Significant diurnal variation of CO2 flux from a shallow eutrophic lake: effects of submerged aquatic vegetation and algae bloom

Abstract

Shallow eutrophic lake with submerged aquatic vegetation (SAV) has high primary productivity, but its diurnal variation of CO 2 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 CO 2 flux and the effects of algae bloom, SAV, and trophic state. Results show that the area with SAV (Site 1) acts as a CO 2 sink during both daytime and nighttime in most months (spanning 5 months), and CO 2 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 CO 2 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 CO 2 sink with CO 2 flux of −7.3 ± 4.73 mmol m −2 d −1 , whereas Site 2 (with algae) plays as CO 2 source with CO 2 flux of 5.54 ± 3.15 mmol m −2 d −1 . Furthermore, monthly variation during 7 months suggests that total daily CO 2 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 CO 2 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.