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Tool Talk: Simulating the Tide

Wednesday, March 16th, 2016

When testing ideas with experiments, scientists have to balance control and realism.

Being able to control conditions (such as temperature, light levels, water movement, etc.) during an experiment allows scientists to isolate cause and effect. However, the techniques required to control experimental conditions results in experiments that do not necessarily reflect what happens out in nature, in the real world. This balance of the need for control vs realism results in a prevalence of field experiments in some scientific disciplines and laboratory experiments in others.

One of the tools that marine scientists can use to balance the competing demands of control and realism are mesocosms. We discussed what a mesocosm was and how they are used in a recent Tool Talk post (to read it again, click here). Essentially, mesocosms are intermediate-sized experimental containers that are large enough to include elements of field realism, but small enough that some experimental conditions can be controlled.

When working in the marine environment, one of the elements of realism a scientist might want to include are the tides. Tides are the ebb and flow of water over a habitat like a mudflat, beach or salt marsh. The area experiencing this movement of water is called the intertidal zone. Tides can be important for bringing in nutrients and removing wastes, bringing in seeds or larvae that develop in the intertidal or even bringing in and removing predators. However, as in the case with the Deepwater Horizon oil spill, with each high tide more oil (or components of oil) was brought into the intertidal habitats in the Gulf, including salt marshes and mudflats.

ACER scientists are using mesocosms at the Dauphin Island Sea Lab to test the effects of oil exposure on salt marsh grass growth rates. Salt marsh grasses are found intertidally and the fluctuating water levels of tides have been shown to affect growth rate. Thus, being able to create an intertidal environment with realistic tidal conditions allows scientists to conclude that the measured growth rates reflect the effects of oil exposure and not another factor, like water level, on salt marsh growth.

This is where DISL’s Technical Support Services group used their ingenuity and construction talents. It is important to note that any water contaminated with oil cannot be released to the Gulf. Thus, not only did Tech Support have to figure out how to make the water level in the mesocosms rise and fall, they had to do it using a closed system, such that no oil was released to the waters around Dauphin Island. They devised a system of water reservoirs, pipes and pumps controlled by programmable electronic boards that allow the water level in the mesocosms to rise and fall, just as it would in the real salt marsh. Each pump is controlled by an Arduino microcontroller board that regulates the pump’s output to a desired flow rate; an Arduino board serving as a relay (like a light switch) to turn the pumps on and off, changing the flow direction. Two mesocosms, holding salt marsh plantings, serve as controls and have no oiling while two others, also holding salt marsh plantings have been treated with oil. Water flows in or out of each mesocosm over a 12 hr period. The rise of open source, user friendly microcontrollers that are relatively inexpensive has simplified and automated the complex process of moving water in and out of the mesocosms on a time schedule.

Nowadays, it really does take a team to conduct research.