High-resolution diurnal variation mechanism of oxygen and acid environments at the water-sediment interface during cyanobacterial decomposition

Changes in the oxygen and acidic environments of water caused by algal blooms in lakes aggravate the uncertainty of the eutrophication process, profoundly influencing water ecosystems and lake biogeochemical cycles. This study aimed to detail the mechanisms of benthic oxygen and acid processes by determining high-resolution and high-quality chemical gradients and acquiring heterogeneous data for diurnal changes in dissolved oxygen (DO)/pH at the water-sediment interface (SWI) during different stages of algal degradation. Planar optode (PO) systems were used to obtain dynamic two-dimensional (2D) images of DO/pH at a laboratory microcosm interface. The benthic gradient and diurnal variation of DO/pH at the SWI were jointly regulated by the photosynthesis/respiration and migration behaviors of algae, through their influence on the water-carbon chemistry. The DO/pH at the SWI showed unique diurnal variation characteristics at different stages of algal degradation; however, characteristics consistent with the diurnal variation in photosynthesis were limited to the algal growth period. The increase in DO/pH during the daytime resulted from O2-rich and carbonate hydrolysis, to compensate for the reduction in CO2 saturation caused by algal photosynthesis. Nocturnal DO/pH decreased owing to O2 depletion and H+ release, because of increased water CO2 content from algal respiration. When photosynthesis and respiration were inhibited, or not smooth, DO/pH showed a limited increase with uncertainty. The sediment was not completely anaerobic, and DO/pH penetrated to - 20 mm and even to the bottom of the interface (- 40 mm) as a result of algal migration, whereas DO was exhausted and high pH areas transferred to sediments following algal decay. Sediments acted as storage pools in alkaline environments, which posed a potential risk for water eutrophication. The trajectory of algal activity at the SWI was highly consistent with the dynamic variation in DO/pH. Our findings provide new insights for in situ tracking and early prediction of algal blooms, and can be used to formulate a theoretical basis for the mechanism of eutrophication processes and for exploring new algal control technologies.