Filtering the Myth: Oysters, Algae, and Misinformation

By: Grace Kingsbury

As a child I, like many others, was initially repelled by oysters: their slick texture, their briny smell. My perspective shifted when I discovered the variety of sauces and preparations that elevate them, and more importantly, when I learned how essential they are to the health of marine ecosystems.

One common misconception about oysters, which often prevents people from embracing them as food, is a misunderstanding about what it means to be a filter feeder. Environmentalists (myself included) celebrate oysters as ecological warriors, citing projects like the Billion Oyster Project as examples of how oysters can dramatically transform marine environments. Oysters stabilize shorelines, provide habitat, and significantly improve water quality (Bruce, 2021). However, this well-earned reputation sometimes creates confusion when oysters are considered as food. If oysters are so good at filtering polluted water, doesn’t that make them toxic?

The truth is: not all oysters are grown equally. Oysters can be used either as tools for water filtration or harvested for human consumption, but the two goals are not always compatible, and they are tightly regulated to prevent overlap. The oysters used to clean our waters are not the same oysters served to you in a restaurant, ever.

Oysters filter water naturally as part of their feeding process. They use cilia: tiny hair-like structures inside their gills, to move seawater through their bodies. The cilia and mucus trap microscopic phytoplankton and organic particles, and the oysters expel cleaner, phytoplankton-free water. This filtration benefits ecosystems by helping to mitigate eutrophication, the over-enrichment of water with nutrients (primarily nitrogen and phosphorus) that fuels algal blooms (Donnelly, 2023). When these blooms collapse, their decomposition consumes oxygen, creating hypoxic, or "dead," zones where few organisms can survive (Barrat, 2015). Oysters disrupt this destructive cycle by consuming algae before they spiral out of control. This differs from a traditional filter, which is designed to capture harmful contaminants. Oysters aren’t targeting pollutants like heavy metals or pathogens, they’re simply feeding on suspended particles like algae and detritus, which incidentally clears the water.

Historically, this natural water cleaning happened without our intervention. But as oyster populations declined dramatically, driven by overharvesting, disease, and habitat destruction, and as agricultural and urban runoff intensified, ecosystems have become unbalanced. On Long Island specifically, nutrient pollution from septic systems, fertilizers, and urban runoff is a major contributor to eutrophication. Excess nitrogen leaches into bays and estuaries, fueling harmful algal blooms like brown tide and red tide (Lan, 2024). To combat these imbalances, organizations like the Billion Oyster Project and our partner SOAR are working to restore oyster populations by planting shellfish on degraded reefs. These restoration oysters help improve water quality at scale by collectively filtering thousands of gallons of water per day, removing excess phytoplankton and suspended particles that contribute to eutrophication.

While oysters don’t actively seek out heavy metals or pathogens, they can still bioaccumulate contaminants like mercury, cadmium, and lead from their surroundings. These risks are mitigated through strict monitoring of approved growing areas, but as pollution spreads and baselines shift, the definition of “clean” water is becoming increasingly compromised. At high concentrations, heavy metals can impair oyster reproduction, shell formation, and survival (Weng, 2019). Oysters can bioaccumulate certain contaminants, such as heavy metals, in their tissues and shells, which allows scientists to use them as long-term indicators of environmental pollution (Carmichael, 2021). In some cases, dense oyster reefs may function as localized pollutant sinks by trapping contaminants in shells, tissues, and surrounding sediments, potentially limiting their mobility in the ecosystem.

In stark contrast to restoration oysters, waters where oysters are harvested for eating meet rigorous standards: they must be classified as "Approved" or "Conditionally Approved" by regulatory authorities based on ongoing water quality testing. These classifications are not cosmetic; they're based on hard metrics like bacterial contamination levels (typically Escherichia coli counts), toxin presence, and proximity to pollution sources. Areas that don't meet these standards are labeled "Restricted" or "Prohibited," and harvesting from them is illegal (FDA, 2019). Restoration oysters, like those deployed by the Billion Oyster Project, are placed in waters that would never qualify for human food; precisely because their mission is environmental repair, not human consumption. 

Nationwide, the U.S. Food and Drug Administration and the National Shellfish Sanitation Program (NSSP) set harvest standards. In New York State, the Department of Environmental Conservation enforces additional protections, such as mandatory shellfish bed closures after heavy rainfall to avoid contamination from stormwater runoff. Long Island Sound, for instance, has several permanent closure areas, and approved waters may be open seasonally under careful monitoring (Pomeroy, 2021). At Sebonac Inlet Oyster Farm, we specifically monitor the Baykeeper's real-time water quality data, which measures parameters like dissolved oxygen, nitrogen levels, and bacterial counts, to ensure compliance.

This deep-seated fear of “dirty oysters” comes from real historical trauma. In the late 19th and early 20th centuries, oysters from polluted urban waters (especially New York Harbor) caused outbreaks of typhoid and cholera (Sharma, 2010). Not because oysters created disease, but because they filtered and concentrated pathogens present in contaminated water. Today, shellfish sanitation programs and routine water quality testing have dramatically reduced these risks. Modern outbreaks are rare and typically result from regulatory breakdowns or illegal harvesting, such as the 2021 norovirus cases linked to oysters from Canada (FDA- Human Food Program, 2024).

An even newer concern, and frankly one I take more seriously, is microplastics. Oysters, like other bivalves, accumulate microplastics, especially synthetic fibers from clothing and industrial runoff (Baechler, 2020). The concentration of microplastics in oysters depends heavily on the surrounding water, but the amount a person ingests by eating oysters remains negligible compared to the amount absorbed daily from drinking water, airborne dust, and processed foods. While research is ongoing, microplastics have been associated with inflammatory responses and potential hormone disruption. Still, the fear of microplastics in oysters is disproportionate to the reality, especially when we consider that microplastics have now been found in human blood, lungs, and even placental tissue (Ragusa, 2021).

As I see it, refusing to eat oysters for fear of microplastics or toxins while continuing to drink bottled water, wear synthetic clothes, and drive cars is the kind of cognitive dissonance that defines our modern environmental anxiety.

If you believe in cleaner oceans, you should seriously consider eating oysters, specifically, sustainably farmed ones. Every oyster consumed supports a cleaner, more resilient coastline. In a world where agriculture, fishing, and industry often ravage ecosystems, oysters are a rare exception: delicious, low-carbon, regenerative. Instead of hand-wringing over theoretical contaminants, we should recognize oysters as one of the few foods that actively reverse environmental degradation, simply by being grown and eaten.

I write this blog for free and spend my spare time learning about oysters because, although they are not traditionally considered charming animals, I am deeply charmed by them. They quietly perform the hard labor of cleaning our oceans, no flash, no reward.

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