• Energy Research

The El Niño/Southern Oscillation is known to strongly impact marine ecosystems and fisheries. In particular, El Niño years are characterized, among other things, by a decrease in tuna catches in the western Pacific and an increase in the central Pacific, whereas these catches accumulate in the far western Pacific during La Niña conditions. However, the processes driving this zonal shift in the tuna catch (changing habitat conditions, currents or food availability) remain unclear. Here, we use an hindcast simulation from the mechanistic ecosystem model APECOSM that reasonably reproduces the observed zonal shift of the epipelagic community in response to ENSO to understand the mechanisms underlying this shift. Although the response of modeled epipelagic communities to El Niño is relatively similar for the different size classes studied, the processes responsible for these changes vary considerably by organism size. One of the major results of our analysis is the critical role of eastward passive transport by El Niño-related surface current anomaliesfor all size classes. While the effects of passive transport dominate the effects of growth and predation changes everywhere for large organisms, this is not the case for intermediate-sized organisms in the western Pacific, where the decrease in biomass is first explained by increased predation and then decreased foraging success. For small organisms, changes in growth rate induced by the influence of temperature on fish physiology is an important process that reinforces the biomass increase induced by passive horizontal transport in the eastern Pacific and the biomass decrease induced by increased predation by intermediate-sized organisms near the dateline. Finally, contrary to what is often assumed, our model shows that active habitat-based movements are not required to explain the westward biomass shifts that are observed during ENSO. This study illustrates the relevance of using a mechanistic ecosystem model to disentangle the role of the different processes controlling biomass changes. It highlights the essential dynamic role of ocean currents in shaping the response of marine communities to climate variability and its interaction with biological (e.g. growth) and ecological (e.g. foraging and predation) processes, whose relative importance varies with organisms’ size and contribute to modify the community structure.

The yellow clam Mesodesma mactroides is a cool-water species that typifies sandy beaches of the Southwestern Atlantic Ocean (SAO), which embraces one of the strongest ocean warming hotspots. The region is influenced by the Rio de la Plata (RdlP), which represents a zoogeographic barrier that restricts its larval exchange. We investigated yellow clam larval connectivity patterns using an individual based model (IBM). The IBM combined outputs from a 3D hydrodynamic model with a clam submodel that considered salinity- and temperature-dependent mortality for the planktonic larvae. Connectivity across the RdlP estuary occurred only for larvae released in spring during a strong La Niña event. Mortality due to freshwater precluded larval transport across the RdlP, whereas larval mortality induced by warmer waters reduced connectivity, leading to self-recruitment in most areas. Warming acceleration in this hotspot could further restrict larval connectivity between populations in the SAO, with conservation implications for this threatened species.

The Mediterranean region has been shown to be particularly exposed to climate change, with observed trends that are more pronounced than the global tendency. In forecast studies based on a RCP 8.5 scenario, there seems to be a consensus that, along with an increase in temperature and salinity over the next century, a reduction in the intensity of deep-water formation and a shallowing of the mixed layer [especially in the North-Western Mediterranean Sea (MS)] are expected. By contrast, only a few studies have investigated the effects of climate change on the biogeochemistry of the MS using a 3D physical/biogeochemical model. In this study, our aim was to explore the impact of the variations in hydrodynamic forcing induced by climate change on the biogeochemistry of the MS over the next century. For this purpose, high-resolution simulations under the RCP 8.5 emission scenario have been run using the regional climate system model CNRM-RCSM4 including the NEMO-MED8 marine component, coupled (off-line) with the biogeochemical model Eco3M-Med. The results of this scenario first highlight that most of the changes in the biogeochemistry of the MS will occur (under the RCP 8.5 scenario) after 2050. They suggest that the MS will become increasingly oligotrophic, and therefore less and less productive (14% decrease in integrated primary production in the Western Basin and in the Eastern Basin). Significant changes would also occur in the planktonic food web, with a reduction (22% in the Western Basin and 38% in the Eastern Basin) of large phytoplankton species abundance in favor of small organisms. Organisms will also be more and more N-limited in the future since NO3 concentrations are expected to decline more than those of PO4 in the surface layer. All these changes would mainly concern the Western Basin, while the Eastern Basin would be less impacted.

AbstractClimate change is anticipated to considerably reduce global marine fish biomass, driving marine ecosystems into unprecedented states with no historical analogs. The Time of Emergence (ToE) marks the pivotal moment when climate conditions (i.e., signal) deviate from pre‐industrial norms (i.e., noise). Leveraging ensemble climate‐to‐fish simulations from one Earth System Model (IPSL‐CM6A‐LR) and one Marine Ecosystem Model (APECOSM), this study examines the ToE of epipelagic, migratory and mesopelagic fish biomass alongside their main environmental drivers for two contrasted climate‐change scenarios. Globally averaged biomass signals emerge over the historical period. Epipelagic biomass decline emerged earlier (1950) than mesozooplankton decline (2017) due to a stronger signal in the early 20th century, possibly related to trophic amplification induced by an early emerging surface warming (1915). Trophic amplification is delayed for mesopelagic biomass due to postponed warming in the mesopelagic zone, resulting in a later emergence (2017). ToE also displays strong size class dependence, with epipelagic medium sizes (20 cm) experiencing delayed emergence compared to the largest (1 m) and smallest (1 cm) categories. For the epipelagic and mesopelagic communities, the regional signal emergence lags behind the global average, with median ToE estimates of 2030 and 2034, respectively. This is due to stronger noise in regional time‐series than in global averages. The regional ToEs are also spatially heterogeneous, driven predominantly by the signal pattern akin to mesozooplankton. Additionally, our findings underscore that mitigation efforts (i.e., transitioning from SSP5‐8.5 to SSP1‐2.6 scenario) can potentially curtail emerging ocean surface signals by 30%.

Ensemble simulations of the ecosystem model Apecosm (https://apecosm.org) forced by the IPSL-CM6-LR climate model with the climate change scenario SSP5-8.5. The output files contain yearly mean biomass density for 3 communities (epipelagic, mesopelagic migratory and mesopelagic redidents) and 100 size classes (ranging from 0.12cm to 1.96m) The model grid file is also provided. Units are in J/m2 and can be converted in kg/m2 by dividing by 4e6. These outputs are associated with the "Assessing the time of emergence of marine ecosystems from global to local scales using IPSL-CM6A-LR/APECOSM climate-to-fish ensemble simulations" paper from the Earth's Future "Past and Future of Marine Ecosystems" Special Collection.

While the physical dimensions of climate change are now routinely assessed through multimodel intercomparisons, projected impacts on the global ocean ecosystem generally rely on individual models with a specific set of assumptions. To address these single-model limitations, we present standardized ensemble projections from six global marine ecosystem models forced with two Earth system models and four emission scenarios with and without fishing. We derive average biomass trends and associated uncertainties across the marine food web. Without fishing, mean global animal biomass decreased by 5% (±4% SD) under low emissions and 17% (±11% SD) under high emissions by 2100, with an average 5% decline for every 1 °C of warming. Projected biomass declines were primarily driven by increasing temperature and decreasing primary production, and were more pronounced at higher trophic levels, a process known as trophic amplification. Fishing did not substantially alter the effects of climate change. Considerable regional variation featured strong biomass increases at high latitudes and decreases at middle to low latitudes, with good model agreement on the direction of change but variable magnitude. Uncertainties due to variations in marine ecosystem and Earth system models were similar. Ensemble projections performed well compared with empirical data, emphasizing the benefits of multimodel inference to project future outcomes. Our results indicate that global ocean animal biomass consistently declines with climate change, and that these impacts are amplified at higher trophic levels. Next steps for model development include dynamic scenarios of fishing, cumulative human impacts, and the effects of management measures on future ocean biomass trends.