Answer: Bacteria.
Step-by-step explanation:
The pollution and poor water quality can make ocean water contaminated which makes a suitable environment for the growth of bacteria.
Growth of pathogenic bacteria can be harmful the native species present their. Bacteria can be both harmful and useful so growth of harmful bacteria inside ocean water can be deadly for the organisms surviving there.
The algae, zooplankton and phytoplankton are the species already present in ocean water and they cannot be harmful.