From Tiny Plankton to Massive Tuna

Read the article found here.  ​Levels of phytoplankton in an ocean area may seem like a good predictor for the amount of fish that can be caught there, but a new study by Nereus Program researchers finds that this relationship is not so straightforward.  Explain this statement using specific examples from the article.  How is climate change expected to impact food chains in the ocean?