By Kelly (originally posted 21st September 2012)
I’ve a couple of things I’d like to cover today that are all related by a common thread: ocean circulation (again). Last week I wrote about the Great Pacific Garbage Patches that describes the refuse that rides the tides until it is bounded by large circulation features in the eastern and western Pacific, aswell as in a newly discovered ‘patch’ of the North Atlantic (thank you Susan). I was prompted to write this post after reading about the Japanese tsnumai debris crossing the Pacific, and collecting on the shores of western Canada (thank you Evan). Well today I’d like to touch on something a little more controversial that also relates to circulation; why propose ‘super-trawling’ off the southeast Australian coastline?
The action of the world’s current systems couldn’t be more beautifully displayed than in this spectacular animation (above) that recently came my way (thank you NASA, courtesy of thank you Bec). And, like yesterdays post (thank you Nick), it is another demonstration of data-visualization at its finest. If I can draw your attention to around 2:10 you will see a series of whirling dervishes, other wise known as eddies, that spin-off the south-east Australian coast, extending beyond the Tasman Front, which is seen as a jet of water that breaks from the coast near Sydney and meanders off toward New Zealand. The eddies (or dervishes) that continue southward toward Tasmania are of interest today. I present to you the East Australian Current Extension: how PhD students and super-trawlers alike, learned to love ocean circulation.
The Australian government recently legislated against allowing Seafish Australia and the Abel Tasman super-trawler to launch in Commonwealth waters, amidst immense public outcry and concern over collapsing fisheries and more sensationally, the issue of by-catch. For today I am not going to enter the debate, or include horrific pictures of net-entwined dolphins and seals, I’ll do that next week. Today I’d like to talk about nutrients, the biological pump and why we are better off understanding a little about ocean circulation.
It all starts at the base of the food web, which in marine ecosystems starts with phytoplankton (or fighterplankton as my nephew calls it). For phytoplankton to grow, like any plant, they require macronutrients (think nitrate and phosphate), as well as micronutrients such as iron and copper. South-east Australian waters are highly productive, support important commercial fisheries and attract super-trawlers in part because they contain a good balance of these macro and micro-nutrients. So where does it come from?
The waters around Tasmania border the Southern Ocean which contains more nutrients than just about anywhere else in the world. However, the Southern Ocean is very low in iron, and this caps, or ‘limits’, the growth of phytoplankton and allows more macronutrients to leak out toward Tasmania. To the north, the East Australian Current is nutrient poor’, or ’oligotrophic’ to those in the business. Primary productivity in this current is largely supported by recycled nutrients and the whirling dervishes you see in the video that bring nutrient rich water from depth to the surface. While the East Australian current is nutrient poor, it has a higher iron content (thank you dust blowing off the Australian continent) and so by mixing the two water masses you get a wonderful fertilizer for the marine vege patch off the south-east coast of Australia. Move along the foodweb* and you can support small pelagic (open ocean) fish that pique the interest of super trawlers (see below)…finally the connection!
It is dynamics of ocean circulation transporting heat and nutrients around the globe that makes our planet such a habitable one….to fish in. (Thank you Earth Science). Of course this is a rather simplistic view, and has ignored the importance and origin of deeper water masses, but I’ll get to those next week.
If you would like to read more coverage of the debate surrounding the super trawler: in the red corner those in favour can be found here, those against here, and the scientific discuassion paper here. Or for those trying to avoid controversy click here for more information on data visualization and the production of ‘The perpetual ocean” animation.
*Oh you can also feed your deep-sea coral, but that is another story all together.