Cyanobacteria bloom dynamics in eutrophic ponds and their toxic effects on Oreochromis niloticus and Hypophthalmichthys molitrix

Abstract

Cyanobacteria are the most ancient form of life and create harmful algal blooms in freshwater, estuarine and marine ecosystems. It produces some secondary metabolites known as cyanotoxins which pose threat to humans, animals and the environment. Cyanobacterial blooms are common phenomenon in freshwater eutrophic ponds and lakes in Bangladesh. A study was conducted to assess the plankton diversity especially the cyanobacteria and their bloom in four eutrophic ponds in Mymensingh Sadar Upazila and their relationship with physicochemical parameters in both spatial and temporal scales. Plankton samples were collected along with water quality parameters from January 2012 to December 2013. A total of 22 plankton genera of six families were identified. Among them, Ceratiacea (81.36%), Cyanophyceae (72.74%), Euglenophyceae (47.23%), Peridiniaceae (28.67%) found highest in number in Pond 1 (P1), Pond 3 (P3), Pond 4 (P4) and Pond 2 (P2), respectively. Species composition of phytoplankton was typical of eutrophic conditions (high PO4-P, N03-N and NH4) was frequently characterized by the presence of Cyanobacterial bloom but it was also found in scarcity of nitrogen (Microcystis aeruginosa, 30,000 colony/L, June 2013, P3). Plankton diversity status was analyzed by using PAST (Paleontological Statistics version 2.17) software. Result showed that Microcystis sp., Ceratium sp., Tracheolomonas sp., Lepocinclis sp., and Spirulina sp. were the major contributing species (17%) for season all basis. Analysis of Similarity (ANOSIM) results showed that spatial differences and low temporal similarity in species community structure with a diverse assemblage. Canonical Correspondence Analysis (CCA) has been carried out to show the relationship among spatial and temporal data. With the CCA analysis, Conductivity, Ammonium, Dissolved Oxygen (DO), Biological Oxygen Demand (BOD), Nitrate Nitrogen had positive correlation with several phytoplankton species Cyanophyceae (Microcystis sp., Spirulina sp., Lepocinclis sp., Merismopodium sp., Anabaenopsis sp.) and Cholorophyceae (Senedesmus sp., Phacus sp., Pandorina sp.). A bloom of Microcystis aeruginosa was occurred in P3 in May 2013. Bloom sample was collected and analyzed with High performance liquid chromatography (HPLC). Five types microcystins (MC) were detected. The concentration of MC-RR was the highest (5.4μg/L) followed by MC-YR (1.14 μg/L), MC-WR (0.46 μg/L), dm-MC-RR (0.36 μg/L) and MC-LR (2.0 μg/L) which was much higher than the WHO provisional guide value (MC-LR 1 μg/l) for drinking water. Another Microcystis aeruginosa bloom sample was collected from P4 during June 2013. Two types microcystins were detected and the highest concentration was MC-RR (1.0 μg/l) followed by and dm-MC-RR (0.04 μg/l). Among the organs tested of fish (Oreochromis niloticus) sample MCs were only detected from the liver and the concentration was MC-RR (0.049 μg/gm) which was much higher than provisional guideline of WHO 0.04 μg/kg TDI (tolerance daily intake). No MCs was detected from the gut and muscle of the fish (Oreochromis niloticus). Histopathological study confirmed the damage of liver cell of fish exposed with Microcystis aeruginosa bloom in vitro experiment. In exposed fish (Oreochromis niloticus), histopathological alternations were characterized by swollen and granular cytoplasm, vascular proliferation, bile stasis, fatty change and focal necrosis. The present study thus suggested that fish farms should be monitored for the presence of toxic cyanobacterial blooms to minimize the exposure of potent hepatotoxins to fish and humans through the food chain. So, sustained integrated monitoring system for aquaculture and domestic ponds is strongly recommended.