use the fitted equations in a production study, you have to tolerate errors of about five-
fold, as indicated by the scatter (Fig. 7.17b). With an oxygen-to-carbon conversion
factor and very broad-scale averaging of biomass estimates by region, Hernández-
León and Ikeda (2005) have used a sweeping compilation of such respiration data to
evaluate the role of mesozooplankton globally in the world ocean carbon cycle. They
came up with ∼13 Gt C yr−1, 25–30% of likely global marine primary production.
Three-quarters of the grand total respiration occurs above 200 m. This is a high
proportion, given recent estimates of bacterial and protistan respiration. However, it is
unlikely that we will ever exactly determine the roles of different heterotrophic
categories in the bulk metabolism that must roughly balance primary production at the
annual and global scales.
Absorption Efficiency
(^) Measures of ingestion rates were discussed above. Food inside the gut is not,
however, yet inside the animal, the topology of which is really toroidal: the lumen is
outside the body like the hole in a donut. To get food inside, the animal breaks it up
with grinding and enzymes into small molecules for absorption. Components that do
not break up, or are grabbed by gut-dwelling microbes, are defecated, D. The ratio of
absorbed to ingested food is the absorption efficiency, AE = (I − D)/I. “Assimilation”
efficiency is a term often used for this ratio, but full assimilation involves more steps
than just getting nutrient through the gut wall. Both I and D can be amounts or rates;
AE is unitless, a fraction. There are only a few studies of AE in mesozooplankton.
Thor and Wendt (2010) showed with the coastal copepod Acartia tonsa that AE varies
with the type and amount of food. To measure AE, they used phytoplankton cells
labeled with two radioisotopes, ^14 C and ^51 Cr. The carbon labels the organic matter and
is partly absorbed, the chromium adheres to the outside of the cells and mostly is not
absorbed, with the AE = (1 − Φfeces/Φprey), in which Φ = ^14 C/^51 Cr ratios in feces and
food (algae). Derivation is left as an exercise. They also measured ingestion rates by
cell counts before and after some grazing. On Rhodomonas (Fig. 7.18) the AE
declined with food availability; on Thalassiosira it was roughly constant; and on
Dunaliella it varied in an odd but definite pattern. For some foods, at least, digestion
is more thorough if less food is available. The range from ∼50 to 85% is broad
enough to distort general estimates of secondary production based on single AE
values, like those generally applied in NPZ models. Besiktepe and Dam (2002) have
shown that fecal pellet size and production rates vary in parallel with the shifts in AE.
Fig. 7.18 Ingestion rates and absorption efficiency of Acartia tonsa on three different
algae.
(After Thor & Wendt 2010.)