01 Okuda Satoumi Okuda IJs

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Kagawa University Hiroshima University

Ministry of the Environment

Theme 1：Management of Nutrient Concentrations in the Seto Inland Sea

Presenter：

Tetsuji Okuda

Ryukoku Univ.

Division delegate：Wataru Nishijima Hiroshima Univ.

Research delegate：Tetsuo Yanagi EMECS center

S13：Development of Coastal Management

Method to Realize a Sustainable Coastal Sea

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Management of nutrients in enclosed coastal sea

Nutrients Input

Phytoplankton blooms Red tide

Negative side

Positive side

Growth of zooplanktons

and fishes

Low transparency，

Growth inhibition of aquatic plants， Increase of COD，

Hypoxia，Death of fishes， Bad smell

Feedback

Act on Special Measures concerning Conservation of the Environment of the Seto Inland Sea Oct. 2015 _{Coastal management for}

“High productivity and sound material cycling”

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Decrease of fish production

Planktivorous fish

All fish species - Planktivorous fish

Total catch of fishes has decreased from mid-1980s to recent years 51% .

_{Decrease in total catch of fishes}

Planktivorous fish：from mid-1980s to 1990s

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Nutrient loading from land

Nutrient loading data from MOE during 1981 and 2010 Seto inland sea

TN and TP loading from land decreased to

40% and 61%, respectively, during last 30 years.

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Zooplanktons

Planktivorous fishes Predatory fishes

Shellfish

. N u t r i e n t s .

Planktonic Benthic

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Grazing food chain

Phytoplanktons Macro / Micro algae

Seagrass Viewpoint of

Regional Characteristics

Viewpoint of

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Target (objectives)

Regional Characteristics of sea area

_{Characterization of extensive enclosed sea} from the viewpoint of “Phytoplankton growth”

Indices：Background Secchi Depth, Vulnerable Index

Identify a key area for nutrient control based on vulnerability

Biological Production

_{Evaluation of transfer efficiencies to higher trophic levels} _{according to phytoplankton abundance}

Relationship between nutrient load curtailment from land and biological productivity

Coastal management

_{Concept of coastal management in vulnerable areas}

Quantification of nutrient uptake capacity in Seagrass beds

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Regional characteristics of sea area

Sunlight

River Influence (nutrients, sands)

Depth and Wetland properties Flow, Temparature, . . . .

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Light environment

Distribution of secchi depth Mean value during 2005-’14

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Background Secchi Depth BSD

Light attenuation coefficient m-1

K_d = K_water + K_CDOM + K_Tripton + K_{phytoplankton}

BSD：Secchi depth without phytoplankton

Regional value eliminating the influence of eutrophication

Data from MOE 1981-2014

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BSD and Secchi depth improvement potential

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Parameters for Vulnerable Index

Salinity 1981-2010 N 2 _×₁₀4 _s−2 _1981-2010

Vertical Fluidity Freshwater (Nutrients) input

Salinity of surface water Autumn and winter data

Indicator of

nature-derived nutrients from the land

N 2₌ g

ρB

× ρB − ρS

Z_B− Z_S

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Vulnerable Index VI

VI

MOE 127 Stns: 1981-2012 MLIT 229 Stns: 2003-2012

Stns where phytoplanktons have bloomed >2 times were removed. 10％ value in Chl-a conc. : 5.8 µg L-1

Coefficients of the standardized parameters in VI were estimated.

N

VI = 0.90×_LogN2_{- 1.05}_×_{Salinity - 1.05}_×_BSD

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Biological production

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Food sources of predatory fish

Hairtail Trichiurus lepturus

：Responsible for 55% of fish

catches of predatory fish in last decade.

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Stable isotope analysis

-21 -20 -19 -18 -17 -16 -15 -14

Engraulis japonicus Ammodytes

personatus

Leptochela gracilis

Co

Stomach contents in T. lepturus

P

A. personatus E. japonicus Other fishes

L. gracilis Decapoda Squillidae Other Crustacea

Cephalopoda Polychaeta

Hiuchi Nada during 2010-2013

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Contribution of microphytobenthos as the food source of short-necked clam]

：56〜76% Stable isotope analysis

Sea lettuce

30%

Microphytobenthos

66%

Short-necked clam

Transfer efficiency : 10～14％

Important food source

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Relationship between Chl.

a

and transfer efficiency

Primary production

Secondary production

Osaka Bay Data collected in summer 2014-2015

Transfer efficiency

Transfer efficiency decreased in high Chl.a concentration.

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Annual mean

Coastal H1-4,7

Offshore H5,6

Primary production

mgC m-2 _d-1 457 272

Secondary production

mgC m-2 _d-1 12.9 27.8

Transfer efficiency ％

P.P. → S.P. 2.7 9.5

Although coastal area possess the high primary productivity due to abundant

nutrient supply,

the transfer efficiency was relatively low.

Spatial difference of biological productivity between coastal and offshore area

Not only topographical classification of the sea area (Bay-Nada), but also Coastal-offshore

classification (management) is needed. Primary production

of phytoplanktons

Geographical difference of biological productivity

between coastal and offshore area

mgC

m

-2

d

-1

Secondary production of zooplanktons

mgC

m

-2

d

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Nutrients

River

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Coastal management in highly vulnerable areas

PhytoPlanktons

Zoo-Planktons

Fishes

Transparency?

Structure (depth?)

Flow?

Mixing?

Temperature?

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Coastal Management

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Regional Individual Management

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Nutrients

Tidal flats

River

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Coastal management in highly vulnerable areas

Microalgae

Benthos

Birds

Shellfish catch

Denitrification

Fishes

Drifting during

autumn and winter

Decomposition

Drifting seagrasses

Nutrient transport by seagrass：

Nutrient uptake during spring and summer

Drifting during autumn and winter

Decomposition in offshore area

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Nutrient removal by seagrass

Seto Inland Sea

Subtidal zone in Ikushima Bay

Area: 42 ha

Nutrients in winter DIN: 3.4 µM

DIP: 0.43 µM

Nutrient budget in Ikushima Bay

May 2014

Seagrass beds

Zostera marina

Ikushima Bay Outer bay

Nitrogen demand

160

Standing stock

0.36 kgN m-2 Z. marina

Porewater (0-10 cm) DIN：0.38 mgN m-2

Unit: mgN m-2_d-1

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Estimation of nutrient uptake rate by

Z. marina

Phytoplankton in seawater

152〜1,080 mgC m-2_d-1

Area-weighted mean: 399

Seasonal survey in 2015

Microphytobenthos in tidal flats

75〜395 mgC m-2_d-1

Nutrient loading from land to Hiroshima Bay 2009

8,590 tN y-1

589 tP y-1

Zostera area covered

in 1960 597 ha

Zostera area covered

in 1996-’97 135 ha

122 tN y-1 _1.4_％

20 tP y-1 _3.5_％

If recovered.. 540 tN y-1 _6.3_％

90 tP y-1 _15.3_％

Line-transect survey July 2016

Zostera biomass in D.L. 0 to -1.5 m Light attenuation in seawater

Relationship between light and photosynthetic rate

C:N:P Weight ratio = 83:6:1

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環境研究総合推進費テマ業務実施体制

国立大学法人広島大学

栄養塩濃度管理法開発サテマ

国立大学法人香川大学

干潟藻場の栄養物質循環生物再生産に果たす機能の解明サテマ

Wataru NISHIJIMA Satoshi NAKI Youichi SAKAI Takeshi TOMIYAMA Tetsuji OKUDA

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