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The development of fisheries in the oceans, and other human drivers such as climate warming, have led to changes in species abundance, assemblages, trophic interactions, and ultimately in the functioning of marine food webs. Here, using a trophodynamic approach and global databases of catches and life history traits of marine species, we tested the hypothesis that anthropogenic ecological impacts may have led to changes in the global parameters defining the transfers of biomass within the food web. First, we developed two indicators to assess such changes: the Time Cumulated Indicator (TCI) measuring the residence time of biomass within the food web, and the Efficiency Cumulated Indicator (ECI) quantifying the fraction of secondary production reaching the top of the trophic chain. Then, we assessed, at the large marine ecosystem scale, the worldwide change of these two indicators over the 1950–2010 time-periods. Global trends were identified and cluster analyses were used to characterize the variability of trends between ecosystems. Results showed that the most common pattern over the study period is a global decrease in TCI, while the ECI indicator tends to increase. Thus, changes in species assemblages would induce faster and apparently more efficient biomass transfers in marine food webs. Results also suggested that the main driver of change over that period had been the large increase in fishing pressure. The largest changes occurred in ecosystems where ‘fishing down the marine food web’ are most intensive. [ABSTRACT FROM AUTHOR]

• A bottom temperature product for the northeast U.S. continental shelf. • Long-term high-resolution bottom temperature between 1959 and 2021. • Three ocean products were combined. • Warming of the northeast U.S. shelf is estimated at + 0.36 °C decade-1. • Large warming variations among seasons and regions. • Potential use for a wide range of applications from fisheries to marine ecology. The northeast U.S. continental shelf is a highly productive and socio-economically important marine ecosystem in which annual and seasonal variations of bottom temperature play a major role in the distribution, phenology, and productivity of its predominately demersal marine taxa. However, bottom temperature measurements are limited spatially and temporally and thus do not provide the required resolution to assess sub-seasonal variability and trends. Here we combined three ocean products, a regional ocean model (ROMS) and two global ocean data assimilated models (GLORYS12v1 and PSY4V3R1) to build a high-resolution, long-term bottom temperature product for the northeast U.S. continental shelf between 1959 and 2021. We bias-corrected ROMS using monthly decadal climatologies from ocean observations and analyzed long-term changes in the combined time series. Model skill was assessed using a large number of in situ observations. The combined bottom temperature product showed a long-term warming of the northeast U.S. continental shelf of + 0.36 °C decade-1 over the past 63 years, with notable variations among seasons and regions. The strongest long-term warming occurred during the summer months and in the Gulf of Maine. Although biases were observed, the bottom temperature product exhibited good performance reproducing seasonal and annual variability in observed temperature. This high-resolution product could be used in a wide range of applications from local to regional spatial scales, from long-term to near-term time scales, and from fisheries to marine ecology. [ABSTRACT FROM AUTHOR]

Transfer efficiency is the proportion of energy passed between nodes in food webs. It is an emergent, unitless property that is difficult to measure, and responds dynamically to environmental and ecosystem changes. Because the consequences of changes in transfer efficiency compound through ecosystems, slight variations can have large effects on food availability for top predators. Here, we review the processes controlling transfer efficiency, approaches to estimate it, and known variations across ocean biomes. Both process-level analysis and observed macroscale variations suggest that ecosystem-scale transfer efficiency is highly variable, impacted by fishing, and will decline with climate change. It is important that we more fully resolve the processes controlling transfer efficiency in models to effectively anticipate changes in marine ecosystems and fisheries resources. Transfer efficiency is a key parameter describing ecosystem structure and function and is used to estimate fisheries production; however, it is also one of the most uncertain parameters. Questions remain about how habitats, food resources, fishing pressure, spatiotemporal scales, as well as temperature, primary production, and other climate drivers impact transfer efficiency. Direct measurements of transfer efficiency are difficult, but observations of marine population abundances, diets, productivity, stable isotope analysis, and models integrating these constraints can provide transfer efficiency estimates. Recent estimates suggest that transfer efficiency is more variable than previously thought, compounding uncertainties in marine ecosystem predictions and projections. Increased understanding of factors contributing to variation in transfer efficiency will improve projections of fishing and climate change impacts on marine ecosystems. [ABSTRACT FROM AUTHOR]

• Experimental study identifying uncertainty sources in FishMIP global model ensemble. • Warming and lower trophic level (LTL) impacts on model predictions isolated. • Coupling of lower and higher trophic levels a key driver of model warming response. • LTL impacts driven primarily by each model's choice of LTL driver. • Overall climate projections mostly a linear combination of warming and LTL impacts. Climate change is warming the ocean and impacting lower trophic level (LTL) organisms. Marine ecosystem models can provide estimates of how these changes will propagate to larger animals and impact societal services such as fisheries, but at present these estimates vary widely. A better understanding of what drives this inter-model variation will improve our ability to project fisheries and other ecosystem services into the future, while also helping to identify uncertainties in process understanding. Here, we explore the mechanisms that underlie the diversity of responses to changes in temperature and LTLs in eight global marine ecosystem models from the Fisheries and Marine Ecosystem Model Intercomparison Project (FishMIP). Temperature and LTL impacts on total consumer biomass and ecosystem structure (defined as the relative change of small and large organism biomass) were isolated using a comparative experimental protocol. Total model biomass varied between −35% to +3% in response to warming, and -17% to +15% in response to LTL changes. There was little consensus about the spatial redistribution of biomass or changes in the balance between small and large organisms (ecosystem structure) in response to warming, an LTL impacts on total consumer biomass varied depending on the choice of LTL forcing terms. Overall, climate change impacts on consumer biomass and ecosystem structure are well approximated by the sum of temperature and LTL impacts, indicating an absence of nonlinear interaction between the models' drivers. Our results highlight a lack of theoretical clarity about how to represent fundamental ecological mechanisms, most importantly how temperature impacts scale from individual to ecosystem level, and the need to better understand the two-way coupling between LTL organisms and consumers. We finish by identifying future research needs to strengthen global marine ecosystem modelling and improve projections of climate change impacts. [ABSTRACT FROM AUTHOR]