Why is the Eastern Mediterranean so blue?

Full details of this major research project by Professor Michael Krom

The eastern Mediterranean is the cradle of western civilisation. Around its shores have developed many of the great ancient civilisations. More recently apart from being the home to millions of people, it is the holiday destination for millions more. Despite this the waters for the most part are crystal clear and very blue. Very different from the murky green-grey waters that one associates with seawater polluted with sewage and other nutrients.

Professor Michael Krom, director of the Leeds Earth and Biosphere Institute, has been leading the CYCLOPS team of marine scientists from 7 countries studying how the Mediterranean marine ecosystem works. The Mediterranean is unusual in that unlike other areas of the ocean, it requires only the addition of phosphate for the microscopic plants (plankton) to grow rather than nitrate and phosphate together.

The CYCLOPS team went out into the Mediterranean to a location south of Cyprus and fertilized the water with 12 tons of phosphate fertilizer together with an inert tracer and then looked to see how the system responded. They found to their surprise that when you add this crucial fertilizer to the system, the amount of plankton actually decreased. This was because the plankton absorbed the phosphate but before they could grow, they were pounced upon by hordes of hungry microscopic predators. These results are important because they show how phosphate limited marine ecosystems work. As a result of global pollution we are adding more and more N gases to the atmosphere which in turn are changing all oceans towards phosphate limitation.

The team was also able to explain why the system is so short of phosphate. It is because there is more nitrogen than phosphate added to the system by external sources, mainly atmospheric pollution. However unlike other areas of the ocean where this happens, the natural biological buffer system which reduces this excess nitrate to nitrogen gas does not work. That is a due to the same reason why the Eastern Mediterranean is so blue. It has a unique circulation which causes water free of nutrients to flow in at the Straits of Sicily while nutrients rich in nutrients flow out in the deeper waters. This export of nutrients means that there are relatively few nutrients available for plankton to grow in the eastern Mediterranean. The waters are as much a desert as the Sahara desert is to the south. Nutrients produce organic matter which causes the surface waters to become greenish-grey and also is required for the process of nitrate reduction which removes Nitrogen but not phosphate from surface waters. No organic matter, no murky waters.