Standing biomass response to joint benthic-pelagic energy niche in a reef fish community from an upwelling system

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Coastal upwelling systems are marked by strong thermal variability and high food-resource availability, yet whether enhanced pelagic net primary productivity (NPP) directly increases reef fish biomass at the community level remains uncertain. We tested two alternative hypotheses: H1 - reef fish biomass is directly and positively driven by pelagic NPP derived from coastal upwelling; H2 - biomass is only indirectly coupled with pelagic NPP, being sustained predominantly by alternative benthic energy pathways. Using a Hierarchical Modeling of Species Communities (HMSC) framework, we analyzed the biomass of 72 reef fish species over five years (2017–2021) in the Arraial do Cabo upwelling system, Southwestern Atlantic. Fish biomass was related to satellite-derived sea surface temperature (SST), surface chlorophyll-a concentration (SCC), and NPP estimated from models based on the MODIS sensor: the empirical VGPM and the mechanistic CAFE model. Functional traits, phylogenetic relationships, and spatial and temporal random effects were also incorporated. Model fit was adequate, with moderate explanatory power (R² = 0.45), but low predictive power (R² = 0.20), indicating strong stochasticity. Among fixed effects, NPP_CAFE explained the largest share of biomass variation (24.2%), followed by SST (11.7%), NPP_VGPM (9.7%), and SCC (7.1%). However, species responses showed a predominantly negative association between biomass and NPP_CAFE, contradicting the expected positive bottom-up relationship. Spatial structure, particularly differences between sheltered and exposed areas, and species depth range were more important than pelagic productivity pulses in shaping biomass. These findings suggest stronger reliance on benthic macroalgal and detrital pathways than directly on pelagic subsidies. Phylogenetic signal was moderate (ρ = 0.34), suggesting evolutionary constraint on biomass responses. Overall, reef fish biomass in this upwelling system was not primarily regulated by directly pelagic forcing, refuting H1 and supporting H2. Future assessments should integrate benthic productivity metrics to better predict climate-driven changes in upwelling reefs.

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