A marine biologist conducted an experiment to evaluate how water flow velocity affects the… — ACT

A marine biologist conducted an experiment to evaluate how water flow velocity affects the peak bioluminescence intensity produced by the dinoflagellate *Pyrocystis fusiformis*. In four separate experimental trials, cell cultures were exposed to flow velocities of $0.1\text{ m/s}$ , $0.5\text{ m/s}$ , $1.0\text{ m/s}$ , and $2.0\text{ m/s}$ . In each trial, the cell density ( $1{}000\text{ cells/mL}$ ), water temperature ( $20^\circ\text{C}$ ), and ambient light intensity ( $0\text{ lux}$ ) were kept constant.

Match each experimental variable category on the left with its corresponding component from the experiment on the right.

Independent VariableWater flow velocity ( $0.1\text{ m/s}$ to $2.0\text{ m/s}$ )
Dependent VariablePeak bioluminescence intensity of *Pyrocystis fusiformis*
Controlled VariablesCell density ( $1{}000\text{ cells/mL}$ ), water temperature ( $20^\circ\text{C}$ ), and ambient light intensity ( $0\text{ lux}$ )

Answer

The independent variable is the water flow velocity, the dependent variable is the peak bioluminescence intensity, and the controlled variables are the cell density, water temperature, and ambient light intensity.

In experimental design, the independent variable is the factor deliberately manipulated by the experimenter (water flow velocity). The dependent variable is the measured response or outcome (peak bioluminescence intensity). Controlled variables are baseline factors held constant throughout all trials to ensure a fair test (cell density, water temperature, and ambient light intensity).

Step-by-Step Solution

Identify the variable that the researcher intentionally alters across experimental conditions.

Water flow velocity is varied from

0.1\text{ m/s}

2.0\text{ m/s}

, so it is the independent variable.

The independent variable is the manipulated cause in an experiment.

Identify the factor measured to observe the response or output of the system.

Peak bioluminescence intensity is measured in response to flow rate changes, so it is the dependent variable.

The dependent variable is the observed effect or measured output.

Identify the conditions that are explicitly maintained at constant values across all trials.

Cell density (

1{}000\text{ cells/mL}

), water temperature (

20^\circ\text{C}

), and ambient light intensity (

0\text{ lux}

) remain unchanged, making them controlled variables.

Controlled variables isolate the direct relationship between the independent and dependent variables.

Key Concept

Identifying Independent, Dependent, and Controlled Variables

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