In the present study, changes in glutathione S-transferase (GST) enzyme activity were investigated in Ulva rigida specimens, collected from four different depths at six sites in the southern region of the Sea of Marmara. GST enzyme activity showed significant increases and decreases at some sites according to depth. Different abiotic factors affecting all the sites were shown on the Principal Components Analysis axis. Canonical correlation analysis (CANCORR) was performed among abiotic factors the measured and GST enzyme activity in U. rigida specimens. CANCORR analysis results showed that ammonia nitrogen values were negatively correlated with dissolved organic nitrogen values while positively correlated with GST enzyme activity. The present data showed that GST enzyme activity in U. rigida was related to water pollution and abiotic factors.