ICES CM 2015/S:11
Zooplankton and micronekton vertical distribution and diel vertical migration
behaviour in the Northern North Atlantic Ocean
Thor Klevjer1, Eirik Norheim2, Dag Aksnes2, Espen Strand1, Tor Knutsen1, Webjørn Melle1, Peter Wiebe3
1. Institute of Marine Research, Bergen, Norway
2. University of Bergen, Norway
3. Woods Hole Oceanographic Institute, Massachusetts, USA
Summary:
During the Norwegian Trans-Atlantic cruise covering the central parts of four north Atlantic ocean basins,
the Norwegian Sea (NS), Iceland Sea (ICS), Irminger Sea (IRS), and Labrador Sea (LS), data on vertical
distributions of mesozooplankton to micronekton components were collected using a combination of hullmounted and towed multi-frequency acoustics, OPC (Optical Particle Counter), and VPR (Video Plankton
Recorder). In addition, data on both surface light, as well as the light regimes in different hydrographical
regions were collected. Daytime depths of mesopelagic scattering layers showed strong correlation with
optical conditions, with shallow daytime distributions occurring in regions with low light penetration, and
deeper daytime distributions in regions with clearer waters. For micronekton, DVM behaviour in the eastern
basins (NS and ICS) was heavily influenced by the short summer nights, with duration of nocturnal descent
coinciding with night lengths, and depths reached during nighttime depending on latitude.
Mesozooplankton showed less pronounced migrations and were generally more shallowly distributed
during the day. Recent works have suggested a strong dependence of daytime depths of vertical migrators
on oxygen conditions, but in the highly oxygenated North Atlantic light conditions at depth appear to exert
a strong control on their vertical distribution. Since factors controlling vertical distribution and migratory
behaviour may affect predator-prey relationships through vertical overlap, we explore vertical overlap
between mesozooplankton prey and micronekton predators under different optical regimes during the 2013
Norwegian Euro-BASIN cruise.
Introduction:
The mesopelagic zone is important to the functioning of the carbon pump, both as a zone of heavy
respiration, but also as home to many of the animals that contribute to the active part of the carbon pump
(Robinson et al. 2010). Some recent modeling studies have suggested that a high proportion of carbon export
may be mediated through mesopelagic animals (Davison et al. 2013), while other studies have concluded
that mesopelagic fish have a moderate influence on carbon export (Hudson et al. 2014). A central point in the
parameterization of these studies are details of the diel vertical migration (DVM), which usually takes the
form of animals residing at depth during daytime, while swimming upwards to feed at shallower depths
under the cover of darkness. While some mesopelagic species have been described as migratory and others
as non-migratory (Watanabe et al. 1999), large-scale studies of mesopelagic DVM behaviour are few. In one
such recent study, mid-water oxygen levels were singled out as a controlling factor (Bianchi et al. 2013),
however, the connection between vertical distribution, vertical migration behaviour and optical properties
have been documented many times (see for instance Dickson 1972). We here describe vertical distribution
and behaviour of scattering layers in 4 basins of the north-atlantic, together with data on environment and
potential prey distribution.
Materials and methods:
Vertical profiles of standard environmental data (salinity, temperature, fluorescence, oxygen) were collected
at predefined CTD stations, with supplementary data on horisontal patterns of temperature, salinity and
fluorescence collected continuously while the ship was underway. Data from the hull-mounted
echosounders gave continuous data on vertical distribution of micronektonic scattering layers. Vertical
distributions of zooplankton were assessed using data from OPC and VPR instruments attached to a towed
body. The towed body was deployed intermittently, but data were collected successfully in all 4 ocean basins.
Data on underway surface light conditions were collected both using a PAR sensor, as well as using a TriOS
hyperspectral radiometer. Subsurface light profiles were collected using another TriOS radiometer. From the
subsurface light profiles we estimated diffuse extinction coefficients at ~500 nm, and environmental
conditions were linked to optical conditions through an empirical regression.
Results and discussion
Optical conditions varied between the basins, and very high light extinctions were recorded in surface
waters in the LS, where also fluorescence values were high. Both chlorophyll levels and salinity were highly
significant in the empirical regression linking extinction coefficients to other environmental conditions.
In the LS vertical distribution of mesopelagic scattering layers were extremely shallow compared to the other
areas, with weighted mean depth (WMD) of 38 kHz backscatter (a proxy for center of biomass of
micronekton) found at ~350 m during daytime, in NS the corresponding WMD was >400m, and both ICS and
IRS had daytime WMD's of ~500 m. In both NS and ICS very little of the total backscatter originated deeper
than ~600 m, whereas in particular IRS showed strong backscatter also from below this depth. However,
using the results from the optical model, the results suggested that the scattering layers in all areas were
distributed at similar light levels in all areas. Our results emphasize the importance of optical conditions to
the daytime vertical distribution of mesopelagic scattering layers in the North Atlantic.
Our results also documents the influence of seasonality on vertical distribution in the mesopelagic: annual
cycles in productivity also affects optical conditions, with optical conditions during a spring bloom situation
being very different to optical conditions during pre- or post-bloom conditions. Different timing of the
spring bloom in different areas may partially explain the observed differences in vertical conditions, in one
case we traversed adjacent water-masses that were probably in different phases, our results demonstrate the
effect of this on the vertical distribution of mesopelagic biota.
The acoustic data recorded during the BASIN cruise document the presence of DVM in all basins visited, but
whereas the first crossing of NS and ICS suggested that a very high proportion of the scattering layers
migrated, comparatively low proportions of the scattering organisms in layers in the both the IRS and LS
basins migrated. Especially in the IRS basin a low proportion of the organisms of the scattering layers were
migrating, and diel changes in WMD were low. Differences in both average daytime residence depths, as
well as the proportion of organisms participating in DVM is likely to have big consequences on the vertical
connectivity between the epipelagic and mesopelagic zones (i.e. strength of carbon pump), but our data
from the NS basin also suggest that both of these factors can vary substantially over relatively short timescales, within the same basin. Further studies are clearly needed.
References:
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