Spectral properties of waters mainly show volume scattering instead of surface scattering.In optically shallow waters

light penetrates through water down to bottom

and then the light reflected from bottom penetrates through water up to surface.So the total water-leaving radiance is a sum of contributions due to water volume and due to bottom vegetation. Taihu Lake

a cover area of 2427.8 km

2

（including 51 islands）

is a large shallow lake with a mean depth of 1.9 m.On 18-29th in October 2004

67 samples were distributed almost all over the Taihu Lake.The apparent optical properties

including remote sensing reflectance

were measured in situ

and the inherent optical properties including backscattering and absorption were also measured either in situ or in the lab according to the instrument user’s manual or the NASA Sea- WiFS protocol.Additionally

we also measured concentrations of chlorophyll-a

suspended particulate matter（SPM）

suspended particulate organic matter（SPOM）

suspended particulate inorganic matter（SPIM）and dissolved organic car- bon（I）OC).Other parameters

including secchi disk transparency

water depth

wind speed and direction

and water temperature

were also measured in situ using their corresponding instruments.According to some literatures describing the relationship between the water depth and Secchi disk transparency

the Taihu Lake is divided into two areas

VIZ

an optically depth water area and an optically shallow water area.And most of the optically shallow water area

with a wholly vertical transparency from water surface to the bottom

have submerged vegetation growth at the bottom.These vegetation bottoms have an effect on the remote sensing reflectance to a certain extent.We used the LEE model to calculate the con- tribution of vegetation bottom to remote sensing reflectance just beneath the surface and the vertical diffuse attenuation co- efficient based under the hypothetical condition of:（a）infinitely horizontal bottom composed of homogeneous vegetation and mush with a Lambert property

and（b）homogeneous water quality in vertical.The results show that the contribution of the vegetation bottom to subsurface remote sensing reflectance has a general tendency to increase with increasing wave- length

and the maximum emerges in the red wavelength range.Then we analyzed the relationships between vertical dif- fuse attenuation coefficient and the secchi disk transparency

total suspended matter concentration suspended inorganic matter concentration.The results show that there is a significant inverse linear function relationship between the vertical diffuse attenuation coefficient and Secchi disk transparency

and there is a significant quadratic function relationship be- tween the vertical diffuse attenuation coefficient and SPM concentration and SPIM concentration.Additionally

the rela- tionship between vertical diffuse attenuation coefficient and the contribution of vegetation bottom was also analyzed quanti- ficationally.These results will improve the retrieval precision of water quality parameters with remote sensing.Worth to note

the calculated value of bottom contribution is approximate and not constant

which may therefore vary little with dif- ferent growth periods of hydrophytes.Additionally

the assumption of BRDF（bidirectional reflectance distribution func- tion）needs to be validated by its measurements especially in vegetation lake area of Taihu Lake.