The deep Mediterranean Sea as a diverse environment

Anastasios Tselepides
Department of Oceanography
Institute of Marine Biology of Crete
Crete
Greece

Abstract

The deep-sea floor has traditionally been considered as a stable unchanging environment inhabited by sparsely distributed organisms which depend upon a constant supply of fine (mostly refractory) particles settling from the euphotic zone. This contention was also considered to be true for the Mediterranean Sea which at the turn of the 20th century was even considered to be “azoic”.

However, during the past three decades, new technological developments, sampling gear and environmental monitoring methods in concert with a proliferation of multidisciplinary research programmes, has gradually changed this notion. We now know that the deep Mediterranean Sea is a dynamic environment linked to upper water column processes through the varying (in both space and time) influx of organic matter. It is also driven and characterised by the dynamics imposed by major geological features (deep abyssal basins, the Mediterranean ridge system, anoxic hypersaline basins, mud volcanoes, cold and hydrothermal seepage, seamounts, submarine canyons, deep-sea Nile fan etc.) which are encountered in certain areas, and especially in the Eastern Mediterranean.

The benthic community has shown to react very rapidly to seasonal or/and episodic sedimentation of organic matter. Small size fractions of benthic organisms (bacteria and protozoa) are known to respond to such inputs, even when the quality and quantity of the sedimenting matter is low or poor. The main processes therefore controlling pelagic-benthic coupling are primarily the export flux of organic matter from the euphotic zone, the quantity and quality of the OM arriving on the sea-bed and of course its rate of oxidation and mineralisation.

The deep eastern Mediterranean also seems to be governed by cataclysmic events which alter the entire biogeochemical balance of the seabed. These events are either the end result of a combination of climatic factors and/or local reactions to a suite of hydrological “perturbations” which cause uplifting of nutrient-rich deep-waters into the photic zone and therefore enhance biological productivity and OM flux to the sea-bed. The large deep-water transient in the Eastern Mediterranean is now considered a classical example of how a “deep-sea desert” can relatively abruptly be converted into an “oasis” or a monoculture of certain opportunistic species (i.e. the polychaete Myriochele fragilis). Species such as Chaceon mediterraneus were once only known from the western Mediterranean basin and completely unknown in the deep eastern basin are now found everywhere and in high numbers.

The questions thus arising are:

The answers to these “ambitious” or “provocative” questions are in the affirmative. There is accumulating evidence that climate change has caused an immediate accumulation of organic matter on the deep-sea floor, altered the carbon and nitrogen cycles and has affected benthic activity in general. Investigations undertaken in the deep Eastern. Mediterranean during the past fifteen years have revealed that it is occasionally a very dynamic environment responding to major climatic or hydrological changes or forcings by altering its biogeochemistry and concomitantly its benthic community structure.