Eutrophication and trophic status using different indices: A study in the Iranian coastal waters of the Caspian Sea

The aim of this study was to assess the eutrophication and trophic status of the Caspian Sea based on different indices and also compare its status with the reference threshold values in the Iranian coastal waters of Caspian Sea (hereafter ICWCS). The water samples (three replicates) were collected along four transects (off the coast from Anzali, Tonekabon, Nowshahr and Amirabad) between spring 2013 and winter 2014. The results show marked significant differences between the mean concentrations of DIN, TP, DO and Chl-a in different seasons (p<0.01) and high significant correlation with different indices in the ICWCS. The mean of Trophic Index of Vollenweider (TRIXvollen.), Trophic Index of the Caspian Sea (TRIXcs) and Technique Quartile Range of Trophic Index (TQRTRIXCS) values ranged from 4.43 to 5.05 (good, high eutrophication risk), 5.21 to 6.00 (moderate, high eutrophication risk) and 0.58 to 0.95 (moderate to high) in different seasons for these three indexes, respectively. The results of nutrients and chlorophyll-a concentrations at the four sampling locations when compared with the reference threshold value indicated that the trophic status of this ecosystem changed from oligotrophic to mesotrophic/meso-eutrophic in 2013-2014. All there indexes (TRIXcs, TQRTRIXCS and Reference Threshold Value methods) produced similar results and therefore, could be used interchangeably for the study area.