Development of water turbidity index (WTI) and seasonal characteristics of total suspended matter (TSM) spatial distribution in Ichkeul Lake, a shallow brackish wetland, Northern-East Tunisia

Abstract

The decrease of fresh water supply from connected rivers induces the reverse of gravity flow from Bizert Lagoon which increases the salinity and the total suspended matter (TSM). Therefore, multi-date MODIS images were analyzed by GIS to calculate and identify the spatial distribution of WTI, TSM indicator. Then, the relationship between wind conditions and TSM’s resuspension in Ichkeul Lake was clarified. After that, the cluster analysis and the temporal fluctuation of daily wind speed; rainfall, and the Potamogeton pectinatus phenology were used to interpret the seasonal distribution of the turbid water. Consequently, the linear correlation between WTI and observed data was found (R2 = 0.66; r = 0.809) and proved by ANOVA test, where P = 0.00143 (< α = 0.05). Besides, the prevailing effects of winds on turbidity were detected, where the daily mean value of WTI was positively correlated with daily wind speed with r = 0.63 and r = 0.65, respectively, for P1 and P2. Thus, the turbidity increases with the increase of wind velocity. Subsequently, wind speed threshold was chosen to be 2.8 m/s. Moreover, the frequent wind with velocity varied between 5 and 7 m/s and may exceed the 9 m/s blew from the WNW direction. Added to this, the seasonal distribution of WTI revealed that the most and moderate turbid days were detected frequently in summer season. Following the prevailing wind direction, the TSM was distributed from the center of the SSE part of Ichkeul Lake.