How do we know the damage possibly caused by such a disaster?

The massive Deep Horizon oil spill of 2010 released enough crude oil
into the Gulf of Mexico to fill an entire swimming pool about 7 thousand
times.
How do we know the damage possibly caused by such a disaster? We could
always plan an experiment….. If you worked for the Dept. of Fish and
Game in the Gulf States, maybe you would…
You could take a part of the sea floor very nearby the site of the
spill, say an area exactly 100 square meters, and count the number of
different species that you find. This would be a measure of
“Biodiversity”, which is generally a good indicator of ecosystem health –
in general, the more Biodiversity, the healthier the ecosystem.
Then you could sample an area of the seafloor exactly the same size, but
further away (say 1 km) from the spill site, and count the number of
species in that site. And then continue sampling the same size area at 1
km intervals, until you reach an area many kilometers away from the
spill site where you know the area was relatively undisturbed by the
spill.
If you run this experiment,
1) What is the dependent variable?
2) What is the independent variable?
3) What is one very obvious control variable?
4) What is another control variable that you can think of?
And….
5) What is a valid hypothesis that you could have for this experiment?