Interacting impacts of climate, eutrophication, and artificial drainage on aquatic greenhouse gas (CO₂, CH₄, N₂O) emissions

Abstract

Coastal areas are increasingly subject to both direct (land-use change) and indirect (climate change) anthropogenic pressures. Land-use change may produce ‘hot spots’ of aquatic greenhouse gas emissions through the combination of increased nutrient run-off and the creation of small artificial waterways (e.g., farm ponds and drains), but predicting future emissions from these landscapes also requires understanding the effects of extreme weather events. Here dissolved CO₂, N₂O, and CH₄ concentrations were measured both across the aquatic continuum (ponds, ephemeral ditches, irrigation drains, streams, tidal rivers, and lagoonal estuaries), and continuously over an extreme winter storm event. Combining these measurements with hydrodynamic modelling revealed the disproportionate greenhouse gas emissions contributed by small, nutrient-loaded waterways: ephemeral drains cover 5% of the total water surface area but produce 50% of emissions (4 – 7 Mmol d¯¹ CO₂-equivalents). However, in-situ storm measurements revealed that high wind winter storms can produce a 16-fold increase in estuary emissions (18 Mmol d¯¹ CO₂-equivalent) that dwarfs these ‘hot spot’ emissions. Wind-enhanced water-air gas transfer (k₆₀₀ from 5 cm h¯¹ under calm conditions to >30 cm h¯¹) was not the primary driver of storm emissions. Instead, storm conditions enhanced pore-water exchange with the eutrophied sediments and decreased primary productivity from floating macrophytes: CH₄ concentrations increased from 110 nM under fair weather conditions to 1,400 nM within hours of winds reaching 90 km h¯¹. Under current climate conditions these types of storms only occur a few days per season, but by triggering coastal emissions they can increase the seasonal greenhouse gas emissions from across the entire catchment by 30-50%. Together these findings demonstrate that including both high-wind predictions and small, artificial drainage features will be critical for accurately predicting future coastal climate feedback loops.