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The oceans have seasons just as the land does. Stretching across the temperate bands located between the freezing poles and the warm tropics lie the seasonal seas. Life is tough in these constantly changing waters, but if animals can persevere and adapt to change, there are myriad opportunities to not only survive but thrive here.

Winter in the seasonal seas is a battlefield. Storms whip up immense waves that can travel for thousands of miles, building to 30 metres in height before making landfall. On the Pacific coast of South America, such waves impact the lives of South America sea lions that have lived on this exposed coast for generations. Newborn pups and mothers are restricted to the colony, but the most intrepid males take a risk, leaving the tempestuous coast for unfamiliar waters and unusual prey - black-necked swans.

Some choose to escape whilst others are undeterred by winters icy grip on the seasonal seas. In Patagonia, a female southern right whale arrives in the protected bay where she was likely born in search of a mate. Testing prospective partners before accepting the best mate, she is rewarded with a gentle belly rub and embrace - a valuable pairing given that these whales were hunted to near extinction in the last century. Thanks to protection, the whaling ban increased this population from just 300 to 25,000, with a surprising twist – white calves.

As daylight lengthens and spring approaches, the greening of the seasonal seas commences. Plankton multiplies, creating huge blooms and feeding many species, including a rather peculiar crustacean, a porcelain crab. In the waters off Chile, over 200 million feast on this bounty, forming the largest-known aggregation of crabs in our oceans. But with limited space and an increased flow of plankton, the crabs do something remarkable - they organise themselves into structured lines, a behaviour that has never been filmed before.

As forests flourish on land, they also do so in the water. In spring, kelp, one of the fastest-growing organisms on earth, can lengthen by half a metre a day, forming submarine jungles that are home to a variety of creatures. In Japan, these lush fronds conceal a rarely seen fish, an eelpout, with a length of just 10 centimetres. Her bold breeding colours signal she is looking for a mate, but late to the party, she must fight for one in the most peculiar way - by engaging in mouth-to-mouth combat.

When spring gives way to summer, temperatures at the coast can rise by 50 Celsius in a matter of weeks. Living for just one year, a female octopus oliveri must capitalise on the summer bounty of Japan’s rock pools whilst surviving the intense heat. But other octopuses dominate the pools close to shore. In an incredible feat, she pulls herself out of the water and walks on land. But as the heat of the day intensifies, she finds herself far from the water she requires to breathe. Can she make it back before she suffocates?

On the Mediterranean coast in summer, the number of people can double. But 50 metres deep in the waters off Corsica, another gathering occurs. Picarel fish in their millions create a supercolony of fish nests, ten million strong across miles of seabed. Here, a male picarel fish father must defend his precious eggs from hermit crabs, sea cucumbers and even other male picarel fish. Against all odds, a few thousand eggs survive an onslaught of cannibals - a job well done by a tenacious father.

At the peak of summer, seagrass meadows can grow ten times thicker than in winter, creating a dense home and feeding ground for many marine creatures. In the lagoon system of the Ria Formosa in Portugal, a long-snouted seahorse tries to find love. But what was once a vast grassland is now an eerily barren world, having been decimated by anchors and fishing boats. The search for a mate seems hopeless, but a lifeline in the form of a ‘seahorse hotel’ created by scientists provides him with a sanctuary and a chance to find a new mate.

The oceans are being changed not just by the seasons but by us. Winter waves hit the coast of Chile, washing newborn pups from their colony. Unable to swim, they become stranded. What’s more, extreme storms are now four times more common there. But there is hope – committed volunteers aid the return of these helpless pups to the colony to be reunited with mum.

It's autumn in New York City. Thanks to fishing quotas being set in 2013, billions of menhaden fish now swarm along America’s busiest coastline, drawing in charismatic marine characters – bottlenose dolphins, sharks and even ocean giants. Once a rare sighting in the waters surrounding the Big Apple, now up to 150 humpback whales feast on the menhaden shoals throughout spring, summer and into autumn. It's a sign that the animals of the seasonal seas show remarkable adaptability and a ray of hope in a rapidly changing ocean.

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