[peepopeep]

I am not an expert on anything but I can give the answer a stab. Let us make many assumptions:

Say plankton dies off, say animals eating plankton die, say humans eating those animals now have less food. Say all the fish die and we are reduced to land-based food. Now we have more space competing for farm land, we have to clear forest. Clearing adds to CO2 emissions, construction, more emissions, less CO2 being taken out. It's clear how this is all one giant feedback loop. Less biodiversity, more overabundance of something else, which leads to less consumption of something else which leads to collapse of ecosystems which leads to use trying to patch things up and doing even more harm.

Just zoom out and think about things that consume things and how that loops back directly to humans, environment, overproduction, overuse and eventually unsustainable usage of resources until there are none left.

[glenneroo]

Also phytoplankton are responsible for 50-85% of the oxygen we breathe, so if they die off it won't just be a food-chain issue.