NEWSLETTER FOR STAKEHOLDERS OF THE DAVIS POND DIVERSION
Vol. 3. June 22, 2005
1. Greetings from the researchers
The Caernarvon diversion, located at the upper Breton Sound, has been operative since 1991 and an LSU research team (head: John Day) has been monitoring the impacts of the river diversion at the Caernarvon site since 2000 through research funding from the EPA (Environmental Protection Agency) and USDA (United States Dept of Agriculture).
This third volume of the newsletter includes the following information for the Caernarvon diversion
1) A short description of the Caernarvon diversion
2) Preliminary results from the PULSES/NUMAN research
We hope that this report of the Caernarvon diversion provides you a picture showing what may happen in the Barataria Basin after the Davis Pond diversion is fully operational sometime next year.
We’d like to say again how much we appreciate your participation.
Thank you.
Dr. John W. Day
Dr. Paul H. Templet
Dr. Jae-Young Ko
Ms. Emily Hyfield
2. A Snapshot of the Caernarvon Diversion
As we mentioned in the last newsletter, the Caernarvon diversion is one of the largest river diversions in Louisiana that have been constructed to combat salinity intrusion and restore coastal wetlands. The project was authorized by the Flood Control Act of 1965, and the Water Resources Development Act of 1974 and 1986. The diversion structure is located on the east bank of the Mississippi River near the St. Bernard - Plaquemines Parish line, and has been operational since 1991.
The primary objective of the Caernarvon project is to divert Mississippi River water containing dissolved nutrients (nitrogen and phosphorus) and sediments to coastal bays and marshes in the Breton Sound estuary. The expected benefits of the diversion were to maintain salinity levels in a desired range for fisheries, especially oysters, and to enhance the conservation and restoration of coastal wetlands. The project was also expected to enhance wildlife populations including alligators and migratory waterfowls. A total of 16,000 acres of marshes are expected to be preserved and 77,000 acres of marshes and bays will be positively impacted by the project. The total average annual benefits were projected to be $8,706,000 for fish and wildlife and $449,000 for recreation, totaling $9,155,000. Construction cost was $26.1 million.
(Source: the website of LaCoast, http://lacoast.gov/programs/Caernarvon/index.htm)
3. Preliminary results from the PULSES/NUMAN research
3.1. The PULSES/NUMAN project
A LSU research team headed by John Day has been studying the impacts of the Caernarvon diversion through fundings from the US Environmental Protection Agency/Dept. of Agriculture/National Science Foundation (PULSES) and US Dept. of Agriculture (NUMAN) since 2000. Supports were also provided by the Louisiana Dept. of Natural Resources, the Louisiana Sea Grant, and the U.S. Geological Survey. The research team studied
1) water flow patterns,
2) the impact of the diversion on wetland loss,
3) the impact of the diversion on water quality,
4) functions of wetland soils and benthic sediments in response to flooding events,
5) impacts of river inputs on fish and shrimp,
6) the potential for eutrophication,
7) development of computer models to help study water quality and wetland habitat change, and
8) an outreach program to determine the opinions of various stakeholders about the river diversion.
One of the main objectives of the study was to determine the effect of discharging water from the diversion structure as large pulses rather than as more constant flows. We called this project PULSES because of this. The idea was to put the same amount of water into the Breton Sound estuary, but to alter the timing of the input. In our experiments, the structure was run wide open for two weeks and then turned off for several weeks and then run wide open for two more weeks. The purpose of the large pulses of water was to raise water levels somewhat so that more of the water flowed over the surface of the marsh rather than flowing in a channel or lake. This would allow more of the sediments to be deposited on the marsh surface and more of the nutrients in the water to be taken up by the marsh plants and soils. These large pulses of water were done in the winter and early spring when they would have less impact on fishery species. The decisions concerning the operation of the structure were done in conjunction with the Caernarvon Interagency Advisory Committee (CIAC). Members of the LSU team made presentations to the CIAC so that they could balance the goals of wetland restoration and fisheries.
Discharge levels ranged from 185 m3/sec (or 6,500 cubic-foot-per-second-cfs) for high flow, and below 15 m3/sec (or 500 cfs) for low flow
(Figure 1). Field measurements showed that above about 4000 cfs, 30% or more of the diverted water flowed over the marsh surface. Below 4000 cfs, most of the diverted water flowed in channels (Figure 2). The diversions led to freshening of the upper basin, but the impact was much less in Breton Sound where most oyster production occurs. A number of other factors other than the diversion affected salinity and water flows. These included tides, the network of channels and lakes, and frontal passages.
3.3. Sediment deposition
We found that the high pulses of water from the diversion led to high rates of sedimentation on the marsh surface. We also found that most of the suspended sediment delivered by the diversion was retained in the first 10 meter from the diversion structure (Figure 3). We never measured large amounts of sediment in diverted river water reaching Breton Sound.
3.4. Water quality
Our results suggest that there are significant reductions of nitrogen and phosphorus as river water flows through the estuary. On average, nitrogen and phosphorus concentrations are reduced by over 50%. This means that by diverting water, nutrient discharge to the Gulf of Mexico can be reduced. The reductions of nitrogen and phosphorus are due to sediment deposition (some nutrients are attached to sediments), uptake by plants during plant growth, burial in the sediments as more and more sediments are deposited, and conversion of nitrate (this is a form of nitrogen) to nitrogen gas in a process called denitrification. This nitrogen gas is then diffuses into the atmosphere and is lost from the system. The 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. Chlorophyll (the green of plants) concentrations near the diversion were low due to the turbid conditions of the muddy water. Higher chlorophyll concentrations occurred in the middle of the estuary in the summer.
3.5. Benthic Sediments
We measured the nutrient removal due to bacteria that live in the mud at the bottom of lakes and in the marsh. These bacteria are the organisms that carry out the process of denitrification. We found that in the winter when temperatures are cold, the rate of nitrogen loss to the air was low. However, when river water flowed over the marsh, the temperature increased rapidly and the loss of nitrogen by denitrification was much higher. So, another benefit of putting in pulses of water is that the water warms up when it flows over the marsh. Most of the loss of nitrogen occurred in the upper estuary near the diversion.
3.6. Impacts on Shrimp
We also made measurements to see how the carbon and nitrogen in river water would affect the food chain of the estuary. We asked the basic question “Does river water affect what animals eat in the estuary?” We studied the small grass shrimp because so many fish eat these tiny shrimp. Grass shrimps are small and clear shrimp about one half to one inch long that is very abundant along the edge of the marsh. We used chemical techniques to measure how much of the nitrogen in the river ended up in the grass shrimp. We found that near the diversion, there were high concentration of river nitrogen in the shrimp, but the amount of river nitrogen decreased as the distance from the diversion structure was larger (Figure 4). We also found that the levels of river nitrogen were highest where the flow of river water was highest. These results showed that material in river water was important to the food chains of the estuary.
3.7. Stakeholder analysis
We conducted an opinion survey of a variety of different stakeholders who have a strong interest in the Caernarvon diversion. The total number of people answering the survey was about 100. Questionnaires were filled out by scientists, local residents, local, state, and federal government employees, land owners, oyster fishers, shrimp fishers, and recreational fishers. The survey was done in 2001-2002. Results of the stakeholder analysis revealed a number of important conclusions:
1) There was a high level of agreement that coastal land loss is a serious problem.
2) We found that people from the local area generally did not use scientific information in making decisions about the diversion. Rather, they formed their opinions based on their own personal experiences and interacting with friends, relatives, and business associates.
3) We found that decision makers, scientists, and local people had very different opinions on the effects of the diversion.
4) We found that the channels to accommodate public opinions and inputs (public meetings, comments to public agencies, etc.) are available, but not actively used.
5) This has resulted in very different opinions and conflicts opinions among stakeholders.
6) There is a need for improved channels of communication, better education about diversions, and better mechanisms to improve stakeholder input to decision making.
These studies revealed a very dynamic behavior of river water as it flows through the Breton Sound estuary. We found that pulsing river water into the system resulted in more water flowing over the marsh and higher uptake of suspended sediments and nutrients. Land loss in the Breton Sound estuary is much lower since the diversion began. The estuary efficiently removed nutrients and sediments and did not lead to persistent algal blooms. These results suggest river diversions enhance marsh stability and reduce land loss. Pulsing seems to be a good way to manage river diversions. Carbon and nitrogen in river water supports the food chain of the estuary.
4. Your Inputs
Please give us your feedback on this newsletter. Let us know if there are specific questions or issues you would like to have addressed in future newsletters. You can contact us by phone, regular mail, or email. We will do our best to get the information and pass it to all of you as a way to keep you well-informed on the Davis Pond diversion as well as other issues in coastal wetlands protection in Louisiana.
In newsletter 4, we will include information on the legal issues, focusing on the Caernarvon diversion in Breton Sound, especially on the oyster suit.
Thanks.
Contact Information:
Emily Hyfield or Jae-Young Ko
225-578-2732 225-578-6505
Mailing Address:
LSU-Coastal Ecology Institute
Baton Rouge, La 70803
Figure 1. Mean daily diversion flow from the Mississippi River to the basin in 2001. Note the large pulse in March 2001.
Figure 2. Breton Sound Basin with main region of estuary influenced by diversion highlighted in gray.
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), shows deposition decreases with distance from the diversion
|
Figure 4. The effect of the Caernarvon diversion of the food chain of the Breton Sound estuary. The circles show the percent of the Mississippi River nitrogen in shrimp tissue for the 12 sampling stations. |
