Lake Qarun is a closed saline lake in the northern part of El- Fayoum Depression of Egypt. The objective of this work is to give a complete picture of lake biotic factors to alarm on dinoflagellates blooming phenomenon and to compare between chlorophyll a values measured by Fluorometer and Spectrophotometer. Monthly patterns in Chlorophyll a concentrations were measured during the period from May 2015 to April 2016, using two different methods; by fluorometric and spectrophotometric techniques. Phytoplankton species in Lake Qarun are heterogeneously distributed all over the year; dinoflagellates were the main dominant group, chlorophyll a frequently accumulated as distinct peak (336 μgl^-1) in July 2015. The phenomenon of dinoflagellated blooming of Prorocentrum micans, Gymnodinium lantzschii and Exuviaell aapora that turning  the lake water to brown in colour, is recorded annually. The two methods used in detection of chlorophyll a during this work are accurate and simplified bring the picture in Lake Qarun. Actually, results of fluorometer are more sensitive and higher than those of spectrophotometer. In fact, high salinity, agricultural drainage water from drains and aquaculture activities turned the lake to variable, sensitive aquatic ecosystem, affecting significantly on phytoplankton structure and blooming dynamics determination. There is no record of toxicity effects or mass fish mortality in the lake due to this phenomenon.