There are just few factors that determine what is affecting plankton in the water, nutrients, light and oxygen.
Most tropical spots get a huge amount sunlight year round, so nutrients, when available they will get used up and into the food chain. The amazing 100’ visibility in the tropics is because those waters are nutrient deserts. Sunlight hitting at a steep angle penetrates relatively deep compared to Northern climes. The clearest water for most temperate divers is fall to spring because the days are short and the sun hits the water at a low angle.
Nutrients get into the water from one of two sources, surface runoff or upwelling from deep water. Deep water has plenty of nutrients, but no light and low oxygen. Undersea mountains ranges or islands like the Galapagos can force this water up towards the surface, allowing for mixing. Tropical waters tend to have much more significant thermoclines, which inhibits that mixing. Northern latitudes (I.e. US, Europe, China, Japan, Canada, etc.) tend to have more surface runoff from human activity, and runoff in general.
These are very broad generalizations, so take them with a grain of salt.
