Spatial patterns in the vertical structure of euphausiids in Gullmarsfjord, Sweden: Identifying influences on bilayer formation and distribution

The formation of two vertically discrete layers (bilayers) at night-time is a commonly observed phenomenon in zooplankton and is regularly found in Gullmarsfjord, a fjord with a 50 m sill depth, deep basin and a three-layered water column. In an acoustic and net sampling survey in September 2003, night-time euphausiid layers occurred at 15 and 45 m, with the deeper layer containing relatively higher concentrations of adult Northern krill (Meganyctiphanes norvegica). The main night-time predatory threat came from the upward migration of demersal fish, which reached the deeper but not the shallower
euphausiid layer. Shoreward advection of coastal waters across the sill creates a layer of resuspended organic matter between 40 and 50 m. The deeper bilayer was located at those depths, particularly at the mouth of the fjord where this organic matter was most concentrated. Krill in the lower bilayer experienced waters that were 4ºC cooler than in the upper bilayer, which can decrease the cost of respiration by around 20%. Accompanying studies have shown significantly higher growth rates in krill consuming sedimentary organic material and benthic filamentous algae. Combined, it appears that energetic benefit and predatory threat were greatest in the deeper rather than the shallower bilayer in Gullmarsfjord. This is the reverse of most other euphausiid habitats, where the highest risk and reward is in the upper bilayer, illustrating that euphausiids adapt
their stereotypic vertical migration pattern to local environmental conditions.