Large Rivers, Nitrogen and Fish: Current and Future Issues

While N is critical in supporting of life on earth, excessive N is environmentally detrimental.  Recent doubling of the global N supply has overwhelmed the natural N-removing biogeochemical processes, contributing to the eutrophication of freshwater and marine systems. Large rivers are often considered mere conduits, carrying land-derived nutrients and sediments to the oceans.  However, rivers with strong floodplain connections provide ideal conditions for N removal. Natural N-removal, via bacterial denitrification, occurs primarily in floodplain wetlands and lakes where carbon-rich, anoxic sediments provide conditions primed for conversion of nitrate-N to non-reactive dinitrogen (N₂) gas.  In addition to removing N, backwaters, wetlands and other floodplain habitats are also critical habitat for fish production in large river systems. Yet, eutrophication and declining N:P ratios (via elevated phosphorus concentrations) promote cyanobacteria dominance in backwater lakes – creating algae of low food quality, potentially leading to reduced fish health. Increasing demand for corn-derived ethanol and increased rainfall due to climate change will likely increase the rate of agricultural N and P delivery to rivers and subsequent downstream waters.  As well, increased eutrophication will likely enhance invasiveness of large rivers by non-native planktivorous fish. Maintenance of floodplain connectivity may ameliorate these impacts to some degree if properly managed.