Chlorophyll underpins ocean productivity yet simulating chlorophyll across biomes, seasons and depths remains challenging for earth system models. Inconsistencies are often attributed to misrepresentation of the myriad growth and loss processes governing phytoplankton biomass but may also arise from unresolved or misspecified photoacclimation or photoadaptation responses. A series of global ocean ecosystem simulations were enlisted to assess the impacts of alternative photoacclimation and photoadaptation assumptions on simulated chlorophyll, primary productivity and carbon export. Photoacclimation alternatives implicitly modulated the premium placed on light harvesting versus photodamage avoidance and other cellular functions, while photoadaptation experiments probed the impact of adding low- and high-light adapted phytoplankton ecotypes. Alternatives generated large chlorophyll responses that addressed prior model biases in ways that simple changes in growth and grazing could not. Simulations with photoadaptation, surface-skewed photoacclimation in deep mixed layers, and acclimation to light levels over mixing depths consistent with photoacclimation time scales in stratified waters were best able to match observed patterns. While chlorophyll was highly sensitive to alternative photoacclimation assumptions, primary production and carbon export were not because chlorophyll changes under near-saturating light at the ocean's surface yielded only modest phytoplankton growth changes that were counteracted by self-shading at depth. Improved photoacclimation and photoadaption constraints and reduced regional uncertainties in satellite-based ocean color estimates are needed to reduce ambiguities in the drivers of chlorophyll change and their biogeochemical implications.

Plankton influences biogeochemical and ecosystem processes, such as sequestration of atmospheric CO2, carbon export to the ocean floor, and the productivity of higher trophic levels. Body size is a proxy for many plankton functional traits, and one means of analyzing its community structure is through the distribution of biovolume across size classes (the size spectrum). To understand how climate forcing can affect plankton communities, we assessed the size spectra in the historical simulations of seven Earth System Models (ESMs) included in the 6th Coupled Model Intercomparison Project (CMIP6) and analyzed projected changes under a high emissions scenario (SSP5-8.5). We compared the historical estimates with the Pelagic Size Structure database (PSSdb), a novel size structure dataset from imaging systems. The median slope from models ranged from -1.66 to -1.07, with shallower slopes from this range falling near both the theoretical expectation and PSSdb observations (-1.05), with variations around the median representing differences in the total biovolume distribution across plankton functional groups. Consistent with the observations, most ESMs show steeper slopes and lower biovolume in oligotrophic subtropical gyres compared to productive ocean regions. There was a lack of agreement between models and observations in the size spectra seasonal cycle, possibly stemming from missing model processes and incomplete sampling. Despite these caveats, the size spectra from ESMs presented here, and their evaluation with PSSdb, provides insights on how climate change will affect ecological processes in the plankton, and highlights areas of improvement in model development and imaging data coverage.

Pelagic tunicates (salps, pyrosomes) and fishes generate jelly-falls and/or fecal pellets that sink roughly 10 times faster than bulk oceanic detritus, but their impacts on biogeochemical cycles in the ocean interior are poorly understood. Using a coupled physical-biogeochemical model, we find that fast-sinking detritus decreased global net primary production and surface export, but increased deep sequestration and transfer efficiency in much of the extratropics and upwelling zones. Fast-sinking detritus generally decreased total suboxic and hypoxic volumes, reducing a “large oxygen minimum zone (OMZ)” bias common in global biogeochemical models. Newly aerobic regions at OMZ edges exhibited reduced transfer efficiencies in contrast with global tendencies. Reductions in water column denitrification resulting from improved OMZs improved simulated nitrate deficits relative to phosphate. The carbon flux to the benthos increased by 11% with fast-sinking detritus from fishes and pelagic tunicates, yet simulated benthic fluxes remained on the lower end of observation-based estimates.

A document by Daniël van Denderen. Click on the document to view its contents.

There is an urgent need for models that can robustly detect past and project future ecosystem changes and risks to the services that they provide to people. The Fisheries and Marine Ecosystem Model Intercomparison Project (FishMIP) was established to develop model ensembles for projecting long-term impacts of climate change on fisheries and marine ecosystems while informing policy at spatio-temporal scales relevant to the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) framework. While contributing FishMIP models have improved over time, large uncertainties in projections remain, particularly in coastal and shelf seas where most of the world’s fisheries occur. Furthermore, previous FishMIP climate impact projections have mostly ignored fishing activity due to a lack of standardized historical and scenario-based human activity forcing and uneven capabilities to dynamically model fisheries across the FishMIP community. This, in addition to underrepresentation of coastal processes, has limited the ability to evaluate the FishMIP ensemble’s ability to adequately capture past states - a crucial step for building confidence in future projections. To address these issues, we have developed two parallel simulation experiments (FishMIP 2.0) on: 1) model evaluation and detection of past changes and 2) future scenarios and projections. Key advances include historical climate forcing, that captures oceanographic features not previously resolved, and standardized fishing forcing to systematically test fishing effects across models. FishMIP 2.0 is a key step towards a detection and attribution framework for marine ecosystem change at regional and global scales, and towards enhanced policy relevance through increased confidence in future ensemble projections.

Phytoplankton stoichiometry modulates the interaction between carbon, nitrogen and phosphorus cycles, yet most biogeochemical models represent phytoplankton C:N:P as constants. This simplification has been linked to Earth System Model (ESM) biases and potential misrepresentation of biogeochemical responses to climate change. Here we integrate key elements of the Adaptive Trait Optimization Model (ATOM) for phytoplankton stoichiometry with the Carbon, Ocean Biogeochemistry and Lower Trophics (COBALT) ocean biogeochemical model. Within a series of global ocean-ice-ecosystem retrospective simulations, ATOM-COBALT reproduced observations of particulate organic matter N:P, and compared to static N:P, exhibited reduced phytoplankton P-limitation, enhanced N-fixation, and increased low-latitude export, leading to improved consistency with observations. Two mechanisms together drove these patterns: the growth hypothesis and frugal P-utilization during scarcity. The addition of translation compensation- differential temperature dependencies of photosynthetic relative to biosynthetic processes- led to relatively modest strengthening of N:P variations and biogeochemical responses relative to growth-plus-frugality. Comparison of the multi-mechanism model herein against frugality-only models suggest that both can capture observed N:P patterns and produce qualitatively similar biogeochemical effects. There are, however, quantitative response differences and different responses across N:P mechanisms are expected under climate change- with the growth rate mechanism adding a distinct biogeochemical footprint in highly-productive low-latitude regions. These results suggest that variable phytoplankton N:P makes some biogeochemical processes resilient to environmental changes, and support using dynamic N:P formulations with the ocean biogeochemical component of next generation of ESMs.

Fish community biomass is generally thought to decline with increasing temperature due to higher metabolic losses resulting in less efficient energy transfer in warm-water food webs. However, whether these metabolic predictions explain observed macroecological patterns in fish community biomass is virtually unknown. Here we test these predictions by examining the variation in demersal fish biomass across 21 productive shelf regions using high-resolution monitoring data from the North Atlantic and Northeast Pacific. We find that biomass per km2 varies 40-fold across regions and is highest in cold waters and areas with low fishing exploitation. We find no evidence that temperature change has impacted biomass within marine regions over time. Yet, the cross-regional patterns suggest that long-term impacts of warming will be negative on biomass. These results provide an empirical basis for predicting future changes in fish community biomass and its associated services for human wellbeing i.e., food provisioning, under global warming.