Studying underwater soils – or subaqueous soils – is a fairly new field. Before the late 90s, soil that was under water was commonly considered to be nothing more than sediment. However, as soil scientists began to study this emerging area further, they realized that underwater soils share many characteristics with soils on land. By 1998, a subaqueous soil survey had begun and subaqueous soils were officially a field of study.
Subaqueous soils are soils that are permanently under water. They are typically under only a few meters of water, but deeper areas are being explored as well. They contribute to healthy ecosystems in marshes and estuaries, as well as tidal basins and coastal areas.
Some underwater soils formed in upland environments. They were later “drowned” by rising sea levels, preserving bright colors and structure in the soil profile. Others formed under water in sediments. With the accumulation of organic matter and minerals like pyrite, they became soils. In either case, it is important to understand how subaqueous soils change across submerged landscapes. This understanding can lead us to best management practices. Ordinary upland soils are mapped and ranked in terms of their suitability to support food crops. Our lab at the University of Maryland is developing similar interpretations for subaqueous soils and how they can support oysters and other shellfish “crops” in Chesapeake Bay.
Gathering soil samples for subaqueous soil research presents more challenges than their more dry counterparts. We have been able to adapt a tool usually used in concrete construction to assist with this. We’ve modified a concrete vibrator, intended to shake the bubbles out of freshly poured concrete. Our repurposing allows the “vibracore” to shake aluminum core tubes so vigorously that they slide into the squishy seafloor. With a tripod and the right boat, they can be pulled out with intact soil samples inside.
Once we have our soil samples, we are able to do some testing on them. These tests help us determine what is going on in the soil and ways to manage the area. We don’t do all of the same testing as for dryland soils, because some of those values don’t pertain. For instance, cation exchange capacity is an important measure in upland soils. It tells us how well a soil can hold most nutrients, but it doesn’t tell us much in a soil that is under water (particularly saltwater, which can hold a lot of nutrients on its own).
On the other hand, we often do additional testing not done on upland soils. We estimate fluidity of subaqueous soils by squeezing handfuls and rating how easily they flow through your fingers. Some subaqueous soil samples resemble a handful of dark-colored mayonnaise. We might even smell soil samples! Our noses can help us to determine if hydrogen sulfide or petroleum odors are present. This can tell us about how the sulfur cycle is operating in these soils, or if they have been contaminated by chemical spills.
Other methods have long been used by other disciplines to study aquatic sediments, including methods from fields such as marine ecology, limnology, and biogeochemistry. Which of these methods should soil scientists adopt as they move into this particularly wet frontier? We’re still learning and improving all the time.
The study of subaqueous soils is a vitally important part of the new Coastal Zone Soil Survey. Soil surveyors weren’t historically very interested in marsh soils, and so the soil maps along both the submerged and the exposed sides of our coasts often convey very little useful information. Map units were often described simply as “Tidal marsh, soft” or “Water.” At the time these surveys were made this was good enough, but now that climate change threatens our coastlines and our agricultural systems it is clear we need more information.
- Can we support rural waterfront communities by identifying prime areas for aquaculture?
- Can we protect and manage our shores by identifying coastal areas most at risk of loss?
- Can we predict which areas of the landscape will sequester carbon and which will release it as the seas continue to rise?
I contend that the answer to these questions, and dozens of similar questions, is a resounding “Yes!” We don’t have all of the information to do this yet, but we’ve learned to conduct soil survey and soil science in the subaqueous environment, and that’s a big step in the right direction.
Answered by Barret Wessel, University of Maryland
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