How are oysters farmed – and what’s the effect on subaqueous soils?

These days, oyster aquaculture is stirring up the conventional definition of agriculture. But exactly how does one grow a crop of oysters? More specifically, how are soils involved in this underwater process? Let’s start by outlining a few key concepts.

“Aquaculture” is the farming of aquatic plants and/or animals for food. Oyster aquaculture has a long history and economic significance worldwide. Oysters are a protein-rich food source commonly farmed in coastal and estuarine waters. While bustling oyster bars are popping up to meet an increasing demand, these tasty morsels are doing far more than just satisfying our cravings for a fun dining experience.

Aquaculture practices maintain oysters underwater in confined containers. Stocking densities can range in quantity of oysters per container or within a given area. Farmers sort oysters by size so larger oysters do not outcompete smaller oysters; this also helps to keep the oysters organized for easy harvest when oysters reach a desirable market size. Credit: Annie Ragan

Oysters are praised as ecosystem engineers. They have the natural capacity to clean major waterways by filtering up to 50 gallons of water a day! During this process, an oyster feeds off particulate matter and nutrients in natural waters. They use both to grow and reproduce.

Their growing process creates waste, though. Excess nutrients, particulates, and poop (yes, oysters poop!), are called “biodeposits.” These fast-sinking biodeposits are rich in nitrogen and carbon. They also have the potential to accumulate on the seafloor. This can affect the underlying subaqueous soils and potentially overwhelm critters that live on – and in – them. For this reason, oyster farming is a methodical process that requires skill, environmental knowledge, and careful consideration.

The oyster farming process begins in hatching and nursery systems. Once oysters are large enough in size, oyster farmers use two primary means to growing-out oysters. Bottom practices allow oysters to grow under natural condition. They are placed onto the subaqueous soil surface and exposed to the elements (e.g. wave action, predators). However, this method poses the risk of significant loss of an oyster crop. Conversely, off-bottom practices keep oysters in confined containers. This includes floating or suspended gear, bags, racks, trays, or cages that sit just above the soil surface.

rectangular oyster cages floating in ocean
A typical off-bottom, rack-and-bag oyster farm at low-tide. This farm is in the subtidal zone of a coastal lagoon, where the subaqueous soils are never exposed to air. While oysters are typically maintained underwater, they can withstand short exposure to air during daily tidal fluctuations. Credit: Chelsea Duball

Off-bottom methods, particularly those kept close to the soil surface, provide several advantages to oyster farmers. These systems are generally located in shallow, walkable areas. This way, routine maintenance and harvest are more easily accessible. Additionally, keeping the oysters suspended off the soil allows for a steady flow of water, which provides a consistent source of food. While these factors make the growing process easier for farmers, maintaining oysters near the soil raises some concerns for environmental quality.

So, how do you assess this critical relationship between oyster farming and subaqueous soils? The answer comes from literally diving in and digging deeper.

I was part of an interdisciplinary team working in coastal lagoons in southern Rhode Island to study oyster aquaculture. We were equipped with an old tin skiff and the underwater version of a shovel (i.e. vibracore). Our study focused on off-bottom oyster farm practices that kept oysters about 8-10 cm above the soil surface, located in three coastal lagoons.

Studying subaqueous soils can be difficult, especially during windy, wavy conditions. Back on the boat where the work surface is steady, researchers from the University of Rhode Island are compositing subaqueous soil core samples taken from oyster aquaculture areas. These soil samples were later analyzed in a laboratory for a suite of physical, chemical, and biological analyses. Credit: Annie Ragan

The goal was to assess the impacts of oyster aquaculture on soils and bottom-dwelling critters after short-term (one week) and long-term (3-21 years) durations of aquaculture activity.

Our results revealed some striking trends, suggesting that oyster aquaculture had no net-negative impacts on soils or the critters. There was no significant accumulation of biodeposit-derived nitrogen or carbon. We suspect the bottom-dwelling critters used up many of these resources. However, there were also many opportunistic species and deposit-feeding organisms in aquaculture sites compared to those not used for aquaculture. This indicates a community shift that favors critters  that thrive under high organic inputs (from the oyster poop) and/or disturbance.

Other factors indicated that the environment below the off-bottom systems was anoxic, or without oxygen. This can be bad for other bottom-dwelling organisms, but there were still plenty of organisms functioning in these soils.

So, while our results suggest oyster aquaculture poses minor changes to soils and bottom-dwelling communities, site-specific factors should always be taken into consideration due to the dynamic nature of coastal environments.

Based on these results, one could propose oyster farming offers a new strategy for sustainable food production and coastal soils management. As the oyster industry continues to succeed and expand, further research will be necessary to investigate the ongoing impacts of oyster farming on coastal ecosystems.

Answered by Chelsea Duball, University of Wyoming

This blog is adapted from a Journal of Environmental Quality paper “Impacts of Oyster Aquaculture on Subaqueous Soils and Infauna.”

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