Effect of sediment resuspension on underwater light field in shallow ...

swedishstreakMechanics

Feb 22, 2014 (3 years and 5 months ago)

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Effect

of

sediment

resuspension

on

underwater

light

field

in

shallow

lakes

in

the

middle

and

lower

reaches

of

the

Yangtze

River
:

A

case

study

in

Longgan

Lake

and

Taihu

Lake


Made

by


Katherine

Bukina

Krasnoyarsk,

2011



INTRODUCTION

1



Effect
of sediment
resuspension

on underwater light field in shallow lakes


Shallow

lakes

are

easily

affected

by

wind
-
induced

sediment

resuspension
.

Sediment

resuspension

plays

an

important

role

in

controlling

structures

and

functions

of

shallow

lake

ecosystems,

and

its

ecological

significance

is

shown

in

two

aspects
:


Wind
-
induced sediment

resuspension

often increases the concentrations of
suspended matter and hence promotes
light attenuation.

The release of nutrients from sediment
and resuscitation of phytoplankton could
increase concentrations of nutrients in
water bodies, promote the growth of
algae and increase primary productivity.


INTRODUCTION

2



Effect
of sediment
resuspension

on underwater light field in shallow lakes

Blom

et al. found that total suspended particulate matter contributed to light
attenuation by 40% annually in open, wind
-
exposed, shallow lakes with
silty

sediment, but by 80% during storm events.



Van
Duin

et al. simulated the relationships between underwater light field and
sedimentation,
resuspension
, water quality, autotrophic growth. Sediment
resuspension

could cause the increase in the concentration of suspended matter
and the decrease in
euphotic

depth, thus primary productivity was inhibited,
especially in winter with weak light intensity. This is an antagonistic process.



The study on the development of antagonism is the core to reveal the effects of
resuspension

on ecosystems and environments further.


INTRODUCTION

3



Effect
of sediment
resuspension

on underwater light field in shallow lakes

In

the

past

20

years,

the

heightening

human

activities

have

caused

serious

lake

water

pollution

in

the

middle

and

lower

reaches

of

the

Yangtze

River
.

Lake

ecosystems

have

been

altering

significantly
.



The

serious

situations

of

eutrophication

and

algal

blooming

in


lakes

have

directly

endangered

the

sustainable

development

of

local

social

economy
.

Not

only

the

study

of

the

internal

source


of

nutrients

from

lakes

should

be

taken

into

account

but

also


controlling

the

external

source

which

enters

lakes

in

order

to

control

eutrophication

of

shallow

lakes
.



One

of

the

effective

approaches

to

control

the

internal

source

of

nutrients

from

lakes

is

the

restoration

of

macrophytes


in

order

to

limit

sediment

resuspension

and

nutrients

release
.


INTRODUCTION

4



Effect
of sediment
resuspension

on underwater light field in shallow lakes


The

macrophytes

can

compete

with

algae

for

nutrients

and

light,


moreover

the

allelopathy

of

submerged

plants

could

be

used


to

inhibit

algal

growth
.

In

these

shallow

eutrophic

lakes

with

high

concentrations

of

suspended

matter,

phytoplankton

and

dissolved

matter,

the

lake

water

transparency

is

very

low

with

a

rather

poor

light

field
.



Therefore,

the

effective

restoration

of

macrophytes

first

requires

a

study


on

the

optical

properties

of

lake

waters,

especially

the

change

of

the

underwater

light

field

resulting

from

sediment

resuspension
,

to


explore

the

dominant

factors

which

affect

light

attenuation,

and

finally

to

take

some

measures

to

improve

underwater

light

condition
.



INTRODUCTION

5



Effect
of sediment
resuspension

on underwater light field in shallow lakes

algal
-
dominated lakes

(
Taihu

Lake )

algal
-
macrophyte

transition lakes

macrophyte
-
dominated lakes

(
Longgan

Lake)

Yang

et

al
.

found

that

algal
-
dominated

and

macrophyte
-
dominated

lakes

possessed

an

evident

difference

in

optical

properties

of

lake

waters

and

underwater

light

fields
.



Longgan

Lake

and

Taihu

Lake

are

selected

to

study

the

effects

of

sediment

resuspension

resulting

from

wind

waves

processes

on

optical

properties

of

water

bodies

to

determine

the

quantitative

relationship

between

sediment

resuspension

and

euphotic

depth

under

different

wind

waves

in

the

middle

and

lower

reaches

of

the

Yangtze

River
.

Three types of shallow lakes could
be divided in the middle and lower
reaches of the Yangtze River:

6



Effect
of sediment
resuspension

on underwater light field in shallow lakes

Fig. 1. Location of sampling sites (MB:
Meiliang

Bay; ST: Station near TLLER; LL:
Longgan

Lake).

Underwater

spectroradiometer
,

wave

meters,

3
D

ultrasound

current

meters

and

automatic

wind

vane

and

anemometer

were

set

up

to

conduct

a

continuous

measurement

under

different

wind

wave

conditions

at

the

center

in

the

Meiliang

Bay,

Taihu

Lake

during

12



䩵ly,

㈰〳

慮d



瑨e

eastern

p慲a



L潮ggan

L慫e

du物湧





䩵汹,

㈰〳

⡆楧
.

1
)
.


䱡Le

water

摥灴h

w慳

2
.
6

m

a湤

瑨t

se摩d敮t

摥灴h

w慳

0
.
8

m




.

Water

depth

was

3
.
4

m,

sediment

depth

was

1
.
8

m,

and

there

was

no

macrophyte

during

the

measurement

period

at

LL
.




MATERIALS

AND

METHODS


Sampling time and location



MATERIALS

AND

METHODS


Underwater irradiance and transparency


measurements

7



Effect
of sediment
resuspension

on underwater light field in shallow lakes

Secchi

disk pattern

Downward

photosynthetically

active

irradiance

(PAR,

400

㜰7

湭n



瑨t

wat敲

surface

and

at

different

depths

(
0
.
2
,

0
.
5
,

1
.
0
,

1
.
5

and

2
.
0

m)

was

measured

using


LI
-
192
SA

underwater

cosine

corrected

sensor

connected

to

Li
-
1400

datalogger
.

Downward

spectra

of

the

PAR

radiation

at

different

depths

were

measured

with

a

scanning

spectroradiometer

(
Macam

SR
9910
,

Livingston,

UK)
.

Secchi

disc

transparency

was

measured

with

a

30

cm

diameter

black

and

white

quadrant

disc
.



The

Secchi

disk,

created

in

1865

by

Pietro

Angelo

Secchi

SJ,

is

a

circular

disk

used

to

measure

water

transparency
.

The

disc

is

mounted

on

a

pole

or

line,

and

lowered

slowly

down

in

the

water
.

The

depth

at

which

the

pattern

on

the

disk

is

no

longer

visible

is

taken

as

a

measure

of

the

transparency

of

the

water
.

Black and white
quadrant
Secchi

disc



MATERIALS

AND

METHODS


Other

measurements

8



Effect
of sediment
resuspension

on underwater light field in shallow lakes

1.
Total absorption coefficient:

a(
λ)=
a
p
(
λ)+
a
CDOM
(
λ)+
a
w
(
λ)


2.
Average wind velocity in half an hour:


less than 4 m∙s
−1
are defined as small wind waves



between 4 and 6 m∙s
−1
defined as middle wind waves



more than 6 m∙s
−1
defined as large wind waves


3.
Diffuse attenuation coefficients for downward spectral irradiance and PAR :




4.
The
euphotic

zone in the water bodies is defined as the upper layer of water
irradiated with sufficient daylight to make photosynthesis possible. Its lower
boundary is often roughly defined. Therefore,
euphotic

depth:

Z
eu

(PAR) = 4.605/K
d

(PAR)



Results


In
Longgan

Lake

9



Effect
of sediment
resuspension

on underwater light field in shallow lakes

Fig. 2. Spectral and PAR diffuse attenuation coefficients under different wind wave conditions at LL.

The

spectral

diffuse

attenuation

coefficients

and

PAR

diffuse

attenuation

coefficients

within

400

㜰7



were



瑨t

r慮ge



0
.


2
.


m

1

and

1
.
74

m

1
,

respectively

under

a

small

wind

wave

condition

on

July

30
.

On

July

27
,

the

values

in
-
creased

up

to

1
.
34

3
.


m

1

and

2
.
02

m

1

under

a

middle

wind

wave

condition,

respectively
.

And

on

July

28
,

they

were

up

to

1
.
82

5
.


m

1

and

2
.
45

m

1

under

a

large

wind

wave

condition
.


PAR

euphotic

depth

under

3

different

wind

wave

conditions

were

2
.
65

m,

2
.
28

m

and

1
.
88

m
.

The

PAR

diffuse

attenuation

coefficients

increased

by

16
%

and

29
%
,

with

a

decrease

of

14
%

and

29
%

for

PAR

euphotic

depth
.




Results


In
Meiliang

Bay of
Taihu

Lake

10



Effect
of sediment
resuspension

on underwater light field in shallow lakes

According

to

the

35

times

of

measurement

of

underwater

PAR,

PAR

attenuation

coefficient

had

an

average

value

of

3
.
63
±
0
.
47

m
-
1
,

and

corresponding

euphotic

depth

had

an

average

value

of

1
.
29
±
0
.
18

m
.


The PAR diffuse
attenuation coefficients


Date


Time

2.63 m
−1

July 13

9:00

3.72 m
−1

July 15

17:00

4.37 m
−1

July 16

17:00

PAR diffuse
attenuation
coefficient was
increased by 66%,
with a decrease of
40% of
euphotic

depth.



Results


In littoral zone near TLLER

11



Effect
of sediment
resuspension

on underwater light field in shallow lakes

Fig. 3. The correlation between PAR diffuse attenuation coefficient and transparency, mean wind velocity within half an hour
at
ST.

PAR

diffuse

attenuation

coefficient

increased

by

28
.
6
%

from

middle

to

large

wind

wave

conditions

during

the

measurement

period
.


The PAR attenuation
coefficients


Conditions

6.74
±
0.35 m
−1

large wind waves

5.24
±
0.50 m
−1

middle wind waves



Discussion

12



Effect
of sediment
resuspension

on underwater light field in shallow lakes

Underwater

light

field

is

very

important

to

the

growths

of

phytoplankton

and

macrophyte
.

Essentially

all

the

light

attenuation

in

water

is

attributed

to

four

components

of

aquatic

ecosystem
:


the water itself

dissolved matter

particulate matter including non
-
algal


phytoplankton

Dissolved

matter

mainly

absorbs

light,

whereas

non
-
algal

particulate

matter

and

phytoplankton

not

only

absorbs

light

but

also

scatters

it
.

Phytoplankton

and

submerged

macrophyte

not

only

compete

with

each

other

but

also

compete

with

other

light

attenuation

components

for

light

energy
.

In

oceans,

deep

lakes

and

eutrophic

shallow

lakes

without

wind
-
induced

resuspension
,

the

largest

contribution

to

light

attenuation

often

originated

from

phytoplankton
.

In

the

shallow

lakes

easily

disturbed

by

wind

waves,

sediment

resuspension

driven

by

wind

waves

played

a

very

important

role

in

controlling

the

concentrations

of

suspended

matter

and

light

attenuation
.



Discussion

13



Effect
of sediment
resuspension

on underwater light field in shallow lakes

All

PAR

euphotic

depths

measured

at

MB

and

ST

were

less

than

1
.
5

m,

so

the

average

thickness

of

the

photosynthetic

zone

was

less

than

1
.
5

m,

which

meant

that

primary

productivity

of

phytoplankton

was

mainly

produced

in

the

surface

1
.
5

m

water

layer
.

At

the

depth

of

more

than

1
.
5

m,

phytoplankton

and

macrophyte

could

not

grow

well

lacking

enough

sunlight

for

photosynthesis
.

Compared

with

MB,

euphotic

depth

in

LL

was

much

higher
.

So,

the

low

euphotic

depth

might

be

one

of

most

important

factors

to

restrict

the

growth

of

submerged

macrophyte
.


The

change

of

suspended

matter

concentrations

causing

by

wind

wave

disturbance

was

the

primary

factor

leading

to

underwater

light

attenuation

in

the

shallow

lakes,

the

maximum

was

generally

in

the

range

of

blue

light

around

400

nm,

reflecting

the

optical

properties

of

shallow

inland

lakes
.




Discussion

14



Effect
of sediment
resuspension

on underwater light field in shallow lakes

These

results

show

that

wind

waves

affected

the

underwater

light

fields

in

shallow

lakes

by

increasing

the

concentrations

of

non
-
algal

particulate

to

enhance

light

attenuation
.


Fig
.

5
.

Relative

contributions

of

the

absorption

coefficients

of

pure

water

(
a
w
),

CDOM

(
a
CDOM
),

non
-
algal

particulate

matter

(a
d
)

and

phytoplankton

(
a
ph
)

under

middle

(a)

and

large

(b)

wind

wave

condition
.




Discussion

15



Effect
of sediment
resuspension

on underwater light field in shallow lakes

Sediment

resuspension

occurs

in

lakes

when

the

shearing

stress

produced

by

wind

waves

and

lake

currents

is

more

than

its

critical

shearing

stress
.

Factors

affecting

sediment

resuspension

include
:

water depth

components of sediments

macrophyte

wind velocity

wind fetch length

the set of the wind

They would affect the distribution of
underwater light fields indirectly.

Lake

PAR
attenuation
coefficient

Water depth

LL

2.45

m
-
1

3.4 m

MB

4.37 m
-
1

2.6 m

ST

6.74 m
-
1

1.8 m

The

deeper

the

water

is,

the

weaker

the

buoyancy

acts

on

bed

sediment,

and

the

less

on

the

suspended

sedimentsunder

the

same

wind

wave

condition
.



Discussion

16



Effect
of sediment
resuspension

on underwater light field in shallow lakes

Sediment

resuspension

resulting

from

wind

wave

disturbance

makes

the

thickness

of

photosynthetic

zone

decease

greatly

in

shallow

lakes

in

the

middle

and

lower

reaches

of

the

Yangtze

River
.

The regression analysis showed that optical
parameters, such as PAR
attenua
-
tion

coefficient,
euphotic

depth, transparency
were all significantly correlated with the
concentrations of suspended matter, wind
velocity and wave height. The absorption
coefficients of CDOM at ST changed slightly,
whereas the absorption coefficients of
phytoplankton to some extent decreased but
the absorption of non
-
algal particulates
resulting from sediment
resuspension

increased significantly.

Lake

PAR
euphotic

depth

LL

0.40 m

MB

0.19 m

ST

0.20

m

Therefore,

when

wind

velocity

is

increased,

the

shearing

stress

is

enhanced

on

lakebed

by

enhancing

waves

leading

to

sediment

resuspension
,

and

then

increasing

light

attenuation

through

increasing

inorganic

particulates

in

water

bodies
.



A time to ask something

17



Effect
of sediment
resuspension

on underwater light field in shallow lakes

Questions?



Adieu

18



Effect
of sediment
resuspension

on underwater light field in shallow lakes

Thank you for
your attention!