Eutrophication is recognized as the main water quality problem worldwide, mainly due to the prevalence of harmful cyanobacterial blooms. Submerged macrophytes can control cyanobacterial biomass by removing nutrients from the water column or producing allelopathic compounds. As a result, they are highlighted as a potential strategy for restoring eutrophic waters. However, studies in semi-arid tropical ecosystems have shown inconsistent results regarding the effects of submerged macrophytes on cyanobacterial communities and cyanotoxin production. The aim of this study was to assess the effect of submerged macrophytes on cyanobacterial community structure and microcystin production in a tropical semi-arid reservoir subject to intense periods of water level drawdown. Submerged macrophyte cover, surface water physical and chemical variables, cyanobacterial community biovolume and microcystin concentration were measured monthly between October 2014 and July 2015 at the entrance of the river (Zone I) and dam zones (Zone II) of a reservoir. Although both zones were dominated by the submerged macrophyte Egeria densa, there were significant differences in macrophyte cover: Zone I had greater submerged macrophyte cover and Zone II had lower submerged macrophyte cover. Higher total cyanobacterial biovolume, potential microcystin producer biovolume and microcystin concentrations were observed in the zone with the greater submerged macrophyte cover. Moreover, our results showed that nutrient concentrations (total phosphorous, total nitrogen and ammonium) and physical variables (water temperature and turbidity) were also associated with the zone with greater submerged macrophyte cover. This study highlights that the water level drawdown of the reservoir appears to have favored an increase in nutrient concentration, especially in the zones with dense vegetation, which may have contributed to the coexistence of the high cyanobacteria biomass, high submerged macrophyte coverage and high microcystin concentration. Thus, developing strategies for the management of submerged macrophyte cover during water level drawdown events is essential for improving water quality and reducing the risk of harmful cyanobacterial blooms at the ecosystem level in tropical semi-arid aquatic ecosystems.

富营养化被认为是世界范围内的主要水质问题，主要是由于有害的蓝藻水华盛行。沉水植物可以通过从水体中去除养分或产生化感化合物来控制蓝藻生物量。因此，它们被强调为恢复富营养化水域的潜在策略。然而，对半干旱热带生态系统的研究表明，沉水大型植物对蓝藻群落和蓝藻毒素产生的影响存在不一致的结果。本研究的目的是评估沉水大型植物对热带半干旱水库中蓝藻群落结构和微囊藻毒素产量的影响，该水库遭受强烈的水位下降。沉水植物覆盖，地表水理化变量，2014 年 10 月至 2015 年 7 月期间，每月在河流入口（I 区）和水库坝区（II 区）测量蓝藻群落生物量和微囊藻毒素浓度。虽然这两个区域都以沉水植物为主Egeria densa，大型植物盖度存在显着差异：I区沉水植物盖度较大，Ⅱ区沉水植物盖度较低。在具有较大沉水大型植物覆盖的区域中观察到较高的总蓝藻生物量、潜在的微囊藻毒素生产生物量和微囊藻毒素浓度。此外，我们的结果表明，养分浓度（总磷、总氮和铵）和物理变量（水温和浊度）也与具有更大沉水植物覆盖的区域有关。这项研究强调，水库的水位下降似乎有利于养分浓度的增加，尤其是在植被茂密的区域，这可能导致高蓝藻生物量、高沉水植物覆盖率和高微囊藻毒素浓度共存. 因此，