BENTHIC METHANOGENESIS IN AMAZONIAN LAKES UNDER WARMER EXPERIMENTAL CONDITIONS: RESPONSES IN FLOODPLAINS OF WHITE- AND BLACKWATER RIVERS.

The greenhouse effect is essential for global thermal regulation, but human
activities have intensified greenhouse gas (GHG) emissions and accelerated
global climate change. Among these gases, methane (CH4) stands out due to its
high global warming potential, approximately 27 to 30 times greater than that of
CO2 over a 100-year timescale. CH4 is produced naturally in anoxic
environments by methanogenic archaea through organic matter degradation.
Amazonian floodplains account for up to 29% of global wetland CH4 emissions.
The biogeochemical dynamics of these ecosystems vary according to water
type (white, clear, or blackwater), influencing fertility and organic carbon quality.
Temperature increases can accelerate organic mineralization and stimulate
methanogenesis, establishing a positive feedback loop in the climate system.
The objective was to evaluate the effects of experimental warming (+4 °C,
simulating the RCP 8.5 scenario) on benthic methanogenesis in surface
sediments of Amazonian floodplain lakes, comparing a whitewater lake
(Cujubim Lake, Madeira River) and a blackwater lake (Redondo Lake, Negro
River). Sediment cores were sectioned into 0–15 cm and 15–35 cm fractions

and incubated under anoxia at 25 °C and 29 °C for 21 days, with CH4
production quantified using gas chromatography (Agilent 7890B). A three-way
ANOVA revealed that sampling site, sediment depth, and temperature
significantly influenced methanogenesis (p < 0.05). The sampling site was the
main driving factor (35.43% of total variation), followed by depth (21.49%) and
temperature (8.90%). Cujubim Lake acted as a CH4 production hotspot,
displaying rates substantially higher than those of Redondo Lake, particularly in
the 0–15 cm layer, driven by a high input of labile organic matter of Andean
origin. In Redondo Lake, dominated by refractory organic compounds from the
forest, production was more constrained. Warming from 25 °C to 29 °C caused
no significant changes in most treatments, except in the deeper layer (15–35
cm) of Redondo Lake, where CH4 production increased significantly at 29 °C,
suggesting higher direct thermal sensitivity in deep anoxic sediments isolated
from CH4 oxidation. In conclusion, sampling site and sediment depth exert
primary control over benthic methanogenesis, overriding the isolated effect of
experimental warming. However, thermal stimulation in deeper, refractory
sediments highlights that the response of Amazonian ecosystems to climate
change critically depends on regional limnological heterogeneity.