Biological Oceanography

(^) The basic vertical migration pattern is simple: down at dawn and up at dusk. A
colorful demonstration of this has been provided by Wade and Heywood (2001) using
an acoustic Doppler current profiler (ADCP), a 153 kHz echo-sounder mounted on a
ship’s hull. It records the shift in frequency of sound reflected from particles in a
series of successive time intervals, and thus distances, after a “ping”. Shifts are
interpreted as velocity relative to the ship. In most applications, ADCPs are used to
provide vertical profiles of horizontal current velocity, but Wade and Heywood used it
to resolve the vertical velocity of the particles. Echo amplitude (backscattering
strength from a volume of suitable size) can also be interpreted as particle abundance.
Most of the particles with a substantial backscattering “target strength” at this
frequency would be larger zooplankton such as euphausiids, or small animals with an
especially strong target strength, like shelled pteropods. Fish with swim bladders
likely are also important. A day-long record of depth-distribution of backscattering
strength (Plate 8.3a), shows a strong layer at ∼350 m (just where the ADCP results
begin to break up) during daytime, which moves up slowly at first, then accelerates,
hitting the surface just after sunset. The same animals returned to depth well before
sunrise. Vertical velocity (both up and down) of “particles” (Plate 8.3b) is maximal
just when the layer is moving fastest, as expected. Sustained swimming at greater than
∼10 body lengths s−1 is unlikely, so ∼6 cm s−1 implies animals on the order of nearly
a centimeter or longer, such as Euphausia or myctophid fish. Enright (1977a) showed
that the strongly migrating copepod Metridia can move vertically at sustained speeds
up to 2.5 cm s−1, about 10 body lengths s−1, so while they often move 350 m or more
in their migrations, they probably cannot precisely follow an isolume. They must start
up earlier than a specific isolume, arrive in the surface later, or both. All available data
(e.g. Plate 8.3) indicate active swimming both up and down. Even a dead fish can’t
sink that fast. Only physonect siphonophores with gas-filled floats rise and sink with
just buoyancy changes – probably quite slowly, since the float is small relative to the
body.
(^) In habitats where some species populations migrate strongly, there are usually
others that don’t migrate. The common supposition that DVM is a typical or nearly
invariant behavior in zooplankton is not correct. Marlowe and Miller (1975)
compared day and night distributions for the entire mesozooplankton community in
the oceanic sector of the Gulf of Alaska, finding that only 10% of species showed
statistically reliable indications of vertical migration. Some groups of zooplankton
migrate more regularly and farther than others, for example the copepod family
Metridiidae (Metridia, Pleuromamma). Others migrate sometimes, sometimes not,
and still others show no variation in vertical distribution coordinated with
illumination.
(^) Reasons for diel vertical migration are commonly divided into two sets: (i) the