The jellyfish Pelagia noctiluca (right); a high concentration of Velella velella and one specimen of Pelagia noctiluca (red arrow) (left). Photos: Wikipedia commons and Lucio Bellomo.
Although some might think – and even say! – that the open sea is a boring desert of water, during La Ruta de la Sal navigation (Port Ginesta, Barcelona – San Antonio, Ibiza and back) the Acrobat made beautiful encounters and could witness the ever-impressive Mediterranean phytoplankton spring bloom.
Besides dolphins, often to be admired playing with the bow of a sailboat, the most surprising encounters we had were surely with the ocean sunfish (Mola mola). We observed two of them on our way to Ibiza, and at least 15 on the way back to Port Ginesta, in both cases at least 30 nm off the nearest coast. Interestingly enough, almost always the ocean sunfish were observed in couples.
Ocean sunfish (Mola mola). Photo: NOAA.
During the night between March 29 and 30, while heading to Port Ginesta, we encountered a huge concentration (about 1 individual/m2) of Pelagia noctiluca, the best known stinging jellyfish in the Mediterranean Sea. This jellyfish performs vertical migrations approaching the surface after sunset for feeding, especially when there is scarce or no moonlight, and going back at depth before the break of day. With this behavior, shared with other inhabitants of the sea such as squids, during the day Pelagia takes advantage of the dark depths to hide from predators.
We also observed other types of jellyfish including juveniles of Velella velella, which indeed usually invade the Mediterranean coasts in April-May, and numerous Salps. These creatures form the so-called plankton (from the Greek πλαντóς meaning errant, drifting), that is, the ensemble of «things» whose motion prevalently depends on marine currents (which actually also includes plastic…). Finally, we spotted a small sea-turtle swimming peacefully at surface only a few miles off Ibiza: wonderful!
One specimen of Salpa maxima. Photo: Amazonaws.
Who said that the Mediterranean Sea is always crystal-clear and transparent? Imagine how puzzled we did feel when the blue water we were admiring while sailing near the coast suddenly became deeply green a few miles offshore, in the heart of the Balearic Sea! Many don’t know that in early spring (mid-March and early-April) some offshore regions of the Mediterranean Sea go indeed from the usual blue color to a greenish one, with a drastic decrease in visibility. So much that people accustomed to the turbid Northern Atlantic Ocean (Spanish northern coast, French Brittany, United Kingdom and so on) would feel totally at home! This surprising phenomenon is due to the particular abundance of phytoplankton, microscopic drifting (plankton) algae (phyto) living out of photosynthesis, a process relying on chlorophyll, a greenish pigment, hence the peculiar water color. The sea has therefore its own bloom just like flowers which, also in spring by the way, paint our countryside.
Average chlorophyll-a (one of the types of chlorophyll present in phytoplankton) concentration between 2003 and 2013 for the spring (left) and summer (right) seasons (http://oceancolor.gsfc.nasa.gov/cgi/l3). The red ellipse shows the portion of the Balearic Sea where the Acrobat sailed during La Ruta de la Sal. It can be seen that chlorophyll-a is approximately 10 times more abundant in spring.
The following – and last! – question is: why the bloom produces in early spring? To perform the photosynthesis, phytoplankton needs 1) energy, which it receives through sunlight, and 2) nutrients, that is, chemical elements such as nitrogen. While the former is naturally all the more present as approaching the sea surface, the latters have a variable vertical distribution according to the seasons. Although nutrients are usually more abundant at depth, they can reach high surface concentrations in those regions where ascensional currents push them upwards. Such upwelling is indeed typical of early spring, when the water column has uniform temperature and salinity. Then, both nutrients and energy are abundant enough at the sea surface, triggering the bloom and the «north-atlantization» of the Mediterranean Sea.
Fortunately, by early June any trace of the bloom has disappeared and the Mediterranean Sea shows its most transparent waters to its tanned people.