Phytoplankton: Fill & Download for Free

While the famously blue and clear waters of the Mediterranean may be attractive, they’re not exactly a good thing - ecologically speaking. In abundance, phytoplankton (which form the base of the food chain in the oceans) can stain the surrounding water green. Look at this satellite image of a plankton bloom off France, for example:In general, these massive blooms occur when local waters are rich in nutrients. If there’s too much nutrients, phytoplankton can experience huge population booms, in turn eating up all the oxygen and potentially causing an ecological disaster. This phenomenon is known as eutrophication.The inverse - oligotrophication - has happened in the Med. It is only connected to the rest of the Atlantic Ocean by the Strait of Gibraltar - which is, at its deepest point, 900 metres deep. In other words, relatively shallow. Most nutrients occur in cold, deep waters, so they are barred from flowing into the inland sea.Without much nutrients, there’s not much phytoplankton, and without much phytoplankton, there’s a very blue, clear sea.Additionally, since phytoplankton are at the bottom of the underwater food chain, their absence means an absence of all the heterotrophic organisms which depend on them. The lack of plankton, I think, is best illustrated by maps like this:This is a map of the world’s oceans showing the milligrams of chlorophyll per square metre, which in essence translates to the abundance of photosynthesizing life. Blue represents very little chlorophyll, and green a lot.As you can see, the Mediterranean - especially the eastern part which includes the Aegean Sea - is as lacking in plankton as some of the most desolate parts of the open ocean. Of course, as some of you may have noticed, the above map raises some other questions. To answer them, we need to factor in the phenomenon of upwelling.In essence, upwelling is when strong winds blow parallel to the coast, thus pushing the surface water offshore and allowing water from the deep to rise up and replace it. Because there’s no light in the deep, there’s no algae to eat up the nutrients, so cold, deep water is typically nutrient-rich.In places like the Baltic, Black, and Caspian Seas, upwelling due to prevailing winds can counter the effect of being enclosed, creating areas rich in phytoplankton. Per unit area, the Black Sea has almost twice as many species than its larger neighbour. The Mediterranean has had no such luck, and remains a sea rich in beauty but lacking in biodiversity.

There are large areas of the ocean which are oligotrophic, meaning that they contain an insufficient amount of nutrients to sustain any significant biomass. The largest of these areas are at the center of the subtropical gyres, in the vicinity of 30-40° north and south latitude.This happens because around those latitudes the large scale wind patterns transition from Westerlies, blowing to the east, to Trade Winds, blowing to the west. To a first order approximation, this means that the large scale winds tend towards zero in this region. If you have studied the Mid-Atlantic trade routes between Africa and North America, you have likely heard of the Horse Latitudes where, due to low winds, ships would become adrift for extended periods of time, often forced to kill or eat their horses due to lack of supplies.The key to understanding the lack of abundant sea life has to do with the winds as well. In most of the ocean, biologically available nitrogen and phosphorus come from relatively deep in the ocean, where biological production is low, and thus these nutrients are not exhausted. The two main mechanisms by which N and P become available in the upper photic (sunlit) zone are strong mixing, due to a strong winter storm or tropical cyclone, and upwelling along the coasts or driven by wind convergence near the equator. The important thing here is that, with little to no wind over a large area of ocean, the nutrient levels are extremely low.Low nutrient levels have the obvious consequence of inhibiting phytoplankton growth, often referred to as primary production, as it involves direct photosynthesis and is how most chemical energy is made available to ocean life. Areas of extremely low primary productivity can be seen in the following satellite image of ocean color:Where deep blues and purples are areas of extremely low chlorophyl concentration, and thus of little to no primary production. This low production cascades up the food chain. Very few phytoplankton means very few animals that feed on phytoplankton, on and on up, until there are very few fish for humans to feed on. While this image is a recent one, remember that these dead zones are a result of large scale atmospheric circulation patterns, which have persisted since before the age of exploration.If you look at a map of Magellan's voyage:you can see that they spent a great deal of time passing through the center of the South Pacific Subtropical Gyre, an area of near zero production in the satellite measurements above. With the above information, we can now infer that the low winds here had the dual effect of "stranding" ships in a region with little to propel them and very little sea life, as they were trapped at the heart of an ocean desert.Images courtesy of NASA's Ocean Color Group (http://oceancolor.gsfc.nasa.gov/) and the Robinson Library (http://www.robinsonlibrary.com/).

Trees are one source, but a bigger one is ocean algae (aka seaweed) and ocean phytoplankton. Together sea-plants generate about half the world's oxygen.UCSB Science Line: If phytoplankton provides 50% of the earth's oxygen what's the other half?