phosphorus ‣ Data Nuggets

12.10.20

Mangroves on the move

mangrove in marsh — A black mangrove growing in the saltmarshes of northern Florida.

The activities are as follows:

All plants need nutrients to grow. Sometimes one nutrient is less abundant than others in a particular environment. This is called a limiting nutrient. If the limiting nutrient is given to the plant, the plant will grow in response. For example, if there is plenty of phosphorus, but very little nitrogen, then adding more phosphorus won’t help plants grow, but adding more nitrogen will.

Saltmarshes are a common habitat along marine coastlines in North America. Saltmarsh plants get nutrients from both the soil and the seawater that comes in with the tides. In these areas, fertilizers from farms and lawns often end up in the water, adding lots of nutrients that become available to coastal plants. These fertilizers may contain the limiting nutrients that plants need, helping them grow faster and more densely.

One day while Candy, a scientist, was out in a saltmarsh in northern Florida, she noticed something that shouldn’t be there. There was a plant out of place. Normally, saltmarshes in that area are full of grasses and other small plants—there are no trees or woody shrubs. But the plant that Candy noticed was a mangrove. Mangroves are woody plants that can live in saltwater, but are usually only found in tropical places that are very warm. Candy thought the closest mangrove was miles away in the warmer southern parts of Florida. What was this little shrub doing so far from home? The more that Candy and her colleague Emily looked, the more mangroves they found in places they had not been before.

Candy and Emily wondered why mangroves were starting to pop up in northern Florida. Previous research has shown nitrogen and phosphorus are often the limiting nutrients in saltmarshes. They thought that fertilizers being washed into the ocean have made nitrogen or phosphorus available for mangroves, allowing them to grow in that area for the first time. So, Candy and Emily designed an experiment to figure out which nutrient was limiting for saltmarsh plants.

mangrove saltmarsh researchers — Candy (right) and Emily (left) measure the height of a black mangrove growing in the saltmarsh.

For their study, Candy and Emily chose to focus on black mangroves and saltwort plants. These two species are often found growing together, and mangroves have to compete with saltwort. Candy and Emily found a saltmarsh near St. Augustine, Florida, in which they could set up an experiment. They set up 12 plots that contained both black mangrove and saltwort. Each plot had one mangrove plant and multiple smaller saltwort plants. That way, when they added nutrients to the plots they could compare the responses of mangroves with the responses of saltwort.

To each of the 12 plots they applied one of three conditions: control (no extra nutrients), nitrogen added, and phosphorus added. They dug two holes in each plot and added the nutrients using fertilizers, which slowly released into the nearby soil. In the case of control plots, they dug the holes but put the soil back without adding fertilizer.

Candy and Emily repeated this process every winter for four years. At the end of four years, they measured plant height and percent cover for the two species. Percent (%) cover is a way of measuring how densely a plant grows, and is the percentage of a given area that a plant takes up when viewed from above. Candy and Emily measured percent cover in 1×1 meter plots. The cover for each species could vary from 0 to 100%.

Featured scientists: Candy Feller from the Smithsonian Environmental Research Center and Emily Dangremond from Roosevelt University

Flesch–Kincaid Reading Grade Level = 8.3

7.24.20

Limit by limit: Nutrients control algal growth in Arctic streams

The Arctic Stream Team. Frances, Breck, Abby, Alex, Jay, and Arial at Toolik Field Station in 2019.

The activities are as follows:

You rely on the nutrients from the foods you eat to grow and thrive. Other organisms, like microbes, do as well! Aquatic algae, a type of microbe that live in the water, need to take in nutrients from their surroundings for growth. Two important nutrients for algal growth are nitrogen (N) and phosphorous (P).

Sometimes the environment does not have all the nutrients that aquatic algae need to grow. When one nutrient is less available compared to others, algae can become nutrient limited. Research on nutrient limitation started with Justus Liebig, a 19^th century scientist who proposed the “law of the minimum.” The law states that the nutrient available in the lowest amount relative to demand will limit overall growth and production. This means that growth is not controlled by all the nutrients, but by the scarcest one (the “limiting factor”). When more than one nutrient limits growth, algae are considered co-limited. This just means that a combination of two nutrients are needed for algae to grow. Knowing what nutrients are limiting growth helps scientists understand how an ecosystem is working.

From other research we know that many ecosystems, including those in the Alaskan Arctic, are phosphorus-limited. Scientists figured this out because they found if they added phosphorus, then algae growth increased. However, climate change could change this. As the Arctic warms, ecosystems on land might start to release nutrients in higher amounts or new proportions into the water. These extra nutrients will likely cause increases in algae growth in streams and ponds, which in turn could change food webs and nutrient cycling. It is therefore important to understand which nutrients are currently limiting algae growth before climate change changes things even more. This starts with tests to see how Arctic algae grow in response to changes in N, P, and N and P in the water.

A team of scientists got to work on this question! Arial, Jay, Frances, Alex, Breck, and Abby are all interested in understanding how climate change may alter nutrient limitations in Arctic streams. Each team member has a unique role in the larger research project. For example, undergraduate researcher Abby spent her 2019 summer at Toolik Field Station in Northern Alaska as part of a research opportunity. She explored nutrient limitation in one particular lake, called Lake I8.

Abby used small cups that placed into the streams that fed into Lake I8. These cups were filled with agar gel, a material used in labs to grow microbes. Each cup contained different nutrient treatments. Abby used four different treatments in her cups: (1) a control (agar only), (2) agar + nitrogen, (3) agar + phosphorus, and (4) agar + nitrogen + phosphorus. On the top of each cup, she placed a glass disk to provide a surface for the algae to grow.

A. Cups before going into the stream. B. Abby putting out her cup treatments into an Arctic stream. C. Cups incubating under water in an Arctic stream. D. Analyzing Chlorophyll a extracted from the cups.

Abby put 5 replicate cups for each treatment at both the Inlet and Outlet streams on the I8 Lake. She left them underwater for 4 weeks. She brought the cups back to the lab to measure the algae that grew on each glass disk. Abby measured how much algae grew on each disk by measuring the amount of Chlorophyll a, the green pigment that helps plants photosynthesize. The more pigment, the more the algae is growing. Abby compared the data from the control to each of the other treatments. When there is more growth in a treatment compared to the control, that means a particular nutrient was limiting at that location. Abby expected that the streams would be limited by the amount of phosphorus, but not the amount of nitrogen. She predicted algae would grow more when they are given additional phosphorus compared to the control treatment.

Featured scientists: Abigail Rec from Gettysburg College; Frances Iannucci, Alex Medvedeff, and Breck Bowden from University of Vermont; Arial Shogren and Jay Zarnetske from Michigan State University

Flesch–Kincaid Reading Grade Level = 8.6

4.16.18

The case of the collapsing soil

An area in the Florida Everglades where strange soil collapse has been observed.

The activities are as follows:

As winds blow through the large expanses of grass in the Florida Everglades, it looks like flowing water. This “river of grass” is home to a wide diversity of plants and animals, including both the American Alligator and the American Crocodile. The Everglades ecosystem is the largest sub-tropical wetland in North America. One third of Floridians rely on the Everglades for water. Unfortunately, this iconic wetland is threatened by rising sea levels caused by climate change. Sea level rise is caused by higher global temperatures leading to thermal expansion of water, land-ice melt, and changes in ocean currents.

With rising seas, one important feature of the Florida Everglades may change. There are currently large amounts of carbon stored in the wetland’s muddy soils. By holding carbon in the mud, coastal wetlands are able to help in the fight against climate change. However, under stressful conditions like being submersed in sea water, soil microbes increase respiration. During respiration, carbon stored in the soil is released as carbon dioxide (CO₂), a greenhouse gas. As sea level rises, soil microbes are predicted to release stored carbon and contribute to the greenhouse effect, making climate change worse.

Shelby collecting soil samples from areas where the soil has collapsed in the Everglades.

Shelby and John are ecologists who work in southern Florida. John became fascinated with the Everglades during his first visit 10 years ago and has been studying this unique ecosystem ever since. Shelby is interested in learning how climate change will affect the environment, and the Everglades is a great place to start! They are both very concerned with protecting the Everglades and other wetlands. Recently when John, Shelby, and their fellow scientists were out working in the Everglades they noticed something very strange. It looked like areas of the wetland were collapsing! What could be the cause of this strange event?

John and Shelby thought it might have something to do loss of carbon due to sea level rise. They wanted to test whether the collapsing soils were the result of increased microbial respiration, leading to loss of carbon from the soil, due to stressful conditions from sea level rise. They set out to test two particular aspects of sea water that might be stressful to microbes – salt and phosphorus.

Phosphorus is found in sea water and is a nutrient essential for life. However, too much phosphorus can lead to over enriched soils and change the way that microbes use carbon. Sea water also contains salt, which can stress soil microbes and kill plants when there is too much. Previous research has shown that both salt and phosphorus exposure on their own increase respiration rates of soil microbes.

A photo of the experimental setup. Each container has a different level of salt and phosphorus concentration.

To test their hypotheses, a team of ecologists in John’s lab developed an experiment using soils from the Everglades. They collected soil from areas where the soil had collapsed and brought it into the lab. These soils had the microbes from the Everglades in them. Once in the lab, they put their soil and microbes into small vials and exposed them to 5 different concentrations of salt, and 5 different concentrations of phosphorus. The experiment crossed each level of the two treatments. This means they had soil in every possible combination of treatments – some with high salt and low phosphorus, some in low salt and high phosphorus, and so on. Their experiment ran for 5 weeks. At the end of the 5 weeks they measured the amount of CO₂released from the soils.

Featured scientists: John Kominoski and Shelby Servais from Florida International University. Written by Shelby, John, and Teresa Casal.

Flesch–Kincaid Reading Grade Level = 9.2

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8.24.15

Urbanization and estuary eutrophication

Charles Hopkinson out taking dissolved O2 measurements.

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.

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 O₂because 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 O₂for 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 O₂in 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

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

6.9.14

Marvelous mud

You can tell that the mud in this picture is high in organic matter because it is dark brown and mucky (in real life you’d be able to smell it, too!)

The activities are as follows:

The goopy, mucky, often stinky mud at the bottom of a wetland or lake is a very important part of the ecosystem. Wetland mud is much more than just wet dirt. For example, many species of microbes live in the wetland mud where they decompose (breakdown) dead plant and animal material to obtain energy. This dead plant and animal material is called organic matter. However, the wetland mud microbes do not have all the oxygen they need to decompose the plant and animal tissues quickly and efficiently. Because of this, the dead material in wetland mud decomposes much more slowly than similar dead material in dry soil.

A successful core! You can see that the tube has mud, as well as some of the water from the wetland that was on top of the mud.

As a graduate student, Lauren became fascinated with wetland mud and its interesting properties. She wanted to know how important all the mud and its organic matter is for wetlands. By talking with other members of her lab and reading scientific papers, Lauren learned that wetland mud can often be high in the element phosphorus and that phosphorus acts as a fertilizer for plants, including wetland plants and algae. However, nutrients, such as phosphorus can build up in wetland mud. Lauren thought it might be possible that the organic matter in the mud was the source of all the phosphorus in some wetlands. She predicted that wetlands with more organic matter would have more phosphorus. If her data support her hypothesis, it could mean that organic matter is very important for wetlands, because nutrients are needed for algae and plants to grow.

Although most mud is high in organic matter and nutrients, not all mud is the same. There is natural variation in the amount of organic matter and nutrients from place to place. Even within the same location mud can be very different in spots. Lauren used this variability to test her ideas. She measured organic matter and phosphorus in mud from 16 freshwater locations (four lakes, five ponds, and seven wetlands). She took cores that allowed her to sample mud deep into the ground. She then brought her cores back to the lab and measured organic matter and phosphorus levels in her samples.

Featured scientist: Lauren Kinsman-Costello from Kent State University

Flesch–Kincaid Reading Grade Level = 9.8

More photos associated with this research can be found here. There is one scientific paper associated with the data in this Data Nugget. The citation and PDF of the paper is below:

Kinsman-Costello LE, J O’Brien, SK Hamilton (2014) Re-flooding a Historically Drained Wetland Leads to Rapid Sediment Phosphorus Release. Ecosystems 17:641-656

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