The zooplankton vertical distribution (VD) has been studied in two areas of the Mallorca shelf
during December 2009 and July 2010. Five and eight, depth levels were sampled at day/night by
using BIONESS an MOCNESS multinet. 17 zooplankton groups were found, where copepods
were the most abundant (70%), being responsible of the whole zooplankton VD pattern.
Copepod high diversity was found in both months (4.3 bits and 3.3bits) and more than 88
species identified. The highest diversity was found in deeper waters (90-120 m) in the afternoon
than during day time (0-90m) in December but in July, always found at deeper layer (100-150
m), suggesting that epipelagic and mesopelagic copepods co-occurred, and diel changes induced
the increased diversity. Clausocalanus arcuicornis (13%), Nannocalanus minor (10%) and
Pleuromamma (P. gracilis and P. abdominalis, 8% and 6%) were the most abundant. However
others such as Acartia clausi and Clausocalanus furcatus appeared in December and
Centropages typicus, Euchaeta acuta and Lucicutia flavicornis in July.
In relation to water conditions the VD exhibited different patterns with a higher difference in
autumn between the two sites than in summer when the zooplankton was concentrated around
the FMD. Species like N. minor, A. clausi, as well as all Clausocalanus and Oithona did not
show clear vertical movements; but others showed weak, intermediate or strong migration, such
as Lucicutia, Euchaeta and Pleuromamma which exhibited clear nocturnal VD. The importance
of these species in the active carbon transfer is discussed in this oligotrophic area of the central
western Mediterranean