The exact ways in which massive disruption of a global ecosystem will become inconvenient are difficult to predict. The desequestering of hundreds of millions of years of carbon production in the space of a couple of generations has broadly predictable climatic effects, but whether they will take the forms of superstorms, massive flooding, new deserts is hard to say, other than the water will rise and currents will change.
Likewise creating oceanic dead spots, while fishing some species out of existence don't have predictable results. Other than, of course, it being unlikely that the species that benefit are unlikely to be as useful as those they replace.
OTOH, people can eat anything, pretty much. So maybe we'll add jellyfish to our fast food menus and everything will be all right.