Size spectrum is critically affected by omnivorous and detritivorous feeding in planktonic foodwebs

Chih-hao Hsieh（台湾大学海洋研究所）

Size spectrum, defined as a plot of biomass density versus biomass class at a logarithmic scale neglecting species identity, is commonly observed in aquatic systems to follow a power-law distribution. The generating mechanisms of this pattern have gained a great amount of attention. Importantly, most of the proposed models assumed a linear positive log(biomass)-trophic level (hereafter, size-TL) relationship. While these models have successfully reproduced a power-law distribution of size spectrum, the linkage between models and empirical data remains elusive. Particularly, natural data often deviate from a power-law distribution to some extent. Such deviations were often considered as results of sampling noise or device artifacts. Such assertions suffer from deficits of sample replicates. To overcome this issue, we sampled time series of size spectra data of plankton in a water reservoir.  In addition, we tested the assumption of a linear positive size-TL relationship using
 size-fractionated stable isotope analysis.  We first identified complex trophic dynamics in the planktonic foodweb as a result of interactions among a grazing foodchain, detritus foodchain, and microbial foodwebs.  As such, an inverse size-TL relationship and omnivorous feeding were often observed.  We further showed that the violation of positive linear size-TL relationship significantly caused the size spectrum to deviate from a power-law distribution and intensified the secondary structure formation in size spectrum.  Specifically, the spectrum of empirical size distribution data fits a power-law distribution closely only when a positive linear size-TL relationship is strong.  Thus, deviations of power-law distribution in size spectrum are not simple results of sampling issues; rather, they provide useful information signaling trophic interactions in plankton foodwebs.

Keywords: grazing food chain, detritus foodchain, microbial foodweb, stable isotope, size-trophic level relationship, time series sampling