the lack of competition would create a good place for an enormous algal bloom to occur. Now this could be good, very short term, for organisms such as whales, but eventually, the dead algal cells would increase the rate of oxygen consumption in the water. If such a phenomenon were to cover the globe, then eventually, all marine organisms would die. this would effect the land because everything in nature is connected and when such a huge link is taken out of the loop, the effects would be disaterous. this would mainly be because of the lack of food and other resources that the ocean gives off. this would eventually climb up the ladder to humans, and that is how and why we would die.
by the way plankton is an animal not a plant and algal blooms are plantonic organisms that grow in mass numbers because of good conditions for them.