On the use of biomass size spectra linear adjustments to design ecosystem indicators

Biomass size spectra describe the structure of aquatic communities ataxonomically. The slope (b) of the normalized biomass size spectrum (NBSS) is often used as an indicator of the impact of perturbations, such as pollution or overfishing. The NBSS intercept (a), has generally been ignored on the basis of a correlation between the NBSS slope and intercept, although this correlation has not been shown to be universal. We assessed whether the NBSS parameters are correlated using: (i) theoretical analysis, (ii) virtual communities randomly generated based only on statistical considerations, and (iii) virtual food webs changing over time following a dynamic bioenergetic model. We also analyzed whether the parameters of the Pareto distribution are correlated or not, using approaches (i) and (ii). We found that when communities change over time there is no single relationship between the two NBSS parameters, due to a dependence on the variation in total community abundance (N). We conclude that to characterize any aquatic system at least two parameters are necessary from the NBSS triad N, a, b. In the case of the Pareto distribution, both N-Pareto and b(Pareto) are necessary.