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In the present monitoring study, the status of biological threats in the southern area of the Caspian Sea (up to 30 m depth) in biological groups (phytoplankton, zooplankton, macrobenthos) and the invasive comb (Mnemiopsis leidyi), daily net primary production (DNPP) of phytoplankton, trophic level indices, water quality indices, eutrophication, petroleum pollutants, toxins heavy metals and detergents as well as the relationship between different parameters based on statistical tests were studied during 2018 (autumn and winter) and 2019 (spring and summer). According to the results, the mean (±SE) parameters of pH, transparency, turbidity, DO%, BOD5, CODMn, NH4/N, DIP, DSi and DNPP were 8.56 (±0.16) mg/L, 2.43(±0.14) m, 9.10 (±0.43) NTU, 98(±2)%, 4.05 (±0.16) mg/L, 4.99(±0.36)mg/L, 2.45(±1.49) µM, 0.22(±0.17) µM, 7.7 (5.8) µM, and 0.65 (±0.45) gC/m2.day, respectively. Also, the mean of water quality index of the Caspian Sea (WQIcs) and its quality class were obtained 70 (fair), 66 (bad-fair), 52 (bad), 80 (fair) in autumn, winter, spring, and summer, respectively. The mean trophic level (TRIXcs) and eutrophication risk (UNTRIX) after a decade of mesotrophic status and high eutrophication risk shifted to meso-oligotrophic and low eutrophication risk.
Bacillariophyta as the first dominant phyla accounted for more than 80% of phytoplankton abundance. Among the dominant species, only the abundance of Pseudonitzschia seriata in winter (187.15±36.47 cells/mL) was in the range of 100-100 units/mL (mean the potential for bloom). Considering three parameters (chlorophyll-a, abundance, and biomass of phytoplankton), was obtained moderate quality (mesotrophic) in cold seasons (autumn 2018 and winter 2018) and good quality (oligo-mesotrophic) in warm seasons (spring 2019 and summer 2019). In this study, four groups of zooplankton such as copepoda, rotifera, cladocera, and protozoa were identified. The copepoda with the presence of an invasive species (Acartia tonsa), formed a major part of the zooplankton community structure. The analysis of Mnemiopsis leidyi abundance (n/m3)showed that 80% of the data were less than 25 and about 97% of the data had a biomass (g/m3) less than 1. Evaluation of major macrobenthos groups showed that in all the seasons except spring, Polychaeta (mainly Streblospio gynobranchiata) accounted for about 70% of the biomass. The minimum (2.2) and maximum (3.6) percent of TOM were recorded in winter and spring, respectively. According to the Abundance and Biomass Curve (ABC), although the biomass line was located above the macrobenthos abundance line, however, its harmony was not the same as to "environment without pollution" pattern.
The mean values of the examined trace elements in water samples during four seasons were obtained in this order Fe>Cu>Zn>As.Pb>Co>Ni>Hg>Cd, respectively. The annual mean concentration of polycyclic aromatic hydrocarbons (PAHs), Organochlorine pesticides (OCPs) and detergents (LASs) in water were 4.24 (±1.42) µg/L, 7.25 (±0.65) µg/L and 0.37 (±0.27) mg/L, respectively. The annual mean concentration of PAHs in sediment samples were recorded at 8.14 (±2.00) µg/kg.dw. The mean values of the examined non-resistance trace elements in sediment samples were obtained in this order Fe>Mn>Cu>Ni>As>Cr>Pb>Cd>Hg. According to the PCA test, the abundance of macrobenthos was significantly correlated with Hg, As, Pb and Cd pollutants and their biomass was significantly correlated with Pb and Cd. The results of CCA test showed that the composition of LASs had a positive correlation with the comb biomass. In the CCA test, the dominant branches of phytoplankton showed a positive correlation with five components of OCPs (Drins, BHC, Heptachlors, DDTs and Endosulfans).
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