49 ml per copepod per hour (over 1 liter per day) feeding on large cells after a period
of starvation, and when collected in particular seasons. Debate about the adequacy of
these rates to provide enough food at natural phytoplankton concentrations appears to
be endless. Many copepods are found where less than 1 μg of particulate carbon is
available in 200 ml; they should be on short rations. Older (e.g. Mullin & Brooks
1976; Derenbach et al. 1979) and newer (Cowles et al. 1998; Benoit-Bird 2009)
observations of thin horizontal layers with phytoplankton concentrations two to five
times above background suggest that, at least in very nearshore areas, copepods may
solve the problem by sojourns in such layers. Acoustic profiles do show strong
associations of zooplankton reflecting 120 kHz sound associated with nearshore thin
layers (Benoit-Bird et al. 2009). The fact that hungry copepods can suddenly filter
very fast when finally given a meal implies that they are equipped to take advantage
of happening into such strata. The acoustic results also suggest that they can stay in
the layers and open wide spaces in their own layering around predatory fish.
(^) Perhaps a hundred worthy papers could be cited here to illustrate various
refinements and extensions of these observations on copepod feeding. We will
consider only a few of the important results:
(^1) The functional response curve is not very fixed. Copepod ingestion
measured soon after collection at a range of food concentrations will tend to
become asymptotic at close to the field concentration of phytoplankton
(Mayzaud & Poulet 1978). More food will not stimulate more eating
immediately. However, copepods collected from high food situations will
have higher asymptotes than those from more dilute situations. Thus, the
functional response is flexible over some interval longer than the term of the
usual feeding experiment (Donaghay & Small 1979).
2 There are feeding thresholds: at low food concentrations, at least of some
foods, filtration rates are reduced (Calanus – Frost 1975; Acartia –
Besiktepe & Dam 2002). Thresholds are potentially important because they
give the phytoplankton a refuge from annihilation, although that is not
likely to be the reason that grazers have thresholds.
3 There are a variety of selection processes. Copepods can eat different sorts
of particles at different rates. Many factors affect which available foods will
be eaten or at least preferred. Particle size is not the primary determinant of
this. Richman et al. (1977) demonstrated selection by Acartia tonsa, from
Chesapeake Bay, feeding on natural particle assemblages. They found that
any peak in the particle-size spectrum could be consumed at significant
rates, while particles just larger and just smaller were ignored completely
(Fig. 7.5). This cannot be done with a feeding mechanism that is essentially
a flour sifter, unless particles can be identified and chucked back out after
sifting. The feeding mechanism proposed by Koehl and Strickler allows