Freshwater Diatoms: Small, Slimy, Stream Quality Monitors
The bottom of this shallow stream is covered with a complex community of algae, comprising many different species. Probably most abundant of all are the diatoms, many of which secrete a slippery mucus as they travel, leaving the rocks very slick.
A few more steps into the current and I will be able to cast into a nice slot. Focused on my fishing, I inch forward. My foot finds a particularly slick rock and I slip, falling into the stream. Pulling myself up, grumbling under my breath, I remember having read about the organisms responsible of the slippery slime covering the rocks.
Diatoms are a type of single-celled algae and are invisible to the naked eye. Small as they are, diatoms play an important role in aquatic ecosystems. They use light to grow, provide food for many other organisms and are sensitive to environmental change. This last attribute allows scientists to monitor water quality by observing diatom communities.
Each diatom is a single cell enclosed in an intricate shell, called a frustule. Made from silica, the frustule is in two parts that fit together one inside the other, kind of like a tin of tobacco. Both sides are patterned with small openings and look almost like fine lace under a microscope. This protects the diatom while still allowing water, gases and light through to the cell.
Diatoms come in a diversity of sizes, shapes and frustule patterns but can be divided into two basic groups: pennate and cennate.
Pennate diatoms are oblong, like a cigar, and some are able to move slowly along surfaces. They propel themselves by secreting mucus through a small opening on the underside of their frustule, and are primarily responsible for the slick covering of rocks in streams and rivers.
Cennate diatoms are symmetrical around a central point; many are circular like a disk. Unlike pennate diatoms, they are immobile and float near the water’s surface.
Between these two diatom groups, there are several thousand species adapted to survive in different habitats. Many species float among plankton in the ocean, some live in freshwater, and a few are found in moist soils. In Montana, diatoms are abundant in our lakes, rivers and streams.
Diatom communities are influenced by environmental change, which makes them good indicators of water quality. A change in water conditions may negatively affect sensitive species with highly specific habitat requirements, while leaving tolerant generalists little affected.
Diatoms are generally used to monitor aquatic habitats by looking for the presence of sensitive species or by measuring total species diversity. If sensitive diatoms are lost due to habitat change, then there will be fewer sensitive species and less total species diversity.
In Montana, the Department of Environmental Quality samples diatom communities across the state. They use these data to look for changes that impair water quality, such as depositing silt and the introduction of nutrients and metals.
A study from 2001 to 2005 sampled 52 sites in rivers ranging from the Clark Fork to the Missouri, looking for changes in diatom communities. The results have provided valuable information on possible threats to important Montana ecosystems.
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"Field Notes" is produced by the Montana Natural History Center.
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