Course:EOSC270/2023/Group 13 - Pros and Cons of Anthropogenic Oyster Reefs

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What are oyster reefs?

Oyster Reef. Credit: Oyster Recovery Project

Oysters begin their lives floating through the ocean as young larvae. As they progress through their life stages, they develop shells and begin their descent to the ocean sediments. Over time, the accumulation of oysters on the sea floor creates oyster reefs that house and protect a large amount of marine biodiversity. The reefs provide essential services to the marine ecosystems such as habitat for organisms, protecting coastal areas, carbon sequestration, combating coastal acidification and filtering water naturally[1]. Despite their importance, oyster reef abundance has declined by 85% from the 1700s through the 1900s due to reckless harvesting methods, disease and low water quality[2] [3]. The recent increase in destruction to the reefs has escalated the restoration efforts due to the urgent need for the services they provide[4].

Oyster reef locations

The location and current condition of oyster reefs around the world. The condition ratings of Good, Fair, Poor, and Functionally Extinct represent the current amount of reef remaining in comparison to its historical abundance. Good: <50% reef lost; Fair: 50-89% reef lost; Poor: 89-99% reef lost; Functionally extinct: >99% reef lost.

Oyster reefs are a structural feature of coastal waters and shorelines.[5] Most of the world’s remaining wild capture of native oysters come from just five ecoregions on the East and Gulf coasts of North America, which together account for more than 75% of the global catch.[5]  These ecoregions are in poor (or worse) condition. In Australia and New Zealand, the native oyster reef ecosystem has largely disappeared. In Asia, specifically China, Japan, and Korea, reefs that were once widespread throughout their regions now have poor conditions. Almost all of the historical reefs located here have disappeared completely.[6] South America has 13 native species of oysters, and 3 introduced species. In 2009 31% of these populations were assessed to be in good condition, 44% in fair condition, 14% in poor, and 6% were functionally extinct.[6] Europe hosts the native European flat oyster (Ostrea edulis) which used to be very abundant throughout the continent. Now most of the reefs located in Europe are in very poor condition or are functionally extinct. In North America, only reefs around the Gulf of Mexico remain in a fair condition.[6] Reefs elsewhere on the continent are either in poor condition or are functionally extinct. The northeast Atlantic coast has the most dramatic habitat loss with less than 6% of historic reefs remaining.[7] Coastal ecosystems are suffering a decline in services that oysters provide.

Anthropogenic reefs vs natural reefs

In core samples of oyster reefs, (b) Bivalves, (c) Resident Crabs and (d) Amphipods were examined for distribution and abundance differences among different reef types, seasons, and landscapes[8].

Oyster reefs are both naturally occurring and anthropogenically created. Naturally occurring reefs consist of oysters that have evolved in the environment and are adapted to abiotic conditions, such as temperature and pH, and biotic interactions, like competition and predation. Building reefs with a specific goal requires consideration of restoration technique, landscape, and availability of the materials[9]. How the oysters interact with different species, optimal conditions for survival and reproduction, and how the changing environment will affect reefs are key to understand in order to ensure effectiveness and longevity[9][8]. Anthropogenically manufactured reefs cannot be guaranteed to fill the exact role that the natural reefs occupied, but the ecological effects can be hypothesized and tested in experiments[8]. To determine if an anthropogenic reef is a good replacement for natural ones, an experiment was conducted in North Carolina, Middle Marsh to observe how fish and invertebrates behave on natural reefs, restored reefs, and a control (no reef)[8]. Results concluded that anthropogenic reefs effectively replenished the lost biodiversity and abundance of organisms occurring on natural reefs[8]. The time it takes for an anthropogenic reef to recover the biodiversity and services of a natural reef is not certain and varies reef-to-reef[9]. A rapid recovery in species and services in anthropogenic reefs occurs at the beginning of its life, but quickly plateaus and may never reach the full potential of a natural reef[9]. The plans for reef development and their success should take into account the loss of services over time[9]. Minimal research has been done on anthropogenic reefs as long-term natural reefs substitutes, but the research provides a good baseline for future studies.

How do humans build oyster reefs?

Volunteers deploying oyster mats in Mosquito Lagoon. © Anne Birch, TN
Volunteers deploying oyster mats in Mosquito Lagoon. © Anne Birch, TN

The restoration of an oyster reef begins with the selection of a site location that satisfies current and future conditions needed for oyster survival, growth, and reproduction.[4] Following a site selection, a hard substrate (most commonly porcelain, concrete, limestone, noncalcium stone, nonoyster shell, dredged shell, or engineered reefs)[10] is chosen to be used as a surface on which the reef can grow.[4] The substrate is anchored and placed on the sea floor of the desired area.[11] After the substrate has been set into place, spat-on-shell juvenile oysters (oyster larvae attached to oyster shells or another surface) will then be placed on the substrate and attach themselves onto the surface.

Oyster mats

Oyster mats are a notable method that have shown successful results in oyster reef restoration. Oyster shells are strongly attached to a mesh material which are then anchored to the bottom of the restoration site with small cement weights and are oriented vertically to simulate a natural healthy oyster reef.[12] At a restoration site in Mosquito Lagoon Florida (2005) oyster mats were blanketed with live oysters in only 2-3 years following the construction.[12] These restorations often utilize very large amounts of volunteers to create mats and bags of oysters and place them on the reef.

How oyster reefs help coastal ecosystems

Simplified illustration of the bivalve filter-feeding digestive process. Courtesy of Brian Leander, BIOL 205, UBC.

Oysters facilitate water filtration

How it works

Oysters belong to a class of marine and freshwater mollusks known as bivalves. Bivalves acquire nutrients by filtering large volumes of water through a structure known as the mantle cavity, which holds their respiratory organ[13].

Membrane-bound organelles called cilia beat in concert to generate a water current through the organism. Consequently, small organisms (i.e. phytoplankton) and organic debris suspended in the water get trapped by sticky mucus coating the surface of the ctenidia (gill). Eventually, these particles are ingested and travel through the esophagus to the stomach where they are decomposed by enzymes. The remaining material passes through the intestine and is incorporated into fecal pellets, which are ejected out of the organism. Overall, oysters retain a large proportion of the nutrients in the water they filter, thus decreasing nutrient concentration in the ambient water.

The significance of filtration via oyster feeding

Role of oysters in the nitrogen cycle. Ashley R. Smyth, Piehler Lab, UNC Chapel Hill Institute of Marine Sciences.
Photograph of coastal algal bloom. NRDC - Aerial Associates Photography, Inc., by Zachary Haslick.

Nitrogen is one of the organic components that oysters filter from the ambient water for their metabolic needs. This anti-nitrogen fixing process is a critical ecosystem service because it converts an excess of usable nitrogen (eutrophication) into bio-unavailable nitrogen. Without oysters to keep usable nitrogen levels in check, algal blooms are stimulated. Algal blooms are of paramount detriment to marine ecosystems due to the consequential cultivation of hypoxic (low oxygen) and turbid environments[14].

An environment with these characteristics is very harsh. A lack of oxygen forces a plethora of marine life to flee or die[15][16]. Moreover, the turbidity associated with copious algae and sediment in the water column limits the depth to which light can penetrate (photic zone). This limits the abundance of primary producers who use light to produce energy and serve as the base of the ecosystem[17].

Oysters possess numerous pivotal features that help prevent these regime shifts and maintain local biodiversity. Firstly, these ecosystem engineers can filter more than 50 gallons of water a day[18] and clutch-planted reefs can have a density of 250 oysters/m2 [19]. This means that one acre of anthropogenic oyster reefs could purify 50,585,750 gallons of water a day. Successfully restored reefs are expected to remove about seven times as much nitrogen each day than unrestored sand/mud bottom areas can[20]. One factor that contributes to their tremendous filtration capacity is that its not tethered by hunger. After satisfying their metabolic needs, they alternatively produce pseudo-feces[21]. Another aspect to consider is that their ability to filter-feed is not contingent on occupying high current areas since they can generate their own water current by ciliary action. Overall, consideration of the conglomeration of oyster traits undoubtedly supports their aquaculture.

Oysters protect shorelines

Coastal communities (human and otherwise) are especially vulnerable to sea-level rise associated with global warming.[22] Issues such as increased flooding risk and coastal habitat degradation call for shoreline protection with two primary goals: wave attenuation (smaller, lower-energy waves) and sediment stabilization.[23]

Oysters as ecological engineers

Bar graph showing sediment erosion/accretion rates
Erosion/accretion rates of sediment on Kutubdia coast over one year in control (left) and reef (right) regions. Figure from paper by Chowdhury et al.

Oyster reefs can help reach these goals. By trapping sediment between the reef and the shore and dissipating incoming wave energy, oyster reefs restore the accretion-erosion balance of coastlines (ensure that sediments accumulate and deplete at appropriate rates).[22][24] Oysters form robust, three-dimensional barrier reefs by excreting a calcium carbonate cement that helps them stick together and withstand turbulence.[25] As an ecology-focused strategy, oyster reef restoration is often preferable to traditional constructions (dams, dikes, etc.) because it minimizes human impact.[23] Furthermore, reefs are dynamic; they grow and adapt with the changing needs of the coastal ecosystem, which gives them an advantage over more rigid erosion barriers.[24] By bearing the brunt of abnormally turbulent waves, oysters allow other (less-resilient) species to live in the intertidal environment.[23] They also lay the groundwork for other protective species, such as mangroves, to take root.[24]

Cartoon of carbonate material in ocean reacting with carbonic acid formed from carbon dioxide and seawater
Calcium carbonate (CaCO3) neutralizes carbonic acid in seawater. Carbon dioxide becomes carbonic acid in ocean: CO2 + H2O ⇌ H2CO3 Carbonic acid reacts with carbonate ion to form bicarbonate: H2CO3 + CO32- ⇌ 2HCO3- Figure from Nature Education.

Case studies in erosion mitigation by oysters

Along the coast of Kutubdia Island, Bangladesh, constructed oyster reefs lessened wave force at tide levels between 0.5-1 m. The region also saw a 54% erosion reduction alongside expansion of the surrounding salt marshes.[22]

Another rapidly-diminishing coast—in Louisiana—also had success with oyster reef experiments, which reduced shoreline retreat and indicated that oyster reefs are likely a sustainable solution.[25]

Oyster shells combat acidification

Oysters, like many mollusks that build calcium carbonate shells, are threatened by the rise of ocean acidity associated with climate change. However, increasing the amount of shell matter in reefs can actually alleviate some of the acidification by re-balancing coastal pH.[26][27]

Researchers from Oregon State studied oyster reef restoration efforts in Chesapeake Bay and found that oyster reefs can combat acidification of coastal waters.[26] Their calcium carbonate shells are broken down by carbonic acid in the water, which produces bicarbonate ions. Bicarbonate, in turn, raises the alkalinity of the seawater.[26][28] This creates a positive feedback loop wherein the growing oyster populations make the environment more habitable for calcifying organisms, including themselves.[26]

The effects on pH of oyster reefs can be amplified by recycling empty oyster shells into the reef habitat. The shells initially use up carbonic acid in the water and produce bicarbonate. The shells also provide a base for new oysters to grow, expanding the reef and compounding its benefits.[27]

Concerns of anthropogenic oyster reefs

Global climate change and the anthropogenic introduction of invasive species lead to drastic changes in global ecosystems. The main concern of Anthropogenic Oyster Reefs is Biodiversity loss and the extinction of certain marine species. Over the past few years, many invasive species have been introduced by humans, knowingly or unknowingly, in many different parts of the world.[29] One such example is the oyster species Crassostrea gigas, commonly known as The Pacific Oyster, which we will discuss in detail.

Invasive oysters: Crassostrea gigas

Crassostrea gigas or The Pacific Oyster - Image taken by Linda Schroeder of The Pacific Northwest Shell Club
Crassostrea gigas or The Pacific Oyster - Image taken by Linda Schroeder of The Pacific Northwest Shell Club

The C. gigas oyster species was introduced in many parts of the world, dominantly the Northwest European Shelf (NWES), because it was thought that they would be able to grow in this region but not complete their life cycle due to the cold temperatures.[29] However, due to recent temperature increases in these regions caused by global warming, the Pacific Oysters can now complete their life cycles in these regions. There are predictions that the C. gigas oyster species will have made these areas part of its niche by 2100 with aid from its enormous capacity to disperse its eggs and reduced time required to reach maturity.[29] This can be extremely dangerous for local biodiversity and negatively impact mussel beds, salt marshes, seagrass beds, polychaete reefs, etc. [30]

Changes in ecosystems due to invasive oyster reefs

A shift in the dominance has been observed with invasive Oyster Reefs overgrowing mussel populations.[30] This shift in dominance could have:

  • top-down effects on phytoplankton
  • bottom-up effects on shellfish predators and parasites and
  • a negative impact on organisms that would dwell on mussel beds but can no longer do the same in oyster reefs[29][30].

Importance of biodiversity of phytoplankton:

Oyster reefs filter feed on phytoplankton, which means an increase in their population will harm phytoplankton populations.

We know that Phytoplankton are crucial components of aquatic ecosystems, serving as the primary producers that form the base of the food web.[31] Ecosystems that have highly varying temperatures need phytoplankton more than other ecosystems because the varying temperatures can affect their growth and distribution. [32] Since phytoplankton is responsible for half of the earth’s oxygen production and play a key role in the biogeochemical cycles of our planet, in addition to being an important part of the carbon cycle as well as a regulator of climate change, it is important to protect their populations.[32] Phytoplankton loss can cause large amounts of loss in Biodiversity since they are a key species of the marine ecosystem.

Biodiversity loss can have a range of negative impacts:

Biodiversity loss in marine ecosystems can have a range of negative impacts such as:

  • changes in ecosystem functioning
  • altered nutrient cycling
  • a negative impact on the goods and services we obtain from these ecosystems. [33]

Summary

References

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