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The microbes are in charge (Pymatuning Wetlands 2015, Day 4)

A student clearly trying to closely examine the obligate anaerobic bacteria.  Or he fell in a hole.

A student clearly trying to closely examine the obligate anaerobic bacteria. Or he fell in a hole. (Photo: AS)

After a breakfast of energy-rich waffles, the Pymatuning wetlanders slowly descended the rungs of the redox ladder into the world of wetland biogeochemistry. The microbes rule this world, and we examined the ways they make living by examining nitrogen, iron, manganese, sulfur, and carbon cycling in wetlands.  Electron acceptors, photosynthesis, oxidation, reduction, aerobic respiration, diffusion, mineralization, nitrification, denitrification, sulfur bacteria, photosynthetic sulfur bacteria, redox potential, ferric iron, ferrous iron, nitrogen fixation, sulfer-reducing bacteria, extended glycolysis, heterotrophs, chemoautotrophs, facultative anaerobes, obligate anaerobes, methanogenic bacteria, cation exchange capacity,  and other trophic-genic-ifications until our brains were full and it was time to cool off in the marsh.

Collecting vegetation cover data at Pymatuning Creek Marsh.

Collecting vegetation cover data at Pymatuning Creek Marsh.

We spent much of the afternoon at Pymatuning Creek Marsh, where the students established transects along the moisture gradient from the edge of the wetland to the interior, and quantified the distribution of vegetation, water-table depth, and pH. While the students collected data I had a little time to quietly explore the marsh a bit, and I took a few pictures…

My least favorite organism in the marsh today. (Deer fly, Chrysops sp.)

My least favorite organism in the marsh today. (Deer fly, Chrysops sp.)

It was hotter than yesterday and the deer flies (Chrysops sp.) were relentless. Much blood was lost. But we obtained the necessary data and managed to collect a few more plant specimens.  This group of students has a fantastic attitude and they are all quite a lot of fun. We returned to the lab to press plants and sort out the unknown plant species that they encountered along the transects.

Tomorrow we will explore the lacustrine wetlands of Pymatuning reservoir, and visit a swamp and some shallow water environments to round out our “must-know” plant list for the first week of class.

And a few more students are now contributing to the twitter feed: #PLEwetlands

"I'll carry this because it looks cool"

“I’ll carry this because it looks cool”

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Down the redox ladder and into the marsh (PLE day 4)

A happy and muddy wetlander.

A happy and muddy wetlander.

The Pymatuning wetlanders started day four of wetland ecology by fearlessly clutching the rungs of the redox ladder and descending into the wetland biogeochemical environment.   They learned about the landlords of these ecosystems – the microbes, and how the activities of these organisms control the way that these ecosystems function.  For many students, this material is the most challenging part of the class.  However, this group of students is full of questions, which keeps things lively and a lot of fun  – not to mention that it keeps the instructor on his toes 🙂

After their brains were swamped with biogeochemical cycles (the students also have started complaining about my bad puns), we headed off to Pymatuning Creek Marsh to collect plant community data as part of a comparative study of marsh and swamp vegetation. The students laid transects from the edge to the interior of the marsh, and used their recently acquired plant identification skills to estimate changes in the abundance of plant species along this environmental gradient.  It was a beautiful afternoon, and the students seemed to enjoy practicing their marsh-plant identification skills.

Collecting vegetation abundance data at Pymatuning Creek Marsh.

Collecting vegetation abundance data at Pymatuning Creek Marsh.

Red-winged blackbird (Agelaius phoeniceus) nesting in cattails (Typha latifolia) at Pymatuning Creek Marsh.

Red-winged blackbird (Agelaius phoeniceus) nest in the cattails (Typha latifolia) at Pymatuning Creek Marsh.

Smelling your way down the redox ladder: wetland ecology in a bottle

“The act of smelling something, anything, is remarkably like the act of thinking. Immediately at the moment of perception, you can feel the mind going to work, sending the odor around from place to place, setting off complex repertories through the brain, polling one center after another for signs of recognition, for old memories and old connection.” – Lewis Thomas

Students experiencing olfactory "thrills" while measuring dissolved oxygen and redox potential of soil microcosms after flooding. The rotten-egg odor was intense in several of these samples.

Students experiencing olfactory “thrills” while measuring dissolved oxygen and redox potential of soil microcosms after flooding. The rotten-egg odor was intense in several of these samples.

Incorporating multiple senses into the learning process is a hallmark of experiential learning, and has long been viewed as a successful education strategy.  In a classroom setting, combining activities like observing, listening, speaking, writing, and drawing can help students to acquire, synthesize, and reinforce their knowledge of the world.  In a field course, the senses of smell and even taste can also inform and enrich the educational experience. Smelling the twig of a black birch, the leaves of spicebush, the flowers of skunk cabbage, or the wonderful rotten-egg aroma of a salt marsh are ecological observations that lead to questions of “why?” and “how?”  Furthermore, the sense of smell seems to be strongly linked to memory, albeit in poorly understood ways (i.e., the Proust effect).  Incorporating these sorts of sensory experiences into laboratory and lecture-based courses is challenging. However, I recently discovered a laboratory activity that was developed to explicitly appeal to the students’ sense of smell.  Well, perhaps “appeal” is the wrong word here.  The activity nicely demonstrates some important aspects of wetland biogeochemistry, a topic that my wetland ecology students often struggle with, and it does this while providing some considerable olfactory “thrills.”

Setup of two experiments. Each experiment included six microcosms, flooded for different lengths of time. Six experiments were done in total, allowing us to assess the influence of sulfate and organic matter quality and quantity on biogeochemical changes induced by flooding.

Setup of two experiments. Each experiment included six microcosms, flooded for different lengths of time. Six experiments were done in total, allowing us to assess the influence of sulfate and organic matter quality and quantity on biogeochemical changes induced by flooding.

The lab was developed for a soil science class by R.S. Dungan, B.D. Lee, and C. Amrhein. It can be downloaded here.  A set of microcosms are created by the students, each containing a soil which is flooded for a different length of time. A simple gaslock is used to prevent oxygen from entering the microcosms. We used six microcosms, representing flooding durations of 20 minutes, 1 day, 7 days, 14 days, 21 days, and 35 days.  In the original activity, the soils were amended with a small amount of gypsum (for a source of sulfate) and nitrogen-rich organic matter (alfalfa).  Students then measure changes in dissolved oxygen, iron, nitrate, and the presence of hydrogen sulfide.

We modified and expanded the lab for an upper-level wetland science course.  For example, we ran experiments with and without an added sulfate source, approximating the chemical environments of a salt marsh versus a freshwater wetland.  Within each of these environments, we also tested the effect that organic matter quality and quantity had on the biogeochemical changes induced by flooding.  To do this, one set of microcosms contained no added carbon (i.e., only the carbon that was present in the soil), one was amended with alfalfa (low carbon:nitrogen ratio), and one was amended with Sphagnum moss (high carbon:nitrogen ratio). In addition to measuring dissolved oxygen, iron, and nitrate, we also measured sulfate, redox potential, and pH.  Changes in concentrations were plotted against time and redox potential.

Photographs of the microcosms, after 35 days, for the different experimental setups.

Photographs of the microcosms, after 35 days, for the different experimental setups.

The results were fantastic, and some are summarized in the video and figures below.  I learned a few things by doing this lab; in particular, I think that with a little practice I could estimate redox potential using only my nose.  Certainly that would be a great skill for a wetland delineator to have!

The short video includes repeat photographs of a single flask, and provides a nice visual summary of the observed changes. Too bad you can’t send smells through the internet…

Figure showing all the data collected by the class, with concentrations plotted against redox potential measurements. Below are student comments along the redox potential gradient.

Figure showing all the data collected by the class, with concentrations plotted against redox potential measurements. Below are student comments along the redox potential gradient.

Biogeochemical changes with soil flooding, showing selected data from the class. Soils included a small amount of gypsum as a sulfate source, and the three lines indicate the results with organic matter of varying quality and quantity.

Biogeochemical changes with soil flooding, showing selected data from the class. Soils included a small amount of gypsum as a sulfate source, and the three lines indicate the results with organic matter of varying quality and quantity.

Biogeochemical changes with soil flooding, showing selected data from the class. No sulfate source was added, and the three lines indicate the results with organic matter of varying quality and quantity.

Biogeochemical changes with soil flooding, showing selected data from the class. No sulfate source was added, and the three lines indicate the results with organic matter of varying quality and quantity.

Questions for the students

A. Write a paragraph for each of the following questions, citing the appropriate figures:

  1. Describe the sequence of biogeochemical changes that occured after soil flooding. What chemical transformations take place?  Why do these changes occur?
  2. Explain the observed differences between the experiments with and without the added sulfate source. Why did these differences occur? What implications do these results have for understanding energy flow in salt marshes and freshwater wetlands?
  3. What is the likely effect of organic matter quality and quantity on the pattern and rate of biogeochemical changes after flooding? Why?

B. Write a sentence (or  equations) for each of the following questions:

  1. Hydrogen sulfide was produced in the experiment that reached a highly negative redox potential. What other gases were likely produced first?
  2. What visual changes occurred in the experiment (added sulfate, low C:N) between day 15 and 20 (see video)? What caused these changes?
  3. Why does nitrate increase in the first few days? What process is taking place?
  4. If we allowed these experiments to continue longer, what gas might be released eventually?
  5. Write the chemical equations for the redox transformations involving oxygen, nitrate, iron, and sulfate.

Literature Cited

Dungan, R.S., B.D. Lee, and C. Amrhein. 1999. Stinking Mud: An Introductory Soil Science Laboratory Exercise Demonstrating Redox Reactions in Flooded Soils. J. Nat. Resour. Life Sci. Educ. 28:89–-92.

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