The Great Southern Ocean is the southern most of the Word’s oceans and is generally taken to be roughly around 55 degrees latitude and that which encircles Antarctica. The cold waters are circumpolar that is, they circle the Antarctic and as matter of interest (great question for those into playing Trivia), therefore comprise the only ocean to circumnavigate the globe without being blocked by land.
Here the cold waters converge with the warmer waters of the north, and in turn create an intermixing which results in a concentration of marine plant life because of its higher than average nutrient concentrations. This increased plant life then results in a greater abundance of animal life.
The entire food chain, ecosystem of the Antarctic is brilliant in it’s simplicity, there being only three or four levels of species: from the basic photosynthetic organisms – the phytoplankton and zooplankton – krill – shellfish and other invertebrates – to the higher flesh-eating levels that include species such as whales, seals, fish, seabirds and penguins.
However, due to this same simplicity and their dependence on this one food source, krill, baleen whales such as humpbacks, fins and blue whales are most susceptible to changes in the krill population.
Today the annual catch of krill is around 400,000 tons annually with over half of it for non-human consumption usually ending up as fish-meal or animal feed. With human consumption it is usually either minced, turned into a paste.
However, human activity such as fishing is not the only threat to krill. As phytoplankton is the main diet of the krill, this main food source is at severe risk from rises in sea temperatures. Rises in sea temperature alters the pattern of the phytoplankton blooms.
In addition, phytoplankton these little marvels of nature are not just as one would assume, the lowest rung on the food chain, this being important enough in itself but are responsible for about half the photosynthesis on the planet. They convert light and nutrients into organic material which is then fed right the way up through the food chain, making them a key elements of life on the planet. After dying, some of the plants drift down into the deep ocean and lock up carbon for very longer periods. That is, the same as land plants, they consume carbon dioxide and release oxygen.
And the bad news about climate change according to New Scientist is that if the top layer of ocean water warms, as it has done, the density contrast between it and the layer below becomes greater. This makes it harder for nutrients to reach the surface, which then starves the phytoplankton.
According to new research has been carried out by the CSIRO’s Dr. Rintoul warming now extends for four kilometres, from the ocean surface to the sea floor.
Researchers have however found that the strongest single predictor for phytoplankton levels was ocean surface temperature due to rising surface temperatures which prevent mixing between the oceans’ oxygen-rich upper layers, where phytoplankton are present, and the colder, more nutrient-rich waters below.
Marine biologist Boris Worm, in a study partly reproduced for Our Amazing Planet concluded that:
“…rising surface temperatures were associated with declining phytoplankton in eight of 10 regions. Of the other two regions, the North Indian Ocean had a stable phytoplankton population and the South Indian Ocean showed growth. The decline in phytoplankton seen around the poles, where it seems logical that increasing warmth would drive more growth, could be driven by increasing winds and ocean mixing, the researcher said.
Local factors such as polar melting and nutrient-rich runoff from agriculture also can influence the phytoplankton, Worm said.
“But all of these effects are unlikely to play out evenly on a global scale,” he said. “The only driver we have that affects phytoplankton everywhere is ocean warming.”