Well, the deep fissure idea doesn't work, as algae need light to grow and reproduce, so once you start getting a couple meters beyond the surface of the water, the space in which they can grow runs out. Most sea life lives near shores and shallows for this reason: the plant life the majority rely directly on (and the prey the predators consume) can only live near the surface.
As for death rates and the impact on the immediate area, this could be a start as to learning a bit about it for yourself, as summarizing in this kind of detail is beyond my personal expertise
Harmful Algal Blooms | Nutrient Pollution | US EPA
However, remember that the chalk cliffs are formed from diatoms, not algae, so if they grew in a sudden bloom, they probably wouldn't be toxic to other creatures, but they'd still choke themselves out with overpopulation. It is the changes in pH and salinity of the water which would cause the most death of sea life during a world wide flood, with temperature contributing some as well.