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PUBLICATIONS Journal of Geophysical Research: Biogeosciences RESEARCH ARTICLE 10.1002/2013JG002477 Key Points: • Floodwater monitored through the Atchafalaya River Basin during the 2011 flood • Large floods and resulting diversions lead to high river-floodplain connectivity • Backwaters remove nitrate resulting in decreased nitrate export to the GOM

Correspondence to: D. T. Scott, [email protected]

Citation: Scott, D. T., R. F. Keim, B. L. Edwards, C. N. Jones, and D. E. Kroes (2014), Floodplain biogeochemical processing of floodwaters in the Atchafalaya River Basin during the Mississippi River flood of 2011, J. Geophys. Res. Biogeosci., 119, 537–546, doi:10.1002/2013JG002477. Received 9 AUG 2013 Accepted 11 MAR 2014 Accepted article online 20 MAR 2014 Published online 21 APR 2014

Floodplain biogeochemical processing of floodwaters in the Atchafalaya River Basin during the Mississippi River flood of 2011 Durelle T. Scott1, Richard F. Keim2, Brandon L. Edwards2, C. Nathan Jones1, and Daniel E. Kroes3 1

Biological Systems Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA, 2School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, Louisiana, USA, 3Louisiana Water Science Center, U.S. Geological Survey, Baton Rouge, Louisiana, USA

Abstract The 2011 flood in the Lower Mississippi resulted in the second highest recorded river flow diverted into the Atchafalaya River Basin (ARB). The higher water levels during the flood peak resulted in high hydrologic connectivity between the Atchafalaya River and floodplain, with up to 50% of the Atchafalaya River water moving off channel. Water quality samples were collected throughout the ARB over the course of the flood event. Significant nitrate (NO3) reduction (75%) occurred within the floodplain, resulting in a total NO3 reduction of 16.6% over the flood. The floodplain was a small but measurable source of dissolved reactive phosphorus and ammonium (NH4+). Collectively, these results from this large flood event suggest that enhancing river-floodplain connectivity through freshwater diversions will reduce NO3 loads to the Gulf of Mexico during large annual floods. 1. Introduction Managing nutrient cycles to protect aquatic resources is one of the greatest engineering challenges in the coming decades. Nitrate (NO3) within the Mississippi River has increased threefold since the 1950s [Goolsby and Battaglin, 2001] and has been strongly linked to summertime hypoxia observed since the 1970s in the Gulf of Mexico [e.g., Rabalais et al., 2002; Scavia et al., 2003]. Diverting river flow and increasing floodplain connectivity is one suggested approach to reduce riverine nitrogen concentrations prior to reaching coastal estuaries [e.g., Mitsch et al., 2001]. However, flood control levees have severed the connection between much of the Lower Mississippi River and the adjacent floodplain, which prevents nutrient processing and removal prior to discharge into the Gulf of Mexico. Diversions from the lower Mississippi River onto the floodplain have been shown to be successful at nitrogen (N) and phosphorus (P) removal [e.g., Lane et al., 1999, 2004; Lindau et al., 2008; Gardner and White, 2010], but these studies have been limited to small diversions of less than 1% of the Mississippi River flood-stage flow. Substantially greater interaction between river flow and the floodplain occurs within the Atchafalaya River Basin (ARB), which receives a significant portion of Mississippi River flow. Most studies on the biogeochemical consequences of this interaction [e.g., Xu, 2006; BryantMason et al., 2013] have focused on main channel flow and have been hampered by water sample collection methods and imprecise stage-discharge relationships that make mass balances difficult to close [Turner et al., 2007]. While wetland sediments of the ARB have high potential for denitrification [Lindau et al., 2008], spatial variability in the floodplain ecosystem [Scaroni et al., 2010], heterogeneity of hydrologic flow paths [Sabo et al., 1999], and stage-dependent river-floodplain connectivity [Amoros and Bornette, 2002] combine to create a complex suite of processes that are not well understood at the basin scale. Furthermore, there are few opportunities to study the effects of large-scale river-floodplain connectivity. During the late spring of 2011, the historic flood in the Lower Mississippi River resulted in the second largest flow ever diverted into the Atchafalaya River. In this study, we observed the evolution of floodwater biogeochemistry throughout the southern ARB during this unprecedented flood, an area that experiences a high degree of river-floodplain interaction. We used repeated, synoptic sampling of water chemistry during the flood to estimate hydrologic connectivity between the main river channel and the floodplain during this large flood pulse. The objective was to understand how hydrologic connectivity throughout the lower basin alters nutrient fate and transport within and ultimately from the basin.

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©2014. American Geophysical Union. All Rights Reserved.

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2. Methods 2.1. Study Area The Atchafalaya River is a major distributary of the Mississippi River in south-central Louisiana, USA (Figure 1). The 2400 km2 Atchafalaya River Basin (ARB), defined by a series of levees, is the largest riverine swamp in the United States. Thirty per cent of the combined flow of the Red and Mississippi Rivers is diverted through the Old River Control structures into the Atchafalaya River and discharges into the Gulf of Mexico through the Morgan City and Wax Lake outlets (Figure 1). The Old River structures are capable of diverting up to 17,800 m3/s of discharge, and an additional 17,000 m3/s can be routed through the ARB via the Figure 1. The Atchafalaya River Basin, in southern Louisiana, receives Morganza Floodway during large events river water from the Mississippi and Red Rivers. Levees (brown lines) (Figure 1). In general, the upper basin is contain water within the basin and control structures at the top of the hydrologically disconnected from basin maintain incoming river flow. Sampling sites ranged from highly riverine flow and consists of rainfallconnected sites to backwater sites only connected during high flows. dominated bottomland hardwoods and agriculture. In the southern ARB, water exchange with the floodplain via channels and overbank flow increases, and vegetation transitions to near permanently inundated cypress-tupelo swamps. 2.2. Flood of 2011

ΔS [cms/1000]

Q [cms/1000]

In April 2011, portions of the central USA received record precipitation, resulting in the highest recorded flow in the Lower Mississippi River (U.S. Geological Survey (USGS) gaging station 07289000 at Vicksburg, MS). At the flood peak, over 19,000 m3/s entered the ARB, of which 94% was from the Mississippi River and the remainder from the Red River (Figure 2). The 20 Morganza Spillway was opened on 14 May—near a Old River Control the peak of the flood—to relieve pressure on Red downstream levees in Baton Rouge, and within 4 Morganza 10 days, a maximum of 5500 m3/s was being discharged through the Morganza Floodway. The diversion into the Morganza Floodway was 0 >2800 m3/s for 2 weeks, and represented over 13% 10 of the total discharge input into the ARB over the 6 b week period beginning mid-May. For scale, this volume of flow is enough to fill the lower ARB to 0 about 1.7 m depth in 2 weeks. Water entering the ARB via the Morganza Floodway mixed with water in the already flooded lower basin. During the −10 event, overland flow occurred in the majority of the 10 01 04 07 10 ARB, excluding dredge-spoil banks and most of the Month upper, western ARB where channel incision and Figure 2. (a) Flow entering the Atchafalaya River Basin from levees prevented flooding. the Mississippi River via the Old River Control structures, the Red River, and from the Mississippi via the Morganza Spillway. (b) The change in storage calculated by total inflow-total outflow. Data for 2011 from U.S. Geological Survey National Water Information System (http://waterdata.ugsg.gov.nwis).

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2.3. Event Sampling Water samples were collected and spot water quality monitoring occurred at 41 stations across


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Figure 3. Conceptual diagram for the water and nitrate-N mass balances performed for the entire basin and at the bottom of the basin.

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the lower ARB to representatively capture the flood pulse from the Morganza Floodway and the Atchafalaya River (Figure 1). Sampling took place weekly over 12 weeks from 19 May through 4 August. Samples were collected synoptically (within 2 days) by five separate teams from Louisiana State University, the U.S. Geological Survey, the U.S. Fish and Wildlife Service, and the Louisiana Department of Wildlife and Fisheries. All samples were collected from the center of the well-mixed swamp channels at 0.3 m below the water surface. Here, swamp channels are characterized by low velocities and no sand that traverse the floodplain [Kroes and Kramer, 2013]. Samples for deuterium analysis were collected at all sites in 20 mL glass containers with zero headspace. At 12 sites, samples for nutrients and carbon analysis were collected in 1 L prerinsed polyethylene bottles, transported on ice, and stored in the dark. All nutrient and carbon samples were transported to Louisiana State University within 24 h of collection and filtered through 0.45 μm syringe filters. Nutrient samples were stored in 125 mL polyethylene containers and frozen until analysis. Samples for carbon analysis (dissolved organic carbon) were stored in precombusted amber glass bottles and refrigerated until analysis (within 4 days). 2.4. Water Analysis

Ammonium (NH4+) and soluble reactive phosphorus (SRP) were analyzed colorimetrically on a segmented flow analyzer (SEAL AA3). Chloride (Cl) and nitrate (NO3) were measured on an ion chromatograph (Dionex ICS-3000). Dissolved organic carbon (DOC) and dissolved total nitrogen were measured on a Shimadzu TOC-Vcph analyzer. Dissolved organic nitrogen was calculated by difference. Dissolved iron (Fe) was measured on an inductively couple plasma atomic emission spectrometry (Spectro Analytical). Water isotopes were measured on an off-axis integrated cavity output spectroscopic liquid water laser isotope analyzer (Los Gatos Research DL-100), with a subset of samples run on a cavity ringdown spectroscopic liquid water laser isotope analyzer (Picarro L1102-i) for quality control. Concentrations were converted to standard notation, which is deviation (δ) of concentration from that of standard mean ocean water, expressed in parts per thousand (‰). 2.5. Flood Connectivity and Nutrient Retention Estimates We coupled measurements of conservative and reactive tracers to quantify floodplain-source water connectivity. We used naturally occurring, stable isotope deuterium (2H or D) because biogeochemical processes do not affect deuterium’s concentration in water. Thus, D can be considered conservative and used to identify water flow paths. Isotopic composition is modified during phase changes, which leads to natural variability and allows identification of water source when sufficient separation in isotopic compositions of the sources exists [Gat, 1996]. Because of strong latitudinal effects in isotopic composition of precipitation, the Mississippi River has a generally lower concentration of heavy isotopes than does local precipitation and runoff [Kendall and Coplen, 2001]. Thus, water in regions of the ARB dominated by local rainfall is expected to be richer in heavy isotopes than in regions with high river connectivity. In addition, evaporation leads to in situ fractionation of D, which increases the difference between Mississippi River water and disconnected backwaters. The proportion of flow through the riparian floodplain was estimated by a mass balance approach (Figure 3). NO3 was selected as the tracer because of the nonconservative behavior within the floodplain, which provided the ability to distinguish river versus floodplain altered water. We assumed that there was no NO3 processing in the Atchafalaya River, which is consistent with little NO3 removal in other large rivers during high flow (e.g., upper Mississippi) [Richardson et al., 2004]. We also assumed incoming river contributions SCOTT ET AL.


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Table 1. Concentrations of NO3 and Dissolved Phosphorus (μM) for the ARB Outlets at Morgan City (USGS 07381600), Wax Lake (USGS 07381590), and the Mississippi River at Baton Rouge (USGS 07374000) From the USGS National Water Information System Water-Quality Database Morgan City Outlet Date

NO3

(μM)

Wax Lake Outlet

Pdiss (μM)

NO3

(μM)

Mississippi at Baton Rouge

Pdiss (μM)

Date

NO3 (μM)

Pdiss (μM)

10/27/10 12/8/10 1/10/11 2/9/11 3/8/11 3/21/11 4/6/11 4/20/11 5/9/11

106 100 107 128 160 131 117 121 79

1.2 0.9 0.6 0.6 0.8 0.6 0.5 0.8 0.6

5/23/11

88

0.7

6/6/11

81

0.8

6/27/11

137

1.0

8/22/11

79

1.2

10/26/10 12/7/10

111 88

1.2 0.8

109 90

1.2 0.8

2/8/11

109

0.7

108

0.7

3/24/11 4/5/11 4/19/11 5/11/11 5/18/11 5/25/11 6/1/11 6/9/11 6/15/11 6/22/11 6/29/11 7/13/11 7/27/11 8/24/11

112 96 112 69 65 66 56 46 69 92 102 110 94 71

0.6 0.6 0.7 0.5 0.4 0.4 0.6 0.9 1.0 1.1 1.2 1.1 1.2 1.2

118 106 118 71 76 77 64 61 84 110 117 121

0.5 0.6 0.7 0.5 0.5 0.5 0.6 0.8 0.9 0.9 1.0 1.0

76

1.5

from the Mississippi River (Old River Control Structure and Morganza Spillway) and the Red River (presenting less than 6% of the flow) were the same NO3 concentration as the main stem Mississippi River at Baton Rouge (site 07374000). The resulting mass balance equations for NO3 and water are the following, where 2 unknowns are flow through the floodplain (QFloodplain) and flow through the lower river (QLowerRiver): QLowerRiver C River þ QFloodplain C Floodplain ¼ Qoutlet-MC C outlet-MC þ Qoutlet-WL C outlet-WL

(1)

QUpperRiver þ QMorganzaFloodway ¼ QLowerRiver þ QFloodplain

(2)

where QLowerRiver = daily discharge in the lower Atchafalaya River bypassing the floodplain (unknown), CRiver = NO3 in the Atchafalaya River (assumed equal to measured USGS NASQAN site 07374000; see Table 1), QFloodplain = daily discharge through the floodplain (unknown), CFloodplain = NO3 at the swamp outlet (measured in this study), Qoutlet-MC = daily discharge at the Morgan City outlet (measured; USGS site 07381600), Coutlet-MC = NO3 at Morgan City (measured), Qoutlet-WL = daily discharge at Wax Lake outlet (measured; USGS 07381590), Coutlet-WL = NO3 at Wax Lake (measured USGS NASQAN), QUpperRiver = daily discharge in the upper Atchafalaya River at Simmesport (measured; USGS site 07381490), and QMorganzaFloodway = daily discharge from the Morganza Floodway (measured; USGS). We used our sampling site at the floodplain outlet as representative for NO3 concentrations of exiting floodplain water. Although river water is not transported uniformly through the floodplain, this sampling site at the floodplain outlet integrates the net biogeochemical processing from the range of hydrologic flow paths (both slow and fast) throughout the floodplain. Net water storage was also calculated as the difference between incoming (QUpperRiver + QMorganzaFloodway) and outlets (Qoutlet-MC and Qoutlet-WL). Net NO3 and P removal within the floodplain was estimated on a daily basis throughout the 2011 water year (1 Oct–30 Sep) using the following mass balance equation and using hydraulic velocity to assume a travel time within the ARB: MFloodplain;i ¼ QUpperRiver;i5 C River;i5 þ QMorganzaFloodway;i5 C River;i5 Qoutlet-MC;i C outlet-MC;i Qoutlet-WL;i C outlet-WL;i

(3)

where MFloodplain is the mass of NO3 -N or dissolved inorganic P reduced within the floodplain, i represents the daily time step, C represents NO3 or dissolved inorganic P concentrations, and Qs are the same as described in equations (1) and (2). Nutrient concentrations over the 2011 water year were obtained from the Wax Lake, Morgan City, and Mississippi-Baton Rouge NASQAN sites (Table 1), and daily concentrations were linearly interpolated between sampling dates. For the purposes of modeling rates of biogeochemical

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transformation, we used the hydraulic residence times estimated from flow peaks. This rough estimate neglects complexities such as stagedependent travel times and mixing with preevent water but is consistent with the fact that the majority of flow was in channels.

−20

δ D (‰)

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−35

−50 06

07

08

Month Figure 4. Variation in deuterium excess (δD ‰) at 41 sites within the basin during and postflood. Each line represents a single sampling station categorized into three site types (orange = backwater; blue = mixed; and grey = riverine).

3. Results and Discussion 3.1. Hydrologic Pathways and Travel Times

The 2011 flood in the lower Mississippi River resulted in the third highest recorded flow through the Atchafalaya River Basin. During the flood, water routed from the Mississippi River via the Atchafalaya River spread across the lower ARB into backwater regions of the floodplain downstream of the main channel levees in the northern half of the ARB. Floodwater also entered the ARB from the Morganza Floodway (Figure 2a), which mixed with existing water and was transported toward the ARB outlets. Water travel time within the eastern ARB (from the outlet of the Morganza Floodway to the outlet of the ARB) estimated from the Morganza flood peak was 3.8–7.3 days (~5 days, which we used subsequently for biogeochemical modeling), which translates to a mean velocity of 0.15–0.3 m/s. In contrast, mean flow velocity in the Atchafalaya River over the same period was 1.9 m/s (< 1 day travel time), as estimated from river gage data between Simmesport and Morgan City. The temporal variation in water storage over the 2011 water year highlights storage within the ARB during the flood peak (Figure 2b). As the flood pulse moved through the basin, water moving from the swamp channels into the floodplain likely filled nonflooded regions and backwaters. Following the flood peak, net storage fluctuates close to zero for the remainder of the water year. Although water balance estimates can be a problem in the ARB [Turner et al., 2007], the cumulative estimates of discharge balance within 0.5% for the 2011 water year. The temporal variation in deuterium was used to classify our samples sites across the river-floodplain gradient. At the flood peak, deuterium across all sites was 36.4 ± 1.4 ‰ but deviated significantly across sites following the flood pulse (Figure 4). During the flood recession, the observed deviation in deuterium is consistent with increased water residence times throughout the floodplain (Figure 4) as less river water entered the floodplain due to a decrease in hydrologic connectivity. During this period, δD regimes across our sampling sites fell into three distinct groups: (1) riverine sites where δD followed the river by decreasing steadily after the peak; (2) backwater sites where δD increased steadily after the peak, indicating mixing with local rain and fractionation by in situ evaporation; and (3) mixed sites with intermediate behavior. We therefore classified each site into these three categories for examining nutrient transformations across our sampling network, based on δD for the final sampling date in August when the differences among sites were most pronounced (Figures 1 and 4). At the peak of the flood (late May through June), our results show that there was extensive mixing and hydrologic connectivity throughout the lower ARB. Deuterium, dissolved organic carbon (DOC) (474 ± 110 μM), and specific conductivity (289 ± 21 μS cm1) were all within narrow ranges across all of our sampling stations through the middle of June (Figures 5a and 5b). During the initial sampling date, there was some differentiation for DOC across our sites, which is consistent with an increase in storage (Figure 2b) and subsequent flushing of preevent water. Our estimated discharge through the floodplain (QFloodplain; see equations (1) and (2)) suggests that up to 50% of river water moved off river and traveled through the floodplain during peak flow and that connectivity remained high through mid-June (Figure 6). 3.2. Nutrients During Peak Discharge (May–June) Although the deuterium data illustrate the river and floodplain were well mixed during the peak of the flood (May–June) (Figure 5a), our spatial and weekly sampling suggest that nutrient transport through the swamp was nonconservative. Despite the short travel time, NO3 was significantly lower during the flood peak within the backwater and mixed water sites (43 μM and 44 μM, respectively) in contrast to the river sites SCOTT ET AL.


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(63 μM) (Figure 5c) and was the dominant form of dissolved nitrogen over the event (NO3 was 70% of the total dissolved nitrogen). Although NO3 was 30% lower within the swamp compared to the river sites, dissolved organic nitrogen (DON) was not significantly different (26 ± 17 μM across all sites during flood peak). The lower dissolved oxygen (DO) concentrations (Figure 5d) within the floodplain (30% lower than river sites) suggest more widespread reducing conditions, which promote denitrification. In the Upper Mississippi River Basin, similar DO depletion in early summer is caused by both aquatic vegetation and microbial activity [Kreiling et al., 2011]. NH4+, which is released during organic matter mineralization, was 20% higher in the backwater sites (Figure 5e). The higher NH4 concentrations are likely from a combination of the inhibition of nitrification (oxidation of NH4+ to NO3) in lowoxygen environments, decomposition and flux out of the organic soil [VanZomeren et al., 2013], and mixing with preevent water from the largely hypoxic backswamp. SRP was also elevated in the backwater sites during the initial flood peak (Figure 5f), likely from mixing with preevent floodplain water.

Figure 5. (a–e) Mean periodic (May–June peak and July– August recession) and weekly concentrations for the three site types identified by isotopic separation in August (b = backwater, m = mixed, and r = riverine). Error bars represent 1 standard error. Mean concentrations within each period that are significantly different at p < 0.05, based on Bonferroni-adjusted post hoc tests, are marked by different roman numerals.

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During the flood pulse, the estimated NO3 removal was significant within the floodplain. This result is best seen from observations at our most southerly floodplain site, which integrates the net biogeochemical processes during transport. Here mean NO3 concentrations at the floodplain outlet (32 μM) were significantly lower than NO3 at the basin outlet (59 μM; p = 0.02) and the Mississippi River NASQAN station (84 μM; p = 0.03) over the same time period. When a NO3 mass balance was performed for floodplain water using this southerly floodplain site, upstream NO3, and the calculated floodplain water discharge (Figure 6), NO3 reduction in the floodplain was calculated to be approximately 75% during the flood peak. In addition to our own measurements, NASQAN measurements at the basin outlets further support the floodplain influence on NO3 transport. The Morgan City NASQAN site, which receives a greater portion of floodplain water from the eastern portion of the basin, had 15% lower NO3 concentrations over the flood period than did the Wax Lake NASQAN site, which receives more direct flow from the Atchafalaya River (see Table 1). Lastly, there is a possibility that a portion of the NH4+ was nitrified within the floodplain (since DO was not below 2.0 mg/L) and subsequently denitrified, which in turn would mean our results underestimate NO3 removal.


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3.3. Nutrients During the Flood Recession (Late June–August)

During the recession limb of the flood pulse, the increased residence times in the backwaters that allowed for isotopic separation (Figure 4) also allowed for greater biogeochemical processing 0 06 07 08 that explain the observed differences in NO3, Month NH4+, and SRP. NO3 all but disappeared in the backwater sites, in contrast to the river sites over Figure 6. Flow moving through the swamp as a percentage of the flood recession, where river NO3 the Atchafalaya River estimated for three scenarios: blue = all concentrations increased during the summer outlet NO3 equal to Wax Outlet; black = all outlet NO3 equal to Morgan City; red = outlet NO3 proportional to respective values. months. For NH4+ and SRP, concentrations were higher at the backwater sites than at the river sites (Figures 5e and 5f). The high dissolved Fe concentrations within the backwaters during the recession (Figure 5g) are consistent with reducing conditions and subsequent iron reduction, resulting in both phosphorus release and the inhibition of nitrification [e.g., Shenker et al., 2005]. 50

The higher dissolved organic matter concentrations (DOM, as measured by dissolved organic carbon or DOC) within the backwater sites during the flood recession are also consistent with enhanced organic matter breakdown and DOM release under reducing conditions [e.g., Grybos et al., 2009]. Generally, less than 25% of wetland-derived DOM is bioavailable [e.g., Fellman et al., 2008; Wiegner and Seitzinger, 2004] and is likely to be consumed during hydrologic disconnection [Hein et al., 2003]. However, this floodplain-derived DOM flushed during reconnection can be photodegraded into bioavailable components during transport [e.g., Sulzberger and Durisch-Kaiser, 2009]. Once in the coastal water of the Gulf of Mexico, the transported DOM may undergo further photodegradation and bacterial mineralization, which results in oxygen depletion [Rabalais et al., 2010]. 3.4. Impacts of Hydrologic Connectivity to Nutrient Export

Pdiss ret. [Mg/day]

NO3- ret. [Mg/day]

Hydrologic connectivity and transport through the floodplain swamp moderated NO3 export from the ARB. The net NO3 reduction for the entire basin during the 2011 flood varied from < 10% to > 37% daily, with the highest rates of retention during the peak of the flood (Figure 7a). The net mass reduction from 15 January to 15 August (which represents over 80% of the total annual N-flux through the basin) was 45.5 Gg, which translates to a total of 16.6% NO3 reduction over the 2011 flood (14% over the entire 2011 water year). 1000

a

500

0

60

b

40 20 0 −20 10

01

04

07

10

Month Figure 7. (a, b) Estimated net nitrate and dissolved phosphorus reduction across the 2011 water year estimated by mass balance at daily time steps between inflow and outflow from USGS. Black dots indicate NASQAN samples collected.

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A recent analysis of the NO3 mass balance across the flood for the entire basin using grab samples collected from the riverbank suggested that net ARB reduction was only 7% from 15 May to 20 July [BryantMason et al., 2013]. In contrast, calculations using USGS NASQAN data suggest NO3 reduction was 20% over the same time period (15 May to 20 July). Multiple sources of uncertainty might account for discrepancies in the two estimates, including poor stage-discharge relationships during this large event leading to incorrect estimation of mass balances, lack of in-channel mixing leading to bias in water samples, in-channel processing of nitrate leading to violations of our assumptions, or assuming all of the incoming water was from the Mississippi River. Because BryantMason et al. [2013] and this study used the same flow data, we believe the largest difference pertains to sample collection (our samples and the USGS NASQAN samples were collected in the center of the channel, in contrast to shoreline


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sample collection in BryantMason et al. [2013]). Our conclusion of relatively high NO3 removal from our detailed spatial and temporal observations of water quality is consistent with previous studies showing highdenitrification potential within the ARB [e.g., Lindau et al., 2008; Scaroni et al., 2011], suggesting our results are more representative of ARB nitrogen removal. Over the course of the flood, dissolved nitrogen was dominated by NO3 accounting for 70% of the total dissolved nitrogen concentrations within our river sites and 60% within the swamp sites. In contrast to previous monthly and annual estimates of nitrogen retention within the basin [e.g., Xu, 2006], our study does not show any NH4+ or dissolved organic nitrogen (DON) removal during transport. In fact, the swamp generated NH4+ (Figure 5e). This is consistent with other studies that show hydrologically disconnected floodplains can accumulate organic matter and NH4+, thus increasing risk of hypoxia to downstream waterbodies upon reconnection [e.g., Tockner et al., 1999; Bechtold et al., 2003; Hein et al., 2003]. Previous studies have also shown that a portion of the NH4+ pool will be nitrified during transport [Forshay and Stanley, 2005], which still provides a nitrogen source to the coastal zone. Further study into the effects of altered hydrologic connectivity within backwater swamps on DOM generation and mineralization, in addition to the subsequent transformations of NH4+, are required to fully understand overall nitrogen retention within these systems across different flow regimes. Our results also suggest that backwater swamps may release dissolved phosphorus if (1) there is connectivity to the river and (2) there is adequate time for development of reducing conditions that result in dissolved P release. The net SRP retention for the entire basin varied from < 20 to 60 Mg/d (Figure 7b). During the rising limb and peak of the flood pulse, this calculation using the limited USGS data suggests there is SRP retention, which was largely due to the net water storage from the water balance (Figure 2b). However, our own SRP observations suggest SRP release over this same period (Figure 5f). During the flood recession, there appears to be a small but measurable dissolved P release (Figure 7b and Table 1), which is consistent with our own water column SRP observations within the floodplain. The Morgan City outlet also had higher dissolved P concentrations than the Wax Lake outlet, which is consistent with this outlet receiving a greater proportion of water from the floodplain (in contrast to the river). Although the ARB is a net sink for sediment [Hupp et al., 2008] and therefore sediment-bound P, release of bioavailable P to the coastal estuary during large events may impact primary productivity [Sylvan et al., 2006; Bianchi et al., 2010; Turner and Rabalais, 2013] including the potential for harmful algal blooms [Bargu et al., 2011]. In an investigation of a restored floodplain wetland in the Upper Mississippi Basin, Kreiling et al. [2013] found that while the wetland was a net sink of N, P, and sediment, both SRP and NH4+ were released during high flow events. Our results using only SRP do not capture the entire pool of phosphorus; further examination including both dissolved and particulate phosphorus is required to determine the potential for both phosphorus release and retention. 3.5. Implications for Large-Scale Diversions Large-scale river diversions of river water onto floodplains have been proposed to reduce downstream nutrient fluxes through a number of biotic and abiotic processes. If floods increase in response to climate variability and climate change [e.g., Diffenbaugh et al., 2005], the opening of the Morganza Spillway and other flood control structures may become more common. Although NO3 diverted into Lake Pontchartrain through the Bonnet Carre Spillway was not significantly reduced relative to the riverine load [Roy and White, 2012], diverting water into the ARB and increasing river-floodplain connectivity resulted in substantial NO3 reduction and a release of SRP and NH4+ during this event. Many of the biogeochemical transformations we observed during this record flood also occur within the portion of the floodplain that floods more frequently, suggesting that the floodplain is likely to impart a signal during more commonly observed floods. Previous research has shown that within portions of the Gulf of Mexico, phosphorus is commonly found to be limiting [e.g., Quigg et al., 2011; Sylvan et al., 2006]. Thus, careful considerations into management decisions to promote nitrogen removal also need to take into account P and NH4+ mobilization that could potentially counteract the benefits of NO3 reduction. The Danube floodplain restoration is one example that illustrates the trade-offs between ecological productivity and nutrient/organic matter retention and or export [Tockner et al., 1999]. Furthermore, increasing floodplain connectivity may alter the floodplain ecosystem, yielding an unintended response (e.g., increased export of organic matter). Although some work has investigated the effect of ecotype on, for example, denitrification [e.g., Scaroni et al., 2011], and other work has investigated effect of hydrologic change on ecosystem structure [e.g., Keim et al., 2006], the interaction between the two

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remains complex and not well understood [Houser and Richardson, 2010]. Ultimately, it is clear that management of connectivity between channelized flow and adjacent floodplains is an essential component of any diversion strategy [Schiemer et al., 1999].

4. Conclusions Our results from this large flood and resulting diversion of Mississippi River water into the ARB documents the wide-scale mixing of river water throughout the ARB, ultimately altering nutrient export downstream. The observed nutrient sinks and sources in the floodplain matched expectations: NO3 was removed from the floodwater during transport within the floodplain, coupled with small increases in both NH4+ and SRP. Spatial variability of water chemistry increased on the falling limb of the flood pulse, presumably due to increased time for biogeochemical processing resulting from hydrologic disconnection from the floodplain. Collectively, these results suggest that enhancing river-floodplain connectivity through freshwater diversions will reduce NO3 loads to the Gulf of Mexico during large annual floods that may become more common due to under a changing climate.

Acknowledgments This work was supported by NSF grant 1141363 to Virginia Polytechnic Institute and State University and NSF grant 1141417 to Louisiana State University. We thank John White for useful comments on the manuscript, and Jody David, Richard Day, Raynie Harlan, Sarah Javed, Brett Rivers, Mike Walker, and David Walther for water quality sampling efforts during the 2011 flood. Data used in preparation of this manuscript are available by contacting the corresponding author. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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