Apart from being an excuse for playing with Corel Draw clipart, this diagram reinforces the point about the degree of convenience of different nitrogen sources for phytoplankton.
Imagine getting up one morning and having to make yourself some sandwiches. Having pre-packed, ready-made sandwiches is equivalent to getting your N from the ever-so-convenient NH4. Having to butter the bread, cut the cheese, etc is akin to getting your N from the marginally more expensive nitrate. But N2 is resorted to only in desperation, like making your sandwiches directly from the most basic of ingredients.