On-board Diary: Mar Menor

Author: Ricardo Aguilar
Date: July 27, 2006



Yesterday was a day of testing. We worked out how to make the best use of the grids, rulers and other equipment that we have with us on board to assess the density of plants and marine algae on the sea floor. In the process, we also tested the coordination between all the team members. For that purpose, we headed towards the coast of Altea, where the extension of a marina has damaged the seagrass meadow of Posidonia oceanica and has also affected some areas of another seagrass species, Cymodocea nodosa. After diving and discussing how to improve our working method, we decided to set course for Murcia and to enter the Mar Menor.

Upon our arrival we have to anchor and wait until 8:00 am for the drawbridge to open, through which we enter this inland sea. Right after passing into the lagoon, groups of jellyfish make their appearance around the boat. The first ones we spot are the innocuous but beautiful Barrel jellyfish (Rhizostoma pulmo), a species whose umbrella can reach a diameter of more than half a meter. The specimens we see here are no bigger than 35-40 cm. Soon after, we also spot "Fried egg" jellyfish (Cotylorhiza tuberculata). The tissue of this jelly contains small, blue, symbiotic algae (Cyanophyceae or Cyanobacteria) which apart from giving these animals a spectacular colour, also provide them with a source of food.

The concentration of jellyfish we come across is enormous. We have made an estimate by counting the jellyfish we encounter within a specific transect from the boat and we have calculated that there may be close to 20 million jellyfish in the top layer of the lagoon, which covers an area of almost 13.500 hectares. And this is a conservative estimate: if we add the jellyfish that remain out of sight because they are at a greater depth, the number must be 2 or 3 times bigger.

Apart from jellyfish, we have also come to film the sea floor here, which is mainly composed of mud and is dominated by meadows of the marine plant Cymodocea nodosa and green algae (Caulerpa prolifera). In this habitat, we find giant pen shells (Pinna nobilis) and ‘razor clams' (Pinna rugosa), oysters (Ostrea edulis), giant gobies (Gobius cobitis), some needlefish, such as the broad-nosed pipefish (Syngnathus typhle), or small anemones that grow on the Cymodocea seagrass (Bunodeopsis strumosa). It just shows that every ecosystem has its surprises.