Balancing carbon fixation and potential rates of sediment methane production in Mediterranean inland shallow lakes and ponds

Abstract Shallow freshwater lakes and ponds are the most abundant lentic ecosystems in the Spanish Mediterranean region. This study quantified the main components of carbon cycling in representative shallow lakes and ponds of different typologies by combining seasonal measurements of planktonic and benthic metabolism, together with ex situ estimates of sediment CH 4 production rates. Metabolic rates varied among ecological types, while trophic status, defined based on chlorophyll- a , was associated to metabolic variations. Overall, the studied ecosystems displayed heterotrophic behaviour with higher aerobic respiration than primary production in both plankton and benthos. However, the net production of helophytes exceeded the water-column metabolic balance, increasing the overall potential for carbon fixation and shifting the system tendency toward net autotrophy. Potential rates of sediment CH 4 production, characterized under controlled incubation conditions, represented a substantial carbon pathway and, due to the high radiative forcing of CH 4 , contributed to the greenhouse gas relevance of these ecosystems depending on subsequent oxidation and emission processes. Fluvial and especially soft water lakes and ponds showed high rates of sediment CH 4 production while simultaneously exhibiting capacity for carbon storage. Potential rates of sediment CH 4 production correlated well with sediment organic matter content but not with other ecological status indicators. These results demonstrate that carbon cycling in Mediterranean shallow lakes and ponds is strongly driven by ecological features such as sediment organic matter, macrophyte cover, and trophic status, which jointly regulate both carbon storage capacity and sediment CH 4 production.