Nitrogen cycling in a freshwater estuary

Abstract

Freshwater estuaries may be important control points but have received limited research attention, emblematic of a general under-appreciation of these ecosystems and the services they provide. These ecotone environments exist at the interface of rivers flowing into large lakes, where seiches cause mixing of lotic and lentic waters within flooded river deltas. We assessed the dissolved inorganic nitrogen (DIN) retention and processing controls in the Saint Louis River Estuary (SLRE), which receives inputs from rivers, urban sources, and Lake Superior. Nitrate (NO3–N) was the dominant form of DIN and was consistently highest in the lower estuary due to seiche-delivered Lake Superior water and nitrification of ammonium from urban sources. The estuary transitioned from a net NO3–N source during high flows to a net sink during summer baseflow conditions. NO3–N availability controlled site-specific denitrification rates while sediment organic matter explained the spatial pattern in denitrification potential. As the estuary shifted from a riverine state to one with more lake influence, seiches delivered Lake Superior NO3–N to the lower portion of the estuary, alleviating the final denitrification control and activating the estuary’s ‘denitrification pump’. This amplified removal condition is maintained by critically delivered NO3–N during periods of warm temperatures and long residence times. Often these controls are unsynchronized in streams where NO3–N is typically lowest during summer baseflow. Similar ephemeral biogeochemical processes are likely found within other seiche-prone lakes where organic-rich sediments accumulate at river mouths and are supplied with chemically distinct lake water during low flow periods.