Figure 1. Overview of the geography, physics, and chemistry of
ETNP Station P2. A. Map of the ETNP Oxygen Deficient Zone and
the location of Station P2. Colors indicate chlorophyll concentrations
at the surface as determined by MODIS satellite in 2012, while the black
outline signifies the region containing <10 μM oxygen at
300 m, according to World Ocean Atlas. The red circle indicates the
location of Station P2 (modified from Fuchsman et al 2019, credit Hilary
Palevsky, Creative Comments License
https://creativecommons.org/licenses/by/4.0/). B-D.Oceanographic parameters collected from a cast at 2017-01-13 12:15 CST
(local time). The thin horizontal green line shows the location of the
base of the photic zone (160 m B-D), defined by the complete attenuation
of the in-situ fluorescence, while the horizontal blue lines show the
surface (90 m, C) and base of the ODZ (900 m, B,D). B. shows
temperature, salinity and oxygen. C. fluorescence and
photosynthetically available radiation (PAR), focusing on the top 200 m
of the water column and photosynthetically active radiation, andD. beam attenuation and density.
Acoustic data reveal diel migration
patterns
Acoustic data, produced by the shipboard EK60 (Andersen, 2001), at ETNP
Station P2, suggest the presence of multiple cohorts of migratory
organisms. We focus initially on backscattering measurements from the
EK60’s lowest frequency 18000 Hz signal, corresponding to organisms the
size of small fish, because it travels furthest into the water column
and has the best resolution of the channels. Most migratory organisms
appeared to leave the surface at dawn and return at dusk, spending the
day between 250 m and 500 m (Figure 2A). There appeared to be two local
maxima in backscattering intensity at mid-day, one at
~300 m and one at ~375 m (Figure 2A).
There also appeared to be organisms that reverse migrated downward at
dusk and upward at dawn, spending the night at ~300 m
(Figure 2B). Just above the base of the photic zone, there was a peak of
organisms that appeared, at mid-day, on some but not all days, without
any visible dawn or dusk migration. (Figure 2C). Some diel migrators
appeared to cross the ODZ and spend the day below the detection range of
the EK60 (Figure 2D), as well as organisms that appeared between 500 m
and 1000 m but did not appear to migrate to or from that depth at our
site, but rather traveled through the EK60’s field of view (Figure 2E).
Similar patterns were evident in each of the other measured frequencies,
with better resolution by the lower frequencies (Figure S3).