Diel vertical migration behaviour in Euphausiids of the northern Benguela current: seasonal adaptations to food availability and strong gradients of temperature and oxygen

Abstract

In the highly productive northern Benguela upwelling system, euphausiids can dominate the mesozooplankton community and may contribute substantially to the vertical flux of organic carbon. The diurnal vertical distribution of four euphausiid species was observed over three seasons from different years. The most abundant, Euphausia hanseni, showed pronounced diel vertical migration (DVM), regularly crossing the thermocline and retreating again to the oxygen minimum zone (OMZ). Nematoscelis megalops was a weak migrant, persisting in the OMZ throughout 24 h. Euphausia recurva showed vertical migration into the OMZ but may have avoided oxygen concentrations below 1 mL O 2 L 21 , Euphausia americana remained in the upper water layers above the OMZ. Thus, euphausiids were divided into different ecological groups using or avoiding the OMZ and were vertically separated, thus avoiding interspecific competition. However, DVM behaviour was adjusted to seasonal variations in water temperature, oxygen and food availability. A conceptual model, combining DVM patterns, environmental parameters such as temperature and food availability and physiological constraints such as species-specific respiration rates, was used to assess the carbon demand of the seasonal DVM behaviours. Energetic considerations based on the DVM model

Quantitative effects of pycnocline and dissolved oxygen on vertical distribution of moon jellyfish Aurelia aurita s.l.: a case study of Mikawa Bay, Japan

“…WMD have been utilized frequently for analyzing the vertical distribution of zooplankton (Pearre, 1973;Makabe et al, 2012;Werner & Buchholz, 2013). However, in the present study, WMD was found not to be a good index for analyzing the relationship between A. aurita vertical distribution and environmental variables, while ULD and LLD showed clear relationships to the pycnocline and low DO levels, respectively (Figs.…”

Spatio‐temporal variability of zooplankton biomass and environmental control in the Northern Benguela Upwelling System: field investigations and model simulation

“…Even though zooplankton biomass was high in hypoxic waters at oxygen concentrations <1 ml·O 2 ·l −1 in March 2008 off Walvis Bay as well as in September 2010 and December 2009 off Kunene River, zooplankton biomass was generally diminished under low oxygen concentrations. However, oxygen depleted water can provide a refuge to many species adapted to these conditions (Childress & Seibel ; Koppelmann & Weikert ; Werner & Buchholz ). In our study, secondary distinct zooplankton biomass maxima were measured in waters below 300 m, in some of the daytime‐hauls even exceeding the near‐ surface maxima.…”

Life in the sea of plenty: Seasonal and regional comparison of physiological performance of Euphausia hanseni in the northern Benguela upwelling system

“…1) during nighttime with a 1-m 2 Multiple Opening and Closing Net with Environmental Sensing System (MOCNESS, Wiebe et al, 1985). Nighttime sampling ensured comparable catch depths, oriented at vertical migration in E. hanseni (see Werner and Buchholz, 2013) and prevented deep hauls causing stress on experimental individuals. A large mesh size (2000 μm) and a large soft closed cod-end bucket were used to further decrease stress on the experimental animals.…”

“…Werner et al, 2012). As E. hanseni is a strong diel vertical migrator (Barange, 1990;Werner and Buchholz, 2013) respiration measurements were conducted in a temperature-controlled water bath at three different temperatures (5, 10 and 15°C) reflecting the water temperatures around Walvis Bay (23°S) between 50, 300 and deep water layers and around 700 m respectively. Regional comparisons of metabolic rates were conducted at 10°C only.…”