Nitellopsis obtusa usually inhabits deep lakes, where the plants display vegetative growth and survive through the winter. In the present study we investigated a shallow lake where Nitellopsis reproduces sexually; we were interested in the effect of light conditions, temperature and water level on sexual reproduction of the species and its ability to survive lakebed drying. We estimated the distribution of all macrophyte species each July from 2009 to 2014 and correlated annual variations with fluctuations in hydrological conditions. We also sampled a variety of morphological characteristics of N. obtusa plants in nine periods throughout the 2009 and 2010 growth seasons and related them to water depth, day length and accumulated heat energy (growing degree-day) at the date of sampling. N. obtusa population was negatively impacted by decrease of water level, probably through the combined effects of competition with fast-growing pioneer species and lack of intrinsic ecological requirements. The fertility of N. obtusa could be interpreted as a response to disturbance in external conditions, leading to the production of long-lived and drought-resistant oospores. Development of male shoots prior to female shoots and their persistence throughout the season probably optimize the fertilization process. Sexual reproduction seems to be controlled by light and temperature conditions. An alternative hypothesis suggests that fertile and sterile populations belong to two distinct ecotypes.