 = 211
particle diameter (mm) Silt and clay contents(%).4 Result4 Changes in particle diameter and silt and clay contents off Ozu. Songda 16.3 Nabi Shanshan 12.2 8.1. Particle diameter Silt and clay contents Usagi J D J D J D J D J D J D J D J D J D J D J D J D J D J D 1997 1998 1999 2 21 22 23 24 25 26 27 28 29 21 211 Year 4 Before Songda From Songda to 28 From 29 to 211 = = < Particle diameter (mm).28±.3.29±.3.32±.2 Silt and clay contents (%) 1.7±2.6 > 6.4±1.8 9.2±2.3
Chemical oxygen demand (mg g -1 ) Total sulfide(mg g -1 ) Result5 6 5 4 3 2 1 Changes in COD and T-S off Ozu..12 COD Sulfide.1 Nabi Songda Shanshan.8.6.4.2 Usagi J D J D J D J D J D J D J D J D J D J D J D J D J D J D 1997 1998 1999 2 21 22 23 24 25 26 27 28 29 21 211 Year. Before Songda From Songda to 28 From 29 to 211 COD (mg g -1 ) 3.5±.9 > 2.2±.6 2.9±.8 T-S (mg g -1 ).61±.38.19±.11.24±.15
Water temperature ( ) Light attenuation coefficients (m -1 ) 3 Result5 Changes in WT and LAC off Ozu. Songda Nabi Shanshan 1.2 25 1. 2.8 15.6 1.4 5 Water temperature Light attenuation coefficients Usagi J D J D J D J D J D J D J D J D J D J D J D J D J D J D J 1997 1998 1999 2 21 22 23 24 25 26 27 28 29 21 211 Year.2. Before Songda From Songda to 28 From 29 to 21 Water tempereture( ) 18.3±.7 18.6±.1 18.8±.1 Light attenuation = = = = coefficients (m -1 ).45±.5.45±.6.51±.8
Water irradiate(μe m -2 s -1 ) Result6 Changes in water irradiate off Ozu. Saturated = 3.314*temperature + 1.5 Compensated =.9712*EXP(.188*temperature) 2 Under water Saturated Compensated Marsh et.al, (1986) 15 Songda Nabi Shanshan 1 5 Usagi J D J D J D J D J D J D J D J D J D J D J D J D J D J D J 1997 1998 1999 2 21 22 23 24 25 26 27 28 29 21 211 Year Before Songda From Songda to 28 From 29 to 211 Saturated > Under water (%) 68 61 65 Compensated > Under water (%)
Solar radiation(mj m -2 ) Water irradiate (μe m -2 day -1 ) 35 3 25 2 15 1 5 Result7 Changes in solar radiation in Hiroshima and water irradiate off Ozu from 29 to 211. 25 2 15 1 5 J MM J S N J MM J S N J MM J 29 21 211 Year 29 21 211 17.2 = 17.8 16.6 = Average From May to July J MM J S N J MM J S N J MM J 29 21 Year 211 29 21 211 5.6 6.4 7.4 < <
Result8 Changes in salinity off Ozu. 35 3 Salinity (PSU) 25 2 15 1 Songda Nabi Shanshan Usagi 5 AADAADAADAADAADAADAADAADAADAADAADAADAADA 1998 1999 2 21 22 23 24 25 26 27 28 29 21 211 Year Before Songda From Songda to 28 From 29 to 211 Salinity(PSU) 31.5±1.2 31.1±2.3 3.2±2.6 = =
Study site 4 Iwakuni Ozu 14 km Kojiro D.L.+1.~-4.5m Seagrass distribution Shoot density Shoot height Water temperature Water irradiate We investigated from April to August in 211.
Vegetated area (m) Z. marina L. shoot density (shoots m -2 ) Result9 Changes in vegetated area of segrass off Kojiro and Z. marina L. shoot density off Ozu and Kojiro. 15 1 4 3 2 Kojiro Ozu 5 Zostera marina L. Zostera japonica Dec Feb Apr Jun Aug 21 211 Year 1 Dec Feb Apr Jun Aug 21 211 Year
Distributed seagrass off Ozu and Kojiro Ozu Shallow Before Songda Shallow Deep August 211 Deep Shallow August 211 Deep Kojiro Z. marina L. Z. japonica
Water irradiate(μe m -2 day -1 ) Water tempereture ( ) 25 2 Result1 Changes in water irradiate and water temperature off Ozu and Kojiro. 3 Ozu Kojiro Ozu Kojiro 15 1 2 5 Apr May Jun Jul 211 Year Ozu < Kojiro 7.4 8.9 1 Average From May to July in 211 Apr May Jun Jul Aug 211 Year Ozu > Kojiro 2.3 18.7
1: 11: 12: 13: 14: 15: 16: 17: 18: 19: 2: 21: 22: 23: : 1: 2: 3: 4: 5: 6: 7: 8: 9: 1: 11: 12: Velocity (cm s -1 ) Direction ( ) Result11 Changes in velocity and direction of sea bottom off Ozu and Kojiro. Velocity at Ozu Velocity at Kojiro 6 Direction at Ozu Direction at Kojiro 36 5 4 27 3 18 2 1 9 16 17 August 211 Ozu < Kojiro Velocity (cm s -1 ) 1.2 1.8
Chlorophyll a contents(μg g -1 cm -2 ) Result12 Z. marina L. and Z. japonica chlorophyll a contents off Ozu and Kojiro. 7 6 5 4 3 2 1 Ozu Z marina L. Kojiro Z marina L. Ozu=Kojiro (Z. marina L.) Ozu Z japonica Z. marina L < Z. japonica (about 8 times)
Discussion1 Photosynthesis conditions off Ozu and Kojiro Temperature : Ozu > Kojiro Water irradiate : Ozu < Kojiro Z. marina L. Chlorophyll a contents : Ozu = Kojiro Chlorophyll a contents : Z. marina L. < Z. japonica Photosynthesis conditions off Ozu Temperature : Almost all year was same Water irradiate : 28 < 29 < 21 Z. japonica vegetated area : Before 26 < 27-28 < 29 < 21-211 Z. japonica shoot density : 21 < 211 Ozu < Kojiro The light condition for Z. marina L. off Ozu was improved because Z. japonica increased from year to year.
Discussion2 Sea bottom was resuspended by wave or flow. Before Songda Typhoon attacked 25-27 Z. japonica will be shaded by Z. marina L. Light condition was improved Z.japonica inhibited resuspension. Z. marina L. will increase Z. japonica will disappear Z. marina L. distributed Z. japonica increased : Z. marina L. : Z. japonica
Conclusions1 1) Z. marina L. off Ozu disappeared after typhoon attacks in 24-27, Vegetated area and shoot density of Z. marina L. decreased. However the vegetated area of Z. japonica increased after 28. 2) After Z. japonica off Ozu appeared in 28, it expanded distribution range which was that of Z. marina L. before typhoon Songda in 24. Shoot density of Z. japonica increased in 211. 3) The difference between before and after typhoon Songda attacked water temperature, light attenuation coefficients, particle diameter off Ozu was not significant. 4) After Z. japonica off Ozu increased in 29, silt and clay contents, chemical oxygen demand, total sulfide of sea bottom increased.
Conclusions2 5) Seagrass bed in Kojiro was dominant species of Z. marina L.. Photosynthetic activity of Z. marina L. off Kojiro was higher than that off Ozu. Because water temperature was lower than off Ozu and water irradiate was higher than off Ozu. The velocity of sea bottom off Kojiro was higher than off Ozu. 6) Photosynthetic activity of Z. japonica was higher than Z. marina L.. Because chlorophyll a contents in Z. japonica was higher than Z. marina L. and water irradiate off Ozu was almost lower than saturating photosynthesis and higher than compensated photosynthesis. 7) The resuspension of sea bottom was inhibited and light condition improved. Because the difference solar irradiate from 29 to 211 was not significant. However water irradiate increased after the vegetated area and shoot density of Z. japonica increased. 8) These results suggest that the resuspension of sea bottom shows significant inhibition of Z. marina L. distribution and Z. japonica has become dominant than Z. marina L. after typhoon attacked. It is reason that photosynthesis activity of Z. japonica is higher than Z. marina L.