Effect of Somatic Growth and Reproduction on Biomass Transfer up Pelagic Food Webs as Calculated from Particle-Size-Conversion Efficiency

Abstract

Biomass or energy transfer up pelagic food webs to larger sized organisms is a function of (1) direct trophic level transfer through predation, (2) somatic growth, a process that augments biomass transfer through predation, and (3) reproduction, which impedes biomass transfer by moving biomass down the food web to smaller sizes. By assuming that particle-size-conversion efficiency (log (food consumed/biomass produced)/log (predator–prey size ratio)) is relatively constant, I derive simple equations to calculate the effect of somatic growth and reproduction on biomass transfer up the food web. This defines the conditions under which somatic growth and reproduction can be ignored and biomass flow can be calculated from predation alone, using a previously developed model. When these conditions are not met, the effect of somatic growth and reproduction can be calculated from data on cohort growth and mortality rates. It is not necessary to identify the food of any species. This eliminates one of the problems often encountered when modeling food webs. I have applied these equations to production of Mysis relicta. If the estimates of Mysis abundance and growth rates are correct, then size-corrected production is about 25% greater for this species when somatic growth is accounted for in the calculations. This is because mortality of young Mysis appears to be low and most production occurs during somatic growth and not during reproduction.