Accelerated carbon cycling after 50 years of warming and increasing hydrologic variability in a temperate stream

Rivers transform and transport much of the organic input they receive from terrestrial ecosystems. This carbon sustains stream food webs and fuels the production and release of carbon dioxide and methane to the atmosphere. Warming water temperatures and intensification of the hydrologic cycle due to climate change are likely to affect these carbon transformations and downstream transport in streams. Here, we examine the natural variability and long-term shifts in the metabolism of New Hope Creek, North Carolina, USA, site of the earliest published estimates of a stream's annual metabolic regime in 1969. We estimated annual ecosystem metabolism over 3 yr (2017–2020) and used the variability observed in the modern dataset to provide context for interpreting long-term change in response to climate drivers. We found that New Hope Creek was heterotrophic in all years, with highly seasonal carbon cycling. Much of the modern variability can be explained by water temperature and flow conditions. Warmer temperatures and longer periods of low flow conditions led to faster carbon cycling and increased heterotrophy, while autumn floods suppressed annual ecosystem respiration by reducing river carbon stocks. Comparing modern estimates to those from 50 yr ago, we find that New Hope Creek is now substantially warmer and has higher metabolic fluxes. Despite the limitations of inferring trends between two distant time points, we use modern data to hindcast metabolism and show how climate change has likely accelerated carbon cycling and shortened carbon residence time in New Hope Creek.