Final Report: PULSES - The Importance of Pulsed Physical Events for Watershed Sustainability in Coastal Louisiana

EPA Grant Number: R828009
Title: PULSES - The Importance of Pulsed Physical Events for Watershed Sustainability in Coastal Louisiana
Investigators: Day, John , Cable, Jaye , Fry, Brian , Justic, Dubravko , Kemp, Paul , Reyes, Enrique , Templet, Paul , Twilley, Robert
Institution: Louisiana State University - Baton Rouge , University of Southwestern Louisiana
EPA Project Officer: Hiscock, Michael
Project Period: February 28, 2000 through February 27, 2003 (Extended to August 27, 2004)
Project Amount: $899,995
RFA: Water and Watersheds (1999) RFA Text |  Recipients Lists
Research Category: Water and Watersheds , Water

Objective:

The overall objective of this project was to evaluate multiple effects of different scales of river inputs in one coastal watershed in the Mississippi delta, the Caernarvon watershed, just south of New Orleans (Figure 1), where river inputs have been ongoing since the 1991 opening of a gated river diversion structure. Specifically, physical science objectives were to: (1) monitor hydrodynamic responses to river inputs and climatic factors, and (2) evaluate marsh accretion responses to different levels of river inputs. Ecological science objectives were to: (1) evaluate marsh plant growth responses to river inputs that bring both sediments and nutrients; (2) monitor water quality changes at the landscape level at different scales of water diversion; (3) evaluate the function of wetland soils and benthic sediments as nutrient sinks in response to repeated flooding events with river water; (4) assay effects of river inputs on fish, shrimp, and oysters using stable isotopes; and (5) monitor phytoplankton responses to pulsed riverine freshwater and nutrient inputs. Subsequently, we developed an integrated physical/biological water quality model of the Caernarvon system and a regional-level simulation model to understand freshwater discharge and nutrient dynamic interactions. The objective of the social science subproject was to provide an interface between the natural and human systems of the region within the context of sustainable development through conceptual model building, cost/benefit analysis, energy analysis, and multicriteria/stakeholder analysis.

Summary/Accomplishments (Outputs/Outcomes):

We studied the multiple effects of different scales of river inputs into the Caernarvon watershed, just south of New Orleans (Figure 1). River inputs have been ongoing since the 1991 opening of a gated river diversion structure. Discharge levels ranged between 185 m sec-1 for high flow, and 15 m3 sec-1 for low flow (Figure 2).

The estuary stretches southeast for approximately 70-80 km from the diversion structure to the Gulf of Mexico (Figure 1). The upper 40 km of the estuary, encompassing an area of about 1,100 km2, is composed of extensive marshes, small- to medium-size water bodies, and channels, whereas the lower estuary is open water in Breton Sound. Thus, the upper estuary is weakly to moderately coupled to the lower estuary because of shallow, sinuous channels and extensive marshlands. Tidal amplitudes at the Gulf of Mexico end are about 35 cm but are much less in the upper basin because of dampening effects of the marsh dominated area. In this upper estuary, winds and diversions cause much higher water level variations than do tides. Salinity is generally fresh (< 1 psu) in the upper basin except during prevailing south winds or very low diversion flow.

Breton Sound Basin With Main Region of Estuary Influenced by Diversion Highlighted in Gray

Figure 1. Breton Sound Basin With Main Region of Estuary Influenced by Diversion Highlighted in Gray

Mean Daily Mississippi River (shaded) and Caernarvon Structure (line) Discharge From 1999 to 2002. Note the large pulses in 2001 and 2002 associated with this project.

Figure 2. Mean Daily Mississippi River (shaded) and Caernarvon Structure (line) Discharge From 1999 to 2002. Note the large pulses in 2001 and 2002 associated with this project.

Sediment deposition on marshes resulted from a complex set of conditions in which prevailing winds, water velocity, water levels or tides, river flow, and suspended solid loads all contribute to marsh surface delivery. Wind direction was a major controlling factor in providing both total suspended solids and water levels high enough for marsh delivery. The Caernarvon diversion delivered sediment into the northernmost reach of the Breton Sound estuary, but strong or sustained south winds dampened diversion flow and sequestered diverted sediment in the northern estuary, thus preventing deposition in the lower reaches. Statistical analysis revealed that deposition in Breton Sound estuary varied by season, with distance from the diversion (“new” sediment source; see Figure 3), and with proximity to a major waterway.

Average Sediment Deposition by Sampling Site Distance, Where D1 = < 6km (n = 5), D2 = 6 to 10 km (n = 6), and D3 = >10 km (n = 3). Background conditions at a reference station gave similar results to the D3 data. Overall deposition was highest within 10 km of the diversion.

Figure 3. Average Sediment Deposition by Sampling Site Distance, Where D1 = < 6km (n = 5), D2 = 6 to 10 km (n = 6), and D3 = >10 km (n = 3). Background conditions at a reference station gave similar results to the D3 data. Overall deposition was highest within 10 km of the diversion.

Calculations based on results from the sediment pads indicated marsh vertical accretion could reach 2.25 cm yr-1, whereas excess 210Pb measurements recorded at the same site showed a much slower rate of 0.11 cm yr-1. The 137Cs sediment activities showed an annual accretion rate of 0.10 cm yr-1, which agreed very well with 210Pb measurements. These data illustrate an important point about marsh deposition. Short-term sediment trap measurements do not capture the effects of compaction and decomposition and thus represent a more ephemeral mode of deposition.

Discharge from the diversion structure controls salinity through much of the estuary, especially during large ‘pulses’ when almost the entire estuary freshens. Temperature of incoming Mississippi River water ranged from 6-32˚C and generally equilibrated to the rest of the estuary within 10 km, but there were several times when cooler water from the diversion propagated through the entire estuary. During most transects, there were substantial reductions in most nutrient forms, especially nitrate, as water flowed through the estuary. Incoming Mississippi River nitrate concentrations ranged from 41-285 µmol -1, and concentrations at mid-estuary ranged from 1-75 µmol L-1. Possible mechanisms for this reduction are dilution with Gulf water, rain, or groundwater, and uptake by phytoplankton, bacteria, and marsh plants, denitrification, or burial. In the upper estuary, maximum estimated removal rates of total nitrogen and nitrate during a 2-week pulse in May 2001 were 44 percent and 57 percent, respectively, and phosphate and silicate were reduced by 23 percent and 38 percent, respectively. During this period, the upper estuary was almost entirely fresh, rainfall was low, and groundwater was negligible. On the other hand, overland flow across marshes may have mixed river water with low nutrient marsh water, diluting river nutrient concentrations, and actual removal rates may be lower than the calculated maximum rates to the extent that such dilution occurred. N burial and denitrification are other important sinks for N in these coastal watersheds.

The changes in nutrient concentrations during the May 2001 river pulse also led to downestuary changes in stoichiometric nutrient ratios, with an overall increase in the dissolved inorganic silicon to dissolved inorganic nitrogen (DSi:DIN) ratio and a decrease in the DIN:dissolved organic phosphorus (DIP) ratio (Figure 4; Table 1; Lane, et al., 2004). At this time and during other spring discharge pulses, chlorophyll concentrations near the diversion were low but increased after suspended sediments decreased below 80 mg L-1 several kilometers from the diversion structure. Overall, chlorophyll levels generally peaked at mid-estuary, and gradually decreased to low levels in Breton Sound (Lane, et al., in preparation).

Molar Ratios of DSi:DIN, and DIN:DIP With Distance From the Caernarvon Structure During the Spring Pulse of 2001. Horizontal dashed lines indicate the Redfield ratio. Distance was determined as a straight-line from the structure to the respective sampling stations.

Figure 4. Molar Ratios of DSi:DIN, and DIN:DIP With Distance From the Caernarvon Structure During the Spring Pulse of 2001. Horizontal dashed lines indicate the Redfield ratio. Distance was determined as a straight-line from the structure to the respective sampling stations.

Table 1. Concentrations of DSi, Total Nitrogen (TN), DIN, Total Phosphorus (TP), DIP, and Salinity With Distance From the Caernarvon Structure During the Spring Pulse of 2001. Most data are from March 22, 2001.

(µM) (µM) (µM)

(µM)

(µM)

(PSU)

117.8 138.9 128.3

5.1

1.2

0

108.3 108.8 112.9

3.4

1.7

0

96.6

99.3

86.8

2.6

1.4

0

76.8

72.9

50.6

2.0

1.0

0

62.1

60.3

35.5

2.2

0.9

1.5

47.9

49.9

16.9

2.0

0.8

4.5

Nitrate fluxes in winter/spring 2002 indicated very low uptake or even efflux of nitrate from the sediment. Denitrification rates increased with increasing distance from the diversion and increasing water temperatures in the field, with maximum rates of up to 325 µmol N2-N flux m-2 h-1 at Grand Lake. In summer 2002, high nitrate fluxes into the sediments of more than 350 µmol NO3 m-2 h-1 were estimated for cores from Big Mar. Nitrate uptake rates decreased regionally from that maximum down to zero with increasing distance from the diversion. This drop is consistent with a reduction of ambient dissolved inorganic nitrate in the water to non-detectable levels (Figure 5). Denitrification rates at Big Mar were higher than 300 µmol N2-N flux m-2 h-1 in summer and even though no nitrate uptake was detected for Grand Lake with our method, denitrification rates of up to 100 N2-N flux m-2 h-1 were measured.

Nitrate Flux Averaged From 3 Replicate Cores Per Location (summer 2002).  Bars show nitrate flux, with flux into the sediment as negative value. Circles give the ambient nitrate concentration in the water column.

Figure 5. Nitrate Flux Averaged From 3 Replicate Cores Per Location (summer 2002). Bars show nitrate flux, with flux into the sediment as negative value. Circles give the ambient nitrate concentration in the water column.

The δ15N values for grass shrimp showed a strong gradient throughout the sampling area, with highest values close to the diversion and decreasing values further away from it (Figures 6, 7). River-derived nitrogen was strongly incorporated into the food web, leading to grass shrimp with elevated δ15N values through much of the estuary. At marsh-influenced sites, however, this effect was much diminished. The simplest interpretation of these results is that there is another nitrogen source present in marsh-influenced sites, with the source having low δ15N values. Use of marsh nitrogen derived from this fixation, or also possible use of dissolved organic nitrogen that may have low δ15N values in river water, possibly could lead to the lower δ15N values in food webs at these stations.

Values of Grass Shrimp Muscle Tissue Versus Average salinity for the 12 Sampling Stations. Samples were collected 11 times between December 2000 and July 2002. Error bars represent 95% confidence levels. The black and white symbols stand for the stations closest to the diversion and marine station, respectively

Figure 6. Average δ15N (‰ ) Values of Grass Shrimp Muscle Tissue Versus Average salinity for the 12 Sampling Stations. Samples were collected 11 times between December 2000 and July 2002. Error bars represent 95% confidence levels. The black and white symbols stand for the stations closest to the diversion and marine station, respectively.

Average Contribution (%; black = 100%, and white = 0%) of Mississippi River Nitrogen to Shrimp Muscle Tissue for the 12 Sampling Stations. Calculations area based on Figure 3 and the assumption that the δ15N values at the station closest to the diversion completely derive from Mississippi River nitrogen and that the  δ15N values at the most marine station are not influenced by the Mississippi River nitrogen.

Figure 7. Average Contribution (%; black = 100%, and white = 0%) of Mississippi River Nitrogen to Shrimp Muscle Tissue for the 12 Sampling Stations. Calculations area based on Figure 3 and the assumption that the δ15N values at the station closest to the diversion completely derive from Mississippi River nitrogen and that the δ15N values at the most marine station are not influenced by the Mississippi River nitrogen.

The habitat model was used to test the effects of different management scenarios. Modeled riverine inputs had strong effects on watershed dynamics, as detailed by Reyes, et al. (2003).

We incorporated information from Caernarvon in an analysis of the relations among natural capital, pollution, and social welfare. Results showed that sustainable functioning of natural systems should contribute substantially to development of societal wealth in Louisiana. Preliminary results of the stakeholder analysis revealed that although there is high-level agreement that coastal land loss is a significant problem, diversions are not always viewed as an appropriate solution to this problem. This is because of a combination of factors, especially that: (1) local people tend to make judgments based on heuristics (e.g., personal experience for generalization); (2) there are significantly different responses among decisionmakers, experts, and local people; (3) channels to accommodate public opinions are available, but not actively used; and (4) diverse opinions and conflicts exist and persist.

The results of the study suggest that there are benefits as well as potential detrimental impacts of diversions. The diversion results in nutrient uptake, marsh accretion, lower salinities, and incorporation of riverine materials into local food webs. There is concern, however, over the potential of high nutrient levels leading to eutrophication, and long-duration pulses of 1 month or more can depress fisheries yields of shrimp and oysters. These costs and benefits need to be carefully considered in the management of diversions.


Journal Articles on this Report : 15 Displayed | Download in RIS Format

Other project views: All 109 publications 18 publications in selected types All 17 journal articles
Type Citation Project Document Sources
Journal Article Day JW, Ko J-Y. Some results from monitoring multiple aspects of the Caernarvon river diversion for spring 2001. CoastWise 2002;11(2):10-11. R828009 (2001)
R828009 (Final)
not available
Journal Article Fry B. Stable isotopic indicators of habitat use by Mississippi River fish. Journal of the North American Benthological Society 2002;21(4):676-685. R828009 (Final)
  • Abstract: JSTOR
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  • Journal Article Fry B. Steady state models of stable isotopic distributions. Isotopes in Environmental and Health Studies 2003;39(3):219-232. R828009 (Final)
  • Abstract: Informaworld
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  • Journal Article Fry B, Ewel KC. Using stable isotopes in mangrove fisheries research - a review and outlook. Isotopes in Environmental and Health Studies 2003;39(3):191-196. R828009 (Final)
  • Abstract from PubMed
  • Full-text: Univesity of Florida PDF
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  • Abstract: Informaworld
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  • Journal Article Hyfield ECG, Day JW, Cable JE, Justic D. The impacts of re-introducing Mississippi River water on the hydrologic budget and nutrient inputs of a deltaic estuary. Ecological Engineering 2008;32(4):347-359. R828009 (Final)
  • Full-text: Science Direct
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  • Abstract: Science Direct
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  • Other: Science Direct PDF
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  • Journal Article Justic D, Turner RE, Rabalais NN. Climatic influences on riverine nitrate flux: implications for coastal marine eutrophication and hypoxia. Estuaries 2003;26(1):1-11. R828009 (2001)
    R828009 (2002)
    R828009 (Final)
    R827785E02 (Final)
  • Abstract: Springer - Abstract
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  • Journal Article Lane RR, Day JW, Justic D, Reyes E, Marx B, Day JN, Hyfield E. Changes in stoichiometric Si, N and P ratios of Mississippi River water diverted through coastal wetlands to the Gulf of Mexico. Estuarine, Coastal and Shelf Science 2004;60(1):1-10. R828009 (2002)
    R828009 (Final)
  • Full-text: Science Direct PDF
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  • Abstract: Science Direct
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  • Other: Science Direct HTML
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  • Journal Article Lane RR, Day Jr. JW, Marx BD, Reyes E, Hyfield E, Day JN. The effects of riverine discharge on temperature, salinity, suspended sediment and chlorophyll a in a Mississippi delta estuary measured using a flow-through system. Estuarine, Coastal and Shelf Science 2007;74(1-2):145-154. R828009 (2002)
    R828009 (Final)
  • Full-text: Science Direct
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  • Abstract: Science Direct
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  • Other: Science Direct PDF
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  • Journal Article Snedden GA, Cable JE, Swarzenski C, Swenson E. Sediment discharge into a subsiding Louisiana deltaic estuary through a Mississippi River diversion. Estuarine, Coastal and Shelf Science 2007;71(1-2):181-193. R828009 (Final)
  • Full-text: Science Direct
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  • Abstract: Science Direct
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  • Other: Science Direct PDF
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  • Journal Article Snedden GA, Cable JE, Wiseman WJ. Subtidal sea level variability in a shallow Mississippi River Deltaic estuary, Louisiana. Estuaries and Coasts 2007;30(5):802-812. R828009 (Final)
  • Abstract: SpringerLink
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  • Journal Article Templet PH. Energy flow diversity and economic development. International Journal of Energy, Environment and Economics 2000;10(1):23-38. R828009 (2000)
    R828009 (Final)
    not available
    Journal Article Templet PH. Externalities, subsidies and the ecological footprint:an empirical analysis. Ecological Economics 2000;32(3):381-383. R828009 (2000)
    R828009 (Final)
    not available
    Journal Article Templet PH. Energy price disparity and public welfare. Ecological Economics 2001;36(3):443-460. R828009 (2000)
    R828009 (Final)
  • Full-text: Science Direct
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  • Abstract: Science Direct
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  • Other: Science Direct PDF
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  • Journal Article Walker ND, Huh O, Babin A, Haag A, Cable J, Snedden G, Braud D, Wilensky D, Prasad K . A role for remote sensing in managing Mississippi River diversions. Backscatter 2003;14:25-28. R828009 (Final)
  • Full-text: LSU
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  • Journal Article Xu Z, Cheng G, Chen D, Templet PH. Economic diversity, development capacity and sustainable development of China. Ecological Economics 2002;40(3):369-378. R828009 (2001)
    R828009 (2002)
    R828009 (Final)
  • Full-text: Science Direct
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  • Abstract: Science Direct
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  • Other: Science Direct PDF
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  • Supplemental Keywords:

    watershed, estuary, restoration, ecosystem, integrated assessment, decision making, survey, ecology, modeling, monitoring, Gulf Coast, sustainable management, EPA Region 6,, RFA, Scientific Discipline, Geographic Area, Water, Ecosystem Protection/Environmental Exposure & Risk, Nutrients, Water & Watershed, Ecosystem/Assessment/Indicators, Ecosystem Protection, State, Ecological Effects - Environmental Exposure & Risk, Southeast, Environmental Monitoring, Ecological Risk Assessment, Watersheds, nutrient transport, coastal ecosystem, eutrophication, ecological exposure, flood plains, coastal watershed, economics, marsh plant growth, river inputs, watershed sustainablity, sediment transport, fisheries, conservation, Louisiana (LA), Louisiana, pulsed physical events, tropical storms, aquatic ecosystems, watershed sustainablility, riverine ecosystems , water quality

    Relevant Websites:

    http://www.lsu.edu/aeg/pulses/pulses.html Exit

    Progress and Final Reports:

    Original Abstract
  • 2000 Progress Report
  • 2001 Progress Report
  • 2002 Progress Report
  • 2003