(Ailing et al. 2008). They consider a topic that we leave readers to pursue if they are
interested.
Lower-Level Trophic Transfers in the Sea
Dilution Experiments
(^) Two methods have been widely used to examine trophic-level processes among
organisms smaller than the larger metazoan zooplankton, often defined as those less
than 200 μm: the micro-, nano- and picoplankton. Both methods were initiated by
Michael Landry and colleagues: dilution experiments and size-fraction outgrowth
experiments. Dilution experiments were explained in Chapter 3, and size-fraction
outgrowth was introduced by Calbet and Landry (1999, 2004). Not surprisingly,
dilution and fractionated outgrowth experiments have lately been combined in
dilution experiments using just biota small enough to pass some small filter pores,
say, 10 μm.
(^) Of course, dilution experiments have been subjected to modeling (e.g. First et al.
2009), which shows some complexities that should be reviewed by those planning
further applications. Those include the effects of grazer growth during incubations,
which make the m vs. μ relationship non-linear, an effect observed in actual
experiments by First et al. For present purposes we ignore that possibility.
(^) Calbet and Landry (2004) reviewed the dilution results (see Box 7.5) of the many
(788) experiments providing estimates of autotroph growth and mortality rates during
the two decades after the method was introduced (Landry & Hassett 1982). These
were not, of course, of uniform quality, but Calbet and Landry exercised some quality
control, and the results are informative. Recall that the regression of apparent growth
rate, call it μa, versus the fraction of sampled seawater, reduced in steps by adding
filtered seawater (0.45 μm or smaller pores), provides estimates of autotroph growth
rate, μ, unaffected by grazing as its intercept and the grazing mortality rate, m, as its
slope. With greater dilution, two things progressively allow μa to increase: (i)
relatively fewer grazers (G) in the incubation container, and (ii) fewer autotrophs (A)
and so greater volume that each grazer must search to find one. The method is, then,
an application of modified encounter theory, m = k[G][A]; the modification being that
[A] changes during the incubation (see Landry et al. 2000 for an approximation). To
get realistic evaluations of the interactions, it is necessary to run the incubations for
24 hours, because photosynthesis occurs only in daylight, but grazing is continuous
and phytoplankton cell division can also occur in, and often favors, the night. It can be
argued that the best procedure is to obtain m from nutrient-enriched dilutions and μ