Submerged aquatic weeds may colonize freshwater shallow lakes by forming dense mats: when this occurs, their invasion interferes with human activities and generates several impacts on aquatic processes. In this paper, we present the preliminary results of an investigation concerning the dissolved gas and nutrient dynamics within a bed of Egeria densa Planch. in a bay of Lacanau Lake (southwestern France). Two 24-h cycles were carried out during summer and autumn 2013, for the measurement of dissolved oxygen (O2), carbon dioxide (CO2), methane (CH4) and dissolved inorganic nitrogen (DIN). Our results show that Egeria densa seasonal dynamics might strongly influence the colonized area: during summer, steady levels of O2 (O2 > 0.15 mM) were coupled to the macrophyte photosynthetic activity, while during autumn respiration processes were mostly pronounced (0.02 mM < O2 < 0.25 mM). High concentrations of CH4 (0.6–53.9 microM) and CO2 (0.03–0.83 mM) were measured within the bed during both sampling campaigns, that indicating a potential contribution to greenhouse gas emissions towards the atmosphere. This study suggests that a shallow environment colonized by an elevated biomass of submerged aquatic weeds is most likely subjected to water hypoxia and nutrient mobilization.