Trophic amplification: A model intercomparison of climate driven changes in marine food webs

2.3 Impact of warming on the biomass response to NPP change We defined a metric, Ri, to quantitatively measure trophic amplification. Ri is calculated by dividing the relative change in total consumer biomass by the relative change in NPP in each cell i between the 2090–2099 decade and the reference period: Ri ¼ Total consumer biomass changei Net Primary Production changei A value greater than 1 indicates an amplification of NPP changes; a value between 0 and 1 describes an attenuation of NPP changes and a negative value indicates an inversion (Fig 2). Since several studies have suggested that trophic amplification arises directly or indirectly from temperature-dependent ecological processes [3, 15, 25, 27], we investigated the impact of SST on the metric Ri. The objective was to test the hypothesis that a greater increase in SST would result in a greater trophic amplification and thus a greater Ri. For each MEM and ESM, the ratio R is represented as a function of the SST increase relative to the reference period 1995–2014. Then, the Ri are averaged across all cells characterised by the same SST increase, regardless of the year. Because ecological processes at play could differ when NPP is expected Table 1. Functioning summary of the 9 MEMs used in the study. For more details on MEMs’ taxonomic scope, key features and drivers see S1 Table (modified from Heneghan et al., 2021; Lotze et al., 2019; Tittensor et al., 2021). MEMs model Brief model description Taxonomic scope APECOSM [36] A 3-D dynamic energy budget based Eulerian model of size-structured

marine populations and communities based on environmentally driven individual bio-energetics, trophic interactions, and behaviours that are upscaled to populations and communities. Generic size-based communities are explicit (typically epipelagic, migratory, mesopelagic, and bathypelagic) and focus species. BOATS [37] Sized-structured model that combines size-based ecological theory and metabolic constraints to estimate the production of fish biomass. This model resolves three size spectra for three separate species size groups. All commercial species represented by three groups, defined in terms of the asymptotic mass. DBEM [38] DBEM defines a bioclimatic envelope (niche) for each species and simulates changes in growth, reproduction and population dynamics including abundance and carrying capacity under a varying environment. Fish and invertebrate species (primarily commercial). Dynamic Benthic–Pelagic Model (DBPM) [39] A functional trait-based size-spectrum model that joins a pelagic predator size-spectrum with a benthic detritivore size spectrum; can include functional groups that do not feed according to size and unstructured resources. Broadly represents “pelagic” fish predators and “benthic” invertebrates but can include herbivorous fish; flexible functional groups. EcoOcean [40] EcoOcean is a global food web model based on species interactions and transfer of energy across trophic levels (EwE framework). It is designed to evaluate the impact of climate change and human pressure on mobile marine ecosystems. All trophic levels and taxonomic groups included as

biomass pools (51 groups). EcoTroph [27, 41, 42] Global scale model representing the biomass flows from primary producers to top predators. Taking in account of metabolism to compute biomass by trophic level and can evaluate climate change impacts Species are not resolved, only trophic level classes. FishErIes Size and functional TYpe model (FEISTY) [43] FEISTY is a spatially explicit, size- and trait-based model of higher trophic level dynamics based on first principles. It describes the three main commercially harvested fish functional types: forage fish, large pelagic fish, and demersal fish. Forage, large pelagic and demersal fish, as well as benthic invertebrates, between 1 mg and 125 kg. Macroecological [44] A static model size-structure model, which uses minimal inputs together with ecological and metabolic scaling theory to predict mean size composition and abundance of animals (including fish) 180 body mass classes, Species are not resolved, only body mass classes. Zooplankton Model of Size Spectra (ZooMSS) [45] ZooMSS is a size- and trait-based model working at the functional group scale. It uses the functional size-spectrum framework to resolve the body size ranges, size-based feeding characteristics and carbon content of the nine most abundant zooplankton groups and three (small, medium and large) fish groups. Nine zooplankton groups, from flagellates to jellyfish and all marine animals between 1 mg and 10 tonnes. https://doi.org/10.1371/journal.pone.0287570.t001 PLOS ONE Trophic amplification revealed

PLOS ONE | https://doi.org/10.1371/journal.pone.0287570 August 23, 2023 5 / 23