Final Report: Changes in Water Conditions and Sedimentation Rates Associated With Construction of the Mobile Bay Causeway

EPA Grant Number: R827072C024
Subproject: this is subproject number 024 , established and managed by the Center Director under grant R827072
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: Alabama Center For Estuarine Studies (ACES)
Center Director: Shipp, Robert L.
Title: Changes in Water Conditions and Sedimentation Rates Associated With Construction of the Mobile Bay Causeway
Investigators: Fearn, Miriam , Haywick, Douglas , Sanders, Justin
Institution: University of South Alabama
EPA Project Officer: Packard, Benjamin H
Project Period: October 1, 2001 through January 31, 2003
RFA: Alabama Center For Estuarine Studies (ACES) (1999) RFA Text |  Recipients Lists
Research Category: Targeted Research


The study area for this project is Chocalata Bay, a floodbasin now isolated from the larger estuary of Mobile Bay by the Mobile Bay Causeway. The objective of this study was to use sediment cores to obtain information on pre- and post-construction conditions in and around Chocalata Bay to help decision-makers determine the best course of action for mitigating the environmental impacts of the Causeway. The study used:

  1. diatom analysis as an indicator of water conditions;
  2. pollen and phytoliths as indicators of vegetation change;
  3. grain size analysis as an indicator of energy conditions;
  4. and cesium-137 (Cs-137) and lead-210 (Pb-210) to provide dating control for critical horizons in the cores.

Summary/Accomplishments (Outputs/Outcomes):

Fieldwork Accomplished During the Summer of 2002

At least three sediment cores were taken at each of four sites in Chocalata Bay. The longest core (Core 1, 190 cm) was retrieved from the northernmost site. Cores in Chocalata Bay north of the Causeway are at least 1 m long; those south of the Causeway are slightly shorter. During 2003, work focused on processing samples for pollen and biogenic silica, counting pollen and diatoms, determining organic carbon content and grain size, and completing Cs-137 and Pb-210 counts. This project provided research experience for one graduate student in marine sciences and eight undergraduate students (two in physics, two in biology, three in geography, and one in geology). Diatom investigations related to this research project will be part of a Ph.D. dissertation for one graduate student.


Cs-137 is present only in the top 20 cm of cores inside Chocalata Bay, whereas analysis of the core south of the Causeway has Cs-137 down to 50 cm. Other cores from subestuaries around Mobile Bay have Cs-137 down to 50-100 cm. This suggests that the sedimentation rate in Chocalata Bay is low because currently it is [was fine as it was] isolated from Mobile Bay, its primary sediment source. Claims that the Causeway made Chocalata Bay fill up faster with sediment were not supported by our cesium data. If we extrapolate that sedimentation rate back in time, the sediments corresponding to Causeway construction are about 40 cm downcore. Pb-210 dating provides a different picture of the sedimentation rate, although it does confirm that sediment about 40 cm down in Core 1 corresponds to the late 1920s, when the Causeway was built. Pb-210 implies that the sedimentation rate before Causeway construction was about 0.15 cm per year; and 0.61 cm per year after construction. Although this indicates that sedimentation is four [remove number] times greater now than before, only a few Pb-210 data points exist to substantiate this scenario.

Grain size analysis of the sediment cores indicates that Chocalata Bay was changing from an estuarine to a deltaic environment before Causeway construction. Mobile Bay and delta regional processes controlled the depositional environment. After construction, more specific local influences dominated. Core 1 at the northern end of Chocalata is most representative of the bay itself, while Core 3 appears to be influenced strongly by a nearby channel. Core 4 between the Causeway and the Interstate 10 Bayway is too short to show pre-Causeway conditions; in addition, it appears strongly influenced by the Apalachee River.

Pollen analysis of Core 1 (and others) shows that pine, bald cypress, oak, and sedges always have [was fine as it was] been the dominant pollen contributing taxa; however, at about 40 cm in Chocalata Bay cores, disturbance indicators become important. This coincides well with our extrapolation from the CS-137 data and from the Pb-210 data that this level marks Causeway construction. Interestingly, water milfoil (Myriophyllum) pollen is not apparent until about 30 cm downcore, some 20 years after the Causeway was built. Although some Mobilians blame the Causeway for the proliferation of this invasive aquatic species, our data do not confirm the relationship.

Phytolith analysis of Cores 1 and 4 shows relatively stable percentages of these grass type indicators. In the upper part of Core 1, there is a slight increase in dumbbell shapes characteristic of drier, better drained soils. This likely is [was fine as it was] related to the habitat available along the Causeway itself. Sponge spicules counted in the biogenic silica samples show a dramatic change across the Causeway boundary. Whether this represents a change in the material transported into Chocalata Bay, or a change in sponges living in the bay itself, is still unknown.

Diatom analysis confirms that Chocalata Bay became calmer and fresher after the Causeway was built. Biraphid diatoms increase in importance in the upper 40 cm of Core 1 inside Chocalata Bay. In general, biraphid diatoms are mobile, whereas all others attach to plants or sediment. Biraphids are more common in lower energy environments. The upper 40 cm also has slightly higher percentages of freshwater diatom taxa. This is consistent with grain size data and with isolation from Mobile Bay. Diatom diversity shows little change pre- and post-Causeway, suggesting that Chocalata Bay remains a productive habitat.

These data all confirm that about 40 cm of sediment has accumulated in Chocalata Bay since the mid-1920s, when the Mobile Bay Causeway was built. Conditions in the bay became calmer and slightly fresher after Causeway construction. With isolation from Mobile Bay, sedimentation rates seem to have increased if Pb-210 data are correct, or the rates are lower than in other estuaries if cesium levels are accurate. Although the Causeway obviously had an effect on Chocalata Bay, no dramatic changes are evident in the sedimentary record except for the change in sponge spicules. Efforts to remove parts of the Causeway will disturb the habitat that now exists and may not lead to restoration of the original conditions. This study focused on Chocaclata Bay itself. Effects of Causeway construction on Mobile Bay is beyond the scope of this research.

An undergraduate student is doing a directed study on sponges to help resolve the questions surrounding the sponge spicules found in Chocalata Bay.

Journal Articles:

No journal articles submitted with this report: View all 5 publications for this subproject

Supplemental Keywords:

estuary, ecosystem, pollen, diatoms, phytoliths, cesium-137, lead-210, indicators, decisionmaking, Alabama, U.S. EPA Region 4,, RFA, Scientific Discipline, ECOSYSTEMS, Water, Geographic Area, Ecosystem Protection/Environmental Exposure & Risk, Ecology, Aquatic Ecosystems & Estuarine Research, estuarine research, Environmental Chemistry, Ecosystem/Assessment/Indicators, Ecosystem Protection, Soils, State, Chemistry, Restoration, Aquatic Ecosystem, Aquatic Ecosystems, Terrestrial Ecosystems, Ecological Monitoring, Ecology and Ecosystems, Aquatic Ecosystem Restoration, coastal ecosystem, wetlands, estuaries, watersheds, erosion, shorelines, waves, Alabama (AL), coastal environments, anthropogenic impact, wetland stabilization, wind-wave models, ecosystem, environmental indicators, estuarine waters, water quality, human modifications, sediment dynamics, breakwaters

Relevant Websites: Exit Exit

Progress and Final Reports:

Original Abstract
  • 2002 Progress Report

  • Main Center Abstract and Reports:

    R827072    Alabama Center For Estuarine Studies (ACES)

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
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    R827072C007 Benthic Study Component
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    R827072C009 Fisheries-induced changes in the structure and function of shallow water "nursery habitats": an experimental assessment
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    R827072C011 Evaluation of Alabama Estuaries as Developmental Habitat for Juvenile Sea Turtles
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    R827072C020 Evaluating Trophic Processes as Indicators of Anthropogenic Eutrophication in Coastal Ecosystems: An Exploratory Analysis
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    R827072C026 Holocene Sedimentary History of Weeks Bay, AL: Human and Natural Impacts on Deposition in a Gulf Coast Estuary
    R827072C027 Shelter Bottlenecks and Self-Regulation in Blue Crab Populations: Assessing the Roles of Nursery Habitats and Juvenile Interactions for Shelter Dependent Organisms
    R827072C028 Predicting Seagrass Survival in Nutrient Enriched Waters: Toward a New View of an Existing Paradigm
    R827072C029 DMSP and its Role as an Antioxidant in the Salt Marsh Macrophyte Spartina alterniflora
    R827072C030 A Preliminary Survey of Aerial and Ground-Dwelling Insects of the Mobile/Tensaw Delta
    R827072C031 Natural Biogeochemical Tags of Striped Mullet, Mugil cephalus, Estuarine Nursery Areas in the North Central Gulf of Mexico
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    R827072C034 Influence of Invasive Plant Species in Determining Diversity of Aquatic Vegetation in the Mobile-Tensaw Delta
    R827072C035 The Influence of Shallow Water Hydrodynamics on the Importance of Seagrass Detritus in Estuarine Food Webs
    R827072C036 Food Web Interactions, Spatial Subsidies and the Flow of Energy Between the Mobile Bay Delta and Offshore Waters: A SGER Proposal to the Alabama Center for Estuarine Studies
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