Vertical distribution of zooplankton is an important biological factor that can modulate zooplankton transport, dispersal, and survival in the ocean. Seawater temperature and the associated formation of a thermocline can affect the spatial distribution of organisms in the water column and possibly modulate larval vertical distribution in coastal waters. In this study, we examined larval vertical distribution over small spatial scales where environmental conditions could exhibit strong and fairly predictable variability.
Diel variation in vertical distributions of barnacle larvae across the thermocline was characterized within the Bay of Cartagena of central Chile. Two intensive 21- and 24-h surveys were conducted in the northern (ECIM) and southern (CTGN) extremes of the bay in summer 2016 and spring 2017. In each survey, vertical tows were conducted at both sites every 3 h above and below the thermocline.
During summer 2016, larval vertical distribution differed between taxa (balanids and verrucids), barnacle stages (nauplii, cyprids), and sites within the bay. Of all taxa analyzed Balanid nauplii at ECIM were more abundant in the bottom layer during the day and at the surface during the night, suggesting they can control their vertical distribution and follow a diel pattern.Acording to the results all barnacle larvae were found in diferent positions in the water column between day and night during periods of high stratification in this survey.. In spring 2017 all barnacle larvae were confined to the upper layer, despite the strength of stratification, potentially as a result of the intrusion of hypoxic water (< 2 mL L−1) at the bottom, which reached up to around 5 m deep.
We conclude that diel vertical distribution is not uniform across different barnacle larvae taxa and stages within the Bay of Cartagena and is not affected by stratification. Of all the taxa analyzed, only balanid nauplii showed a pattern of diel vertical migration across the thermocline under certain hydrographic conditions.