Resting vs active stages: the up and down in plankton dynamics

Composition of the marine coastal plankton undergoes seasonal fluctuations and interannual variations also due to the alternance of active and resting stages (cysts) in the water column and sediments. To fully understand the functioning of planktonic systems in marine coastal areas we need to integrate information coming from pelagos and benthos into a continuum that helps us to quantify the biological transfer between these two domains. To achieve this goal we have set up an ad-hoc experimental design in a confined marine basin at Taranto (southern Italy, Ionian Sea). We considered: active stages in the water column; cyst production (sediment traps); cyst accumulation in the sediments and in situ cyst germination (inverted traps). The whole study was structured in two "times", one in autumn, to observe the encystment dynamics, and one in the following spring, to observe the excystment. A total of 222 taxa were found in all the components of the system. In particular, 150 were active in the water column, 88 were cysts sunk in the sediment traps, 72 constituted the "cyst bank" in the sediments, and 63 were found as newly excysted active stages in the inverted traps. Only 17 taxa resulted common to all the sampling types. Scrippsiella trochoidea (Dinoflagellata), Ciliophora spp. and Acartia sp. (Copepoda) were selected as examples of life-cycle strategies. Different behaviours of taxa and their tuning with the environmental conditions result in the apparent unpredictability of plankton dynamics and composition in confined marine coastal areas.