Assimilation efficiency in herbivorous aquatic organisms—The potential of the ratio method using ¹⁴C and biogenic silica as markers1

A method for measuring assimilation efficiencies based on the change in the Si : 14 C ratio in food and feces is proposed. Since this indicator method uses biogenic silica as the inert substance, the method is restricted to aquatic herbivores that consume food containing it. The method has been demonstrated on adult female Calanus hyperboreus fed the diatom Thalassiosira anguste‐lineata ; assimilation efficiency decreases from 87.7 to 84.2% during a threefold increase in ingestion rate. The effect of errors in the estimate of Si : C ratios is slight when assimilation efficiency is high, but with decreasing values of assimilation efficiency, the sensitivity to errors in the estimates of silicon and carbon is increased.