Seasonal variation of water isotopes in different forest ecosystems in the Central Amazon

The Central Amazon comprises mosaics of forest ecosystems with different water dynamics and soil characteristics. The water dynamics from each ecosystem affect the evaporation signal seasonally expressed in the water isotopes (δ¹⁸O and δD). The recognition of the evaporative signal from different forest segments is essential for the development of hydrological and eco-hydrological studies in the complex Amazon biome. In this study, we used stable isotopes to evaluate how the water dynamics of different forest ecosystems affect seasonal water evaporative signals in each environment. We monitored water isotope signals from 2018-2020 in different compartments (precipitation, soil, stream, groundwater, river, lake, and flooded areas) of two non-flooded forests (clay soils - “plato” and sandy soils – “campinarana”) and three flooded forests (pristine igapó, disturbed igapó and várzea). We found that the soil water sampled by lysimeters in the upland forest seasonally expresses the isotope signal from the rainwater (Local Meteorical Water Line-LMWL) without a strong evaporative overprint. The water isotope signal from flooded forests is more variable. The isotopic composition of pristine rivers has an overlapping signal from the rain isotope signal (LMWL). However, water from the river downstream from the very large hydropower dam (Balbina) has a strong evaporative signal. During the flooded period, the water within the flooded forests has a more evaporated signal than the signal from the source (such as the river or lakes). During the non-flooded period, the water isotope signal from the soil inside the flooded forest corresponds to the rainwater signal. To the best of our knowledge, these represent the first description of the water isotope signals from the compartments in different Central Amazonian forest ecosystems. They illustrate and identify the high variation of the evaporative signal from the complex Amazon biome, knowledge that is essential to understanding how different forest ecosystems influence water recycling in the Amazon hydrological cycle.