Question

most ocean food chains depend on phytoplankton as primary producers. where in the ocean do phytoplankton live?
a throughout all ocean waters, because phytoplankton can tolerate a wide range of temperature and light conditions
b within about 200 meters of the ocean shoreline, because phytoplankton depend on nutrients that wash into the water from land
c within about 200 meters of the ocean surface, because phytoplankton require light to survive
d attached to shells along the ocean floor, because phytoplankton cannot float

Explanation:

Phytoplankton are photosynthetic organisms, so they need light to survive. Light penetrates the ocean surface up to about 200 meters (the photic zone). Option A is incorrect because while they can tolerate some range, they need light which isn't available everywhere. Option B is incorrect as their nutrient source isn't solely from land runoff. Option D is incorrect as phytoplankton can float (they are plankton, meaning they drift). So the correct reason is their need for light, limiting them to within about 200 meters of the ocean surface.

Answer:

C. within about 200 meters of the ocean surface, because phytoplankton require light to survive