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).