(^) The approach remains the subject of review and argument. Huntley and Lopez in
their development chose some data rather cannily to show that attempts at
“physiological” estimation of growth rates were wildly variable. For example, from
some studies examining the effects of food, they included data in their evaluation
ranging from full nutrition down to none. This gave a much wider range of g
estimates than could fairly be expected from field results. In a response to critics,
Huntley (1996) made an interesting point: that the goal of the 1992 paper had been to
move the issue from laboratory growth studies to getting good estimates of biomass.
We cannot do the latter, yet, since net-tow, acoustical and other estimates of
zooplankton abundance are strongly affected by patchy distributions. Growth-rate
estimates are unlikely to be off much more than two-fold, while biomass estimates are
generally uncertain by a factor greater than two-fold, sometimes 10-fold.
“Hirst” Models
(^) Were Huntley and Lopez right? Is temperature the dominant control on a global scale?
Several papers by Andrew Hirst and colleagues have looked at the literature from a
different perspective. These authors began with a critique of the Huntley–Lopez
analysis, saying that it assumes that growth is always food saturated. In fact, Huntley
and Lopez only claimed that, over the full range of latitude, the data show strong
dominance of variation in g by temperature. If you examine the Huntley–Lopez graph,
there is still more than two-fold variability on the g scale at any temperature,
providing lots of room for nutritional and body-size effects. Hirst and Bunker (2003)
extracted from the literature all of the stage-specific growth rates that they could find.
The usual technique in the papers that they reviewed was related to the artificial
cohort method discussed above: sample the zooplankton, count out a large number of
living individuals of a given stage from one species (say, copepodite 3 of Calanus
finmarchicus), dry and weigh a few (Wstart), let the rest feed and grow for a day or two
(t), then dry and weigh those (Wend). Alternatively, carbon content was measured by
combustion. Usually, growth, g, in copepods is roughly exponential over intervals
ranging from the whole of development (Acartia) down to a stage or so (e.g. results
for Calanus in Vidal 1980), so they calculated exponential growth rates as g = 1/t
ln(Wend/Wstart). The assembled results show the expected temperature dependence
(Fig 7.20a).
Fig. 7.20 Relationships of development rates and growth of pelagic copepods to
habitat factors and body mass. (a) Inverse of time from spawning to final molt to
adulthood (“development rate”) vs. temperature”. Circles, broadcast spawners;