Seasonal changes of size spectra of the Benguela offshore mesopelagic ecosystem compartment in relation to primary production

Abstract

Seasonal differences in marine size spectra of micronekton at the shelf-ocean interface of the northern (NBUS) and southern Benguela upwelling system (SBUS) in Feb-Mar 2019 and Sep-Oct 2021 were analysed for mesopelagic fishes and total micronekton, the latter also including invertebrates. A resource dependent population model based on the metabolic theory of ecology (MTE) containing resource and temperature terms and a term representing a transfer function was applied to test three different types of size spectra slope estimates. The model fitted best with linear slopes calculated of log-binned averaged community biomass (LBNbiom method), while maximum likelihood and quantile regression estimates proved less effective. The best model for total micronekton contained significant effects both for resource term and transfer function, but not for temperature, and was 3.6 times more effective explaining the data than a non-MTE model. Normalized biomass size spectra (NBSS) slopes of the total micronekton were in the theoretical range between -0.80 and -1.37, where the near-equilibrium slope of -0.80 was obtained for the SBUS under oligotrophic conditions in 2021. Seasonally, NBSS slopes were steeper in the NBUS than in the SBUS. The slopes for the fishes' subcomponents ranged from -0.23 to -0.92, where values > -0.75 fall outside the theoretical range, suggesting that selecting taxonomic subsets for size spectrum analysis is problematic. The importance of the productivity regime shaping the biomass spectrum directly through the resource level and indirectly through the transfer function is highlighted. For mesopelagic fishes, generation time and fecundity are applied to explain slopes > -0.75.