Your hypothesis is incomplete...
"if sediment from surrounding areas near aquatic habitat contain compounds that are harmful to aquatic organisms."...then what? Will they have a lower survival rate? Smaller size/weight? The first part of an experiment is to come up with a proper hypothesis (or "question" you want to answer).
The second part is to do a good experiment. Typically you'll want to have a positive control, a negative control and the quantity you'll be testing (the sediment near aquatic areas), though it might be tough to have controls in this case, since I guess you don't really know what is "harmful" to water fleas. And even if you did know, then what may be harmful to them would not necessarily be harmful to other aquatic organisms. Some aquatic organisms thrive on phosphorus, others nitrogen, while others are harmed by both (and other things like that). Maybe water fleas are damaged by the chemicals in the sediment whereas fish, algae and whatever else lives there are not affected by the chemicals. In that case you aren't really finding that that the sediment is toxic to the aquatic organisms, you're finding out that water fleas can't survive in this new environment. You should probably use a species native to that area.
Also, you would have to show that the sediment near the aquatic area is the same as in the pool of the water, or that the suppposed toxins are in fact leeching their way into the water where the aquatic organisms live.
I know nothing about these water fleas - are they freshwater or saltwater organisms? Is the water you're testing freshwater or saltwater? If they were different, and you were measuring mortality rate, then you would get a false positive. And how exactly are you going to measure if there is a negative effect on the water fleas? Do they show a certain colour in water? Is there a particular chemical they excrete/secrete which you are going to measure?
These are some things you'll have to think about, in order to do a proper science experiment. The best thing is probably to look into what types of things you are interested in answering.
When I was in elementary school, I used to hate brushing. I did an experiement on brushing teeth to see if there was anything behind brushing, or if my parents/dentists were trying to scare me. It had something about coke and brushing. I used eggs to simulate teeth (enalmel coat) and had jars filled with I think tap water, purified water and coke. I had two samples of each - one sample I brushed with a toothbrush two times a day and the other sample I left alone. I looked at them after a couple of weeks and the coke one without brushing was nasty (yellow and black and stuff) whereas the coke toothbrush one was okay.
I know this was a lot to read and I hope I didn't discourage you, but in order to do a proper science experiment, you have to keep a lot of things in mind. Anyways, best of luck.
