Comments on the analysis of grazing in Lake Erken 1

Nauwerck’s (1963) data on the relationship between zooplankton and phytoplankton in Lake Erken have been partly reanalyzed and reinterpreted by Cushing ( 1976). Cushing’s work is motivated by Nauwerck’s unsettling conclusion that the zooplankton of Lake Erken cannot be nourished exclusively by the phytoplankton, nor can the zooplankton constitute the principal source of mortality for the phytoplankton. Cushing’s analysis, although quite ingenious, is entirely founded on the fallacious assumption that the major phytoplankton taxa have identical ( average) growth rates at any given time (p. 349). Although we must grant some license to anyone who attempts this kind of analysis, the key assumption is in this case highly questionable and thus seems to invalidate the work. The net growth rates of phytoplankton species change seasonally as do the growth rates of division or class-level groupings of species, which are typically controlled by one or at most a few dominants. This variation results in succession. The net growth rates are of course a function of both biomass accretion and biomass loss. Differential biomass loss by sinking (Hutchinson 1967; Smayda 1970) and by grazing (Burns 1968, 1969; Wilson 1973; Porter 1973) has been documented in nature and experimentally. Differential biomass accretion, while more difficult to quantify, has also been demonstrated (Watt 1971; Stull et al. 1971). While these points seem widely accepted, Cushing seems to have simplified reality by the assumption that differential biomass accretion is not significant. The probable inaccuracy of this assumption is well illustrated by the data of Stull et al., which show variations in biomass renewal rate of three orders of