Abstract:

The migration of freshwater harmful cyanobacteria into estuarine brackish waters poses a significant threat to local ecosystems. The different forms in which these cyanobacteria produce algal toxins, such as microcystins (MCs), can exert varying impacts on zooplankton communities. This study aimed to investigate the predominant forms of MCs produced by Microcystis in brackish waters and their subsequent effects on the survival and community composition of metazoan zooplankton. An in situ mesocosm experiment was conducted in a nearshore brackish wetland. Rectangular mesocosms (1.5 m depth, 150 L volume) were established in the open water area, filled with ambient wetland water filtered through a No. 25 plankton net, and inoculated with Microcystis colonies collected from Dianshan Lake to achieve an initial chlorophyll a concentration of 60 μg/L. The experiment included three replicate mesocosms and ran for 14 days. Data were processed using Microsoft Excel and statistically analyzed with SPSS 23.0. One-way analysis of variance (ANOVA) and t-tests were used to determine statistical significance. The survival of Microcystis and changes in microcystin forms (particulate vs. dissolved) were analyzed, alongside the effects of these toxin fractions on zooplankton abundance and community structure. The results demonstrated that Microcystis survived at a salinity of 4‰, with MCs predominantly existing in the intracellular (particulate) form. Within the mesocosms, intracellular microcystin concentrations initially increased and subsequently decreased, while the proportion of extracellular (dissolved) microcystins remained low (3%~8%) and presented a decreasing trend. Low salinity (≤7‰) appeared to promote intracellular microcystin synthesis. Conversely, both algal growth and intracellular microcystin synthesis were significantly inhibited when salinity was increased to 10‰, accompanied by a significant increase in microcystin release. Differential sensitivity and tolerance to MCs were observed among metazoan zooplankton groups. Particulate MCs were associated with slower-acting but longer-lasting toxicity, resulting in final zooplankton abundance significantly lower than initial levels, with copepods becoming proportionally more dominant in the community structure. In contrast, conditions favoring dissolved MCs led to substantial acute mortality, followed by a recovery phase where final abundance significantly exceeded initial levels, and rotifers emerged as the dominant group. These findings indicate that the partitioning of Microcystis toxins in brackish environments directly influences zooplankton abundance and community composition, thereby impacting the aquatic ecosystem. This research provides a crucial scientific basis for assessing the ecological risks of Microcystis toxins in brackish waters and for informing the development of effective water management strategies.

Key words: harmful cyanobacterial blooms, brackish waters, microcystins, zooplankton, ecological risk