By Tom Baird
On first inspection of our salt marshes, the visitor’s attention is captured by the tall grasses. Cordgrass flourishes along the banks of tidal creeks and dark Needle Rush dominates the higher marsh. The visual beauty of vast expanses of marsh grasses has inspired poets like Sidney Lanier and become a favorite of nature photographers. Yet in winter we may think the marsh is dormant and that there is little productivity. The marsh grasses are not flourishing and producing organic matter to sustain the marine life in the bay or Gulf.
However, much goes on in a salt marsh that escapes the glance of the casual visitor. Biologists document a teeming community of organisms thriving at the surface of the marsh. Many of these organisms are microscopic plants which take nutrients brought in by the tides and with sunlight transform them into carbon compounds that support grazers in the marsh and other life forms in the nearby sea. These tiny plants create a dynamic mosaic of forms that nourish the marsh.
One of the most interesting groups living on the surface of the muds in a marsh is the diatoms. Diatoms are a type of brownish or yellowish algae. Diatoms living on the surface of muds are termed edaphic, and large numbers of them can color the marsh substrate a golden brown. Scrape the surface of the mud and examine the material under a microscope. The slimy material is active with life forms. The diatoms are easy to distinguish from the other microscopic plants. The diatoms have a stiff glass-like outer capsule called a frustule. That capsule is made of silica and is a characteristic shape and size for each species of diatom. In addition, the frustule of each species of diatom has its own characteristic set of markings such as bumps, pores and ridges. While other algae have varying shapes, the diatoms appear uniform, gem-like.
All of the soil diatoms and many of the freshwater species and marine benthic species belong to a group called the pennate diatoms. The pennate diatoms have a frustule that is generally oblong, elongated or boat-shaped. Worldwide there are 2900 species of pennate diatoms, and all of the edaphic diatoms of salt marshes are pennate. In addition, many of the pennate diatoms are motile, that is they can move. This enables them to migrate to optimum light conditions in the marsh.
In addition to diatoms, the marsh surface is home to a variety of filamentous algae that often form mats at the water’s edge and between the stalks of marsh grasses. Important in this group are the blue-green algae, so called because they contain chlorophyll and photosynthesize, yet structurally they are bacteria, lacking the internal membrane structure of algae. Because blue-green bacteria often possess tolerances for extremes of temperature and drying, they are often found in the salt pans or salt barrens – those large, flat areas of high soil salinity caused by the receding tide. In these areas algal mats sometimes form. The blue-green bacteria thrive in the bright sunlight and as the water evaporates, form black sections that look like broken pieces of pavement. The blue-green bacteria are important not only as producers, but many of the species found in salt marshes are nitrogen fixers – taking the gaseous nitrogen from the atmosphere and incorporating it into a stable and usable form for the higher plants. Much as you may fertilize your garden, the blue-green bacteria fertilize the marsh.
The two components of marsh production, edaphic algae and marsh grasses, interact with one another to create alternating peaks of production. In spring, grass growth is slow while light intensity increases. The more rapidly metabolizing single-celled algae utilize the increasing light and algal production steadily increases. By summer, the grasses shade the substrate and algal production decreases while the standing crop of grasses increases until fall. When the grass cover diminishes in the fall, the algae use the still high light intensities for another peak of production. On sunny winter days, algal production can still be high. The algae even out productivity in the marsh by functioning in winter when the marsh grasses die back and sunlight reaches the substrate. The interaction of these two components ensures maximum production of the marsh over the year and over the entire marsh surface. You can see this without a microscope, at least in mass, when the surfaces of the marsh muds are colored pinks and greens.
Studies of salt marshes have found the production of algae to be about 25 percent of the production of marsh grasses. This is a significant and often overlooked aspect of the marsh ecology. Additionally, algal production occurs when grasses are dormant and may provide more easily assimilated food for marine life than that provided by higher plants.
Herbicides, pesticides, hydrocarbons, and heavy metals are inhibitory to algae and different algal species differ in their susceptibility. Runoff, oil spills, and urbanization near marsh lands can visibly affect the grasses and animal communities of a marsh. Less visible, but no less important, are these impacts on the edaphic diatoms and filamentous algae of the marsh surface. The productivity of our marshes is dependent on the health of this fascinating assemblage of microscopic plants gliding at the mud’s surface. The next time you visit or look out over our salt marshes, think about all the unseen gems working out of sight to keep our marshes beautiful, healthy and productive.
Tom Baird has been a fisheries biologist, high school and community college teacher (oceanography and microbiology), director of a science and environmental center, teacher of science and principal in Pinellas County as well as an educational consultant. He retired from the Florida Department of Education and he and his wife divide their time between Tallahassee and Cape San Blas.