Whole‐lake methane emissions from two temperate shallow lakes with fluctuating water levels: Relevance of spatiotemporal patterns

Abstract

AbstractGlobally, shallow lakes are an important source of methane (CH4) emissions to the atmosphere. Previous studies of such lakes have rarely focused on the within‐lake spatiotemporal variability, which is critical for generating representative whole‐lake fluxes, and better understanding and constrain large‐scale emissions. To address this issue, we determined the variability of CH4 fluxes and CH4 concentrations in two small shallow (≤ 150 cm) lakes in Central Europe biweekly over almost 2 years. We found that both lakes were a source of CH4, mainly by ebullition. At the shallower Lake Heideweiher, which temporarily dried out, the average flux was 7.2 mmol m−2 d−1, the average flux from Lake Windsborn that never dried out, was 3.5 mmol m−2 d−1. The spatial differences (between and within lakes) were most strongly related to sediment C‐content and quality, which in turn was linked to depth or distance to shore. The highest fluxes occurred in the central parts of both lakes. The temporal variability of the fluxes was primarily correlated with sediment temperature and degree of drying measured as the time since drying up. The whole‐lake estimates were dominated by low water periods and the warm summer months. Overall, we show that short‐term and small‐scale measurements cannot account for the high variability of CH4 fluxes from small lakes, and that reliable large‐scale assessments need to consider such spatiotemporal variability.