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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. - PowerPoint PPT Presentation

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Page 1: Made by  Katherine  Bukina Krasnoyarsk, 2011
Page 2: Made by  Katherine  Bukina Krasnoyarsk, 2011

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 BukinaKrasnoyarsk, 2011

Page 3: 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.

Page 4: Made by  Katherine  Bukina Krasnoyarsk, 2011

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.

Page 5: Made by  Katherine  Bukina Krasnoyarsk, 2011

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 sourceof nutrients from lakes is the restoration of macrophytes in order to limit sediment resuspension and nutrients release.

Page 6: Made by  Katherine  Bukina Krasnoyarsk, 2011

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.

Page 7: Made by  Katherine  Bukina Krasnoyarsk, 2011

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:

Page 8: Made by  Katherine  Bukina Krasnoyarsk, 2011

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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, 3D 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 - 17 July, 2003 and at the eastern part in Longgan Lake during 27- 31 July, 2003 (Fig. 1).

Lake water depth was 2.6 m and the sediment depth was 0.8 m at MB. 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

Page 9: Made by  Katherine  Bukina Krasnoyarsk, 2011

MATERIALS AND METHODS Underwater irradiance and transparency measurements

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Effect of sediment resuspension on underwater light field in shallow lakes

Secchi disk pattern

Downward photosynthetically active irradiance (PAR, 400 - 700 nm) at the water surface and at different depths (0.2, 0.5, 1.0, 1.5 and 2.0 m) was measured using LI-192SA 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 SR9910, 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

Page 10: Made by  Katherine  Bukina Krasnoyarsk, 2011

MATERIALS AND METHODS Other measurements

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Effect of sediment resuspension on underwater light field in shallow lakes

1. Total absorption coefficient: a(λ)=ap(λ)+aCDOM(λ)+aw(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:

Zeu (PAR) = 4.605/Kd (PAR)

Page 11: Made by  Katherine  Bukina Krasnoyarsk, 2011

Results In Longgan Lake

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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- 700 nm were in the range of 0.98 - 2.97 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 - 3.95 m−1 and 2.02 m−1 under a middle wind wave condition, respectively. And on July 28, they were up to 1.82 - 5.40 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.

Page 12: Made by  Katherine  Bukina Krasnoyarsk, 2011

Results In Meiliang Bay of Taihu Lake

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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.

Page 13: Made by  Katherine  Bukina Krasnoyarsk, 2011

Results In littoral zone near TLLER

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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

Page 14: Made by  Katherine  Bukina Krasnoyarsk, 2011

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.

Page 15: Made by  Katherine  Bukina Krasnoyarsk, 2011

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.

Page 16: Made by  Katherine  Bukina Krasnoyarsk, 2011

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 (aw), CDOM (aCDOM), non-algal particulate matter (ad) and phytoplankton (aph) under middle (a) and large (b) wind wave condition.

Page 17: Made by  Katherine  Bukina Krasnoyarsk, 2011

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.

LakePAR

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.

Page 18: Made by  Katherine  Bukina Krasnoyarsk, 2011

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 depthLL 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.

Page 19: Made by  Katherine  Bukina Krasnoyarsk, 2011

A time to ask something 17

Effect of sediment resuspension on underwater light field in shallow lakes

Questions?

Page 20: Made by  Katherine  Bukina Krasnoyarsk, 2011

Adieu 18

Effect of sediment resuspension on underwater light field in shallow lakes

Thank you for your attention!