Nitrification and denitrification rates were estimated simultaneously using a 15N dilution technique in sediment-water columns of Lake Cataouatche that receives diverted Mississippi River water. Labeled and unlabeled NO3 were added to surface water of replicated sediment-water columns and changes in the concentrations of labeled and non-labeled nitrate in water were determined over time. The average rate of total nitrite plus nitrate NO2 + NO3 decrease from the water columns was 16.2 mg N m−3 d−1, whereas 15N-labeled (NO2 + NO3)-N decreased at the rates of 5.8 mg 15N m−3 d−1 over the 57 d incubation period. The averaged rates of nitrate reduction and nitrification were 51.8 μmol N m−2 h−1 (43.5 mg N m−3 d−1) and 30.8 μ mol N m−2 h−1 (25.9 mg N m−3 d−1), respectively. Results indicate the lake sediment has capacity to process nitrate entering the system via denitrification. Nitrification occurring simultaneously at the sediment-water interface was also a significant process representing 59.4 percent of the denitrification rate. Water quality issues associated with nitrate levels in diverted Mississippi water entering Lake Cataouatche should consider both the coupling of nitrate reduction of river water nitrate and nitrification of nitrogen in nitrogen-enriched lake sediment. Nitrification in bottom sediment is a significant nitrate source and should be regarded as important factor in the determination of the maximum daily load of nitrate that the lake can effectively assimilate without adversely affecting water quality.

Introduction

Mississippi River water is currently being diverted into Louisiana coastal wetlands for slowing or reversing marsh deterioration attributed to the rapid subsidence (DeLaune et al., 2003). The re-introduction of Mississippi River water into Louisiana coastal wetlands and estuaries also introduces nutrients. There has been controversy about possible eutrophication impacts of such diversion to water quality change within Louisiana coastal zone (Turner and Rabalais, 1994). The primary nutrient associated with water quality in freshwater systems is usually phosphorus. However, coastal systems including Louisiana Barataria Basin, which receives diverted river water, are N-limited. Current freshwater input into Barataria Basin is almost exclusively from rainfall. A 30% increase in freshwater inflow associated with diversion containing higher concentration of nutrients can potentially contribute to eutrophication of the Barataria Basin watershed. Nitrate in Mississippi River waters during spring is 50 times higher than nitrate in Barataria Basin waters. Silicate and phosphorus levels are several times higher. Nitrate-N in the Mississippi River at New Orleans is on the order of 1 mg N l−1 (Battaglin et al., 2001).

The potential negative impact associated with nutrient enrichment creates interest in quantifying N-processing in wetlands receiving diverted Mississippi River water. There is evidence that agricultural runoff and wastewater from human activity in tributary watersheds can degrade coastal ecosystems. Nuisance algal blooms and low oxygen in bottom waters can kill fish and shellfish, and has been well documented (Anderson et al., 2002) off the coast of Louisiana in the so-called Gulf Dead Zone (Rabalais et al., 1996). Denitrification, which removes N in such systems, is important because it represents a direct loss to the atmosphere (Knowles, 1982). Denitrification occurs mainly at the sediment-water interface. Nitrate in river water entering Louisiana wetlands, when in contact with anaerobic soil or sediment surface, can be biologically reduced to gaseous N (Gale et al., 1993). However, the sediment may also serve as a source of nitrate to the water column. Mineralization of organic N to ammonium-N and the subsequent nitrification in the surface oxidized layer, can also be a source of nitrate to the water column. Louisiana coastal restoration efforts and estuaries' N-budgets are intertwined. In this study, we quantify nitrate removal and production in sediment-water column from a freshwater lake through which Mississippi River water is being diverted into Louisiana Barataria Basin.

Materials and methods

Study location

The Davis Pond Diversion is located near Luling, Louisiana in St. Charles Parish on the west bank of the Mississippi River. The freshwater diversion structure is capable of delivering a maximum discharge of 300 m3s−1 through four 4.3 m × 4.3 m gated box culverts (Addison, 1999). The diversion will potentially benefit 134,000 ha of marsh in Barataria Basin estuary, a 314,400 ha wetland complex hydrologically bound on the east by the Mississippi River levee and to the west by Bayou LaFourche. The diversion discharge follows a course south through a 3,400 m long channel under the Highway 90 underpass and into a 3,700 ha ponding area bound by constructed levees before entering Lake Cataouatche (Fig. 1). Lake Cataouatche is a shallow (mean depth ∼ 2 m) freshwater lake in the northern portion of the Barataria Basin estuary with a mean tidal range of 0.03 m. Lake Cataouatche has open connections with Lake Salvador to the south, through which discharge water will reach the brackish and salt marshes in the lower reaches of Barataria Basin estuary and finally the Gulf of Mexico.

Laboratory technique

Surface sediments were collected with a Peterson dredge on January 23, 2004 from two sites in Lake Cataouatche (Site 1 and Site 2). Site 1 is approximately one half kilometer into the lake, while Site 2 is in the center of the lake (Figure 1). The sediments were kept in black plastic bags and sent to laboratory.

In the laboratory, the sediment was thoroughly mixed and composited. Subsamples of the surface sediment were measured and placed into large plexiglass columns (62 cm in height by 14 cm inside diameter). Sediment-water columns (3 replicates for each site) consisted of an approximately 6.0 cm layer of sediment and 40 cm of overlying water. The sediment-water columns were preincubated at 21°C for several months in order to allow for equilibration and establishment of a thin surface-oxidized layer at the sediment-water interface. After the sediment-water columns had equilibrated, overlying water was removed and fresh lake water added with the volume of water added recorded. After equilibration for several days, non-labeled and 15N-labeled NO3 (NaNO3, minimum 98 atom% 15N) were added to the water column to produce an initial concentration of about 107 μmol N (32 atom% 15N). The systems were incubated in the dark at 21°C for 57 d. At scheduled times (1, 9, 15, 22, 29, 36, 43, 50, 57 d), water samples (110 ml for each column) was removed from each column and filtered through 0.45 μm filters. The filtrate was stored in a refrigerator at 0°C and kept frozen prior to analysis. Concentrations of the dissolved inorganic NO2 + NO3 were estimated by using a Lachet QuickChem (LACHAT Instrument, QuickChem ® 8000, Automated Ion Analyzer, Zellweger Analytics, Inc.). The isotope ratio analysis was determined at the University of Illinois.

Isotopic dilution calculation using labeled and non-labeled N added in sediment-water columns were used to determine both nitrate reduction and nitrification rates (DeLaune and Smith, 1987; Lindau et al., 1988). If only nitrification and NO3 reduction are responsible for changes in the NO3 pool in the sediment-water columns, their rates can be calculated using the equations outlined by Koike and Hattori (1978). Briefly the equations are

formula
formula
where Y = NO3 reduction; Z = production of NO3 by nitrification; 1 and 2 were the different observation times, and a are the average isotopic content of the NO3 and NH4 between observations t1 and t2, respectively. Natural 15N abundance (0.37 atom % 15N) was assigned for a, because the amount of natural NH4 present in the microcosm was large compared with that of 15N-NH4 produced by dissimilatory NO3 reduction to NH4. Chemical characteristics of sediments were determined by the Soil Testing Laboratory at Louisiana State University.

Results and discussion

Chemical characteristics

The chemical characteristics of sediments for Sites 1 and 2 are shown in Table 1. The sediment was slightly acidic with a pH between 6.5 and 6.7. The Ca and Mg content were relative higher than that of other elements determined, such as Na, K, and P. The total N contents were 0.62% and 0.76%, respectively, for the two sites.

Nitrate reduction and production

Concentrations of inorganic NO2 + NO3 in the water of sediment-water columns, with their incubation time, and their combined 15N content are shown for Site 1 and Site 2 (Figure 2 and Figure 3). Results showed that decreases in 15N-labeled and the total (NO2 + NO3)-N were linear. Correlation coefficients ranged from 0.881 to 0.973, and all were highly significant (P < 0.001). The rates of total NO2 + NO3 decrease were 15.2 and 17.1 mg N m− 3 d−1, whereas 15N-labeled NO2 + NO3 decreased at rates of 6.0 and 5.5 mg 15N m− 3 d−1, respectively. The results suggest that the Lake Cataouatche sediment has the capacity to process significant nitrate via denitrification. The results also demonstrated that nitrification is a significant process in the system.

Nitrate reduction (denitrification) averaged 51.83 μ mol N m− 2 h−1 (43.53 mg N m− 3 d−1) over the 57 d incubation period in the two sediment-water columns for the two sites. Nitrification (nitrate production) averaged 30.79 μ mol N m− 2 h−1(25.86 mg N m− 3 d−1), representing about 59.4% of the denitrification rate (Table 2).

The measured nitrification (30.79 μ mol N m− 2 h−1) and NO3 reduction (51.83 μ mol N m− 2 h−1) in the sediment-water column for this study are within the range (ca. 10–90 μmol N m− 2 h−1 and 50–134 μ mol N m− 2 h−1, respectively) of values previously reported for coastal and estuarine sediments (Nishio et al., 1983; DeLaune and Smith, 1987; Seizinger et al., 1984). Using the 15N dilution method, DeLaune and Smith (1987) reported that nitrate reduction in sediment from a coastal Louisiana lake averaged 134 μ mol N m− 2 h−1 over a 50-d incubation period in the sediment-water columns and nitrification averaged 90 μ mol N m− 2 h−1, about 70% of the denitrification rate. Lindau et al. (1988) reported the NO3 production rate in soil-water columns averaged 32 μ mol N m− 2 h−1and reduction 101 μ mol N m− 2 h−1 also using the same 15N dilution. Seizinger et al. (1984) reported NO3 release by sediments to the flood-water, which suggested that the measured denitrification rate (50-100 μ mol N m− 2 h−1) was dependent on nitrification. Nishio et al. (1983) reported nitrification and denitrification inferred by NO3 disappearance to be in the range of 10 to 40 μ mol N m− 2 h−1. Wang et al. (2003) reported that nitrification was the major source of NO3 for denitrification, representing 56% to 79% of total denitrification rates among the three estuary stations in Québec.

Water being diverted from the Mississippi River at Davis Pond first passes through a 3,764 ha ponded freshwater marsh before entering Lake Cataouatche. It has been recently shown that amount of nitrate in river water passing through the ponded freshwater marsh is governed by discharge rate. At a discharge rate of 35 m3 s−1, the ponded wetland effectively removed N in N-enriched lake sediment from diverted river water found in concentration of 1 ppm NO3-N. At a discharge of about 100 m3 s−1, less than 30% of the nitrate was removed by the ponded freshwater marsh (Johnson, 2004).

During high discharge or pulsing events, a significant amount of nitrate in the diverted Mississippi river water passes through the ponded wetland into Lake Cataouatche (Johnson, 2004). The amount of nitrate-N in water entering the lake at discharge rate of 100 m3 s−1 is on the order of 660 mg N m3 s−1, an amount equivalent 5700 kg NO3-N per day. Assuming that the area of the Lake is 8,000 ha, this would be a N-loading rate of 0.7 kg ha−1 d−1 or 70 mg N m−2 d−1. This study showed NO3-N removal was in the range of 10.3–17.4 mg N m−2d−1, an amount insufficient to process all the N passing through the ponded freshwater marsh into the lake at high discharge or pulsing rates. There are other removal mechanics such as uptake by submerged aquatic and phytoplankton; however, results suggest that discharge or pulsing should perhaps be regulated so that the ponded wetlands removes most of the nitrate limiting the amount entering Lake Cataouatche. Denitrification is an anaerobic process, occurring in the sediment. On the other hand, nitrification is an aerobic process that can occur in water column, or at the thin surface oxidized layer at the sediment-water interface where redox conditions may fluctuate over time. These two processes (nitrification and denitrification) are coupled and can occur simultaneously at this interface where redox condition may support both nitrification and denitrification. Nitrate produced in the oxidized surface layer diffuses down into the reduced sediment layer where it undergoes denitrification. Data from this study showed that there is a significant amount of nitrification occurring in surface sediment of Lake Cataouatche, which contributes a significant amount of nitrate to the water column. The two processes are important factors necessary for quantifying the amount of nitrate that the Lake can effectively assimilate without impacting water quality of the Lake.

Acknowledgments

This work was supported by Louisiana Sea Grant Program and Louisiana Department of Natural Resource. We would like to thank Dr. Richard Mulvaney's group at the University of Illinois for analyzing the 15N samples.

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