Absence of winter and spring monsoon changes water level and rapidly shifts metabolism in a subtropical lake
We investigated how the lack of usual winter and spring monsoons, effectively representing consecutive drought events, affected the dynamics of ecosystem metabolism in a shallow mesotrophic seepage lake in northeastern Taiwan. An instrumented buoy provided high-frequency free-water dissolved oxygen measurements, water temperature profiles, and meteorological data, which we used to estimate daily values of gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP). Results revealed that the disappearance of monsoons decreased lake level and volume, concentrated dissolved nutrients, stimulated the development of algal biomass, promoted stratification, and resulted in a major shift in lake metabolism. Offshore GPP and R were both initially stimulated but then decreased due to shallower mixing depths in the water column. The lake rapidly shifted from a heterotrophic state to a highly autotrophic status when the water level dropped to the lowest level. A return to autotrophy was caused by a greater decline in R than an increase in GPP. This study demonstrates the dramatic effect that drought events can have on lake ecosystem function and suggests that nutrient control may be important in mitigating the effects of a predicted warmer and drier climate and increased water withdrawal in this region.