The present study focuses on variations in day and night copepod community structure in a meromictic Mediterranean ecosystem (Lake Faro). Because of the high salinity in the lake, this environment is defined as a coastal-marine rather than true-lagoon ecosystem. Copepod abundances showed wide oscillations, with marked spatial and seasonal heterogeneity. The copepod assemblage was dominated by coastal and estuarine species Paracartia latisetosa and Acartia margalefi, in late summer–autumn and spring, respectively. A remarkable change in species composition was the occurrence of the alien species Oithona brevicornis, never having been reported in this environment before. O. brevicornis is progressively replacing the congener species O. nana. The discovery of the hyperbenthic calanoid species ‘ecological group’ contributes to increased functional diversity of this ecosystem, despite their relatively low abundances. All six hyperbenthic calanoid species occurred in the plankton community only at night, with spatial distribution patterns being related to substrate more than water physico-chemical parameters. Among all hyperbenthic calanoid species, Pseudocyclops xiphophorus was the only one able to inhabit the whole lake because of its daytime habitat (fouling material, attached to submerged ropes and mooring posts, widely distributed all around the lake). In this habitat, the temporal abundance pattern resembled the night-time one in plankton community.

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