富营养化水体中光学活性物质的垂向分布及其对遥感反射光谱的影响

富营养化水体中光学活性物质的垂向分布对水下光场分布具有重要影响,决定了水色参数定量遥感反演的精度.基于2011年8月和11月在太湖进行的垂直分层采样,分析了水体中光学活性物质含量的垂向分布特征,并探讨了其影响因素及其对水体遥感反射光谱的影响.结果表明,风速是影响富营养化水体中光学活性物质垂向分布的重要因子之一.风速较小(小于3.0 m·s~(-1))或无风时,表层叶绿素a浓度急剧增加,易形成水华;而风速大于5.0 m·s~(-1)时,水体发生垂直混合,底层叶绿素a浓度最大;其它风速条件下,叶绿素a浓度在某一水深处出现极大值.在风速小于5.0 m·s~(-1)的情况下,悬浮物浓度和有色溶解有机物(CDOM)的垂向分布相对均一;当风速超过5.0 m·s~(-1)时,底泥再悬浮过程对两者的贡献较大.在未发生藻华情况下,0~0.5 m水深范围内的叶绿素a浓度均值与波段比值的相关系数最大(r=0.86),对水体遥感反射比的影响较大;而表层悬浮物浓度对水体遥感反射比的影响最大,研究结论可为水色参数含量的精确估算提供方法论支持.

Vertical distribution of optically active water components and its influence on remote sensing reflectance in a eutrophic water

The vertical distribution of optically active water substances (chlorophyll a,suspended solids and CDOM) is important to control underwater light climate in eutrophic waters,which fundamentally affects the estimation of water color variables from remote sensing.Using vertical bio-optical dataset in August and November,2011 from Taihu Lake,analyses of vertical variation of optically active water components and its influencing factors were performed.Furthermore,the effect of the vertical profiles of water components present on remote sensing reflectance was also examined.The results demonstrated that wind speed was an extremely important factor influencing the vertical distribution of optically active water components in eutrophic lakes.Under conditions when low wind speeds (< 3.0 m·s^-1) or no wind occurred,chlorophyll a concentrations rapidly increased in surface water,leading to algae blooms.At high wind velocities (> 5.0 m·s^-1),vertically mixing occurred and chlorophyll a moved downwards with a maximum in the bottom water.Under other wind speeds conditions,chlorophyll a concentration had a maximum value at a water depth in Taihu Lake.The suspended solids and CDOM were quasi-uniformly distributed in the vertical profiles on condition that the wind speed was less than 5.0 m·s^-1.However,resuspension processes were important in controlling the sources of suspended solids and CDOM when wind velocity was higher than 5.0 m·s^-1.The average chlorophyll a concentrations at 0~0.5 m water depth was significantly related to the band-ratio(R_rs(709)/R_rs(681)) with the correlation coefficient r=0.86,which showed its relatively high impact on the remote sensing reflectance when no algae blooms appeared,however,for shallow waters,the surface suspended substance showed an important influence on remote sensing reflectance of water.These results will be benefit for providing methodology for better estimates of water color variables in inland waters.