Habitat Shift for Plankton: The Living Side of Benthic-Pelagic Coupling in the Mar Piccolo of Taranto (Southern Italy, Ionian Sea)

Resting stages represent the answer for species to the variability of environmental conditions. In confined marine habitats, variability of conditions is high, and bottoms host plankton resting stages in the so-called "marine cyst banks". The Mar Piccolo of Taranto was chosen as a pilot site in which to investigate how marine cyst banks and plankton affect each other in the living part of the benthic-pelagic coupling. The attempt was based on the use of multiple devices for integrated sampling of benthic and pelagic stages and allowed us to identify 207 taxa/categories in the whole system (127 as active forms, 91 as resting stages). The sediments added 80 taxa to the plankton list obtained only from the water column, thus confirming the importance of this kind of approach in perceiving the actual diversity of the studied site. The sediment cyst bank involved 0.15-1.00% of its content in daily benthic-pelagic exchanges, in terms of cyst germination and import, respectively. In addition, the cyst production, which was higher than the cyst germination, is responsible for the existence of a permanent biological reservoir in the sediments. The benthic-pelagic coupling, however, was completely depicted in the present investigation only for seven taxa. This result is due to the still scant knowledge of the life cycles and life histories of single species. Apart from the identification difficulties that still have to be clarified (which cysts belong to which species), the cycle presence/absence is also characterized by the diversification of strategies adopted by each species. The observation of plankton dynamics from the benthos point of view was useful and informative, unveiling a huge assemblage of resting forms in the sediments only minimally affected by cyst import/export, because it is more devoted to a storing role over long periods. Consequently, the continuation of life cycle studies appears necessary to understand the diversity of strategies adopted by the majority of plankton species.