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Taking Stock of Ocean Productivity


 
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Emeritus Professor John Raven, from Dundee University, is visiting the School of Life Sciences at the University of Technology Sydney and has a warning about the productivity of our the oceans.    
In 1798 Malthus published the first edition of his book on population, warning that human population growth could not grow indefinitely because resources would run out. Technological fixes to this problem are hailed as the answer but there are limits.
Mobile phones, for instance, need trace elements such as indium which have to be mined and are a finite source. Just like land-based systems like forests, the oceans have productive plants, from giant kelps 50 long to cyanobacteria less than a millionth of a metre in diameter.
While algae are widespread in oceans they do face a lack of nutrients. There needs to be a constant input of nutrients from the land to keep the oceans productive. Rock weathering produces phosphorous and nitrogen fixation produces nitrogen.

 
Some of this is taken up by algae, an important part of the food chain in oceans. Other algae sink to the bottom, taking carbon (a good thing) along with phosphorus and nitrogen out of the system, meaning fewer nutrients are left. Plants on land have developed into trees which increase their capacity for photosynthesis but for marine life this only happens in coastal ocean systems, productive sea grass beds and kelp forests rivalling the productivity of land-based forests. While coastal systems have a relatively high concentration of nutrients, regions such as the centre of the Pacific Ocean have very little nitrogen and phosphorus available to the phytoplankton and these waters have very low productivity, and so these waters are clear and appear blue because there is little chlorophyll and other pigments in the sparse phytoplankton. 

 
Phytoplankton - the foundation of the oceanic food chain. Date before 6 November 2009 Source http://www.photolib.noaa.gov/bigs/fish1880.jpg [1] Author NOAA MESA Project    

Some very long-lived and slow-growing algae are found growing to about 280m below sea level. While the 280m available for algal growth is a small fraction of the mean sea depth of about 3,700m, it is similar to the terrestrial layer of photosynthetic productivity; the tallest trees being 100m high. In the sea, however, the water also absorbs light so that at its lower level there is very little light for photosynthesis. The water competes with the algae, creating a double whammy for it – lack of nutrients and competition for light absorption.


 
One good outcome from ocean warming is that the mixing depths become shallower, so the algae get more light for photosynthesis. However, the increased temperature gradient between the productive layer and the more nutrient-rich deep waters means less of the nutrients get into the surface water.
There is also an increased carbon dioxide availability ('ocean acidification') - about a third of the carbon dioxide mankind has released into the atmosphere since about 1750 has dissolved in the ocean. How increased light and carbon dioxide, but decreased nutrients such as nitrogen and phosphorus, influence ocean productivity will be seen over the next few hundred years.
Professor Raven believes we are approaching a maximum of how much fish we can remove from the oceans, just as Malthus warned. Aquaculture is not helping because small fish are often fed into aquaculture facilities, short circuiting the natural links between algal production and fish stocks.

It is a global warning.
We must take stock of the productivity of our oceans.

Emeritus Professor John Raven was interviewed for A Question of Balance by Ruby Vincent. Photo of Professor Raven by Dr Lisa Roberts. Summary text by Victor Barry, November 2015.

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