Marine Sediments' Dirty Little Secret: Using Sediments to Study Human-altered Ecosystems

DEBRA WOODALL (WoodalD@daytonastate.edu) is a professor at the Institute of Marine and Environmental Studies at Daytona State College in Daytona Beach, Florida.

Students enrolled in Introduction to Oceanography (OCE1001) at Daytona State College (DSC) are discovering the dirty little secret hidden within their sediment samples. DSC is located in Daytona Beach, Florida, where Atlantic beaches provide a very popular tourist destination for sun, fun, and...beach driving. These same beaches are also important habitats for certain sea turtle species, harboring one of the densest sea turtle nesting areas found in the Western Hemisphere, as well as a species of mole crab, those tiny but important little morsels of food that burrow themselves into the sand to escape from predators such as birds and fish. For sea turtles and mole crabs, the type and condition of sandy sediments may play a role in species number.

Daytona Beach also is near the northern end of the Indian River Lagoon (IRL)—a shallow-water estuary stretching across 40 percent of the east coast of Florida. Seagrass beds within the IRL are some of the more important habitats, serving as spawning and nursery grounds for many species of oceanic and lagoon fish and shellfish. However, sediment type, as affected by the energy within a marine system (e.g., waves, tidal currents), may impact the photosynthetic capabilities of these seagrass beds.

OCE1001 is a flipped-hybrid course allowing for in-class learning activities that help to increase critical-thinking skills; class time is 1.3 hours per week. Students are typically fulfilling their Gen-Ed requirements and have very little background in the marine and/or geologic sciences; course enrollment is limited to 35 students.

Learning about Sediments

I have students complete an activity in which marine sediments from local beach and IRL environments are used to bring awareness to the fact that by altering a sediment source, we are altering its type and, more decisively, species habitat! Prior to the activity, students review online information about differences between lithogenous, biogenous, and hydrogenous sediments and are introduced to the Wentworth scale. They also learn about sources and importance of sediments and how the water's energy affects their resuspension and movement. The review continues in class by having students describe sediments as depicted on Powerpoint images using textural terms associated with their shape, size, and maturity.

Students are then divided into four groups and given a sediment sample (sediment A, B, C, or D), a scoopula and a "water column" (a 1.5-liter, waterfilled soda bottle). To maintain sediment secrecy, students are only told that the sediment sample in Group A is related to Group B's and that Group C's sample is related to Group D's. Students work within their groups to describe their "dry" sample and complete a comparative description for their related sample (e.g., is it more/less mature?). Each group then transfers a representative sample into their water column and applies energy (i.e., shakes it). Resuspending the sample helps to separate the sediments into various grain sizes thus allowing students to approximate the percent of clay-, silt-, sand-, and pebble-size sediments. It also helps to highlight the possible environmental impact caused by resuspended sediments. When groups are done describing their sediments, the class rejoins to discuss the differences between the related groups.

General Sediment Descriptions

Here's how the groups are regularly described.

Group A—generally described as a mixture of lithogenous (coarse sand-size) and biogenous (small, broken shells) sediments, moderately-well sorted.

Group B— lithogenous, very well sorted, medium sand-size, and more mature than Group A.

Group C—described as a poorly sorted mix of lithogenous and biogenous sediments with an approximate proportion of 25 percent clay- and silt-size, 65 percent sand-size, and 10 percent gravel-size particles.

Group D—mostly lithogenous with an approximate proportion of 65 percent muck-, clay-, and siltsize and 35 percent fine sand-size particles.

The secret? Both sets of sediments come from similar environments. However, one sample within each group is more representative of a natural-source sediment while the source of the other has been affected by human activity.

The Dirty Little Secret

The secret? Both sets of sediments come from similar environments. However, one sample within each group is more representative of a natural-source sediment while the source of the other has been affected by human activity. Group A and B sediments are from our local beaches. Group A represents a natural beach sand that, due to its sand/shell mix, is well known for being "mushy" to walk on. Sediment B is also from our beach but from an area that has experienced beach nourishment (the process by which sediment, e.g., sand, lost through longshore drift or erosion is replaced from outside sources). Very well sorted, this sediment is preferred for beach-nourishment projects due to its tendency toward hard compaction, allowing for easier beach driving.

Now that students understand why there are differences in the sands they encounter on our beaches, I pose these questions: How does changing the source and type of sand impact the habitat of sea turtles and mole crabs? And what impact might beach driving have on the condition of the sediment and, thus, on habitat?

Group C and D sediments are from areas within the IRL. However, Group C sediment was sampled from an area far removed from most human development (e.g., homes and cities) while Group D sediment was sampled within their vicinity. Sediments altered by human development are typically high in fine-size sediments, including muck, that easily resuspend and increase IRL turbidity levels. Students now visually understand why many of our waterways are "murky." The question posed here: How might the introduction of finer-size muck impact the health and growth of our seagrass beds?

The take-home message for this lesson: The source of a sediment affects the type of sediment found at each location. In turn, the sediment type affects the type of habitat— change the source and you change the habitat!

Answering questions posed in this exercise helps students to realize that humans hold some responsibility for changing sediment sources and thus habitats. This realization typically results in a student-led discussion critically evaluating the environmental impact versus economic benefits resulting from these actions and/or choices.

REFERENCES

• Garland, E., "The Indian River Lagoon: An estuary of national significance," Palatka, Florida: St. Johns River Water Management District, http://floridaswater.com/indianriverlagoon/. Last updated on December 23, 2015, accessed January 11, 2016.
• Thurman, H.V. & Trujillo, A.P. (2004). Introductory Oceanography (10th Edition), Upper Saddle River, N.J.: Pearson Prentice Hall.