The sentence was constructed like this:
A skilled reader has no trouble matching the word phytoplankton with its definition. After all, the two ideas are right next to each other in the the sentence!
But what about less skilled readers? I know from watching my students try to copy text quotes that they often overlook pieces of punctuation. In fact, I learned that many of my students are reading the sentence as this:
Without the first comma, the meaning of the context clue is completely lost. Actually, it's worse than lost--by logical reasoning phytoplankton couldn't be tiny ocean plants. On a test question, students would confidently choose anything but "tiny ocean plants" as the meaning of phytoplankton, because by their reading the sentence clearly states that phytoplankton is something different from tiny ocean plants.
Once I realized that my students didn't know the meaning of the first comma, I knew what to do next. I had to show them lots and lots of examples of appositive context clues! I also had to make sure that they were carefully reading and attending to punctuation.
After just a few examples taken from our read alouds and written on the board, students were able to answer questions related to the context clues with confidence. They noticed the commas setting off the appositives as the important signposts that they are!