The community data was analyzed using principal response curves (PRC) method as described by Van den Brink and Ter Braak (1998, 1999). This method is based on redundancy analysis ordination techniques, the constrained form of principal component analysis (PCA) and is specially designed to evaluate the effects of stressors at the community level taking time into account. The significance of the PRC diagrams was tested by Monte Carlo permutation tests by permuting whole time series in the partial RDA, from which the PRC was obtained. All data were ln transformed. PRC analysis was performed using the software package CANOCO version 4.5 (Ter Braak and Smilauer, 2002)