Distribution and composition of mesopelagic macroplankton and micronekton in the North-east Atlantic.

Abstract

This study investigates the distribution and abundance of mesopelagic organisms in relation to environmental and biological conditions. A multidisciplinary cruise was conducted on board the G.O. Sars during summer 2018 in the North-east Atlantic, east of the Reykjanes Ridge. During the cruise multiple methods were used to measure zooplankton and micronekton: fine meshed trawls and acoustic methods. Macroplankton and micronekton were sampled with two types of fine meshed trawls (mouth opening of 36 and 800 m2 and 8 and 20 mm stretched mesh size, respectively) in oblique hauls with a maximum depth corresponding to the maximum depth of the mesopelagic zone (1000m). A time- and depth-referenced camera system mounted inside the trawl provided fine-scale information on species distribution within trawl hauls. Towed echosounder data were collected at frequencies of 38, 70, 120, 200 and 330 kHz that was operated between 10 and 1000m, as well as from hull mounted transducers. Macroplankton and micronekton individuals were identified and counted. Post-processing of the acoustic data were conducted with LSSS according to standard IMR routines. To identify the main prey items, stomach contents of key predators were analyzed. Distribution and biomass of macroplankton and micronekton were related to sea surface fluorescence, temperature and salinity (measured with a thermo-salinograph with fluorometer), primary production (sampled with and Fast Repetition Rate Fluorometry) and bottom topography. Finally, comparison between acoustic backscatter and biomass estimates obtained by the trawls was conducted, which is an indispensable step to better understand the role of the mesopelagic community in the oceans.

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Species composition
In the trawls, a total of 120 taxa were identified:
o13 crustaceans
o87 fishes
o10 molluscs
o4 gelatinous organisms
Vertical distribution and relationship with environmental factors
Acoustic observations showed 2 Deep Scattering Layers (DSL):
•Upper layer (between ~200-500 m depth), which moved to the surface at night, probably to feed.
This layer generally was associated with water masses around 8°C and 5.6-6.1 ml/l O2.
•Lower layer (between ~500-800 m depth) generally associated to water masses between 6-8°C
and O2concentration from 4.6-6.0 ml/l.
There is another scattering layer (between 50-250m) that showed higher intensity levels of acoustic
backscatter when chlorophyll of surface waters was ˃1 µg/L. This chlorophyll concentrations are also
associated with high and wide acoustic backscattering in the surface waters.
Stomach contents of key predators will be analyzed to identify the main prey items.
Vertical distribution of organisms from the in-trawl camera system.
Mesopelagic organisms play an important
role in the vertical carbon flux, because
most of them feed in surface layers at night
and staying between 200-1000 m depth
during daylight [1]. To estimate mesopelagic
abundance net sampling has been used [2].
However, sampling with nets leads to high
bias in the assessment of marine
communities [3], but it has the advantage
of permitting a precise taxonomic
identification as well as length
measurements of catch [4]. Acoustic
techniques have the advantage of lack of
avoidance and large volume sampled [3].
The cruise was conducted
from 6-24 June 2018 on
R.V. “G.O. Sars”.
Equipment:
•ACTD, measuring: temperature, salinity,
fluorescence and dissolved O2 .
•Tw o trawls lined with fine meshes:
•A time-and depth-referenced camera
system mounted inside the trawl.
•Ahull mounted echo sounder.
•Atowed echo sounder, operating
between 10 and 1000m.
•A drifting echo sounder, operating
between 67 and 460 m.
To investigate the distribution and
composition of the mesopelagic
macroplankton and micronekton.
To compare the trawls and
acoustic methods to estimate
biomass.
Eva García-Seoane,Thor Klevjer, Tor Knutsen, Gavin Macaulay, Kjell Arne Mork, Shale Rosen, Espen Strand, Rupert Wienerroither, Melanie Underwood and Webjørn Melle
Institute of Marine Research (IMR)-P. O. Box 1870, 5817 Nordnes-Bergen, Norway.
1. INTRODUCTION
2. OBJECTIVES
3. METHODS
5. FUTURE WORK
Distribution and composition of mesopelagic macroplankton
and micronekton in the North-east Atlantic
References:
1. Robinson C., D.K. Steinberg, T.R. Anderson, J. Arístegui, C .A. Carlso n , J . R . F ro st, J.-F. Ghiglione, S. Hernández-León, G.A. Jackson, R. Koppelmann, B . Quéguiner, O. Ragueneau, F. Rassoulzadegan, B.H. Ro b i s o n , C. Tamburini, T. Tanaka, K.F. Wishner and J. Zhang. 2010. Mesopelagic zone
ecology and biogeochemistry –a synthesis. Deep Sea Research Part II: Topical Studies in Oceanography 57 (16): 1504-1518.
2. Gjøsæter J. and K. Kawaguchi. 1980. A review of the world resources of mesopelagic fish. Fao Fish Technical Papers 193 (193): 1-153.
3. Koslow J.A., R.J. Kloser and A. Williams. 1997. Pelagic biomass and community structure over the mid-continental slope off southeastern australia based upon acoustic and midwater trawl sampling. Marine Ecology Progress Series 146: 21-35.
4. Béhagle N. , C . Cotté, T.E. Ryan, O. Gauthier, G. Roudaut, P. Brehmer, E. Josse and Y. Cherel. 2016. Acoustic micronektonic distribution is structured by macroscale oceanographic processes across 20–50°s latitudes in the south-western Indian ocean. Deep Sea Research Part I:
Oceanographic Research Papers 110: 20-32.
Fig 2: Echogram (after
noise removal) at 38
kHz (from the LSSS
software) from 9 June
at 00:00 to 18 June at
00:00. The scale on the
right indicates the
volume backscattering
strength (dB re 1m-1) in
the echogram. The
yellow arrow indicates
the top scattering layer,
and the red and green
the upper and lower
deep scattering layers,
respectively.
Theoretical
mouth
opening (m2)
Measured
mouth
opening (m2)
Mesh
size
(mm)
Small 36 30-34 8
Large 800 400-500 20
Fig 1: Study area.
Anoplogaster cornuta
Chirostomias pliopterus
Valenciennellus
tripunctulatus
Lampadena
speculigera
Cryptopsaras couesii
Scopelogadus beanii
ICES ANNUAL SCIENCE CONFERENCE 2018: 24-27 September 2018, Hamburg (Germany)
100
200
400
500
600
700
800
900
300
Messor
equipped with
acoustic,
optical and
oceanographic
sensor
packages.
Deep Vision in-
trawl camera
system
Drifting
SIMRAD
W B AT
Comparison between acoustic backscatter
and trawl biomass estimates.
Top scattering layer (NASC -m2nmi-2-)
Upper Deep scattering layer (NASC -m2nmi-2-)
Lower Deep scattering layer (NASC -m2nmi-2-)
Tr aw l
track
Benthosema glaciale (42 mm standard
length -SL-) imaged by the Deep Vision
system at 578 m depth.
4. RESULTS AND DISCUSSION
Examples of
mesopelagic
fauna collected
during the cruise.
(kg) (kg)
Stylocheiron
maximum