20 m. However, fourth copepodites with substantial gut contents were present at all
depths from near the surface to below 80 meters from at least 03:00 h. To make this
descent of ∼85 m while pigment was eliminated from the gut at a rate of −0.022 min
−1 and still retain 3 of the original 5.7 ng pigment copepod−1, requires the distance to
have been covered in 29 min, nearly 5 cm s−1 or 25 body lengths s−1. That is fast,
perhaps a quarter of the predator escape velocity, rather than a typical steady
swimming velocity less than 10 body lengths s−1. More work is needed, but if descent
on satiation actually occurs, it appears from the data of Durbin et al. to have started
well before midnight.
(^) These observations of surface-acquired food in gut content at depth require that
some individuals are en route up or down throughout the night, and perhaps in
daytime as well. Some daring individuals may leave the security of the dim depths to
grab a noon lunch; some safety-conscious night diners may leave for deeper layers out
of the moonlight, even starlight, as soon as they have eaten. Trapping at night in a
Washington State (USA) fjord of just upward-bound and just downward-bound
individuals (Pierson et al. 2009) shows that many more copepods of several grazing
species that are swimming down have full guts than do those swimming up. It is of
related interest that non-migrating zooplankton, particularly species that are primarily
herbivorous, often cease feeding during the day, even though they remain around the
clock in the stratum with the most phytoplankton. Durbin et al. (1990) demonstrated
with a time-series of gut-content pigment estimates for the copepod Acartia tonsa
from Narragansett Bay that it ceases feeding altogether near dawn and does not
resume feeding until dusk. This cycling of behavior also is certainly a predation
avoidance adaptation. Casual observation of copepods in glass containers shows that
those with their guts full of dark phytoplankton are far more visible than those that are
empty.
(^) In addition to its importance in understanding the individual lives and population
dynamics of zooplankton (and much nekton), diel vertical migration has been
suggested to be the largest mass movement of biomass on Earth. It moves significant
amounts of carbon from surface layers deeper into the ocean, where all migrators
respire and some die. Longhurst et al. (1990) suggested that the carbon transport is on
the order globally of 2.7 × 10^14 g C yr−1, in their estimates about 20% of flux to
sediment traps at ∼150 m. Ontogenetic downward migrations of animals to their
diapause depths add to this, particularly at high latitudes. Bollens et al. (2011) found a
range of migratory transport estimates in the literature from 10 to 50% of particulate
flux, and Hernández-León et al. (2010) suggest that nocturnal feeding on surface
mesozooplankton by migratory midwater fish, particularly when aided by moonlight,
generates substantial transfers to depth. Variation and uncertainty in these estimates
remain large. Of course, some of what goes down each dawn comes back up at dusk,
confounding the difficulty of evaluating net transfers via vertical migration. Like