Aquatic Organic Carbon and Nutrient Fluxes, Water Quality, and Aquatic Productivity in the Atchafalaya Basin, Louisiana 1

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1 This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. Aquatic Organic Carbon and Nutrient Fluxes, Water Quality, and Aquatic Productivity in the Atchafalaya Basin, Louisiana 1 Victor W. Lambou 2 Abstract.--Aquatic carbon and nutrient fluxes, water quality, and productivity in the Atchafalaya Basin, Louisiana are reviewed" Overflow areas had large areal net exports of nitrogen and dissolved organic carbon but acted as a sink for phosphorus. Ammonia levels increased dramatically during the summer. Primary production in the overflow areas was primarily above the water surface and energy chains within the water are basically dependent upon heterotrophic production. INTRODUCTION The Atchafalaya Basin contains one of the largest remaining floodplain bottomland hardwood forests in the United States and supports major sport and commercial fisheries and outdoor recreational uses dependent upon the annual flooding of the forest (Lambou 1985). This paper reviews aquatic carbon and nutrient fluxes, water quality, and productivity in the Basin. DESCRIPTION OF AREA The Atchafalaya Basin (fig. 1) comprises an 8,345 km 2 lowland floodplain area confined between natural levee ridges that delineate the present and former courses of the Mississippi River. Its overall dimensions are approximately 72 by 193 km with elevations ranging from 15 m to sea level. There are six segments in the Atchafalaya Basin which have integrity due to manmade levee systems. These are: (1) the 287 km 2 Morganza Floodway, (2) the 611 km 2 West Atchafalaya Floodway, (3) the 340 km 2 Pointe Coupee Sump Area, (4) the 2,129 km 2 Atchafalaya Basin Floodway--historically subject to fre uent and prolonged natural flooding, (5) the 259 km 1 leveed Atchafalaya River and other segments located mainly between the upper Atchafalaya River and levees and Old River, and (6) the 4,719 km 2 East and West Basin areas 1presented at The First North American Riparian Conference, Riparian Ecosystems and their Management: Reconciling Conflicting Uses, Tucson, Arizona, April 16-18, Victor W. Lambou is with the Environmental Monitoring Systems Laboratory, United States Environmental Protection Agency, Las Vegas, Nevada. isolated by levees. Of most value from a fisheries and recreational standpoint is the Atchafalaya Basin Floodway, which during normal years has in excess of 1,619 km 2 flooded by overflow from the Atchafalaya River. Water normally enters the Basin from two major sources. A portion of the Mississippi River's flow enters through the Old River Control structures and eventually joins with the Red River to form the main stem of the Atchafalaya River. The main stem flows are then confined by levees until the river enters the Atchafalaya Basin Floodway. There the water spreads out through distributaries and during high water by overbank flows over almost the entire Atchafalaya Basin Floodway. The water exits through ~he Wax Lake and Lower Atchafalaya River outlets. The Atchafalaya River receives approximately 30% of the combined flows of the Mississippi and Red Rivers at the latitude of the Old River Control structures. Atchafalaya River discharges show both seasonal and annual variation. The "average shifted hydrograph" for the Atchafalaya River at Simmespart indicates the seasonality of flows (fig. 2). The hydrograph was computed from daily discharges at the latitude of Old River and adjusted to 30 percent of the total flow to account for the presence of the Old River structure~. Each year's hydrograph was shifted to peak on April 15 (the day the unshifted average hydrograph peaked) before averaging, since daily averages of unshifted hydrographs result in considerably lower peak stages than is representative of actual conditions. The cyclic nature of the flows shown in figure 2 also describes the typical annual regime of overflow for the Atchafalaya Basin Floodway. The other segments of the Basin are not subjected to the same type of prolonged overbank flooding. 180

2 Figure 1.--The Atchafalaya Basin, Louisiana. ~ u M A M A S 0 N 0 Figure 2.--Average daily shifted hydrograph for the Atchafalaya River at Simmesport, Louisiana, for the period , adopted from U.S. Army Corps of Engineers (1982). Detailed comparisons are made in this report for three distinct hydrological subunits within the Basin. Two of these, Fordoche and Buffalo Cove, are part of the Atchafalaya Basin Floodway and show seasonal water-level patterns that mimic those of the Atchafalaya River. The Fordoche subunit receives some local runoff from outside the leveed Atchafalaya system which is not true of other hydrological subunits within the Atchafalaya Basin Floodway. The third subunit, Pat Bay, is isolated from the historical influence of the river by manmade levees. Water-level patterns in this unit are now determined exclusively by local runoff. Hydrological patterns of the three subunits studied are compared in table 1. MATERIALS AND METHODS A total of 148 sampling stations were located throughout the Atchafalaya Basin. Locations included the major inlet and outlets to the Atchafalaya Basin Floodway and major water exchange ~ E 0 points within the Atchafalaya Basin Floodway. Helicopters were employed to collect water and biological samples on 14 separate occasions from at which time 80 to 130 stations throughout the Basin were sampled in approximately 10 days. Intensive sampling of five areas (Fordoche, Buffalo Cove, Pat Bay, inlet of the Basin proper, Atchafalaya River main stem, and the outlets of the Basin proper - Wax Lake and Morgan City) was conducted using small boat crews on approximately a monthly basis. Integrated samples for chlorophyll a (CHLA) and organic carbon were collected by pumping. CHLA samples were integrated from the lower limit of the photic zone to the surface while carbon samples were integrated from top to bottom. Samples for total phosphorus (TP), dissolved orthophosphate (OP), nitrite-nitrate nitrogen (N02N03N), ammonia (NH3N), total kjeldahl nitrogen (KJEL), dissolved oxygen (DO), total alkalinity (TALK), and ph were collected at selected depths to represent the water column. In extremely shallow water or in fast moving main channel areas, samples were collected only from the surface. In a previous study, it was determined that in fast flowing waters in main channel areas physical and chemical constituents are uniformly mixed (U.S. DI 1969). Samples were collected on a total of 1,301 station-day combinations. The sampling methods and design are more fully described by Hern et ale (1980). Analyses were performed in acccordance with the procedures described in EPA (1971, 1974) except for chlorophyll a and water transparency. Chlorophyll a analyses-were performed according to the fluoro;etric procedure described by Yentsch and Menzel (1963) and corrected for pheophytin while water transparency was measured in the field by Secchi disk readings. Total organic carbon (TOC) 181

3 Table 1.--Comparison of three hydrological subunits within the Atchafalaya Basin Period Area Area Average maximum depth Hydrological subunit (area) flooded (months) flooded (km2 ) flooded (%) of flooding (m) Type of flooding Buf f a 10 Cove Within Atchafalaya Basin Floodway (91 km 2) Fordoche Within Atchafalaya Bas in F loodway (270 km2 ) Pat Bay Outside Atchafa1aya Basin F100dway (45 km2 ) < >1.1 < >1.0 < >0.4 Overbank flooding: a) headwater to backwater; h) active exchange of water between overbank areas, permanent water bodies and major channels. Overbank flooding: a) backwater to headwater; b) active exchange of water between overbank areas, permanent water bodies and major channels. Flooding associated with local precipitation events: a) flooding generally short-lived and involves relatively small percent of the area; b) shallow "rainwater swamps" created with flooding periods highly variable ranging from 1 to 11 months; c) minor water exchange between "rainwater swamps" and permanent water bodies; d) water loss from "rainwater swamps" largely by evapotranspiration. was determined from unfiltered samples while dissolved organic carbon (DOC) and dissolved nutrients were determined from samples filtered through type HA 0.45-micron Millipore filters. Particulate organic carbon (POC) was determined by subtraction. Total nitrogen (TN), organic nitrogen (ON), and inorganic nitrogen (ION) were determined by addition or subtraction as follows: KJEL + N02N03N, KJEL - NH3N, and NH3N + N02N03N, respectively. See Hern et ale (1980) for more details. In order to determine ambient parameter means, calculations were performed in such a way to give equal weight to each depth sampled at a station on a particular day, each day sampled at a station during a time interval, and each station sampled in a given geographical area during a time interval. If a parameter was not sampled on at least three different days during i time interval, the station was not used in determining the mean value for the time interval. The ratio of TP to ION (Nip) was determined only when data points for TP, NH3N, and N02N03N values were all available. Seasons were defined as follows: winter, December 21-March 20; spring, March 21-June 20; summer, June 21-September 20; and fall, September 21-December 20. High water was defined as any time flows in the Atchafalaya River at Simmesport were> 4,799 m 3 s-1 (the mean discharge for a 9-year period, ) and low water as any time flows were < 4,799 m 3 s-1. Using topograhic data from U.S. Army Corps of Engineers and U.S. Geological Survey maps, a storage curve was developed for the Fordoche, Buffalo Cove, and Pat Bay hydrological subunits. These curves relate water-level changes at key gauging stations to the net volume of water added or released from the subunit during a given time interval. Time intervals over which net storage changes were determined extended over one or more days during which water-level changes were in a single direction. Storage changes due to precipitation and evapotranspiration were determined by using daily precipitation values from the nearest U.S. weather station and average monthly evapotranspiration rates as obtained through water balance calculations from long-term records ( ) at those stations. The remaining part of the storage change then represented the net change in water volume as a result of inflow, outflow, or a combination of the two. Annual net inflow and outflow volumes determined for the period July 1, 1976 through June, 1977 are: Buffalo Cove Fordoche Pat Bay 254 1, , These volumes were then proportioned to the various water exchange points within the Fordoche or Buffalo Cove subunit. For the Pat Bay subunit, intermittent discharge measurements over the period did not allow for apportioning of outflow or inflows and did not suggest simultaneous occurrence of inflow or outflow. This subunit was therefore treated as a single inlet-outlet system in which net storage change due to the flow was equated to actual inflow or outflow. Carbon fluxes were determined by multiplying carbon concentration times flow volumes at each major opening of the various hydrological subunits for the period July 1, 1976 through June, ~977. Carbon and nutrient fluxes for the Atchafalaya Basin Floodway were determined by using published daily flow records for the Atchafalaya River (U.S. GS 1976, 1977). As flow data were only available for the inlet to the Floodway (115, m 3 for the period July 1, 1976 through June, 1977), it was assumed that all water entering the system must exit through the two outlets. This is reasonable considering the fact that changes in volume due to precipitation and evapotranspiration are negligible compared to the volume of water (fig. 2) entering and leaving the system. The inflow water was proportioned to 60 percent exiting through the Lower Atchafalaya River outlet and 40 percent through the Wax Lake outlet, based on data collected by the U.S. Corps of Engineers. Fluxes were determined by multiplying concentrations times flow volumes during the period July 1, 1976 through June, A measured concentration on a particular sampling day was assumed to be representative of the concentration in the inflow or outflow for a period of time halfway back to the previously and forward to the next measured concentration. 182

4 RESULTS AND DISCUSSION Carbon and nutrient fluxes are presented in table 2. Even though the entire leveed Atchafalaya Basin drains directly or indirectly through the Atchafalaya Basin Floodway, I feel it is inappropriate to use the Basin's total area to calculate or determine areal net export for the Atchafalaya Basin Floodway. The processes which control internal input to the system, as well as deposition and conversion, take place mainly within the area subject to overflow, and this area is totally within the Atchafalaya Basin Floodway. Fundamental differences exist in carbon fluxes between those areas which are subject to extensive overflow (Atchafalaya Basin Floodway, Buffalo Cove, and Fordoche) as compared to the non-overflow subunit (Pat Bay). The overflow areas had large annual areal net exports of DOC while the non-overflow subunit had a relatively low areal net export. Areal export of POC was high in the non-overflow subunit while the overflow areas acted as sinks for POCo Weighted mean DOC concentrations in the outflow of the overflow areas increased over the concentrations in their inflow. There were some differences in carbon fluxes between the Atchafalaya Basin Floodway as a whole and its two overflow subunits which were sampled. The Atchafalaya Basin Floodway acted as a sink for TOC mainly through the loss of POCo Quantities of POC decreased by 35 percent in its outflow over that in its inflow, while quantities of POC decreased by only 17 percent and 16 percent in the outflow of its two overflow subunits. Net export of DOC was very similar for all three areas. The greater deposition rate of POC in the Atchafalaya Basin Floodway can be explained by historical changes in sedimentation patterns and water flows. Flows from the Mississippi River began to be diverted on a regular basis to the Atchafalaya River in the mid-1900s (Fisk 1952). Diversion steadily increased until 1963 when the Old River Control structures (fig. 1) were placed into operation in order to prevent the complete capture of the Mississippi River by the Atchafalaya River. The deposition of sediments associated with the increase flows have almost eliminated Grand and Six Mile lakes which originally occupied a significant portion of what is now the Atchafalaya Basin Floodway (Fisk 1952, Roberts et al. 1980). Presently, the main area for sedimentation is rapidly changing to the Bay area below Morgan City. However, significant amounts of sediments are still being deposited in remanent lakes and backswamp areas as well as on developing natural levees in the lower portion of the Atchafalaya Basin Floodway. I believe that as part of this process, large quantities of POC are being deposited causing the Basin to act as a sink for TOC and, Import Weighted mean (mg 1-1 ) Atchafalaya Basin 1Fl oodway (2,129 km 2 ) TOC DOC POC TP OP TN NH3N N02N03N KJEL ON ION 1,119, , ,762 31,746 5, ,734 12,713 71,792 61,942 49,230 84, Buffalo Cove (91 km 2 ) TOC 2,173 DOC 652 POC Fordoche (270 km2) TOC 13,513 Table 2.--Annual nutrient and organic carbon fluxes for the Atchafalaya Basin Floodway and organic carbon fluxes for Buffalo Cove, Fordoche, and Pat Bay Gross Export Weighted kg l03 mean (mg l-1) 899, , ,443 24,427 6, ,260 16,969 76,914 80,346 63,378 93,882 2,881 1,621 1,260 15,738 10,286 5, Net export kg ,534 23, ,319-7,319 1,517 23,526 4,256 5,122 18,404 14,148 9, ,225 3, Ratio of net export to import (%) Aeral net export (kg km-2 ) -103,116 11, ,288-3, ,050 1,999 2,406 8,644 6,645 4,405 7,776 10,650-2,873 8,240 11,636-3,396 DOC POC 7,145 6,369 Pat Bay (45 km 2 ) TOC , ,490 DOC ,105 POC ,385 1If the area is considered to be the entire leveed Atchafalaya Basin (3,626 km 2 ) the areal net export in kg km-2 for TOC, DOC, POC, TP, OP, TN, NH3N, N02N03N, KJEL, ON, and ION would be -60,544; 6,560; -67,104-2,018; 418; 6,488; 1,173; 1,413; 5,076; 3,902; and 2,586, respectively. 183

5 when sediment deposition decreases sufficiently, the Atchafalaya Rive~ will become a net exporter of TOC to the estuarine area below Morgan City. The Atchafalaya Basin Floodway had a large areal net export of TN mostly in the form of ON (table 2). Weighted mean concentrations increased in the outflow over the concentrations in the inflow for all forms of nitrogen. The Atchafalaya Basin Floodway acted as a sink for TP, mainly in the form of particulate phosphorus. The quantity of OP increased by 29 percent in the outflow over that in the inflow. Taylor et al. (1984) has reviewed inputoutput nutrient studies in forested wetlands reported in the literature. They found that for all four studies reviewed, the wetlands acted as sinks for TN. However, Mattraw and Elder (1984) found that the overflow floodplain of the Apalachicola River in Florida exported TN. The outflow mass was 20 percent greater than that in the inflow which is similar to the 18 percent for Atchafalaya Basin Floodway. Taylor et al. found that for eight studies, the forested wetlands served as a sink for TP which is consistent with the decrease of 23 percent found in the outflow over that in the inflow of the Atchafalaya Basin Floodway. However, Mattraw and Elder found 23 percent more TP in the outflow from the Apalachicola River's floodplain. Undoubtedly the sedimentation in the lower portion of the Atchafalaya Basin Floodway discussed previously has had some effect on nutrient export from the Atchafalaya Basin Floodway. Ambient parameter means for selected geographic areas within the Atchafalaya Basin Floodway are given in table 3. Both TP and nitrogen levels found in the Atchafalaya Basin are high. NH3N levels dramatically increase during the summer in the Atchafalaya Basin Floodway undoubtedly due to the mineralization of ON via ammonification. Nip ratios are relatively low in the Atchafalaya Basin suggesting nitrogen limitation for autotrophic production (Lambou et al. 1976). However, both organic and inorganic forms of nitrogen and phosphorus are biologically available to bacteria. Complete metabolism (mineralization of carbon) at optimal rates by bacteria requires carbon:nitrogen:phosphorus ratios of approximately 100:10:1 to 100:5:1 dependent upon bacterial community composition and environmental conditions (Alexander 1961). In addition, many of the constituents of the total organic pool do not undergo rapid breakdown and therefore do not represent a significant drain upon the available nitrogen pool. Herbaceous material and leaf litter flooded during high water stages in the Basin contain sufficient nitrogen to support their breakdown by bacteria. Also, undoubtedly nitrogen fixation is an important source of nitogen to the Atchafalaya Basin as evident by the relatively large areal export of TN. Significant fixation would be expected under the conditions of fairly low Nip ratios that occur in the Atcha alaya Basin. Dierberg and Brezonik (1983) found that nitrogen fixation was in important source of nitrogen in natural cypress swamp domes in Florida. CHLA levels outside the leveed Atchafalaya Basin were much higher than those inside the leveed Atchafalaya Basin system (table 3) even though nutrient levels were high in both areas. The relatively lower SD readings within the Table 3.--Ambient parameter means areas within the Atchafalaya for selected geographic Basin Louisiana TOC DOC POC TP OP NH3N N02N03N KJEL DO CHLA SD TALK (mg' I-I ) (mg'i-1 ) (mg' I-I) (~g'i-1 ) (~g'1-1 ) (~g'rl) (~g'r1) (~g '1-1) (mg'i-1 ) (~g '1-1) (I") ph (mg '1-1 ) Nip 1Leveed Atchafalaya Basin System (79) Atchafalaya Basin Floodway (57) Winter Spring Summer Fall High water Low water Fordoche (16) Winter Spring Summer , Fall High water Low water , Buffalo Cove (16) Winter Spring Summer , Fall High water Low water , Outs ide leveed Atchafalaya System (37) Pat Bay (5) Winter Spring Summer Fall Number of stations located within a geographic area is given in parentheses. 2Atchafa1aya Basin F100dway exclusive of the Fordoche area. 184

6 Atchafalaya Basin Floodway reflect higher mineral turbidity levels. Primary production within the overflow area is primarily above the water surface and on dry land during low water periods while energy chains within the water are basically dependent upon heterotrophic production. Hern and Lambou (1977) found phytoplankton to be approximately five times as high in Pat Bay (8,000 cells'ml- l ) as in Buffalo Cove (1,800 cells'ml- l and Fordoche (1,500 cells ml- l. Aquatic primary productivity estimates based on oxygen production and carbon dioxide uptake were found to be in the order of two to five times ~~ea~ir in Pat Bay ( and 26.8 C02 mmol'm 'h ) as c~mpa~ed to Buffalo Cove (5.2 O 2 and 5.4 CO 2 mmol'm =;h =1 and Fordoche (7.5 O 2 and 16.3 CO 2 mmol'm 'h ). Based on data presented by Hern and Lambou, aquatic production/ respiration ratios for Buffalo Cove and Fordoche were 0.19 and 0.34, respectively, indicating an heterotrophic system. DO levels within the Atchafalaya Basin Floodway are sometimes relatively low because of decomposition of litter in the flooded forests (table 3). The mean DO saturation was only 61 percent inside the leveed Atchafalaya Basin system while outside the leveed system it was 80 percent. Insufficient DO in the water column can cause fish kills and/or anaerobic decomposition of organic material with concomitant releases of ammonia and other noxious gases such as hydrogen sulfide. However, before the onset of anaerobic conditions DO levels became too low to provide for the maintenance and well-being of the fish population as a whole. DO levels of 2 and 4 mg'l- l were selected to compare conditions which may be detrimental to fish populations (table 4). These levels have been reported to impact growth and development of fish or to produce lethal Table 4.-- the time DO concentrations were below 2 and 4 mg'l-l by water level and selected geographic area Season Water Level Winter Spring Summer Fall Hi~h Low Total 1Atchafalaya Basin Floodway No_ of days measured time (2 mg-i time (4 mg-i Buffalo Cove No_ of days measured time (2 mg-i time (4 mg-i Fordoche No- of days measured 29 ~ time (2 mg-i time (4 mg-i Atchafalaya Basin Floodway exclusive of Fordoche_ conditions even though finfish and crawfish have been found in DO levels as low as 1 mg'1-1 in the Fordoche area (Lambou 1985). The majority of the low DO problems occurred in the spring. The Atchafalaya Basin Floodway is extremely productive of aquatic life, and finfish and crawfish harvest rates of 8,797 km'km-2 'yr-1 per maximum area flooded have been documented (Lambou 1985). The key to the high productivity of the Atchafalaya Basin Floodway is the short, efficient, bacteria-detritus food chain (Lambou 1985). Given sufficient nutrients (nitrogen and phosphorus) and adequate DO levels, carbon becomes the lifeblood of the aquatic system. Prolonged overbank flooding, with the inundation of additional land areas containing herbaceous material and forest litter renew the carbon resources needed to drive the bacteria-detritus system. LITERATURE CITED Dierberg, F. E., and P. L. Brezonik Nitrogen and phosphorus mass balances in natural and sewage-enriched cypress domes. Journal of Applied Ecology 20: Fisk, H. N Geological investigations of the Atchafalaya Basin and the problem of Mississippi River diversion. U.S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, Miss. Vol. 10, 145 pp. Hern, S. C" and V. W. Lambou Productivity responses to changes in hydrological regimes in the Atchafalaya Basin, Louisiana. In: Proc. Int. Symp. on Environmental Effects of Hydraulic Engineering Works, pp Tenn. Valley Authority, Knoxville, Tenn. Hern, S. C., V. W. Lambou, and J. R. Butch Descriptive water quality for the Atchafalaya Basin, Louisiana. EPA-600/ , Environmental Monitoring Series. 168 pp. Lambou, V. W., L. R. Williams, S. C. Hern, R. W. Thomas, and J. D. Bliss Prediction of phytophlankton in lakes. In: Proceedings of the Conference on Environmental Modeling and Simulation, EPA-600/ pp Lambou, V. W Importance of bottomland hardwood forest zones to fish and fisheries: the Atchafalaya Basin, a case history. To be published in the Proceedings of Bottomlands. Hardwoods Workshops, St. Francesville, La. 63 pp. Mattraw, H. C., and J. F. Elder Nutrient and detritus transport in the Apalachicola River, Florida. U.S. Geological Survey water - supply paper 2196-C. 62 pp. Roberts, H. H., R. D. Adams, and R. H. W. Cunningham Evolution of sand-dominant subaerial phase, Atchafalaya Delta, Louisiana. Am. Ass. Petrol. Geol. Bull. 64: Taylor, J. R., M. A. Cardamone, and W. J. Mitsch Bottomland hardwood forests, their functions and values. Mitsch and Associates. Louisiana, Ky. 87pp. 185

7 u. S. Department of the lnte rior Endrin pollution in the Lower Mississippi River Basin. Federal Water Pollution Control Administration, Dallas, Tex. 213 pp. u.s. Environmental Protection Agency Methods for chemical analysis of water and wastes. Analytical Quality Control Laboratory, Cincinnati, Ohio. 312 pp. u.s. Environmental Protection Agency Methods for chemical analysis of water and wastes. National Environmental Research Center, Cincinnati, Ohio. 298 pp. u.s. Geological Survey Water resources data for Louisiana water year U.S. Geological Survey Water-Data Report LA pp. u.s. Geological Survey Water resources data for Louisiana water year U.S. Geological Survey Water-Data Report LA pp. Yentsch, C. S., and D. W. Menzel A method for the determination of phytoplankton chlorophyll and phaeophytin by fluorescence. Deep Sea Res. 10: