By Dr. Chanagun Chitmanat
Nutrient pollution, especially the over-enrichment of nitrogen and phosphorus in coastal waters, has led to widespread issues of eutrophication in marine and estuarine ecosystems. Eutrophication can cause harmful algal blooms, hypoxia (oxygen depletion), and a loss of biodiversity, posing significant threats to aquatic life and water quality (Lan 2024). Traditional water treatment methods are often costly and limited in scope, and as such, there is a growing interest in nature-based solutions, such as shellfish aquaculture, to help mitigate these environmental challenges.
Shellfish, particularly filter-feeding species such as oysters, mussels, and clams, naturally improve water quality by removing suspended particulate matter, algae, and excess nutrients from the water column. This review explores the scientific basis for the role of shellfish aquaculture in water quality improvement, its environmental benefits, challenges, and the prospects for scaling up these efforts.
Mechanisms of Shellfish Filtration
Shellfish, as filter feeders, play a key role in marine ecosystems by filtering large quantities of water to obtain food in the form of plankton and organic particles. This filtering process has significant implications for water quality improvement.
Shellfish remove excess nitrogen and phosphorus from the water, including effluent from shrimp ponds (Jones 2001), which are key contributors to eutrophication. Nitrogen is primarily absorbed by shellfish and used for growth, particularly for shell and tissue production. In contrast, phosphorus is generally removed through the deposition of shells and excreted matter. This nutrient removal can reduce the likelihood of harmful algal blooms, which are often triggered by nutrient imbalances.
Shellfish filter out suspended particles, including organic detritus and plankton, which improve water clarity. Triangle sail mussels (Hyriopsis cumingii) promoted the establishment of a clear-water state by grazing on phytoplankton and detritus and by stimulating submerged macrophyte growth according to Gao (2017). This not only enhances the aesthetic quality of water, but also promotes light penetration to submerged aquatic vegetation, such as seagrasses, which rely on adequate sunlight for growth.
As shellfish feed, they also contribute to the bioturbation of the seabed, disturbing sediment and enhancing the microbial activity of the benthic layer. This process can accelerate the breakdown of organic matter, further contributing to nutrient cycling and improving the overall health of the sediment. Mussel and oyster reefs modulate water flow and transport, allow attachment of algae and invertebrates, and provide cover and refugia from predation (Dumbauld 2009).
Environmental Benefits of Shellfish Aquaculture
The integration of shellfish aquaculture into water quality management strategies offers numerous environmental benefits.
Shellfish farming directly contributes to the reduction of excess nutrients in coastal waters, thereby mitigating the effects of eutrophication. Clements (2019) reported that shellfish harvesting has the capacity to remove a mean annual total of 99,088 kg and 204,571 kg of nitrogen from two Atlantic Canadian provinces, New Brunswick and Prince Edward Island, respectively. Removing large-scale oyster aquaculture from a nutrient-rich bay in Taiwan caused the chlorophyll levels and phytoplankton production to increase four times in a less flushed area of the bay (Huang 2008).
Shellfish reefs, such as oyster beds and mussel beds, can serve as important habitats for marine life, providing shelter and foraging grounds for a variety of species. The addition of mussels to the seafloor resulted in a general reduction of infaunal abundance, but an increase in epifaunal and pelagic faunal abundances, specifically from those species that benefit from benthic habitat complexity and an increase in food availability (Benjamin 2022). These habitats also contribute to enhancing biodiversity by supporting marine organisms that depend on hard substrates for settlement and growth.
Shellfish farming can also contribute to carbon sequestration, a process in which carbon dioxide is removed from the atmosphere and stored in shells and sediments. In China, approximately 6.23 Mt CO2-eq a−1 was fixed via net carbon sequestration during shellfish growth from 2015 to 2019 (Feng 2023). Shellfish aquaculture, especially when integrated with other coastal habitat restoration projects like seagrass beds or mangroves, may play a role in offsetting greenhouse gas emissions.
By enhancing the resilience of coastal ecosystems, shellfish aquaculture can help buffer the effects of climate change. Shellfish reefs can act as natural barriers against storm surges and coastal erosion, protecting shorelines from the impacts of extreme weather events. However, climate-induced hazards such as changing patterns of precipitation, disease, and invasive species are some of the key factors limiting oyster production globally (Neokye 2024).
Case Studies
Numerous case studies have highlighted the success of shellfish aquaculture in water quality improvement.
Bandon Bay, Thailand: Oysters filter large volumes of water daily, removing excess nutrients like nitrogen and phosphorous which are responsible for eutrophication and harmful algal blooms. The filtration capacity of oysters helps in mitigating the negative impacts of agricultural runoff, shrimp farm effluent, and untreated wastewater from coastal urban areas (Kaewnern 2008).
Quang Ninh, Vietnam: In the northern coastal regions of Vietnam, mussel farming has been implemented as part of efforts to restore water quality and improve local ecosystems. Mussels filter plankton, organic particles, and nutrients from the water, helping to stabilize water conditions. The establishment of mussel beds also enhances sediment stability and reduces the risks of coastal erosion (Nguyen 2024).
Tasmania, Australia: Mussel farming in Tasmania has helped reduce the amount of nutrients entering the waters of sensitive estuarine ecosystems. These farming practices have led to increased water quality and healthier marine environments, benefiting both the aquaculture industry and local biodiversity (Crawford 2003).
Lysefjord, Norway: Mussel farming in Norway has been successfully integrated into nutrient management strategies for coastal waters. Mussels are cultivated in areas where nutrient loads are high, providing an effective means of removing excess nitrogen and phosphorus, while also supporting local fisheries (Filgueira 2010).
Chesapeake Bay, USA: Oyster farming has been shown to significantly reduce nutrient levels and improve water clarity in Chesapeake Bay. In some areas, oysters have filtered out enough nitrogen to contribute to the restoration of seagrass beds, which further enhances water quality (Schulte 2017).
Challenges and Recommendations
While shellfish aquaculture offers significant environmental benefits, several challenges and considerations must be addressed to maximize its effectiveness and ensure its sustainability as a water quality improvement tool.
Overharvesting of wild shellfish populations can compromise the ecological balance of marine ecosystems. Effective management practices are essential to ensure that shellfish farming operations do not lead to over-exploitation or depletion of natural populations.
While shellfish farms can improve water quality at local scales, poorly managed farms can have negative environmental impacts. So, although shellfish aquaculture can provide significant benefits for water quality improvement in coastal ecosystems, it also has the potential to accumulate contaminants including heavy metals, marine toxins, pathogens, and persistent organic pollutants which can pose serious health risks to humans. To ensure that shellfish remain a safe and nutritious food source, it is essential to monitor water quality, implement sustainable aquaculture practices, and adhere to food safety regulations. By doing so, the benefits of shellfish aquaculture can be maximized while minimizing risks to human health.
Shellfish farming is vulnerable to diseases and pathogens, which can spread rapidly within farms and affect local ecosystems. Effective biosecurity measures are needed to prevent the introduction and spread of harmful organisms, such as Vibrio bacteria or invasive species.
The impacts of climate change, such as ocean acidification, rising sea temperatures, and changing salinity patterns, could affect shellfish health and survival. These changes could potentially alter the filtration capacity of shellfish, reducing their effectiveness in improving water quality. Monitoring and adaptive management strategies are needed to address these challenges.
Conclusion
Shellfish aquaculture represents a powerful nature-based solution for improving water quality in coastal ecosystems. Through the filtration of excess nutrients, suspended solids, and organic matter, shellfish farming can help mitigate the effects of eutrophication, promote biodiversity, and enhance the resilience of coastal environments. However, to maximize its benefits, careful management and monitoring are essential to avoid localized environmental impacts, manage diseases, and ensure the sustainability of shellfish populations. As the world faces growing environmental challenges, the integration of shellfish aquaculture into broader water quality management strategies presents an innovative and promising path towards achieving sustainable and resilient marine ecosystems.
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