Seasonal Seas | The Blue Planet | BBC | Episode 5

The fifth instalment surveys the effects of the seasons on the world’s temperate seas – the most productive on Earth. Sable Island near Nova Scotia boasts the largest colony of grey seals, which breed there when the weather is at its worst. The pups remain marooned for weeks until the spring, when they are strong enough to swim. Spring also heralds the bloom of phytoplankton: it provides food for copepods, and they in turn are prey to jellyfish, which assemble in vast, million-strong swarms. On the Californian coast, giant kelp also flourishes and by summer, grows at the rate of a metre a day. It provides a sanctuary for shoals of fish and sea otters, the latter anchoring themselves to the seaweed when resting and keeping its grazers in check by eating them. Late summer in Alaska sees Pacific salmon heading inshore to breed. However, the level of their favoured river is too low and they are forced to wait in the open sea, where they fall prey to a salmon shark.