This photo does not show the amazing amount of life in a single drop of seawater. At Deep Sea News, Miriam Goldstein has an excellent debunking of this viral photo. What makes the debunking excellent? Goldstein doesn't just say "that's wrong", she shows you the even cooler reality behind the error.
This is NOT a “drop” of seawater. The ocean is not a thick zooplankton soup, except for in some rare and special circumstances. This is the result of towing a zooplankton net around to concentrate seawater enough to actually look at the zooplankton. Basically, this photo is a swimming-pool amount of ocean concentrated down into about a half-pint of goo.
At Deep Sea News, you can read more about the zooplankton nets and see pictures that demonstrate how they work.