水华（Water bloom）或藻华（Algal bloom），学术名词为“水体富营养化”，是发生在淡水中，由水体中氮磷含量过高导致藻类突然性过度增殖的一种自然现象，同时也是一种二次污染。通常水的颜色呈现出绿色或蓝色。

“水华”是淡水中的一种自然生态现象，涉及到的藻类有蓝藻（即蓝细菌）、绿藻、硅藻等。自然形成的水华现象会很快消失，并不会带来环境影响。

而人为的往水体中排放氮素（主要是铵盐、硝酸盐和亚硝酸盐）和磷素（主要是正磷酸盐和各种形态的磷酸盐），使得淡水富营养化，超出环境容量和自净能力，“水华”频繁出现，面积逐年扩散，持续时间逐年延长。中国的太湖、滇池、巢湖、洪泽湖都有水华现象。

An algal bloom is a rapid increase or accumulation in the population of algae (typically microscopic) in a water system. Cyanobacteria blooms are often called blue-green algae. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number of phytoplankton species are involved, and some blooms may be recognized by discoloration of the water resulting from the high density of pigmented cells. Some algal blooms can result in extreme issues, such as the Lake Erie blooming in 2011, which lead to a nearly full-scale trophic cascade.