New research shows that lakes play a key role in the carbon cycle by influencing how much carbon is released into the atmosphere versus how much flows downstream. Published in Geophysical Research Letters, the study analyzed 32 connected stream-and-lake networks in northern Sweden. Scientists measured carbon emissions and how much dissolved carbon moved downstream. They found that networks with 10 times more lake surface area had nearly twice the carbon emissions, emphasizing the need to view lakes and streams as interconnected systems rather than isolated parts. To understand this better, the research team measured carbon dioxide emissions from over 360 stream sections and 42 lakes. They also tracked how much dissolved carbon was transported downstream. Networks with more aquatic surface area had longer water retention times and higher carbon emissions compared to the amount of carbon that flowed downstream. One possible reason is that when water stays in the system longer, microbes have more time to break down dissolved organic carbon, eventually releasing it as carbon dioxide into the atmosphere. The balance between carbon emissions and downstream transport also varied by season. Emissions were lowest in spring and autumn and highest in summer. This is significant because summer conditions were warmer and drier, with lower stream flow. When less water is moving through a network, carbon remains in the system longer. Warmer temperatures also speed up the chemical processes that convert dissolved organic carbon into carbon dioxide. The study found that these patterns remained consistent even during an unusually dry summer, showing that both the amount of water and how it's distributed between lakes and streams affect carbon dynamics. However, the research does not suggest that lakes are the main source of carbon emissions. In fact, streams still produced more emissions overall. But in lake-rich networks, a greater proportion of the carbon was released into the atmosphere rather than carried downstream. Lakes can also store carbon in their sediments, adding another possible fate for carbon entering the system. The researchers argue that understanding the global carbon cycle requires looking at all parts of inland water systems together. Lakes, streams, wetlands, reservoirs, and floodplains all influence how carbon moves through a landscape—whether it's transported, stored, or released into the atmosphere. As the Arctic warms and hydrological conditions change, these connections may become even more important. The study suggests that the structure and shape of a water network might be just as important as the amount of water flowing through it when determining the fate of carbon.