2.2 Environmental variables
A total of 19 environmental variables were measured at each sampling station, including temperature, salinity, pH, total organic carbon (TOC) i, total nitrogen (TN) , total phosphorus (TP) , ammonium nitrogen (NH4+-N) , nitrate nitrogen (NO−3-N) , nitrite nitrogen (NO−2 -N) , Organic matter , Sulfate (SO4-) , Petroleum and sulfide. In addition, seven heavy metals, including Hg, Cd, Pb, Cr, As, Cu, and Zn. Of these, the pH value was measured by adding 5 ml of distilled water to 2 g of the precipitate and recording the pH value using a pH electrode (STARTER 300, OHAUS, Beijing, China). Total nitrogen (TN) content was analyzed by Kjeldahl method(J. M. Lynch & Barbano, 1999). Total phosphorus (TP) content was determined by molybdenum blue colorimetry at 660 nm after hydrofluoric and perchloric acid digestion. Ammonium nitrogen (NH+4 -N), nitrate nitrogen (NO−3 - N), nitrite nitrogen (NO−2 -N) were quantified by automated discrete analysis (CleverChem 380, Germany)(Islam, Sarker, Yamamoto, Wahab, & Tanaka, 2004). Heavy metal element content was determined by inductively coupled plasma mass spectrometry (ICP-MS, Optima, 2000 DV, Perkin Elmer, USA).