38 years of declining and stable chlorophyll-a trends in northern US rivers

Human activities, extreme weather events, increasing temperatures, and changes in rainfall patterns have led to drastic alterations to rivers, globally. These changes can profoundly affect primary producers, which are the foundation of food webs and aquatic community stability. Portions of the northern United States have experienced long-term environmental change and support a variety of land uses, yet productivity patterns in large rivers remain poorly understood. Our objectives were to (1) estimate chlorophyll-a concentrations, a surrogate for primary productivity, for large rivers across the northern United States, (2) evaluate 38-year trends in chlorophyll-a concentrations in each river, and (3) assess regional patterns in chlorophyll-a trends. We analyzed 33 sites across 18 rivers located in the northern half of the continental United States. We used a boosted regression tree with Landsat imagery and in-situ environmental variables to estimate chlorophyll-a concentrations when observed data were lacking (R² = 0.79). We found that over the 38-year span, chlorophyll-a concentrations decreased in nine of the eighteen rivers analyzed, occurring in regions of intensive agricultural watershed land-use. Rivers with no long-term trends (9 of 18 rivers) were in regions dominated by forested land cover. The long-term decreases in river primary productivity we observed may be attributed to the increased precipitation and river discharge through time and use of regenerative agricultural practices. Human alterations to river basins have changed the biogeochemical aspects of many rivers. However, conservation and restoration initiatives can mitigate the adverse effects of these changes and reduce the ecological consequences of eutrophication.