“Mediterranean” is a name steeped in history – cradle of civilization or a land of conflict, a surface of storms or dizzying calms. The Mediterranean is a semi-enclosed sea made up of several basins separated by straits, thresholds or only administrative boundaries. Its dynamics depend on exchanges of water, matter and also on living beings, with neighboring seas. Are these exchanges negligible or decisive in annual or long-term evolution of the Mediterranean? Within the Mediterranean, evaporation is not compensated by river inflows; as a result, increases in density, combined with decreases in temperature in winter, lead to surface waters sinking to the bottom in some areas of the Mediterranean. Outside the polar zones, this phenomenon of deep-water formation is found only in the Mediterranean where the thermohaline cell reproduces the global circulation process in miniature. For the Mediterranean basin, this results in a reverse estuarine circulation, which is characterized at Gibraltar by incoming Atlantic waters at the surface and outgoing Mediterranean waters at depth. The Atlantic water flow is therefore crucial. Its slightly salty water leaves a trace as far away as the Ligurian Sea, far from Gibraltar. However, the Atlantic flow is, above all, involved in the eastern basin in the formation of new water that will then irrigate all the regions of the Mediterranean, before mixing in specific places, into the deep water that will finally emerge in the Atlantic Ocean. Entry and exit imply a budget of non-living material, and also of living beings. As the open sea is never further than 300 km from the coast, the impact of continental inputs on pelagic areas is particularly significant. It must be taken into account that the shores of the Mediterranean are heavily populated and are the focus of important touristic, agricultural and industrial interests; continental emission sources are intense and also varied with, overall, the North responsible for anthropogenic influences and the South responsible for detritic (Saharan) natural influences. While African or European winds lift the sea, mix and carry away the surface layer, they also bring loads of matter that disrupt surface marine ecosystems. This sea, and each basin as well, appear to be the complex drivers ensuring the annual dynamics of marine ecosystems. Hydrodynamics, biology and chemistry combine locally and are subject to near or distant, rapid or long-term regulations, which are not yet fully known or predictable.

For several areas of the Mediterranean Sea, knowledge has been accumulated and attempts have been made to generalize it. The Ligurian Sea is a basin in which the topography of the coasts and bottoms produces simple situations. It is an “ocean model” as described by early hydrologists. Cyclonic water circulation and central divergence result from topography. The absence of a continental shelf provides access to the deep sea, and the absence of large tides simplifies temporal dynamics. These simple conditions are the main elements of the “model”. The Ligurian Sea was a laboratory to test methods and analyze different phenomena. However, with the advent of recent apparatus and research techniques, new instruments, new research strategies and new properties have been discovered. The various examples of the dynamics of offshore ecosystems, more or less enriched by the nutrient resources stored at the sea bottom, illustrate the situations encountered in vast territories of the world ocean.

The continuation and amplification of regular observations of water and planktonic populations, which began around 1895, have made it possible to identify long-term effects, particularly on a climatic scale for the entire Mediterranean basin. Knowledge about the Ligurian Sea has generally been obtained independently by laboratories, by young established researchers, during oceanographic campaigns and now, using autonomous instruments. This sea has been a training and teaching ground. The accumulation of data over time has allowed long-term series of its properties. International research programs and cooperation with other institutions have resulted in the creation of new scientific teams and large, well-equipped research vessels. It was a good opportunity to export oceanic knowledge, on the dynamics and “know-how”, capitalized in the Ligurian Sea, to the whole world. Although these snapshots on various areas are important because they are tests of hypotheses and theories, they are complementary to the exploration of all aspects of marine ecosystems, on the neighboring body of water that is easy to sample frequently, such as the Ligurian Sea. However, these distant expeditions have made it possible to define the relative place of the dynamics of the Ligurian Sea in the context of the world ocean, and to observe the amplitude of variations in the variables measured locally, with respect to those possible elsewhere. Now, the coverage capacity of all oceans with satellites and autonomous vehicles allows the continuous acquisition of new variables that must be identified and validated. The Ligurian Sea then becomes an area of testing and calibration, a kind of field laboratory giving access to the open sea. This knowledge has gradually been accumulated under various influences, within the framework of French university work, theses, master reports, or under the impetus of research agencies including CNRS, CNEXO, and also by Italian CNR, or international universities, EURATOM and IAEA. All this work and all these results deserve to be consolidated. This book is then a first step in presenting the functioning of an oceanic region, from the perspective of forming or identifying particularities and similarities with other regions of the world ocean. It also highlights the research effort that has been devoted to this part of the Mediterranean for more than 150 years. In addition to the evolution of data acquisition, we must also take into account the environmental, climatic and meteorological changes of the last three decades. Climate and meteorological changes have most likely affected the physical ocean pump. Furthermore, the supply of nutrients through external inputs has undergone significant changes, yielding modifications in plankton dynamics. A shift in phytoplankton populations in favor of species adapted to oligotrophy has indeed been observed in the Ligurian Sea.

It is within this framework that the ambitions of this book are set.

Christophe MIGON
Paul NIVAL
Antoine SCIANDRA
November 2019