When scientists identify a problem they must sometimes find new methods to study the problem. Sometimes scientists come up with a new method based on some other method they know about or have used before. One new field of study is called sclerochronology (skler ə krə nä lə jē). This new field of study is based on dendrochronology.

Dendrochronology is a field of study in which scientists learn about trees by studying the tree’s growth rings (figure 1). The field of sclerochronology similarly enables scientists to learn about any organism that leaves growth rings in hard tissues like bones or shells. Some scientists examine fish scales and mussel shells to learn more about the animal (figure 2). The growth rings in mussel shells can provide information about how fast the animal grew, how old it is, and how growth changes with changes in climate.

Bottomland hardwood forests are unique ecosystems (figure 3). These ecosystems have a lot of diversity and are highly endangered. Bottomland hardwood forests are river swamps. These forests are found along rivers and streams and typically include large floodplains. The way in which water moves and flows through these rivers and forests has an effect on the entire ecosystem. Hydrology is the study of how water moves and flows. Changes to the hydrology of the rivers and streams may cause problems for the entire ecosystem.

In earlier studies, scientists learned that periods of flooding are important to this ecosystem. One reason these periods of flooding are important is because carbon deposits from the land, in the form of leaves and other organic matter, are transferred into the river during these periods. Carbon is important for the plants and animals in and along the river. The scientists in this study wondered if flooding provides other benefits for this ecosystem.

One way to study flooding is to look at the amount of water in a stream or river at different times. The amount of water and the velocity, or speed, of water in a stream or river is called streamflow (figures 4a, 4b, and 4c). You will learn more about streamflow in the “Introduction” section. The scientists were particularly interested in how freshwater mussels responded to changes in streamflow.

An ecosystem is a community of plant and animal species interacting with one another and the nonliving environment. Ecosystems may change over time. Ecosystem changes are
recorded in unique ways. For example, a tree’s growth rings show scientists which years of the tree’s life included drought and fire. Similarly, the shell of a mussel can show scientists the age and growth rate of the animal. In this study, the scientists wanted to use information from the mussels’ shells to help them better understand the environment in which the mussels lived.

The scientists in this study were interested in studying mussels in bottomland hardwood forest ecosystems. Streamflow is an important factor in bottomland hardwood forest ecosystems. Streamflow is the volume of water flowing through a stream at any particular place. The scientists wanted to figure out how streamflow affected mussels and plants near and in the rivers in these ecosystems. The scientists also wanted to compare streamflow with cypress trees along the river to see if streamflow affected mussels and trees the same way (figure 5).

North America has the highest diversity of freshwater mussels in the world (figure 6).

Mussels are mollusks. They have two shells that are joined by a ligament (figure 7). Mussels have two small siphons used to draw in and expel water. This siphoning action helps to filter water of bacteria, algae, and other small particles, and it makes this material available to other organisms that live on the bottom. Mussels, therefore, help improve water quality.

The outer shell of a living mussel is a home to some organisms. Mussels are also a source of food for many animals. Some animals that eat mussels are muskrats, raccoons, otters, turtles, and fish (figure 8). In addition, the empty shells become homes for animals after the mussels are no longer living.

Mussels are very sensitive to changes in rivers or lakes, and this sensitivity makes them good indicators of the health of aquatic ecosystems. Mussels are also interesting because they have a unique reproductive cycle. Due to the importance of mussels in the ecosystem, the scientists wanted to know how streamflow in the bottomland hardwood forest ecosystems affected the mussels. The scientists wanted to know specifically how streamflow affected mussel growth.

The scientists studied 13 mussel species. The scientists collected the mussels from Alabama, Arkansas, and Mississippi (figure 10). They collected mussels from two sites in Alabama on the Sipsey River, one site on the St. Francis River in Arkansas, and one site on the Little Tallahatchie River in Mississippi (figure 11).

The rivers that the scientists collected from were different from each other in several important ways. Some of the rivers were regulated and some were unregulated. A regulated
river is a river whose water is controlled by dams or other water storage methods (figure 12a). An unregulated river does not have any dams or other water storage methods (figure 12b). Unregulated rivers are free flowing.

Streamflow can be different for regulated and unregulated rivers. For example, a dam can be opened to release more water, which causes higher streamflow, or it can be closed, reducing the amount of water in the river. Changes in streamflow at certain times of year can have negative impacts on aquatic organisms. The scientists wanted to see if the difference between regulated and unregulated streams affected the mussels (figure 13).

In order to understand how streamflow affected the mussels, the scientists had to collect a variety of data. The scientists used microscopes to examine thin sections of mussel shells (figures 14 and 15). The scientists took pictures of these shell sections with a digital camera, which allowed them to measure how fast the mussels grew in different years. They also took cores from cypress trees to measure how fast the trees grew.

The scientists gathered streamflow data for each of the rivers they studied. The scientists were able to obtain streamflow data for each day over several decades from data collected
by the U.S. Geological Survey and other government agencies. They put all of this information into a computer program to help them analyze the data.

In the unregulated Sipsey River, mussels grew faster during times of low streamflow and slower during times of high streamflow (figure 17).

Some mussel species in the St. Francis River also showed this relationship. The strongest and most consistent trend in the St. Francis River was a strong, positive relationship between mussel growth and hydrologic reversals.

A hydrologic reversal is when the river changes from a rising river to a falling river or from a falling river to a rising river. A greater number of reversals can mean that the water falling on the land winds up in the river and flows downstream very quickly. This rapid movement of water often happens in an agricultural landscape like the St. Francis River because much of the forest is gone. In a forested landscape like the Sipsey River, the water is absorbed by the forest and enters the river more slowly. This absorption of water influences things like the frequency and severity of floods.

Unlike in the other rivers, mussel growth in the regulated Little Tallahatchie River was not significantly related to any streamflow measurements. This finding suggests that regulation of streamflow can disrupt the normal cycles of mussel growth found in unregulated streams. Interestingly, cypress trees behaved exactly the opposite from mussels. Cypress trees grew faster in high-streamflow years but slower in low-streamflow years.

The scientists found that in unregulated streams, mussels showed higher growth in low-streamflow conditions. Low-streamflow conditions enable the growth of microbes and
algae in the water, which make up much of an adult mussel’s diet. During periods of high streamflow, mussel growth slows.

The scientists think that the mussels’ growth pattern may be a result of several factors. One factor is that during times of high streamflow, mussels may require more energy to maintain their position in the riverbed. Another factor is that high streamflow might make it more difficult for mussels to process their food. This difficulty is because the water is turbid and carries a lot of sediment that mussels must separate from edible material like algae and microbes. Finally, high water may dilute the food particles that mussels need to grow. The scientists would need to conduct additional studies to find out exactly why mussel growth is related to streamflow.

Even though the mussels experience lower growth during high streamflows, higher streamflows are needed periodically. Higher streamflows help keep the mussel habitat in good condition by removing fine sediment from the streambed. For some species of mussel, too much fine sediment may impact growth, feeding, and survival of young mussels. The different response of cypress trees also shows
that a wide range of streamflows is necessary for the best growth for different organisms in the ecosystem.

The scientists found no relationship between streamflow and mussel growth in regulated streams. This finding suggests that dams or other forms of regulation disrupt the normal cycles of mussel growth. The scientists said that the potential relationships found between mussels and streamflow should be studied in other landscapes and rivers. This additional research is necessary so that the scientists will know if these relationships occur in many different places or only in the rivers they studied.