A redução de volume leva a um aumento nas taxas de produção primária e respiração em um lago tropical natural

Abstract

Lakes are important centers of carbon transformation, and due to that, are subjected to the effects of climate change, while they are also capable of regulating these changes. Aquatic metabolism is considered a good sentinel of climate, once the metabolic rates respond quickly to a series of alterations induced or caused by climate change. Therefore, identifying and understanding the alterations in lake gross primary production (GPP), respiration (R), and net ecosystem production (NEP) dynamics is vital to better characterize the vulnerabilities of these environments to natural and to human-induced disturbances. The study aimed at comparing the changes in physical, chemical, and, especially, in the dynamic and in the magnitude of metabolic rates of a tropical lake that lost about 60% of its volume in the last 10 years. It is believed that the decrease in volume can lead to an overall increase in nutrient and dissolved organic matter concentrations, reducing water transparency and light availability. These changes may influence metabolic rates directly and indirectly. Therefore, two predictions were assessed: i) epilimnetic PPB and R would increase; and ii) the role of seasonality in epilimnetic metabolism would diminish due to the physicochemical changes induced by water loss. Two distinct periods were sampled, the first one, between 2011-2012 (P1), and the second one, between 2017-2019 (P2), when the lake volume was ca. 60% lower than during P1. Each period was also evaluated between seasons (rainy and dry season). The metabolic rates were estimated using high-frequency measurements and the inversing modeling approach. Moreover, the atmospheric flux was calculated using 10 different equations to understand the role of physical processes in metabolic rates estimates. In general, during P2 there was an increase in concentrations of total nitrogen, total phosphorus, chlorophyll-a, dissolved organic carbon, total suspended solids, and a decrease in light availability in the upper mixed layer. Both GPP and R increased during P2 (40% and 38%, respectively). While R rates were different between seasons, with the highest rates registered during the dry season, GPP was similar in both dry and rainy seasons, contrasting with P1, when both rates responded to seasonality. In relation with NEP, during P1 the lake was close to atmosphere equilibrium (-0.18 ± 0.29 mmol O2 m-3 d-1; mean ± standard error), however, in P2 there was a switching between autotrophy during the rainy season and heterotrophy in the dry season (6.44 ± 0.51 mmol O2 m-3 d-1 e -18.98 ± 1.81 mmol O2 m-3 d-1, respectively). Moreover, the decrease in lake volume during P2 turned the mixed layer depth more variable during the entire period. Since the physical processes also affect the DO oscillation in the water column, the change in the stability of stratification also contributed to the changes observed in metabolic rates. The results of this study showed that ecosystem metabolism changed during the period of persistent hydrological drought, and it may continue to change in response to the ongoing variation in rainfall and temperature patterns.