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Examine a large shell from the pile and notice the layers of calcium carbonate that have accumulated to form the shell. How many layers have accumulated?
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From the top of the oyster pile, look out at Hood Canal. What color is the water?
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Based on the color of the water when you visit, do you think there is currently a bloom of calcium carbonate-plated coccolithophores present in the Canal? Why or why not?
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Geology Lesson:
At this location you are standing on a pile of oyster shells, primarily composed of calcium carbonate. Enjoy the view as you look out over the Hamma Hamma River and its delta flowing into Hood Canal. Hood Canal's geology profoundly affects oyster habitats and shell development, which we'll explore in this lesson. Oysters undergo a captivating developmental process shaped by surrounding geological and environmental factors. In this lesson, we'll explore these geological factors and focus on calcium carbonate's crucial role in forming oyster shells.
Geological factors impacting oyster formation:
- Sedimentation: Oysters begin their life as larvae, which are typically planktonic and float in the water column. Sedimentation plays a crucial role in their development as they settle onto the seafloor or another suitable substrate. The type of sediment and its composition can affect the success of oyster settlement.
- Substrate Selection: Oyster larvae need a hard substrate to attach and grow on. This substrate can be natural, such as rocks or shell beds, or artificial, like oyster reefs, oyster mesh bags, or piers. The availability and composition of these substrates in a specific location influence where oysters will develop.
- Tidal and Current Patterns: Tidal currents and water flow patterns in coastal areas can impact the distribution of oyster larvae and their settlement. Oysters often settle in areas where currents are suitable for bringing food and nutrients.
- Salinity and Water Quality: Geological factors also affect the chemical composition of the water, including salinity. Oysters require a certain range of salinity levels for optimal development. Geological formations, such as underground aquifers, estuaries, or in the case of this specific location, the clean, cold, aerated water of the Hamma Hamma river influence the growing conditions of these local oysters.
- Mineral Availability: Oysters require specific minerals, particularly calcium carbonate, for shell formation. The geological composition of the substrate and the surrounding environment can influence the availability of these essential minerals.
Calcium Carbonate and oyster shells:
Calcium carbonate is a chemical compound with the formula CaCO3. It is a mineral found abundantly in nature and is the primary material in oyster shells. It consists of calcium (Ca), carbon (C), and oxygen (O) atoms bonded together. Its molecular structure includes calcium ions (Ca⊃2;⁺) and carbonate ions (CO3⊃2;⁻).
Calcium carbonate, a key mineral for oyster shell development, is extracted from the surrounding seawater and plays a vital role in oyster growth and the overall health of the oyster ecosystem in Hood Canal. It is instrumental in the biomineralization process that facilitates the development of these shells.
Oyster shell development and decomposition:
- Initial Shell Formation: When an oyster is in its larval stage, it starts with a tiny, soft, and translucent shell called a "prodissoconch." This shell provides some protection but is not suitable for the oyster's adult life.
- Calcium Carbonate Deposition: As the oyster grows, it begins to extract calcium ions (Ca2+) and carbonate ions (CO3⊃2;⁻) from the surrounding water. These ions combine within the oyster's mantle tissue to form calcium carbonate (CaCO3), which is the primary material of the shell.
- Layered Growth: The oyster secretes layers of calcium carbonate onto the inner surface of its existing shell. Each layer consists of tiny calcium carbonate crystals held together by a protein matrix. These layers accumulate over time, gradually thickening the shell.
- Shell Shape: The oyster's mantle tissue, which lines the inner shell, also plays a role in shaping the shell. The mantle has specialized cells that control the deposition of calcium carbonate, allowing the shell to grow in the specific shape and pattern characteristic of that oyster species.
- Reinforcement: Oysters can reinforce their shells as needed. For example, if the shell becomes damaged or thin, the oyster will deposit additional layers of calcium carbonate to repair and strengthen the shell.
- Iridescent Layer: The innermost layer of the shell is often iridescent and is composed of nacre, also known as "mother-of-pearl." Nacre consists of alternating layers of calcium carbonate and organic material. It gives the shell its lustrous appearance.
- Complete Shell: Over time, the layers of calcium carbonate accumulate, creating a fully formed and hardened shell that provides protection to the oyster's soft body. The rate of shell growth can vary depending on factors like water temperature, food availability, and the oyster's age. Oysters continue to add layers to their shells throughout their lives, with older oysters generally having thicker shells.
- Shell Decomposition: After an oyster dies various physical and chemical processes begin to break down the shell. These processes include physical weathering from waves and currents, as well as the actions of organisms such as burrowing worms and small organisms that may bore into the shells. In the case of the oyster pile you are standing on, you also play a role in their decomposition!
- Calcium Carbonate Release: As the oyster shell decomposes, calcium carbonate ions are released. This dissolution process gradually increases the concentration of calcium and carbonate ions in the water. The released calcium carbonate becomes available to other marine organisms in the vicinity. It can be utilized by various organisms, including young oysters, shellfish, and in Hood Canal, microscopic plankton called coccolithophores that are plated with white calcium carbonate. When there are blooms of these plankton in Hood Canal their plates can impart a milky, turquoise hue to the water that is often visible from space.
Sources
- Hama Hama Oysters webpage, All things oysters
- Earth.com, Brilliant-non-toxic-bloom-in-hood-canal
- https://earthobservatory.nasa.gov/images/88454/bloom-in-hood-canal-washington
- https://en.wikipedia.org/wiki/Marine_biogenic_calcification
- Smithsonian Institution, https://ocean.si.edu/ocean-life/invertebrates/ocean-acidification
- Florida State University,The Oyster Life Cycle https://marinelab.fsu.edu/absi/community-engagement/aboutoyster/lifecycle_anatomy/
- Science Direct, Journal of Hydrology, Relating estuarine geology to groundwater discharge at an oyster reef in Copano Bay, TX. Volume 564, September 2018, Pages 785-801 Nicholas Spalt, Dorina Murgulet, Xinping Hu