well-worked example. Part of the difficulty is uncertainty about what and how much
animals actually eat in the field, i.e. which of the available particles they actually
ingest. Thus, the “IA” term is unreliable. For example, an ingestion rate obtained from
the phytoplankton pigment method will neglect nutrition obtained from colorless
herbivorous protists, which are major components in the diets of many
mesozooplankton. Another problem is the necessity to rely upon laboratory data,
considered here mainly for respiration, which in this context is a general term
covering catabolism of organic matter and excretion.
Respiration
(^) The respiration term is generally measured as oxygen consumption rates in sealed
containers over intervals of hours or a day. Change in oxygen concentration is
determined by Winkler titrations, with electrodes or (lately) with optodes. There are
complications in selecting and interpreting oxygen consumption data. Immediately
after capture there is generally a strong spike in oxygen consumption. This may be
due to capture stress, or it may be close to the real metabolic rate. In laboratory
containers of any size small enough for a reasonably low density of animals to
provide measureable oxygen uptake, their ambits are restricted and no predators can
give chase. These effects may possibly explain the drop in rates, rather than recovery
from capture. There is no certain way to tell which rates are applicable to the field.
Fed and unfed animals have different rates, a well understood effect. Tsutomu Ikeda,
who generated a great part of the available data, has generally preferred to ignore (or
not measure) the high initial values, assuming they represent stress, and starts
measurements 12 hours after capture. Subsequent oxygen utilization rates are
reasonably steady in fed animals, slowly declining in starved animals. Nitrogen and
phosphorus excretion rates are also high initially, but they continue to drop for several
days, even weeks, after oxygen use stabilizes. The difficulty of assuring normal
measurements makes the physiological approach to secondary productivity
problematical. However, useful approximate measures can be produced.
(^) Many studies have been done of zooplankton respiration rates, which vary with
temperature and body size. Ikeda (1974, 1985) has shown these relations over wide
ranges of plankton type (seven phyla) and size in both the tropics and polar zones
(Fig. 7.16).
Fig. 7.16 Weight-specific oxygen consumption of zooplankton individuals from seven
phyla as a function of body weight, grouped by region. Tropical plankton respire
faster at a given weight and respond more strongly to body size than boreal plankton.
(After Ikeda 1974.)