Summer nitrate uptake and denitrification in an upper Mississippi River backwater lake: the role of rooted aquatic vegetation

Abstract

In-stream nitrogen processing in the Mississippi River has been suggested as one mechanism to reduce coastal eutrophication in the Gulf of Mexico. Aquatic macrophytes in river channels and flood plain lakes have the potential to temporarily remove large quantities of nitrogen through assimilation both by themselves and by the attached epiphyton. In addition, rooted macrophytes act as oxygen pumps, creating aerobic microsites around their roots where coupled nitrification-denitrification can occur. We used in situ 15 N-NO 3 -tracer mesocosm experiments to measure nitrate assimilation rates for macrophytes, epiphyton, and microbial fauna in the sediment in Third Lake, a backwater lake of the upper Mississippi River during June and July 2005. We measured assimilation over a range of nitrate concentrations and estimated a nitrate mass balance for Third Lake. Macrophytes assimilated the most nitrate (29.5 mg N m -2 d -1 ) followed by sediment microbes (14.4 mg N m -2 d -1 ) and epiphytes (5.7 mg N m -2 d -1 ) . Assimilation accounted for 6.8% in June and 18.6% in July of total nitrate loss in the control chambers. However, denitrification (292.4 mg N m -2 d -1 ) is estimated to account for the majority (82%) of the nitrate loss. Assimilation and denitrification rates generally increased with increasing nitrate concentration but denitrification rates plateaued at about 5 mg N L -1 . This suggests that backwaters have the potential to remove a relatively high amount of nitrate but will likely become saturated if the load becomes too large.

Vegetated canals mitigate nitrogen surplus in agricultural watersheds

“…Primary producers (e.g. benthic microalgae, epiphytes, submerged and emergent macrophytes) have been shown to strongly affect Nrelated microbial transformations (Eriksson, 2001;Forshay and Dodson, 2011;Kreiling et al, 2011;O'Brien et al, 2014;Soana and Bartoli, 2014). Benthic vegetation can inhibit or stimulate N loss via denitrification coupled to nitrification in the rhizosphere.…”

Geomorphic Approaches to Integrated Floodplain Management of Lowland Fluvial Systems in North America and Europe

“…In the Finger Lakes project described above (Johnson et al 1998), increased water inputs to isolated backwaters used to stimulate fish production also increased nitrogen removal. Nitrogen carried by the inflowing river water was rapidly lost through denitrification (James et al 2008a, b) and assimilation by periphyton (Kreiling et al 2010). The rate of nitrate reduction in a given lake was related to the rate of nitrogen loading to the lake and the sediment surface area for nitrate uptake.…”

“…During floods, when loading and flow-through rates were high, the hydraulic retention time was too short for more complete nitrate removal, and some nitrate was transported downstream. Further, the presence of dense macrophytes enhanced nitrate removal by providing a substrate for nitrate-assimilating periphyton, by increasing water retention and by promoting coupling of nitrification and denitrification (Kreiling et al 2010). Clearly, reconnection of backwaters to channels improves nitrogen-removal rates but only up to a certain nitrogen load.…”

Elevated levels of δ15N in riverine Painted Turtles (Chrysemys picta): trophic enrichment or anthropogenic input?

“…Differences in nitrogen cycling between rivers and ponds may compound the effects of nitrogen input in the river, as increased rates of benthic denitrification within a river result in elevated ␦ 15 N levels relative to ponds and lakes (Sebilo et al 2003;Piña-Ochoa and Álvarez-Cobelas 2006). Input of nitrates from inorganic fertilizer in the Mississippi River can also accelerate denitrification rates relative to nearby ponds or lakes (Richardson et al 2004;Kreiling et al 2011). The significantly elevated ␦ 15 N in river populations highlights the compounding effect of denitrification of inorganic fertilizer in riverine systems and suggests that this process can influence the nitrogen assimilated by higher vertebrates.…”

“…Nitrogen loading in freshwater systems has significant implications for ␦ 15 N levels, especially in agricultural systems. Because denitrification rates are greater in streams and rivers relative to ponds and lakes (Richardson et al 2004;Kreiling et al 2011), higher primary producer ␦ 15 N is expected in rivers relative to populations in standing water (Jansson et al 1994;Saunders and Kalff 2001;Piña-Ochoa and Álvarez-Cobelas 2006). Given that both trophic position and sources of biologically available nitrogen may cause variation in consumer 15 N among ecosystems, assessing the factors underlying spatial variation in consumer ␦ 15 N levels among aquatic ecosystems may be valuable in understanding stable isotopic variation in natural systems.…”