Final answer:
The variation in NPP in estuaries is correlated with the intensity and duration of sunlight exposure, which affects the population changes of phytoplankton.
Step-by-step explanation:
The variation in NPP (Net Primary Productivity) in estuaries is correlated with several factors. One of these factors is the intensity and duration of sunlight exposure, which affects the population changes of phytoplankton in the estuary. Phytoplankton are primary producers that play a crucial role in NPP by converting sunlight into chemical energy through photosynthesis. The availability of sunlight determines the growth and productivity of phytoplankton, which in turn affects the overall NPP in estuaries.