Our Local Landscape, Weeks Bay
In his previous post, Sandy Page taught us all about land judging competitions. This time, we are learning about a local environmental treasure, the Weeks Bay estuary. Be sure to scroll down to see further explanation of estuaries and the evolution of Weeks Bay.
Sandy’s article continues below with more in-depth information…
Estuarine Characteristics
Estuaries we see today are primarily a landscape feature created within the past 20,000 years or so. In a geologic time frame this is a blink of an eye. As a human that mostly considers events within a human timeframe, conceptually it is difficult to imagine geologic time frames that often speak of events in scales of a billion, million, or hundreds of thousands of years. The geologic record supports the idea of sea level rising and falling hundreds of feet relative to sea level today. These events can be caused by tectonic shifts that are related to the structure of the earth’s crust and forces that take place within it. For example, the path taken by the confluence of the Tombigbee and Alabama Rivers, which merge to form the Mobile/Tensaw River System, is constrained by an area that is defined by a “graben” fault. This kind of crustal fault occurs when a section of the earth’s crust subsides relative to the sections of crust adjacent to this fallen block. The age of this occurrence is uncertain but ancient. The resulting valley has produced a narrow deltaic system bounded by hilly uplands on both sides. For this discussion of Weeks Bay as an evolving landscape feature, however, we are referring to a period that began around 18,000 years ago. This time period marks the end of the last ice age when massive continental glaciers began to melt.
Image by Jim Lacefield, PhD*1 from drawing by W. Everett Smith*2
Many people are now aware of the submerged cypress forest 60 feet underwater located offshore of the Alabama coast in the Gulf of Mexico (thanks to Ben Raines, local author, film maker, and historian, 2017). This forest existed during a time when climate and the hydrologic cycle of the planet had contributed to forming large continental ice sheets the world over. Because so much of the earth’s water supply was tied up in these glaciers, sea level was much lower than it is today. Hence, the shoreline of the northern Gulf of Mexico existed closer to the edge of the continental shelf roughly 60 miles offshore. Concurrent with these events, normal weathering and erosion of the landscape worldwide carved hills into valleys and produced features similar to our modern landscape. But these landscape features of the prehistoric Alabama coast and Baldwin County were developing many miles south of the present shoreline, at elevations below sea level of today. Around 20,000 years ago the planetary cycle of climate and hydrology that produced the ice age went into reverse. As the continental ice sheets melted, sea level rose, and as a result, deposition of sediment from fresh water streams began to fill in the valleys that are currently part of the sea floor of the Gulf of Mexico. Over time, the infilling of sediments created deltaic systems that migrated towards the present coastline.*1, 2
Mobile Bay is a dominant feature of a nationally recognized estuarine system. The Mobile/Tensaw watershed actually contributes water, energy, and sediment from the drainages of the Tombigbee and Alabama Rivers, including large tributaries such as the Blackwater, Coosa, Tallapoosa, and Cahaba Rivers. Weeks Bay is also a similar but smaller version. It is a tributary estuary of the much larger Mobile/Tensaw watershed system. The major streams contributing sediment to the Weeks Bay estuary are the Fish and Magnolia Rivers. Both estuaries are considered drowned valleys that have filled in with sediment and are relatively shallow. Mobile Bay is part of a bar-built estuary*3 where the sandy Fort Morgan Peninsula and Dauphin Island narrows the opening of the bay and creates a barrier to the saline seawater of the Gulf. The narrow mouth of Weeks Bay also constrains and influences salt and fresh water mixing.
Consider the force of gravity delivering fresh water from streams down slope encountering the unrelenting force of the planet’s ocean tides. Mixing of fresh and salt water produces brackish water. The tendency towards producing brackish water is influenced by the geomorphic character of the estuarine landscape. In other words, the constraining landforms such as barrier islands, peninsulas, and size of the bay opening and the extent of the contributing watersheds affect the amount of mixing. Fresh and salt water have different densities. They often do not blend well unless stirred by currents. The amount of salt water from the Gulf introduced into the system is constricted when entering Weeks Bay by the narrow opening into Mobile Bay. Fresh water will dilute salt water but is also reliant on the amount and current of the fresh water as it encounters a body of salt water. The amount of mixing is dependent on dynamic competing forces of fresh water amounts, amount of salt in the receiving body of water, direction of winds, evaporation, and tidal periods. When energy potential – such as slack currents, calm winds, and low tides - is low in estuaries, the more dense salt water will sink below the lighter density fresh water. These dynamic and temporal conditions support a complex and vitally important web of life.
Ecosystem services are benefits provided to people. They may be of an economic, cultural, or ecological nature. Ecosystem services of estuaries enrich biodiversity and exhibit distinctive habitats. They provide filtration of contaminants from upland runoff, stimulate nutrient cycling, and supply sediment deposits to maintain emergent marsh that affords protection of coastal areas. Estuaries contain varied habitats that support diverse communities of plants and animals. For example, shallow nutrient-rich sediments are a major characteristic of marshes and seagrass bed nurseries that support young fish as well as shellfish. Marshlands are breeding grounds for birds and aquatic species that also provide food and shelter; they are a haven for migratory birds. Coastal zone protection is provided as wind and wave energy is dissipated by absorbing energy from storm surge waves as marshes and maritime forests buffer against onshore winds of severe tropical storms and hurricanes. Estuarine vegetation such as marsh grasses provide filtration of sediment containing water-borne pollutants such as herbicides, pesticides, and heavy metals. Nutrients are recycled and carbon sequestration occurs as organic matter is slowly decomposed under anaerobic conditions of continuous saturation and inundated soils. Carbon trapped in estuarine habitats is not in the atmosphere warming the planet. Additionally, estuaries support tourism and related commercial and recreational fishing, along with aquaculture of oysters, crabs, crawfish, and shrimp. Healthy estuaries benefit local economies and offer popular recreational opportunities.
*1 Lacefield, J. 2013. Lost Worlds in Alabama Rocks. Alabama Museum of Natural History. 2nd addition.
*2 Smith, W.E. 1988. Geomorphology of the Mobile Delta: Geological Survey of Alabama Bulletin 132.
*3 https://oceanservice.noaa.gov/education/tutorial_estuaries/est04_geology.html
*Smith, W.E. 1986. Geomorphology of coastal Baldwin County, Alabama. Geological Survey of Alabama Bulletin 124.