Ingestion
Marine animals often mistake many man-made pollutants for food. Animals will either choke on the pollutants or ingest the pollutants and die. Plastic bags commonly cause this problem.
Entanglement
Ordinary items such as plastic, six-pack soda can holders and fishing line are harmful pollutants to marine wildlife. They can ensnare marine animals, effecting their ability to move, breath and eat. If the animals cannot release themselves from the pollutants, they may starve or drown.
Productivity
Certain chemicals, such as plant fertilizer, greatly increase an organism's ability to reproduce. When these pollutants are introduced to marine ecosystems they trigger an increased production or bloom of certain organisms. This can especially lead to algae blooms, such as red tide. Blooms must consume more nutrients and oxygen from the water, leading other organisms to starve or drown.