Answer:
Dissolved oxygen.
Step-by-step explanation:
The dissolved oxygen in the water could cause algae to grow, and too much algae will choke out the entire underwater organism population. The algae will change the pH of the water, making organisms that need a particular pH level struggle to survive.
Also, if you know about the red algae blooms in particular areas, almost no other organisms can survive in the the area at that time, decreasing the diversity in that ecosystem until the algae levels eventually go down.
I hope I didn't go off on a tangent. I hope this helps. :)