The phenology of an ecosystem, or its seasonal timing of carbon, water and energy exchange is both a cause and a consequence of the phenology of all component organisms. While the magnitude of peak and cumulative ecosystem productivity can vary considerably across terrestrial and open water ecosystems as a function of nutrient supply, global variation in the phenology of ecosystem GPP in lakes, oceans, forests, deserts and grasslands is fairly easy to predict from climate data. The same cannot be said for river ecosystems for several reasons. First, in many rivers seasonal variation in light is uncorrelated with seasonal variation in temperature, because the reduction in light supply through canopy interception, sediment loads or colored organic matter can overwhelm differences in seasonal light regimes. Second, in the majority of rivers the algae, mosses and macrophytes that fuel aquatic ecosystem productivity are subjected to intense disturbance multiple times each year that may significantly reduce their biomass through scouring, burial or desiccation. Finally, most rivers receive energetic subsidies in the form of detritus and dissolved organic matter from their surrounding watersheds. These allochthonous inputs can match or exceed in situ GPP and thus decouple the seasonal and annual patterns of GPP and ER. For each of these reasons we expect a reduced coherence between climate drivers and the productivity of river ecosystems.
Dissolved oxygen data are collected continuously in many ongoing monitoring and research projects in rivers. Over the last two years, StreamPULSE collaborators have turned these valuable records into continuous metabolism estimates for several hundred (and counting) rivers. These records vastly expand our ability to examine annual patterns of metabolism within and across river ecosystems. Our emerging understanding suggests that hydrologic disturbance, riparian shading and river sediment loads are key determinants of seasonal and annual river productivity. All three of these critical drivers are affected by land use and climate change in ways that are likely altering both total annual productivity and ecosystem phenology (the timing of productivity peaks).