Marine heatwaves can occur again before organisms and communities have recovered from an earlier event. However, most plankton experiments end when warming stops and describe resistance more clearly than recovery. Here, I distinguish recovery debt (a residual physiological, reproductive, demographic, or community deficit after temperature returns toward reference conditions) from thermal memory (any persistent effect of prior exposure on a later response, whether protective or detrimental). A structured Web of Science search returned 50 candidate records; 9 met the eligibility criteria, and citation chaining added 7 more, giving 16 focal studies. Nine concerned copepods, and, to my knowledge, none followed an isolated free-living heterotrophic protist after heating. Only one study, a mesocosm experiment, applied two finite heatwaves, separated by 5 days and without a Heatwave-2-only treatment. No study manipulated the inter-event interval. Two copepod experiments compared naive and previously exposed lineages under the same later heatwave, in both cases across generations. The available studies show that post-event trajectories can differ among traits, that delayed reproductive or molecular effects can persist after cooling, and that community function can return without restoration of composition. This evidence does not establish an interval effect, but it suggests a testable hypothesis: the response to a second heatwave depends on how far protective regulation has decayed and how far energetic repair, reproduction, and demographic turnover have progressed since the first. Testing this requires factorial event histories, repeated sampling during recovery, and intervals expressed in biological as well as chronological time.
Field campaigns consistently document bottom-intensified turbulent fluxes near the seafloor, whose vertical divergence drives abyssal densification and diapycnal downwelling. This appears paradoxical under the classical assumption of steady abyssal stratification, because dense bottom waters continuously enter from Antarctic source regions. Motivated by evidence of evolving Indo-Pacific stratification, we use idealized simulations to investigate abyssal overturning circulation under transient and steady-state conditions. Using a kinematic rather than a thermodynamic water-mass-transformation approach, we diagnose advective overturning transport, isopycnal volume tendency, and kinematically diagnosed diapycnal transport in two cooling experiments with different mixing intensities, one warming experiment, and one quasi-equilibrium experiment. Advective upwelling occurs in all four experiments, whereas the direction of diagnosed diapycnal transport varies. In Type-I cooling, the isopycnal volume tendency exceeds the advective overturning in magnitude, yielding downward diagnosed diapycnal transport; in Type-II cooling, the advective overturning exceeds the isopycnal volume tendency, yielding upward diagnosed diapycnal transport. The warming and quasi-equilibrium experiments also exhibit upward diagnosed diapycnal transport. These relationships remain qualitatively consistent in potential-temperature coordinates. The Type-I cooling experiment also spontaneously produces an observationally documented bottom-intensified buoyancy-flux profile, with an e-folding length scale comparable to previous estimates. Thus, transient cooling does not uniquely determine the direction of diagnosed diapycnal transport; rather, its direction depends kinematically on the relative strengths of advective overturning and isopycnal-volume adjustment. Within the idealized framework examined here, these results provide a transient kinematic interpretation of the classical interior-downwelling conundrum while showing that diagnosed diapycnal downwelling is not an inevitable consequence of abyssal cooling.
Global climate change is profoundly affecting the ocean environment, leading to alterations in sea temperature, salinity and nutrients. The Western Pacific Warm Pool (WPWP), due to its unique thermal characteristics, provides an ideal natural laboratory for studying the impacts of climate change on surface planktonic microbial communities. We hypothesize that microbial eukaryotes and bacteria differ in their sensitivity to environmental variation and that abundant and rare taxa show divergent responses to future climate change. Therefore, this study compares the diversity and distribution of microbial eukaryotes and bacteria between the WPWP and non-WPWP areas, examining their responses to global warming and predicting future trends. Our results show significant differences in both microeukaryote and bacterial communities between the WPWP and surroundings. Overall, species replacement is the primary response mechanism to environmental changes for both groups. The diversity of microeukaryotes is more closely related to environmental factors, and they experience greater environmental selection pressure. Interestingly, there is no significant relationship between temperature and the primary producers. Projections indicate that microbial diversity may decline in the WPWP by 2100, while high-latitude regions may experience higher diversity. Suitable habitats for high- and moderate-abundance taxa are projected to contract from the WPWP toward coastal areas and the Eastern Pacific, whereas rare taxa may retain suitable habitats within the WPWP. Overall, this study highlights that species attributes and abundance shape the future trajectories of diversity and provides a scientific basis for predicting biodiversity trends and developing conservation strategies under global climate scenarios.
Biological and physical dynamics are often coupled in the ocean, yet the impacts of internal solitary waves (ISWs) on mesopelagic organisms remain unclear. Here, using high-resolution acoustic backscatter and hydrographic measurements from the northern South China Sea, the deep scattering layer (DSL), formed by aggregations of scattering organisms, exhibits a pronounced vertical displacement during ISW passage. The depth of the DSL closely follows isotherm displacements, indicating a strong influence of wave-induced vertical motion. This correspondence suggests that the DSL can serve as a useful proxy for ISW arrival time and potentially for wave amplitude, although amplitude estimates exhibit greater uncertainty. In addition, multiple groups of vertically migrating scatterers are resolved within the diel migration layer, with more than half completing their migration within one hour. In the single reliably tracked ISW-migration overlap case, one migration occurring entirely during ISW passage shows slower speed and a longer duration than a reference migration without ISWs. For the migration beginning before ISW arrival, part of the associated acoustic feature extends downward into the wave trough during the ISW passage and overlaps vertically with the other migrating acoustic feature. These findings highlight the potential of DSL tracking to complement ISW monitoring and provide an observational basis for understanding how energetic internal waves reorganize mesopelagic scattering layers and modify apparent migration trajectories.
Climate change is exerting multiple and pervasive effects on marine ecosystems, including alterations in environmental conditions, declines in ocean productivity, and modifications in trophic structure. Climate-induced changes have also been suggested to affect fish life history traits. So far, multispecies assessments addressing the propagation of climate effects through food webs remain scarce. Using the individual-based ecosystem model OSMOSE, including temperature and oxygen-dependent bioenergetics and eco-genetic sub-models, we simulated three major climate-induced drivers based on historically observed trends: increased sea surface temperature (SST), reduced oxygen saturation, and a decline in mean plankton size. We assessed their effects on species biomass and on gonad weight, size-at-age and age-at-maturation. The model was parameterized and fitted to the Bay of Biscay ecosystem dynamics over the period 2000-2022. Our results indicate that recent SST increases and oxygen declines had limited direct physiological impacts on biomass and life history traits. In contrast, recent reduction in mean plankton size emerged as the main vector of change. These alterations directly affected small pelagic fish and propagated through the food web, leading to a reduction in ecosystem carrying capacity and widespread modifications of life history traits. Overall, our findings highlight the central role of plankton size structure in climate-induced response of fish life history traits. These changes should be better considered in management frameworks, as they may affect the resilience of future fish population and ecosystem dynamics.
