Urbanization and Estuary Eutrophication

Charles Hopkinson out taking dissolved O2 measurements.

Charles Hopkinson out taking dissolved O2 measurements.Student activity, Graph Type A, Level 4

The activities are as follows:

An estuary is a habitat formed where a freshwater river or stream meets a saltwater ocean. Many estuaries can be found along the Atlantic coast of North America. Reeds and grasses are the dominant type of plant in estuaries because they are able to tolerate and grow in the salty water. Where these reeds and grasses grow they form a special habitat called a salt marsh. Salt marshes are important because they filter polluted water and buffer the land from storms. Salt marshes are the habitat for many different kinds of plants, fish, shellfish, and birds.

Hap Garritt removing an oxygen logger from Middle Road Bridge in winter.

Hap Garritt removing an oxygen logger from Middle Road Bridge in winter.

Scientists are worried because some salt marshes are in trouble! Runoff from rain washes nutrients, usually from lawn fertilizers and agriculture, from land and carries them to estuaries. When excess nutrients, such as nitrogen or phosphorus, enter an ecosystem the natural balance is disrupted. The ecosystem becomes more productive, called eutrophication. Eutrophication can cause major problems for estuaries and other habitats.

With more nutrients in the ecosystem, the growth of plants and algae explodes. During the day, algae photosynthesize and release O2 as a byproduct. However, excess nutrients cause these same algae grow densely near the surface of the water, decreasing the light available to plants growing below the water on the soil surface. Without light, the plants die and are broken down by decomposers. Decomposers, such as bacteria, use a lot of O2 because they respire as they break down plant material. Because there is so much dead plant material for decomposers, they use up most of the O2 dissolved in the water. Eventually there is not enough O2 for aquatic animals, such as fish and shellfish, and they begin to die-off as well.

Two features can be used to identify whether eutrophication is occurring. The first feature is low levels of dissolved O2 in the water. The second feature is when there are large changes in the amount of dissolved O2 from dawn to dusk. Remember, during the day when it’s sunny, photosynthesis converts CO2, water, and light into glucose and O2. Decomposition reverses the process, using glucose and O2 and producing CO2 and water. This means that when the sun is down at night, O2 is not being added to the water from photosynthesis. However, O2 is still being used for decomposition and respiration by animals and plants at night.

The scientists focused on two locations in the Plum Island Estuary and measured dissolved O2 levels, or the amount of O2 in the water. They looked at how the levels of O2 changed throughout the day and night. They predicted that the upper part of the estuary would show the two features of eutrophication because it is located near an urban area. They also predicted the lower part of the estuary would not be affected by eutrophication because it was farther from urban areas.

A view of the Plum Island estuary

A view of the Plum Island estuary

Featured scientists: Charles Hopkinson from University of Georgia and Hap Garritt from the Marine Biological Laboratory Ecosystems Center

Flesch–Kincaid Reading Grade Level = 9.6

The ground has gas!

Measuring nitrogen (N2O) gas escaping from the soil in summer.

Measuring nitrogen (N2O) gas escaping from the soil in summer. Photo credit: Julie Doll, Michigan State University

The activities are as follows:

If you dig through soil, you’ll notice that soil is not hard like a rock, but contains many air pockets between soil grains. These spaces in the soil contain gases, which together are called the soil atmosphere. The soil atmosphere contains the same gases as the atmosphere that surrounds us above ground, but in different concentrations. It has the same amount of nitrogen, slightly less oxygen (O2), 3-100 times more carbon dioxide (CO2), and 5-30 times more nitrous oxide (N2O, which is laughing gas!).

Measuring nitrogen (N2O) gas escaping from the soil in winter.

Measuring nitrogen (N2O) gas escaping from the soil in winter. Photo credit: Julie Doll Michigan State University.

Nitrous oxide and carbon dioxide are responsible for much of the warming of the global average temperature that is causing climate change. Sometimes soils give off, or emit, these greenhouse gases into the earth’s atmosphere, adding to climate change. Currently scientists are working to figure out why soils emit different amounts of these greenhouse gasses.

During the summer of 2010, researchers at Michigan State University studied nitrous oxide (N2O) emissions from farm soils. They measured three things: (1) the concentration of nitrous oxide 25 centimeters below the surface of the soil (2) the amount of nitrous oxide leaving the soil (3) and the average temperature on the days that nitrous oxide was measured. The scientists expected that the amount of nitrous oxide entering the atmosphere would depend on how much nitrous oxide was in the soil and on the temperature.

Featured scientist: Iurii Shcherbak from Michigan State University

Flesch–Kincaid Reading Grade Level = 9.2

More information on the research associated with this Data Nugget can be found hereInformation on the effects of climate change in Michigan can be found here.

Data associated with this Data Nugget can be found on the MSU LTER website data tables under GLBRC Biofuel Cropping System Experiment. Bioenergy research classroom materials can be found here. More images can be found on the LTER website.

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Fertilizing biofuels may cause release of greenhouse gasses

An aerial view of the experiment at MSU where biofuels are grown

An aerial view of the experiment at MSU where biofuels are grown. Photo credit: K. Stepnitz, MSU

The activities are as follows:

Greenhouse gases in our atmosphere, like carbon dioxide (CO22) and nitrous oxide (N22O), trap heat from the sun and warm the earth. We need some greenhouse gases to keep the planet warm enough for life. But today, the majority (97%) of scientists agree that the levels of greenhouse gases are getting dangerously high and are causing changes in our climate that may be hard for us to adjust to.

Scientist Leilei collecting samples of gasses released by the growing of biofuels

Scientist Leilei collecting samples of gasses released by the growing of biofuels. Photo credit: K. Stepnitz, MSU

Greenhouse gases are released when we burn fuels to heat and cool our homes or power our cars. Most of the energy we use today comes from fossil fuels. These energy sources are called “fossil” fuels because they are made from plants that grew hundreds of millions of years ago! After these plants died, their tissues were slowly converted into coal, oil, and natural gas. An important fact about fossil fuels is that when we use them, they release CO2 that was stored millions of years ago into our atmosphere. The release of this stored carbon is adding more and more greenhouse gases to our atmosphere. In order to reduce the effects of climate change, we need to change the way we use energy and think of new ways to power our world.

One potential solution could be to grow our fuel instead of drilling for it. Biofuels are a potential substitute for fossil fuels. Biofuels, like fossil fuels, are made from the tissues of plants. The big difference is that they are made from plants that are alive and growing today. Unlike fossil fuels that emit CO2, biofuel crops first remove CO2 from the atmosphere as the plants grow and photosynthesize. When biofuels are burned for fuel, the CO2 is emitted back into the atmosphere, balancing the total amount that was removed and released.

Scientists are interested in figuring out if biofuels make a good replacement for fossil fuels. It is still not clear if the plants that are used to produce biofuels are able to absorb enough CO2 to offset all of the greenhouse gases that are emitted when biofuels are produced. Additional greenhouse gases are emitted when producing biofuels because it takes energy to plant, water, and harvest the crops, as well as to convert them into fuel. In order to maximize plant growth, many biofuel crops are fertilized by adding nitrogen (N) fertilizer to the soil. However, if there is too much nitrogen in the soil for the crops to take up, it may instead be released into the atmosphere as the gas nitrous oxide (N2O). N2O is a greenhouse gas with a global warming potential nearly 300 times higher than CO22! Global warming potential is a relative measure of how much heat a greenhouse gas traps in the atmosphere.

Leilei is a scientist who researches whether biofuels make a good alternative to fossil fuels. He wondered whether there were steps that farmers could take to reduce the amount of N2O released when growing biofuel crops. Leilei designed an experiment to determine how much N2O is emitted when different amounts of nitrogen fertilizer are added to the soil. In other words, he wanted to know whether the amount of N2O that is emitted into the atmosphere depends on how much fertilizer is added to the field. To test this idea, he looked at fields of switchgrass, a perennial grass native to North America, that is one of the most promising biofuel crops. These fields of switchgrass were first planted in 2008 as a part of a very large long-term study at the Kellogg Biological Station in southwest Michigan. The researchers set up eight fertilization treatments (0, 28, 56, 84, 112, 140, 168, and 196 kg N ha−1) in four replicate fields of switchgrass, for a total of 32 research plots. Leilei measured how much N2O was released by the soil in the 32 research plots for many years. Here we have two years of Leilei’s data.

Featured scientist: Leilei Ruan from Michigan State University

Flesch–Kincaid Reading Grade Level = 10.1

More information on LTER climate change research can be found hereInformation on the effects of climate change in Michigan can be found here.

Data associated with this Data Nugget can be found on the MSU LTER website data tables under GLBRC Biofuel Cropping System Experiment. Bioenergy research classroom materials can be found here. More images can be found on the LTER website.

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Fair traders or freeloaders?

Measuring chlorophyll content in the greenhouse

Measuring chlorophyll content in the greenhouse

The activities are as follows:

When two species do better when they cooperate than they would on their own, the relationship is called a mutualism. One example of a mutualism is the relationship between a type of bacteria, rhizobia, and legume plants. Legumes include plants like peas, beans, soybeans, and clover. Rhizobia live in bumps on the legume roots, where they trade their nitrogen for sugar from the plants. Rhizobia fix nitrogen from the air into a form that plants can use. This means that legumes that have rhizobia living in their roots can get more nitrogen than those that don’t.

Under some conditions, this mutualism can break down. For example, if one of the traded resources is very abundant in the environment. When the plant doesn’t need the nitrogen traded by rhizobia, it doesn’t trade as many sugars to the rhizobia. This could cause the rhizobia to evolve to be less cooperative as well. Less-cooperative rhizobia may be found where the soil already has lots of nitrogen. These less-cooperative bacteria are freeloaders: they fix less nitrogen, but still get sugars from the plant and other benefits of living in nodules on their roots.

Photo by Tomomi Suwa, 2013

Rhizobia nodules on plant roots. In exchange for carbon and protection in the nodules from plants, rhizobia provide fixed nitrogen for plants.

One very important legume crop species is the soybean. Soybeans are used to produce vegetable oil, tofu, soymilk, and many other food products. Soybeans trade with rhizobia for nitrogen, but often farmers add more nitrogen into the field as fertilizer. Since farms use a lot of nitrogen fertilizer, researchers at KBS were interested in how different types of farming affected the plant-rhizobia mutualism.

They grew soybean plants in a greenhouse and added rhizobia from three different farms: a high N farm, low N farm, and organic farm that used no N fertilizer. After four weeks, the researchers measured chlorophyll content of the soybean plants. Healthy plants that have lots of nitrogen will have high chlorophyll content, and plants with not enough nitrogen will have low chlorophyll content. Because high nitrogen could lead to the evolution of less-cooperative rhizobia, they expected that rhizobia from organic plots would be most cooperative. They predicted rhizobia from high N plots would be the least cooperative, and rhizobia from low N plots would fall somewhere in the middle. More-cooperative rhizobia provide more nitrogen, so the researchers expected plants grown with cooperative rhizobia to have higher chlorophyll content than plants receiving less-cooperative rhizobia.

Featured scientist: REU Jennifer Schmidt from the Kellogg Biological Station

Flesch–Kincaid Reading Grade Level = 10.1

For more information on the evolution of cheating rhizobia, check out these popular science articles:

If you are interested in performing your own classroom experiment using the plant-rhizobium mutualism, check out this paper published in the American Biology Teacher describing methods and a proposed experimental design: Suwa and Williamson 2014