Wow! You have done a lot of great planning of the experiments. I hope I can help you avoid any pitfalls along the way to producing some great data.
The dinos are pretty sensitive to their growth conditions--the temperature and the growth medium especially--so let them acclimate and grow for a while before you do the experiment.
Single-celled plankton like dinos respond quickly to changes in their environment so a five-day exposure to sunscreen should be long enough to see an effect on their ability to produce light.
There are a couple of things I'd like to suggest for you to do as part of your plan. Scientists have to think carefully about the conditions, the independent variables, that they choose for their test organism. They also have to define their hypothesis carefully as other scientists are going to review their work and pick out any errors.
For example, if a researcher were planning to do a test of sunscreen on plankton they would try to obtain the product with and without the test compound--oxybenzone in your case. In that way the dinos that will be the control are exposed to the same product as the test dinos except for the sunscreen chemical. If this is not done and you see an effect on bioluminescence then you cannot be certain that it was due to the oxybenzone and not some other ingredient in the sunscreen product. What I would recommend (and other experts please respond with your suggestions!) is that you mimic the way sunscreen would enter seawater by applying a measured amount to the inside of a small glass container, filling it with 100 ml of dino medium (
https://www.carolina.com/biological-med ... /153757.pr) and allowing the liquid to absorb whatever leaches out from the sunscreen over a certain period--say two hours. Then you would pour the medium into a clean glass container and use that to expose the dinos.
Since you can't get oxybenzone by itself, why don't you try to get several sunscreen products with different amounts of oxybenzone or no oxybenzone and try the experiment with each. You still won't be able to prove that an effect is caused by oxybenzone because other chemicals are present in sunscreen products, but if the effect increases with increasing concentration of oxybenzone then you at least have suggestive evidence for its being the causative agent.
Let us know what you think of this idea. Also, remember to plan to have at least three cultures of dinos for each treatment so that after you do the light readings you can average the results and get a standard deviation. This statistical information will tell you whether or not the readings are valid. If you have time and enough materials I would suggest that you also plan to repeat the experiments. This really helps to strengthen the reliability of your data and convince people of your conclusions.
Keep posting and we'll try to answer your questions so you can get this project underway.
Good luck!
Sybee