ADCP observations of migration patterns of zooplankton in the Cretan Sea

Abstract

Abstract. The lack of knowledge of the mesopelagic layer inhabitants, especially of those performing strong vertical migration, is an acknowledged challenge as its incomplete representation leads to the exclusion of an active carbon and nutrient pathway from the surface to the deeper layers and reversely. The vertical migration of mesopelagic inhabitants (macro-10 planktonic and micro-nektonic) was observed by acoustical means in the epi-and mesopelagic layer of the open oligotrophic Cretan Sea (Eastern Mediterranean) for almost 2.5 years at the site of an operational fixed-point observatory located at 1500m depth. The observed organisms were categorized in four groups according to their migration patterns. The variability of the migration patterns was inspected in relation to the physical and biological environmental conditions of the study area. The stratification of the water column does not act as a barrier for the vertical motion of the strongest migrants, moving up to 400 15 m every day. Instead, changes of light intensity (lunar cycle, daylight duration, cloudiness) and the presence of prey and predators seem to explain the observed daily, monthly and seasonal variability. The continuous presence of these organisms, yet capable of vertical motion and despite the profound seasonal circulation variability at the site of the observatory, implies their presence in the broader study area. The fundamental implications of the above for biogeochemical processing in oligotrophic seas due to the intimate link of the C and nutrient cycles, are discussed. 20

Zooplankton diel vertical migration in the Corsica Channel (north-western Mediterranean Sea) detected by a moored ADCP

“…ADCPs have been used in previous studies to investigate DVM in the Mediterranean Sea, in particular in the Ligurian Sea (Tarling et al, 2001, andBozzano et al, 2014), in the Ibiza Channel (Pinot and Jansá, 2001) and in the Cretan Sea (Potiris et al, 2018). Bozzano et al (2014) used acoustic backscatter data from a moored ADCP to investigate zooplankton dynamics in the upper thermocline and to infer the composition of the community in the Ligurian Sea.…”

“…Pinot and Jansá (2001) studied DVM in the Ibiza channel, where they described light irradiance as the primary factor that controls DVM on the daily and seasonal basis. Potiris et al (2018) studied the role of DVM for the functioning of the biological pump in the Cretan Sea, using a moored ADCP, CTD casts, net samples and other auxiliary information on environmental conditions, finding four different patterns of nocturnal DVM (divided by depth ranges). Other studies that Ocean Sci.…”