Significant Diurnal Variation of CO2 Flux from a Shallow Eutrophic Lake: Effects of Submerged Aquatic Vegetation and Algae Bloom

Shallow eutrophic lake with submerged aquatic vegetation (SAV) has high primary productivity, but its diurnal variation of CO2 flux remains unclear. Moreover, algae bloom has become a serious environmental problem; however, its effects are still unclear. Thus, monthly measurement in Lake Ulansuhai, a shallow eutrophic lake in China, was conducted throughout the ice-free period (April–October, 2019), to study monthly and diurnal variation of CO2 flux and the effects of algae bloom, SAV, and trophic state. Results show that the area with SAV (Site 1) acts as a CO2 sink during both daytime and nighttime in most months (spanning 5 months), and CO2 absorption during nighttime accounts for 25–45% of the total daily absorption. In contrast, the area with both SAV and algae (from May to September; Site 2) during whole ice-free period acts as a CO2 source during nighttime all the time, accounting for 30–100% (average of 80%) of the total daily emissions. Site 1 (without algae) during whole ice-free period acts as a net CO2 sink with CO2 flux of −7.3 ± 4.73 mmol m−2 d−1, whereas Site 2 (with algae) plays as CO2 source with CO2 flux of 5.54 ± 3.15 mmol m−2 d−1. Furthermore, monthly variation during 7 months suggests that total daily CO2 emission flux in April (after ice-melt period) at Site 1 (without algae) and Site 2 (with algae) reached 17.78 and 34.75 mmol m−2 d−1, respectively, indicating that the period plays an important role for CO2 emission budget during the whole ice-free period. Results show that the effects of algae bloom and SAV need to be paid more attention in further works.