Interrelation of Remotely Sensed Water Quality Parameters in Thermaikos Gulf (Greece)
Monachou, Styliani1; Kalopesa, Eleni2; Alexandridis, Thomas K.1; Adrianopoulos, Agamemnon2; Tsirika, Anastasia1; Zalidis, George C.1
1Aristotle University of Thessaloniki, School of Agriculture, GREECE; 2Balkan Environment Center, GREECE
The European Water Framework Directive (2000/60/EU) is designed to preserve, restore and improve the water environment. A key element in identifying and classifying water bodies according to the guidelines of the Directive is the monitoring of water quality parameters. Frequent and accurate data on water quality parameters is an important consideration for any monitoring project. However, the cost of data collection in the field is large and prohibitive for extended water bodies, in particular coastal ones. Satellite remote sensing can monitor several water quality parameters accurately, frequently, and at low cost. Investigation of the interrelation between water quality parameters in a water body is very helpful in understanding geochemistry of the studied area; it helps to assess the overall water quality and also to quantify relative concentration of various pollutants in water and provide necessary indications for implementation of rapid water quality management programmes. Having a time series of complete wall-to-wall coverage of a water body from satellite remote sensing offers a unique possibility for investigating the spatial distribution of these relationships. The aim of this work is to use remotely sensed water quality maps of chlorophyll-a (Chl-a), total suspended matter (TSM) and sea surface temperature (SST) of Thermaikos gulf (Greece) in order to investigate the relationship between them. ENVISAT MERIS images were acquired for two years with a time step of 8 days. The satellite images were processed with BEAM software (Toolbox for analysis and processing of EO data, Version 4.10.1) and in order to compute the concentration of Chl-a and TSM the Case-2 Regional Processor (MERIS) Algorithm has been applied. Case-2 is an algorithm for coastal waters that is used to estimate the water quality parameters of water which absorb and scatter light in the visible spectrum. Aqua MODIS satellite images with a time step of 8 days were used for the estimation of SST, as provided by the MODIS Science team. Temporal interpolation was applied to fill-in the data gaps due to persistent cloud cover. Then, cross correlation and time series analysis were applied to identify the relationship between the parameters in pairs, and with successively increasing time lag between them. A set of six test sites were selected to monitor the various water bodies in the study area. The preliminary results of this work in progress show a positive correlation between TSM and Chl-a concentration in the areas influenced by river discharges. This is due to high nutrient concentrations in these areas which has enhanced algal growth. Seasonal variations of SST values coincide with the fluctuations of Chl-a indicating the possible development of blooms during spring and autumn and the low levels of Chl-a concentrations in summer and winter. The results of this work may provide input to a decision support system that can help policy makers in the area improve their management. ESA is acknowledged for providing the ENVISAT MERIS data (C1P.7542). USGS provided the Aqua MODIS data.