Climate-induced decrease in biomass flow in marine food webs may severely affect predators and ecosystem production

Climate change and overfishing combine to drive the population decline of the Indo-Pacific humpback dolphins in the Pearl River Estuary from the Northern South China Sea

Overfishing and climate change pose significant threats to cetacean populations, yet the specific impacts on individual species, particularly cetaceans inhabiting complex coastal areas, are not well understood due to limited data. This study utilizes five years of field survey data, alongside fishery activity and climate change scenarios, to assess the population dynamics of the Indo-Pacific humpback dolphin in the Pearl River Estuary from the Northern South China Sea. Our results indicate a dramatic and ongoing decline in the humpback dolphin population over recent decades, decreasing from over 1000 to 742 individuals. The individual impact of climate change on biomass is moderate, showing changes between -1.3 % and + 11.97 %. Projected climate change scenarios reveal further population reductions, exacerbated by increasing fishing pressures, with declines ranging from 6.17 % to 20.39 %. Notably, our simulations highlight the detrimental effects of unrestrained socioeconomic development on humpback dolphins' viability and population. The dolphins exhibit adaptive dietary strategies to maintain energy levels in changing ecosystems; however, total energy intake still declines across all age classes, requiring increased foraging efforts. This may lead to decreased group sizes, altered distribution patterns, and reduced reproductive success, further increasing their vulnerability to additional stressors. The complex interplay between human activities and environmental changes in marine ecosystems, which significantly impacts cetaceans, provides crucial insights for developing integrated management strategies to safeguard the biodiversity and resilience of coastal marine ecosystems.

Chlorophyll dynamics in low latitudes unveils a new partition of the ocean

Understanding ecosystem services and assessing the impacts of climate change and human activities on the global environment require accurate partitioning of the biosphere. Unlike terrestrial biomes, which tend to have relatively clear boundaries, ocean ecosystems-accounting for over 95 % of the biosphere's volume-have indistinct boundaries and can be divided in multiple ways with varying degrees of fragmentation. Satellite observations of surface chlorophyll a concentration (Chl) have greatly enhanced our understanding of the oceanic environment. In this study, we propose a new, straightforward partition of the global ocean between 40° N and 40° S (~68 % of the ocean's surface, covering nearly half of the Earth's surface) based on seasonal Chl dynamics and the relative amplitude of Chl peaks. This partitioning method differs from existing schemes and identifies ecoregions with high Chl peaks (22 % of the area), low peaks (55 %), and no peaks (22 %). Chl peak amplitude is mainly driven by the seasonal deepening of the mixed layer and the associated nutrient enrichment. This process explains the Chl dynamics in over 96 % of the low-latitude ocean, while the remaining 4 % is influenced by local factors such as monsoons, upwelling, and large river plumes. Overall, low-latitude regions are predominantly characterized by a seasonal decoupling of production and consumption. Deep-sea ecosystems are likely shaped by both this pulsatile nature of surface productivity and the overall downward carbon flux, which is consistent with existing partitions of deep-sea pelagic and benthic fauna.

Potential Spatial Mismatches Between Marine Predators and Their Prey in the Southern Hemisphere in Response to Climate Change

Global change is rapidly reshaping species' habitat suitability ranges, hence leading to significant shifts in the distribution of marine life. Contrasting distributional responses among species can alter the spatial overlap between predators and prey, potentially disrupting trophic interactions and affecting food web dynamics. Here, we evaluate long-term changes in the spatial overlap of habitat suitability ranges for trophically related species, including crustaceans, fish, penguins, and pinnipeds across 12 Large Marine Ecosystems from the Southern Hemisphere, merged into three primary regions: South America, Southern Africa, Australia and New Zealand. To this aim, we first use Boosted Regression Trees (BRTs) to hindcast and project species-specific changes in suitable habitat from 1850 to 2100 under two future climate scenarios: SSP1-2.6 (low climate forcing) and SSP5-8.5 (high climate forcing). We then analyze changes in species habitat suitability and potential predator-prey spatial overlaps. Findings reveal that marine species generally exhibit changes in their suitable habitats, with pronounced shifts towards higher latitudes under the SSP5-8.5 scenario. However, contrasting trends emerge among predators across functional groups and regions of South America, Southern Africa, Australia and New Zealand. These variations highlight the need for species and regional-specific management responses. We also project contrasting spatial mismatches between predators and prey: predators experiencing declines in suitable habitat tend to exhibit greater overlap with their prey in future scenarios, whereas those with expanding suitable habitat show reduced spatial overlap with their prey. This study provides valuable insights that can inform spatial management strategies in response to climate change and illustrate how climate change may weaken species' ability to adapt to climate-driven environmental changes due to trophic disruptions.

Temperature-dependent responses and trophic interaction strengths of a predatory marine gastropod and rock oyster under ocean warming

Predator-prey interactions are important in shaping ecosystem structure. Consequently, impacts of accelerating global warming on predators will have notable implications. Effects are likely to be particularly marked for tropical organisms which are anticipated to be sensitive to further thermal stress. Here, we investigated effects of future ocean warming on the predatory dogwhelk Reishia clavigera and its predation of Saccostrea cucullata. Mortality of the predators rapidly increased under the extreme elevated temperature, while those exposed to moderate elevated temperature displayed similar mortality as the ambient. Predators that survived moderate temperature increases altered their oxygen consumption patterns, increased average feeding rates, and functional responses, although condition index and energy reserves were unchanged. Overall, we show extreme ocean warming scenarios can remove predators and their consumption of prey from an ecosystem, whereas moderate warming can intensify predator-prey interactions. Such temperature-dependent alterations to predator-prey interactions would lead to fundamental changes of ecosystem structure as the ocean warms.

Large potential impacts of marine heatwaves on ecosystem functioning

Ocean warming is driving significant changes in the structure and functioning of marine ecosystems, shifting species' biogeography and phenology, changing body size and biomass and altering the trophodynamics of the system. Particularly, extreme temperature events such as marine heatwaves (MHWs) have been increasing in intensity, duration and frequency. MHWs are causing large-scale impacts on marine ecosystems, such as coral bleaching, mass mortality of seagrass meadows and declines in fish stocks and other marine organisms in recent decades. In this study, we developed and applied a dynamic version of the EcoTroph trophodynamic modelling approach to study the cascading effects of individual MHW on marine ecosystem functioning. We simulated theoretical user-controlled ecosystems and explored the consequences of various assumptions of marine species mortality along the food web, associated with different MHW intensities. We show that an MHW can lead to a significant biomass reduction of all consumers, with the severity of the declines being dependent on species trophic levels (TLs) and biomes, in addition to the characteristics of MHWs. Biomass of higher TLs declines more than lower TLs under an MHW, leading to changes in ecosystem structure. While tropical ecosystems are projected to be sensitive to low-intensity MHWs, polar and temperate ecosystems are expected to be impacted by more intense MHWs. The estimated time to recover from MHW impacts is twice as long for polar ecosystems and one-third longer for temperate biomes compared with tropical biomes. This study highlights the importance of considering extreme weather events in assessing the effects of climate change on the structures and functions of marine ecosystems.

Integrative biological analyses of responses to food deprivation reveal resilience mechanisms in sea urchin larvae

A fundamental question in ecology is how organisms survive food deprivation. In the ocean, climate change is impacting the phenology of food availability for early life-history stages of animals. In this study, we undertook an integrative analysis of larvae of the sea urchin Strongylocentrotus purpuratus-an important keystone species in marine ecology and a molecular biological model organism in developmental biology. Specifically, to identify the mechanisms of resilience that maintain physiological state and the ability of organisms to recover from food deprivation, a suite of molecular biological, biochemical, physiological and whole organism measurements was completed. Previous studies focused on the importance of energy reserves to sustain larvae during periods of food deprivation. We show, however, that utilization of endogenous energy reserves only supplied 15% of the metabolic requirements of long-term survival (up to 22 days) in the absence of particulate food. This large energy gap was not supplied by larvae feeding on bacteria. Estimates of larval ability to transport dissolved organic matter directly from seawater showed that such substrates could fully supply metabolic needs. Integrative approaches allowed for filtering of gene expression signatures, linked with gene network analyses and measured biochemical and physiological traits, to identify biomarkers of resilience. We identified 14 biomarkers related to nutrition-responsive gene expression, of which a specific putative amino acid transporter gene was quantified in a single larva experiencing continuous nutritional stress. Advances in applications of gene expression technologies offer novel approaches to determine the physiological state of marine larval forms in ecological settings undergoing environmental change.

Warming and pollution interact to alter energy transfer efficiency, performance and fitness across generations in zebrafish (Danio rerio)

Energy transfer efficiency across different trophic levels, from food to new biomass, can determine population dynamics and food-web function. Here we show that the energy needed to produce a unit of new biomass increases with warming and exposure to bisphenol A (BPA), an endocrine disrupting compound. These environmental effects are at least partially transmitted across generations via DNA methylation. We raised parental (F0) and their offspring (F1) zebrafish (Danio rerio) of two genotypes (DNA methyltransferase 3a knock-out [DNMT3a-/-] and wild type [DNMT3a+/+]) at different temperatures (24 and 30 °C), with and without BPA (0 and 10 μg l-1) to test whether the effects of BPA are i) temperature specific, ii) mediated by DNA methylation, and iii) transmitted across generations even if offspring are not exposed. All experimental factors interacted to influence growth in length and mass, and metabolic rates with the result that wild-type F0 and F1 fish experienced the greatest energetic cost of growth under warm conditions in the presence of BPA. However, this response was not observed in DNMT3a-/- fish, indicating that DNA methylation is at least partly responsible for mediating these effects. Under the same conditions (warm + BPA) wild-type parents had reduced swimming performance, and reduced fecundity, and offspring embryonic survival was reduced significantly; genotype affected these responses significantly. Our results indicate that the conditions that are becoming increasingly common globally - warming and endocrine disrupting compounds from plastic pollution and production - can have detrimental effects on energy transfer efficiency and thereby potentially on food-web structure. These effects can be transmitted across generations even if offspring are not exposed to the pollutant, and are likely to have ramifications for conservation and fisheries.

Steeper size spectra with decreasing phytoplankton biomass indicate strong trophic amplification and future fish declines

Under climate change, model ensembles suggest that declines in phytoplankton biomass amplify into greater reductions at higher trophic levels, with serious implications for fisheries and carbon storage. However, the extent and mechanisms of this trophic amplification vary greatly among models, and validation is problematic. In situ size spectra offer a novel alternative, comparing biomass of small and larger organisms to quantify the net efficiency of energy transfer through natural food webs that are already challenged with multiple climate change stressors. Our global compilation of pelagic size spectrum slopes supports trophic amplification empirically, independently from model simulations. Thus, even a modest (16%) decline in phytoplankton this century would magnify into a 38% decline in supportable biomass of fish within the intensively-fished mid-latitude ocean. We also show that this amplification stems not from thermal controls on consumers, but mainly from temperature or nutrient controls that structure the phytoplankton baseline of the food web. The lack of evidence for direct thermal effects on size structure contrasts with most current thinking, based often on more acute stress experiments or shorter-timescale responses. Our synthesis of size spectra integrates these short-term dynamics, revealing the net efficiency of food webs acclimating and adapting to climatic stressors.

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

Marine animal biomass is expected to decrease in the 21st century due to climate driven changes in ocean environmental conditions. Previous studies suggest that the magnitude of the decline in primary production on apex predators could be amplified through the trophodynamics of marine food webs, leading to larger decreases in the biomass of predators relative to the decrease in primary production, a mechanism called trophic amplification. We compared relative changes in producer and consumer biomass or production in the global ocean to assess the extent of trophic amplification. We used simulations from nine marine ecosystem models (MEMs) from the Fisheries and Marine Ecosystem Models Intercomparison Project forced by two Earth System Models under the high greenhouse gas emissions Shared Socioeconomic Pathways (SSP5-8.5) and a scenario of no fishing. Globally, total consumer biomass is projected to decrease by 16.7 ± 9.5% more than net primary production (NPP) by 2090-2099 relative to 1995-2014, with substantial variations among MEMs and regions. Total consumer biomass is projected to decrease almost everywhere in the ocean (80% of the world's oceans) in the model ensemble. In 40% of the world's oceans, consumer biomass was projected to decrease more than NPP. Additionally, in another 36% of the world's oceans consumer biomass is expected to decrease even as projected NPP increases. By analysing the biomass response within food webs in available MEMs, we found that model parameters and structures contributed to more complex responses than a consistent amplification of climate impacts of higher trophic levels. Our study provides additional insights into the ecological mechanisms that will impact marine ecosystems, thereby informing model and scenario development.

How can physiology best contribute to wildlife conservation in a warming world?

Global warming is now predicted to exceed 1.5°C by 2033 and 2°C by the end of the 21st century. This level of warming and the associated environmental variability are already increasing pressure on natural and human systems. Here we emphasize the role of physiology in the light of the latest assessment of climate warming by the Intergovernmental Panel on Climate Change. We describe how physiology can contribute to contemporary conservation programmes. We focus on thermal responses of animals, but we acknowledge that the impacts of climate change are much broader phylogenetically and environmentally. A physiological contribution would encompass environmental monitoring, coupled with measuring individual sensitivities to temperature change and upscaling these to ecosystem level. The latest version of the widely accepted Conservation Standards designed by the Conservation Measures Partnership includes several explicit climate change considerations. We argue that physiology has a unique role to play in addressing these considerations. Moreover, physiology can be incorporated by institutions and organizations that range from international bodies to national governments and to local communities, and in doing so, it brings a mechanistic approach to conservation and the management of biological resources.

Marine protected areas, marine heatwaves, and the resilience of nearshore fish communities

Anthropogenic stressors from climate change can affect individual species, community structure, and ecosystem function. Marine heatwaves (MHWs) are intense thermal anomalies where water temperature is significantly elevated for five or more days. Climate projections suggest an increase in the frequency and severity of MHWs in the coming decades. While there is evidence that marine protected areas (MPAs) may be able to buffer individual species from climate impacts, there is not sufficient evidence to support the idea that MPAs can mitigate large-scale changes in marine communities in response to MHWs. California experienced an intense MHW and subsequent El Niño Southern Oscillation event from 2014 to 2016. We sought to examine changes in rocky reef fish communities at four MPAs and associated reference sites in relation to the MHW. We observed a decline in taxonomic diversity and a profound shift in trophic diversity inside and outside MPAs following the MHW. However, MPAs seemed to dampen the loss of trophic diversity and in the four years following the MHW, taxonomic diversity recovered 75% faster in the MPAs compared to reference sites. Our results suggest that MPAs may contribute to long-term resilience of nearshore fish communities through both resistance to change and recovery from warming events.

Four decades of climatic fluctuations and fish recruitment stability across a marine-freshwater gradient

Investigating the effects of climatic variability on biological diversity, productivity, and stability is key to understanding possible futures for ecosystems under accelerating climate change. A critical question for estuarine ecosystems is, how does climatic variability influence juvenile recruitment of different fish species and life histories that use estuaries as nurseries? Here we examined spatiotemporal abundance trends and environmental responses of 18 fish species that frequently spend the juvenile stage rearing in the San Francisco Estuary, CA, USA. First, we constructed multivariate autoregressive state-space models using age-0 fish abundance, freshwater flow (flow), and sea surface temperature data (SST) collected over four decades. Next, we calculated coefficients of variation (CV) to assess portfolio effects (1) within and among species, life histories (anadromous, marine opportunist, or estuarine dependent), and the whole community; and (2) within and among regions of the estuary. We found that species abundances varied over space and time (increasing, decreasing, or dynamically stable); and in 83% of cases, in response to environmental conditions (wet/dry, cool/warm periods). Anadromous species responded strongly to flow in the upper estuary, marine opportunist species responded to flow and/or SST in the lower estuary, and estuarine dependent species had diverse responses across the estuary. Overall, the whole community when considered across the entire estuary had the lowest CV, and life histories and species provided strong biological insurance to the portfolio (2.4- to 3.5-fold increases in stability, respectively). Spatial insurance also increased stability, although to a lesser extent (up to 1.6-fold increases). Our study advances the notion that fish recruitment stability in estuaries is controlled by biocomplexity-life history diversity and spatiotemporal variation in the environment. However, intensified drought and marine heatwaves may increase the risk of multiple consecutive recruitment failures by synchronizing species dynamics and trajectories via Moran effects, potentially diminishing estuarine nursery function.

Potential impacts of climate change on agriculture and fisheries production in 72 tropical coastal communities

Climate change is expected to profoundly affect key food production sectors, including fisheries and agriculture. However, the potential impacts of climate change on these sectors are rarely considered jointly, especially below national scales, which can mask substantial variability in how communities will be affected. Here, we combine socioeconomic surveys of 3,008 households and intersectoral multi-model simulation outputs to conduct a sub-national analysis of the potential impacts of climate change on fisheries and agriculture in 72 coastal communities across five Indo-Pacific countries (Indonesia, Madagascar, Papua New Guinea, Philippines, and Tanzania). Our study reveals three key findings: First, overall potential losses to fisheries are higher than potential losses to agriculture. Second, while most locations (> 2/3) will experience potential losses to both fisheries and agriculture simultaneously, climate change mitigation could reduce the proportion of places facing that double burden. Third, potential impacts are more likely in communities with lower socioeconomic status.

Endocrine disruption from plastic pollution and warming interact to increase the energetic cost of growth in a fish

Energetic cost of growth determines how much food-derived energy is needed to produce a given amount of new biomass and thereby influences energy transduction between trophic levels. Growth and development are regulated by hormones and are therefore sensitive to changes in temperature and environmental endocrine disruption. Here, we show that the endocrine disruptor bisphenol A (BPA) at an environmentally relevant concentration (10 µgl^-1) decreased fish (Danio rerio) size at 30°C water temperature. Under the same conditions, it significantly increased metabolic rates and the energetic cost of growth across development. By contrast, BPA decreased the cost of growth at cooler temperatures (24°C). BPA-mediated changes in cost of growth were not associated with mitochondrial efficiency (P/O ratios (i.e. adenosine diphosphate (ADP) used/oxygen consumed) and respiratory control ratios) although BPA did increase mitochondrial proton leak. In females, BPA decreased age at maturity at 24°C but increased it at 30°C, and it decreased the gonadosomatic index suggesting reduced investment into reproduction. Our data reveal a potentially serious emerging problem: increasing water temperatures resulting from climate warming together with endocrine disruption from plastic pollution can impact animal growth efficiency, and hence the dynamics and resilience of animal populations and the services these provide.

Interactive effects of light, CO2 and temperature on growth and resource partitioning by the mixotrophic dinoflagellate, Karlodinium veneficum

There is little information on the impacts of climate change on resource partitioning for mixotrophic phytoplankton. Here, we investigated the hypothesis that light interacts with temperature and CO₂ to affect changes in growth and cellular carbon and nitrogen content of the mixotrophic dinoflagellate, Karlodinium veneficum, with increasing cellular carbon and nitrogen content under low light conditions and increased growth under high light conditions. Using a multifactorial design, the interactive effects of light, temperature and CO₂ were investigated on K. veneficum at ambient temperature and CO₂ levels (25°C, 375 ppm), high temperature (30°C, 375 ppm CO₂), high CO₂ (30°C, 750 ppm CO₂), or a combination of both high temperature and CO₂ (30°C, 750 ppm CO₂) at low light intensities (LL: 70 μmol photons m^-2 s^-2) and light-saturated conditions (HL: 140 μmol photons m^-2 s^-2). Results revealed significant interactions between light and temperature for all parameters. Growth rates were not significantly different among LL treatments, but increased significantly with temperature or a combination of elevated temperature and CO₂ under HL compared to ambient conditions. Particulate carbon and nitrogen content increased in response to temperature or a combination of elevated temperature and CO₂ under LL conditions, but significantly decreased in HL cultures exposed to elevated temperature and/or CO₂ compared to ambient conditions at HL. Significant increases in C:N ratios were observed only in the combined treatment under LL, suggesting a synergistic effect of temperature and CO₂ on carbon assimilation, while increases in C:N under HL were driven only by an increase in CO₂. Results indicate light-driven variations in growth and nutrient acquisition strategies for K. veneficum that may benefit this species under anticipated climate change conditions (elevated light, temperature and pCO₂) while also affecting trophic transfer efficiency during blooms of this species.

Next-generation ensemble projections reveal higher climate risks for marine ecosystems

Projections of climate change impacts on marine ecosystems have revealed long-term declines in global marine animal biomass and unevenly distributed impacts on fisheries. Here we apply an enhanced suite of global marine ecosystem models from the Fisheries and Marine Ecosystem Model Intercomparison Project (Fish-MIP), forced by new-generation Earth system model outputs from Phase 6 of the Coupled Model Intercomparison Project (CMIP6), to provide insights into how projected climate change will affect future ocean ecosystems. Compared with the previous generation CMIP5-forced Fish-MIP ensemble, the new ensemble ecosystem simulations show a greater decline in mean global ocean animal biomass under both strong-mitigation and high-emissions scenarios due to elevated warming, despite greater uncertainty in net primary production in the high-emissions scenario. Regional shifts in the direction of biomass changes highlight the continued and urgent need to reduce uncertainty in the projected responses of marine ecosystems to climate change to help support adaptation planning.