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Background: Widespread common carp (Cyprinus carpio) mortalities have been recorded in Khuzestan province fish farms in recent years. In summer of 2017 two cases of harmful algal bloom were encountered that led to massive mortality in common carp in Khuzestan, Iran. Aims: The aim of this study was to identify the possible etiologic agent of two mortalities with characteristic symptoms of gill lesions due to harmful algae. Methods: Water samples were collected and 5 moribund fishes were examined by histophatologic, scanning electron microscopic and PCR examination. Results: In wet smear preparations, a lot of algal cells and fragments, and sloughed, necrotic epithelial cells were observed between the lamellae. In histopathologic examination of gills, hyperplasia, necrosis and algal cells surrounded by hyperplastic cells were seen in tissue sections. No inflammatory cell aggregation was noticed. In scanning electron microscopic examination the algae was found attached to the gill surface (cell diameter: 8.5 ± 4.2 μm) with 2 equal flagella. Conclusion: Phytoplankton analysis using direct microscopy and electron microscopy, morphologically resembling Heterosigma was identified, however, in PCR tests, Heterosigma analysis showed negative results, therefore the causative agent was called "Heterosigma-like" algae. [ABSTRACT FROM AUTHOR]

It is well known that algae play an important role in treatment of municipal wastewater with facultative ponds. The objective of the present study was to determine the ratio of effluent algal BOD concentration in facultative ponds to influent BOD concentration. This descriptive analytical study was carried out on a full-scale wastewater stabilization pond system located in Islamabad Gharb, northeast Iran. A total of 138 grab samples were collected from Primary and Secondary Facultative Ponds (PFPs and SFPs) twice per month over a period of one year and analyzed for algal cell number, chlorophyll a, and BOD5 concentrations. The mean ratios of effluent algal concentration of PFP5 and SFP7 to influent BOD5 of PFP5 were 1.11 ± 0.53 and 1.59 ± 0.66, respectively. Also, the mean ratios of effluent algal concentration of PFP6 and SFP8 to influent BOD5 of PFP6 were 1.20 ± 0.63 and 1.57 ± 0.68, respectively. Except PFP6, significant differences were found in the means of the respective ratio in PFP5, SFP7, and SFP8 between cold and warm seasons (p < 0.05). Although there was a significant difference in the means of the respective ratio between primary and secondary facultative ponds (p < 0.05), there was no significant difference between the same ponds in modules (p > 0.05). Lower effluent algal concentration to influent BOD5 ratio found in cold months indicates strong dependency of algal photosynthetic activity to sunlight and temperature. Also, the effluent algal concentration in secondary facultative ponds was higher than that in primary facultative ponds, since better condition is provided by secondary facultative ponds for algal-bacterial activity. [ABSTRACT FROM AUTHOR]