When we think of the ocean we largely picture a vast, blue wilderness, as witnessed in last week’s episode of Blue Planet 2 – Big Blue. This week we’ve been promised a glimpse inside the ocean’s ‘Green Seas‘ – the most productive, and arguably most important expanses of the marine environment.

During my time studying at the University of Southampton, I’ve learned a great deal about our ‘green seas’. Phytoplankton, or microscopic marine algae, drives all photosynthetically-derived life in our ocean and forms the basis of global marine food webs, playing a role in the oceans similar to that of plants on land. Phytoplankton blooms occur seasonally and are especially characteristic of the temperate North Atlantic Ocean, coastal waters, and sub-polar regions – some phytoplankton blooms are even seen from space!  These blooms provide temporary, bountiful feasts for zooplankton (consisting of gelatinous animals, larvae, and microscopic invertebrates), and in turn, smaller plankton-feeding ‘bait’ fish like anchovies, sardines, and herring. As we saw in the Big Blue’s ‘boiling seas’, large shoals of small fish can attract the ocean’s largest and greatest predators including dolphins, whales, seals and sharks, so expect to see another spectacular feeding extravaganza in this episode.

Another ‘green sea’ likely to make an appearance are the great giant kelp forests of the north-east Pacific, a personal favourite of mine. During our first-year marine ecology lectures, we learnt about a phenomenon coined the trophic cascade, and a specific case study from the kelp forests off the west coast of the USA. A trophic cascade occurs when a top predator is removed from the food web, thereby freeing the lower trophic levels from predation and allowing their herbivorous populations to thrive. In the kelp forest case study, the predacious sea otter was removed from the system, hunted almost to extinction by humans during the 18th and 19th century for their thick fur pelts. As a result, herbivorous sea urchins were allowed to thrive in numbers, grazing on the giant kelp to such an extent that vast areas of the dynamic and diverse kelp forests were cleared, and ‘urchin barrens’ left in their wake. Thankfully, legislation was put in place and conservation programmes successfully established to protect the remaining sea otters, allowing sea otter populations, and consequentially the health of the entire kelp forest ecosystem, to fully recover. This is possibly one of the greatest conservation success stories of all time, so I fully expect the tale to be told by the Blue Planet 2 team (not to mention that sea otters are undeniably adorable!).

Green seas also play an important role in the global carbon cycle and in controlling atmospheric carbon dioxide levels. Phytoplankton absorb carbon dioxide for photosynthesis on a scale equivalent to that of terrestrial forests, and transfer about ten gigatonnes of carbon from the atmosphere to the deep sea each year, via the biological carbon pump. I hope this episode not only highlights the importance of our ‘green seas’ on an ecological scale, for example with mangrove forests providing nurseries for many fish species, but also on a global climatic scale, with each ‘green sea’ ecosystem a significant and altogether vital carbon sink.

Feel free to share any comments or questions regarding Green Seas – I hope you enjoy the episode!

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