基于MERIS影像的洪泽湖叶绿素a浓度时空变化规律分析

叶绿素a(Chl-a)浓度是衡量藻类生物量及评价水体营养状态的重要指标.基于洪泽湖2016年7月、2016年12月共49个实测水质参数与同步光谱数据,验证了5种可应用于MERIS/OLCI数据的Chl-a遥感估算模型(包括波段比值模型、三波段模型、FLH模型、MCI模型以及UMOC模型)在洪泽湖水域的适用性.结果表明,UMOC模型是最适用于洪泽湖水域的Chl-a浓度估算模型,其平均相对误差为32.30%,低于波段比值模型的75.17%,三波段模型的62.44%,FLH模型的45.87%和MCI模型的56.95%.进而利用UMOC模型,结合MERIS数据,获取了洪泽湖2002~2012年Chl-a浓度遥感估算产品,并分析了洪泽湖Chl-a浓度的时空变化规律.洪泽湖Chl-a浓度具有明显的时空差异性.依据水体像元长时间序列月平均Chl-a浓度的差异,将洪泽湖水体分为了区域A、区域B和区域C这3种类型.区域B和区域C水体无明显的变化趋势,区域A则显著增加.与气象因子的相关性分析表明,区域B和区域C年平均Chl-a的波动主要受年降水量的影响,反映了该2个区域Chl-a浓度的变化主要受湖流强度的控制,区域A年平均Chl-a浓度的变化与年平均风速呈显著负相关性,风速下降的气候大背景可能会加重这一区域的富营养化程度,威胁南水北调的水质安全.此外,在汛期(7~9月)洪泽湖水体Chl-a浓度与离淮河入湖口的距离呈显著的正相关关系,证明了这一时期淮河对洪泽湖藻类浓度具有明显的抑制作用.

Remote Sensing of Chlorophyll-a Concentrations in Lake Hongze Using Long Time Series MERIS Observations

Chlorophyll-a (Chl-a) concentrations are usually measured as the proxy of phytoplankton biomass and used to evaluate the trophic status of inland waters. Based on 49 in situ samples taken from two measurement campaigns in Lake Hongze in 2016, we evaluate the performance of five Chl-a estimation algorithms (including the band ratio, three-band, FLH algorithm, MCI, and UMOC algorithms). The results showed that the UMOC model was the most suitable model for the estimation of Chl-a in Lake Hongze. The mean relative error (MRE) of UMOC was 32.30%, much lower than the band ratio algorithm (75.17%), three-band algorithm (62.44%), FLH algorithm (45.87%), and MCI algorithm (56.95%). The best-performing UMOC model was applied to the atmospherically corrected 689 MERIS images between 2002-2012 and long time series MERIS Chl-a concentration estimation products were acquired. Between 2002 and 2012, the mean Chl-a concentration in Lake Hongze was 19.560 mg·m^-3 with substantial spatial and temporal variability. Based on the variability of monthly mean Chl-a concentrations in each pixel, the Lake Hongze waterbody was divided into three water types, Region A, Region B, and Region C. The annual mean Chl-a concentrations of Region B and Region C showed no significant changes, while the concentrations in Region A increased markedly. The analysis of the meteorological factors showed that the fluctuations of the annual mean Chl-a concentrations in Region B and Region C were mainly affected by annual precipitation, suggesting that the Chl-a concentrations of these two regions are dominated by the intensity of the lake flow. The annual mean Chl-a concentrations of Region A showed a strong negative correlation with the annual mean wind speed. The descending trend of the annual wind speed may enhance the eutrophication degree of this region, threatening the safety of the water quality of the South-North Water Transfer Project. The Chl-a concentrations showed a strong positive correlation with the distance from the Huaihe Estuary in the wet season suggesting that the Huaihe River has an obvious inhibitory effect on algal biomass in Lake Hongze during this period.