The answer is light.
One of the factors that affect photosynthesis is light. Without light, the photosynthetic rate is reduced or even stopped which consequently affect algal growth. With increasing depth in the ocean, there is less sunlight present. Still, algae have accessory photosynthetic plastids other than the chloroplasts that can absorb different light in greater ocean depths. This helps them to maximum use sunlight in such conditions. But, in the end, at some particular depth, they would not be able to survive.