The blue waters of the Mediterranean

Back in 2005, I published an article in BlueSci, Cambridge University's Science magazine, which publishes science-related articles on various topics to "provide fascinating yet accessible science to everyone". It explains why the waters of the Mediterranean are usually blue and crystal clear. I reproduce it here, enriched with some photographs. The original article can be found on page 19 of BlueSci issue 3.

Have you ever wondered why the waters of the Mediterranean Sea are so blue and crystal clear? Is it because the sun is shining so brightly whenever you go on holiday there? Or because there is little pollution? Well, these factors may have something to do with it, but the main reason is that the Mediterranean Sea is oligotrophic. The word ‘oligotrophic’ comes from the Greek words ολίγον (oligon) meaning "little/few", and τροφή (trophi) meaning "food/nutrition", so it means "little, or not enough, food". Essentially, the waters of the Mediterranean don’t contain enough nutrients to support massive growth of phytoplankton (algae). The word phytoplantkon again comes from the Greek words φυτόν (phyton), meaning "plant", and πλαγκτός (planktos), meaning "wanderer" or "drifter", so it means "wandering plant". Most phytoplankton are too small to be seen with the naked eye. However, when present in high enough numbers, they may appear as a green or redish/brown discoloration of the water due to the presence of chlorophyll and other pigments within their cells.

Many people are more familiar with the opposite effect 'eutrophication' (from the Greek for ‘plenty of food’) which causes excessive phytoplankton growth, turning the water a turbid green. Eutrophication often occurs in lakes and coastal areas when high levels of fertilisers are discharged into the water as waste from nearby human activities. Fertilisers and other organic waste contain high levels of phosphorus and nitrogen which phytoplankton need to grow. Phytoplankton grow by photosynthesis so, very much like plants on land, they need light and carbon dioxide (CO2). There’s always enough carbon dioxide present in the water and enough light, at least in the spring and summer, for plankton to grow efficiently. However, along with the light and carbon dioxide needed for photosynthesis, algae also require nutrients; the two most important, which are often in short supply in marine waters, are nitrogen and phosphorus (in the form of nitrates or ammonia, and phosphates, respectively). When nutrients are in short supply, the organisms are limited in their growth, no matter how abundant light and carbon dioxide. In eutrophic lakes, fertilisers and other organic waste bring high levels of nitrogen and phosphorus into the water. The phytoplankton feast on these nutrients, grow and divide rapidly, and so the population expands. When the phytoplankton die as part of their natural life cycle, they sink to the bottom of the lake where they are broken down by bacteria. Many bacteria use oxygen to release energy from their food by respiration and, soon enough, the bacteria use up all the available oxygen and the bottom of the lake becomes anaerobic — without oxygen. Many of the plants and fish that normally grow in the bottom waters can no longer survive in these anaerobic conditions. Anaerobic respiration by the bacteria also produces foul smelling by-products, such as hydrogen sulphide and methane. The effects of eutrophication are dramatic and can only be reversed by laborious cleaning efforts.

The processes involved in eutrophication point to another factor important to oligotrophication: the water column is stratified both in lakes and in the sea. Most of the time the top of the body of water is effectively separated from the bottom. This is partly because the surface waters tend to be warmer, making them less dense than the colder bottom waters. Mixing of these two layers of water only occurs due to severe weather conditions, or in areas of upwelling and downwelling; this respectively forces the bottom water layers up, or the surface waters down, due to a combination of geographical features and ocean and atmospheric circulation.

So, how does this explain why the Mediterranean is so blue? We have seen that algae need nutrients to grow, and that the lower waters are, for the most part, separated from the surface waters. Since algae need light to grow, they prefer to be in the top waters where the sun shines, but most of the nutrient supply is in the bottom waters, where bacteria decompose organic matter and release nutrients. Looking at a map of the Mediterranean Sea reveals that it’s really more like a big lake — almost landlocked — with very limited water exchange through the Suez Canal to the Red Sea, or to the Black Sea through the Bosphorus Strait.

The main point of water exchange for the Mediterranean is through the Strait of Gibraltar to the Atlantic Ocean. The Atlantic has plenty of nutrients to offer, but these are mostly found in the deep waters because algae out in the ocean greedily consume the surface nutrients. The Gibraltar Strait is relatively shallow though, so very little deep-water exchange — and thus nutrient exchange — takes place. In addition, the Mediterranean surface waters are more salty than those of the Atlantic because the Mediterranean is relatively warm and its surface water tends to evaporate in the summer leaving the salt behind. (You can definitely taste this if you swim at a beach off the Atlantic coast, compared to Mediterranean waters.) This more salty water ‘attracts’ freshwater, which means surface waters from the Atlantic rush into the Mediterranean at the Strait of Gibraltar, and in return bottom waters from the Mediterranean exit into the Atlantic. A cross-section of the Gibraltar Strait looks something like this:

Hence the vital nutrients required for growth by algae are constantly depleted from the waters of the Mediterranean. This makes the waters of the Mediterranean oligotrophic, so they don’t support high growth of phytoplankton. In turn fewer predators that feed on these, such as zooplankton, can survive. Thus there are fewer zooplankton in the Mediterranean, and the fish tend to be smaller than in the open ocean. With fewer plankton present, the waters of the Mediterranean don’t turn green and murky, and are crystal clear and stunningly blue instead. Perfect for swimming and taking lovely photos!