Studies on temporal and spatial variations of circulation and hydrographic characteristics of Panangad region of the Vembanad lake

Abstract

The objective of the study was to find the seasonal and spatial variations of different hydrographical factors of the Vcmbanad Lake around Panangad region and compare the situation with that of the other parts of the lake. Salinity, temperature, current velocity and direction, dissolved oxygen, primary production and nutrients like phosphate, nitrite and nitrate were the parameters studied. For this monthly observations of these factors were made from nine stations located in and around Panangad region of Vembanad Lake. Beside this tidal cycle observations were carried out from two stations to study the tidal variation of the factors. S.T.D. meter was used to measure salinity, and temperature and a current meter for the current measurements. Water samples from surface and sub surfaces were collected with a clean bucket and a Van Dorn bottle. The estimation of dissolved oxygen, primary production and nutrients were done by standard methods. Marked seasonal fluctuation was noticed in the temperature of different stations. Annual variation showed two maxima. The temperature was minimum during southwest monsoon and maximum during pre monsoon months. In general, the vertical thermal stratification was small. The salinity exhibited marked seasonal fluctuation and generally the monsoon months recorded very low salinity values. After that there was an increase, reaching the maximum in pre monsoon months. Some vertical stratification was noticed during certain months. The Dissolved oxygen values were low during pre monsoon months and the content was more at surface than at the bottom layers. There was high seasonal fluctuation In the phosphate concentration. It showed high concentration during pre monsoon period. Generally bottom layers showed higher phosphate concentration than the surface layers. In the case of nitrite high values were observed durin

118

the monsoon and low values during dry season. Nitrate concentration also showed seasonal variation and high values were observed during the monsoon and low values during pre monsoon. The nutrient concentration was more in the inner smaller arms of the study area, in general. These inner arms were vertically homogenous also possibly due to higher mixing generated by the friction of the sides. In the tidal cycle observations it is found that salinity showed an increase during flood tide and decrease during ebb tide. At both stations considerable reduction in flood current speed was observed during post monsoon, compared to pre monsoon. During pre-rnonsoon a comparatively well mixed condition was prevailed. Post monsoon observations indicated very clear salinity stratification. This was due to the effect of freshwater discharge, which continued during this season though at reduced rate compared to monsoon season. It is found that the tidal effect on temperature was not well marked. It can be seen that different hydrographic factors of Panangad region of the Vembanad Lake exhibited much seasonal and spatial variations and the trend was in general, same to that shown by other parts of the lake.