The Spatial Distribution and Bioaccumulation of Anatoxin-A in Hulun Lake

The intensification of eutrophication in global water bodies has exacerbated the occurrence of cyanobacterial blooms, whose secondary metabolites can have detrimental effects on animals, humans, and ecosystems. This study analyzed and statistically evaluated the species composition and distribution of phytoplankton, assessed the concentration of anatoxin-a (ATX-a) in environmental and biological samples (n = 261), and explored the spatial distribution and bioaccumulation of ATX-a in Hulun Lake (Inner Mongolia, China). In late July 2024, the cyanobacteria Dolichospermum spp. comprised 85.5% of the total phytoplankton density. ATX-a levels were vertically distributed with higher concentrations in deeper water (3 m; 146.69 ± 11.84 ng·L−1) and sediments (3.28 ± 0.45 ng·g−1 dry weight) as compared to surface layers (0.5 m; 132.46 ± 8.19 ng·L−1). In fish, bioaccumulation of ATX-a was greatest in the liver (2.37 ± 1.85 ng·g−1), followed by intestinal contents (1.83 ± 0.74 ng·g−1), with minimal accumulation in muscle tissues (1.74 ± 0.77 ng·g−1). ATX-a levels were higher in smaller fish (minnows) than larger fish (Predatory carp, Gibel carp, and European carp). Additionally, all fish tissue samples contained ATX-a, suggesting that aquatic organisms were continuously exposed to ATX-a throughout the summer. A biodilution of ATX-a was observed from phytoplankton (384.82 ± 176.82 ng·L−1) to zooplankton (1.27 ± 0.12 ng·g−1), followed by biomagnification from zooplankton to fish.