Episode 35
Copyright © John Page Williams, Jr. all rights reserved.
This is John Page Williams with another reading from Chesapeake Almanac. The month is November and this chapter is entitled "Diatoms: Cold-water Jewel Boxes."
The water's getting colder and the days are getting shorter. From the upper rivers to the open Bay, plants are responding to the change. Cordgrasses in the salt marshes have turned from gold to brown. In the fresh marshes, pickerelweed has withered away, the wild rice has fallen over to become part of the stubble on the surface of the marsh. Many of the perennial plants like cattails have reabsorbed nutrients from red leaves and stored starch in their root systems. The annuals like millet and smartweed have left heavy crops of seeds.
In the open waters, most groups of phytoplankton—those microscopic plants that live in the upper layers and drift with the currents—are decreasing as the cooling water slows their metabolism. One group, however, is just building up. The Chesapeake's diatoms reach high concentrations in the fall and drop only a little through the winter.
Diatoms are the dominant group of phytoplankton for much of the Chesapeake's year, and they make major contributions of food and oxygen to the Bay's waters. They comprise a group of algae called Bacillariophyta, all microscopic and either unicellular (that is single celled) or colonial (that is single cells attached together). There is a large group of benthic, or bottom-dwelling, diatoms in the Bay, but even more are planktonic. In numbers and tonnage, these drifting diatoms are stunningly abundant. Concentrations can run up to 10 million cells per liter.
Diatoms are best known for their frustules, cell walls of silica. The walls are made in two halves, with one fitting over the other like a shoebox. Silica is a common element in sand and quartz. We humans extract it mechanically from these minerals to make glass. Diatoms extracted physiologically from river water and Bay water, somewhat in the way that oysters and crabs extract calcium, then they secrete it into all sorts of complex shapes in the frustules. They end up looking like intricately carved glass jewel boxes. Ladies and gentlemen interested in natural history during England's Edwardian period made a hobby of collecting diatoms, looking at them under microscopes and marveling at the designs.
The shapes are wonderful to look at, but recent and current research is directed more to how diatoms fit into the Chesapeake system. Their siliceous frustules raise particularly interesting questions. They offer the diatoms both advantages and disadvantages.
All phytoplankton, diatoms included, have two basic needs: nutrients and sunlight. Sunlight is needed at least part of the time to photosynthesize food, so they must spend a significant portion of their lives (which last a day to a week or more depending on temperature), they spend that time in the upper layers of the water.
But they need nutrients, too, essential building blocks. Because phytoplankton can bloom to dense concentrations, they can use up dissolved nutrients very quickly. A stationary algae cell suspended in the water can easily run out of them. By sinking, the cell can come into contact with more water and thus increase the nutrients available to it. Sometimes there's an advantage in sinking all the way to the bottom, where nutrients collect. But sinking takes the cell away from sunlight, so it needs a way to get back to the surface, too.
Silica is relatively dense, giving diatoms the potential to sink. But the diatom community has developed a strategy to rise to the surface again. Some species develop frustules that are broad flat disks. Others build complex arrays of ribs, folds, horns, and spines, all of which tend to slow their sinking rate. The basic result is a group of plants that sink slowly in still water but rise to the surface with turbulent water.
A key part of the strategy is the diatoms' ability to grow well at lower water temperatures than most phytoplankton. Their populations are highest in fall, winter, and spring, when periodic stormy weather keeps the water stirred up. In summer, when the Bay tends to stratify, with little mixing taking place between surface and deep water, the diatom community lead yields its dominance to the mobile dinoflagellates. For the rest of the year, however, the diatoms depend on sinking to provide them with nutrients, and on turbulence in the Chesapeake's relatively shallow waters to bring them back to the sunlight.
Studying the roles of diatoms in the Bay system brings together an elegant array of scientific disciplines. Physics and mathematics, calculus and statistics, together attempt to describe the transport mechanisms of turbulence and thus of cell movement. Sophisticated biochemical techniques offer insight into nutrient uptake and release. Both laboratory and field studies provide data on grazing of diatoms by zooplankton such as copepods and fish larvae. Computer modeling gives a sense of the ways the community responds to environmental alterations due to man's activities and varying weather patterns.
This kind of systems approach will gradually uncover general principles to make the Bay easier to understand and manage. It is quite different from Edwardian collecting, exciting because it is close to the heart of understanding the Chesapeake system, and unsettling because it is enormously complex. For now, though, just think of all those jewel boxes drifting back into the light every time it storms this fall.
For more happenings around the Bay in November see our other Chesapeake Almanac podcasts and read our blog post "From Fall Colors to Dead Leaves."
