While warming and eutrophication have increased the frequency and magnitude of harmful cyanobacterial blooms globally, the scenario for many eutrophic tropical freshwaters is a perennial year-round bloom. Yet, the drivers of persistent blooms are less understood when conditions such as light, temperature, and nutrients favor cyanobacteria growth year-round, and especially in regions facing recurrent periods of drought. In order to understand the drivers of cyanobacteria dominance, we assessed the abiotic conditions related to the abundance and dominance of the two dominant bloom-forming genera Raphidiopsis and Microcystis, in six shallow, man-made lakes located in the semiarid Northeastern region of Brazil during a prolonged regional drought. Lower water level corresponded to increased phosphorous and nitrogen concentration and, consequently, phytoplankton biomass. Cyanobacterial biomass was also proportional to phosphorus concentrations during year-round blooms. Yet, the two dominant cyanobacterial genera, Raphidiopsis and Microcystis, seldom co-occurred temporally and the switch between them was driven by water transparency. Our results illustrate the effects of drought induced water level reductions on the biomass and composition of cyanobacterial blooms in tropical shallow man-made lakes. Given the ideal year-round conditions (i.e., high light and temperature), droughts may be expected to intensify the risk and multitude of problems associated with eutrophication.

尽管全球变暖和富营养化增加了有害蓝藻水华的发生频率和强度，但许多富营养化热带淡水的情况却是常年常年水华。然而，当诸如光照，温度和养分等条件有利于全年蓝藻的生长时，尤其是在面临旱灾频发地区的情况下，持久性开花的驱动因素却鲜为人知。为了了解蓝藻优势的驱动因素，我们评估了与两个显性花蕾形成属Raphidiopsis和Microcystis的丰度和优势相关的非生物条件。，在长时间的区域干旱期间，位于巴西东北半干旱地区的六个浅层人造湖中。较低的水位对应增加的磷和氮浓度，因此对应于浮游植物生物量。全年开花期间，蓝细菌生物量也与磷浓度成正比。然而，两个主要的蓝细菌属：蝴蝶兰和微囊藻，很少在时间上同时发生，它们之间的切换是由水的透明性驱动的。我们的结果说明了干旱引起的水位降低对热带浅层人造湖中生物量和蓝藻水华组成的影响。在全年理想的条件下（即高光照和高温），干旱可能会加剧与富营养化有关的问题和风险。