Complex sentence: Plants find it easier to use the lighter isotopes (12 C) when they convert sunlight and carbon dioxide into food, thus large blooms of plankton (free-floating organisms) draw large amounts of 12 C into the oceans.
Split into simpler sentences: Plants find it easier to use the lighter isotopes (12 C) when they convert sunlight and carbon dioxide into food. So, for example, large blooms of plankton (free-floating organisms) absorb large amounts of 12 C from the oceans.