Spatial and temporal variability of greenhouse gas ebullition from temperate freshwater fish ponds

Fish ponds with their typically high carbon and nutrient inputs are relevant sources of greenhouse gases. However, not much is known about gas bubble emissions (ebullition) and their high spatiotemporal variability. This is the first study which quantified diffusive and ebullitive greenhouse gas emissions from temperate fish ponds. To improve greenhouse gas estimates, we investigated the diurnal and spatial variability of diffusive and ebullitive fluxes in 12 extensively to semi-intensively managed fish ponds near Bautzen, Germany. Emissions differed greatly between the different ponds but methane was consistently the predominant greenhouse gas. The feeding sites were hotspots with one order of magnitude higher ebullition rates compared to other parts of the ponds. At these hotspots, ebullitive fluxes of up to 38 L/m2d were measured with a mean bubble methane content of 79%, corresponding to a methane flux of 1.24 mol/m2d. Methane accounted for 90% of the global warming potential in one fish pond but carbon dioxide emissions of up to 242 mmol/m2d at the feeding sites were also significant. Nitrous oxide fluxes, in contrast, were low with 5 ± 9 μmol/m2d. Greenhouse gas ebullition decreased exponentially along a transect from the feeding site into the pond and showed some diurnal fluctuations. While diffusion was higher during night, ebullition rates increased in the morning, presumably caused by higher benthivorous fish activity. Our results highlight the potential of temperate fish ponds as significant greenhouse gas sources and ebullition as a significant pathway. For robust quantification, both small scale spatial and temporal variability as well as the hotspot of the feeding area must be considered.