ingestion at the surface, where food is abundant but it is warm, and assimilation at
depth where it is cold. Thus, they could gain in fecundity from the larger terminal
sizes that can be achieved in the cold. This may be important to some species, but it
always comes at the expense of longer generation times, which can offset much of the
gain in fecundity. Some species, particularly in high-altitude lakes, can avoid light-
and ultraviolet-light damage by leaving surface layers while the sun beats directly
down on the water surface. Detrimental effects of UV are readily demonstrated
experimentally, and perhaps migrations to a few meters from the surface help to avoid
those. Such slight displacements are hard to demonstrate in field data. There are also
arguments (e.g. Enright 1977b) that phytoplankton are most nutritious at the end of
the day and should be eaten then, after photosynthesis has had maximum time to
increase cell organic content and quality.
(^) Study of hundreds of species in hundreds of locations and situations has revealed
many variants on the basic down-at-dawn, up-at-dusk theme. An early observation
(Michael 1911) of DVM in chaetognaths seemed to show that they swim up at dusk
into a fairly restricted near-surface layer, presumably having followed a rising isolume
until it passed out through the surface, then slowly scatter downward through the
middle of the night, a phenomenon called “midnight scattering”. In the following
dawn, when there is again visible irradiance, but at less than the preferred intensity,
the animals rise toward the source, the surface, possibly seeking the ideal isolume.
This is called the “dawn rise”, and is shown explicitly by the Daphnia experiments of
Harris and Wolfe (1955) discussed above. While they can be modeled in the
laboratory, the importance of neither midnight sinking nor dawn rise is well
established in the ocean. Even though the data suggesting the phenomena and their
mechanism came from the ocean, Michael’s original results were not strongly
convincing. Some of Wade and Heywood’s (2001) ADCP data show something like
midnight sinking, but only well after midnight, long after any preferred isolume was
absent from the upper ocean. This suggests operation of a food saturation effect.
(^) Pearre (1973) suggested that the subarctic chaetognath Sagitta elegans exhibits
something akin to midnight sinking as a result of satiation. The arrow worms rise at
dusk, then begin to hunt in the well-stocked surface layers. Once they have ingested a
copepod or two, the food quota for the day is essentially met, so they can return to
depth early, gaining the greater safety of the depths sooner. That this happens is
indicated by the presence of surface-dwelling copepods in the guts of chaetognaths
captured at depth in the middle of the night or later, but long before the slowest
individuals, or at least those unlucky in the hunt, descend at dawn. Similar early
departures for deep layers have been claimed for Calanus (Simard et al. 1985; Durbin
et al. 1995) and sundry other plankton and fish. Durbin et al. showed data for fourth
copepodites of C. finmarchicus at one southern Gulf of Maine station in May at which
phytoplankton suitable for ingestion, i.e. cells >7 μm, were only available in the upper