Phenology of gross primary productivity in rivers displays high variability within years but stability across years

Modeling and sensor innovations in the last decade have enabled routine and continuous estimation of daily gross primary productivity (GPP) for rivers. Here, we generate and evaluate within and across year variability for 59 US rivers for which we have compiled a 14-yr time series of daily GPP estimates. River productivity varied widely across (median annual GPP 462 g C m⁻² yr⁻¹, range 19–3445 g C m⁻² yr⁻¹) and within rivers (CV_GPP-Inter 5.7–37.3%). Within this dataset, we found that five rivers have become consistently more productive over time, while 11 rivers have become consistently less productive. Furthermore, trends in ecosystem phenology were identified, where cumulative annual GPP was reached earlier (n = 3) and later (n = 13) in the year across the 25^th, 50^th, 75^th, and 95^th percentiles. Understanding the drivers of productivity trends in rivers will elucidate patterns in river food webs and the functional role of river biogeochemistry.