Organic matter and seasonal flooding control mercury and methane distribution in sediments of a floodplain lake in the Madeira River basin (western Amazon)

We investigated the spatial distribution and environmental controls of total Hg (THg), methane (CH 4 ), and nutrient accumulation in the sediments of an Amazonian lake during low-water and high-water periods. THg concentrations were similar between hydrological periods (0.103 ± 0.035 mg kg −1 in low water; 0.104 ± 0.039 mg kg −1 in high water) and were consistently associated with organic matter (TOC) and nitrogen (N). In contrast, CH 4 exhibited strong seasonal variability, with significantly higher concentrations during high water (7.94 ± 4.35 mg kg −1 ) compared to low water (4.43 ± 3.05 mg kg −1 ). Phosphorus (P) and water depth also increased markedly during high water, reflecting enhanced hydrological connectivity. Geostatistical analyses revealed that THg, TOC, and N shared similar spatial patterns, with higher concentrations in central and distal portions of the lake, whereas CH 4 was more spatially variable. Generalized least squares models indicated that THg was primarily controlled by TOC and pH during low water and by N during high water, whereas CH 4 was influenced by TOC in low water and by N and water depth in high water. Spatial congruence between THg and TOC (54.1%) and between THg and N (51.7%) was moderate, increasing substantially under relaxed criteria (>93%). Methane showed weaker spatial correspondence with its predictors. These results demonstrate that hydrological seasonality decouples Hg and CH 4 dynamics, with Hg strongly linked to organic matter and CH 4 responding to shifts in nutrient availability and water column conditions.