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The transfer of organic carbon from the upper to the deep ocean by particulate export flux is the starting point for the long-term storage of photosynthetically fixed carbon. This ‘biological carbon pump’ is a critical component of the global carbon cycle, reducing atmospheric CO 2 levels by ~200 ppm relative to a world without export flux. This carbon flux also fuels the productivity of the mesopelagic zone, including important fisheries. Here we show that, despite its importance for understanding future ocean carbon cycling, Earth system models disagree on the projected response of the global export flux to climate change, with estimates ranging from −41% to +1.8%. Fundamental constraints to understanding export flux arise because a myriad of interconnected processes make the biological carbon pump challenging to both observe and model. Our synthesis prioritizes the processes likely to be most important to include in modern-day estimates (particle fragmentation and zooplankton vertical migration) and future projections (phytoplankton and particle size spectra and temperature-dependent remineralization) of export. We also identify the observations required to achieve more robust characterization, and hence improved model parameterization, of export flux and thus reduce uncertainties in current and future estimates in the overall cycling of carbon in the ocean. A synthesis of recent work on marine carbon export fluxes finds that many processes that are key to understanding the effects of a warming climate on ocean carbon cycling are missing from current climate models.

The daily vertical migrations of fish and other metazoans actively transport organic carbon from the ocean surface to depth, contributing to the biological carbon pump. We use an oxygen-constrained, game-theoretic food-web model to simulate diel vertical migrations and estimate near-global (global ocean minus coastal areas and high latitudes) carbon fluxes and sequestration by fish and zooplankton due to respiration, fecal pellets, and deadfalls. Our model provides estimates of the carbon export and sequestration potential for a range of pelagic functional groups, despite uncertain biomass estimates of some functional groups. While the export production of metazoans and fish is modest (∼20 % of global total), we estimate that their contribution to carbon sequestered by the biological pump (∼800 PgC) is conservatively more than 50 % of the estimated global total (∼1300 PgC) and that they have a significantly longer sequestration timescale (∼250 years) than previously reported for other components of the biological pump. Fish and multicellular zooplankton contribute about equally to this sequestered carbon pool. This essential ecosystem service could be at risk from both unregulated fishing on the high seas and ocean deoxygenation due to climate change.

Many zooplankton and fishes vertically migrate on a diel cycle to avoid predation, moving from their daytime residence in darker, deep waters to prey-rich surface waters to feed at dusk and returning to depth before dawn. Verticalmigrations also occur in response to other processes that modify local light intensity, such as storms, eclipses, and full moons. We observed rapid, high-frequency migrations, spanning up to 60 m, of a diel vertically migrating acoustic scattering layer with a daytime depth of 300 m in the subpolar Northeastern Pacific Ocean. The depth of the layer was significantly correlated, with an ∼5-min lag, to cloud-driven variability in surface photosynthetically available radiation. A model of isolume-following swimming behavior reproduces the observed layer depth and suggests that the high-frequency migration is a phototactic response to absolute light level. Overall, the cumulative distance traveled per day in response to clouds was at least 36% of the roundtrip diel migration distance. This previously undescribed phenomenon has implications for the metabolic requirements of migrating animals while at depth and highlights the powerful evolutionary adaptation for visual predator avoidance.

Shear-induced migration of elongated micro-swimmers exhibiting anisotropic Brownian diffusion at a population scale is investigated analytically in this work. We analyse the steady motion of confined ellipsoidal micro-swimmers subject to coupled diffusion in a general setting within a continuum homogenisation framework, as an extension of existing studies on macro-transport processes, by allowing for the direct coupling of convection and diffusion in local and global spaces. The analytical solutions are validated successfully by comparison with numerical results from Monte Carlo simulations. Subsequently, we demonstrate from the probability perspective that symmetric actuation does not yield net vertical polarisation in a horizontal flow, unless non-spherical shapes, external fields or direct coupling effects are harnessed to generate steady locomotion. Coupled diffusivities modify remarkably the drift velocity and vertical migration of motile micro-swimmers exposed to fluid shear. The interplay between stochastic swimming and preferential alignment could explain the diverse concentration and orientation distributions, including rheological formations of depletion layers, centreline focusing and surface accumulation. Results of the analytical study shed light on unravelling peculiar self-propulsion strategies and dispersion dynamics in active-matter systems, with implications for various transport problems arising from the fluctuating shape, size and other external or inter-particle interactions of swimmers in confined environments.

As Arctic sea ice deteriorates, more light enters the ocean, causing largely unknown effects on the ecosystem. Using an autonomous biophysical observatory, we recorded zooplankton vertical distribution under Arctic sea ice from dusk to dawn of the polar night. Here we show that zooplankton ascend into the under-ice habitat during autumn twilight, following an isolume of 2.4 × 10−4 W m−2. We applied this trigger isolume to CMIP6 model outputs accounting for incoming radiation after sunset and before sunrise of the polar night. The models project that, in about three decades, the total time spent by zooplankton in the under-ice habitat could be reduced by up to one month, depending on geographic region. This will impact zooplankton winter survival, the Arctic foodweb, and carbon and nutrient fluxes. These findings highlight the importance of biological processes during the twilight periods for predicting change in high-latitude ecosystems.The authors used an autonomous biophysical observatory to estimate the light intensity triggering seasonal zooplankton vertical migration under Arctic sea ice. Considering this trigger, they project future reductions in time spent in the under-ice habitat, with implications for Arctic ecosystems.

Diel vertical migration (DVM) has generally been assumed to cease during the polar night in the high Arctic, although recent studies have shown the occurrence of lunar vertical migrations (LVMs) and shallow DVMs. Here, we quantified when and where full-depth (>20 m), solar-mediated DVM exists on a pan-Arctic scale. We observed the scattering population, most likely to be comprised of zooplankton, using 300 kHz acoustic Doppler current profilers (ADCPs). We quantified the presence/absence of DVM, and found that DVM continues throughout the year to at least 20 m at all locations south of 74° N. North of 77° N, DVM ceases for a period of time during the polar night. The dates of this cessation accurately align with the date of the winter solstice (±2 d). Between 74 and 77° N, DVM presence/absence is variable. Acoustic data sampled at 89°N, however, showed no evidence of DVM at any time during the year—a new observation. Using indicators of presence/absence of sea ice from ADCPs and satellite-derived sea ice concentration data, we revealed that local variations in sea ice cover directly determine the continuation or cessation of DVM during the polar night. Earlier-forming and higher-concentration sea ice causes a cessation in DVM, whereas low-concentration or late-forming sea ice results in continuous DVM when comparing migrations at similar latitudes.

Zooplankton play a key role in the cycling of carbon in aquatic ecosystems, yet their production of carbon-rich fecal pellets, which sink to depth and can fuel benthic community metabolism, is rarely quantified in estuaries. We measured fecal pellet carbon (FPC) production by the whole near-surface mesozooplankton community in the York River sub-estuary of Chesapeake Bay. Zooplankton biomass and taxonomic composition were measured with monthly paired day/night net tows. Live animal experiments were used to quantify FPC production rates of the whole community and dominant individual taxa. Zooplankton biomass increased in surface waters at night (2- to 29-fold) due to diel vertical migration, especially by Acartia spp. copepods. Biomass and diversity were seasonally low in the winter and high in the summer and often dominated by Acartia copepods. Whole community FPC production rates were higher (3- to 65-fold) at night than during the day, with the 0.5–1 mm size class contributing 2–26% to FPC production in the day versus 40–70% at night. An increase in the relative contribution of larger size fractions to total FPC production occurred at night due to diel vertical migration of larger animals into surface waters. Community FPC production was highest in fall due to increased diversity and abundance of larger animals producing larger fecal pellets, and lowest in summer likely due to top-down control of abundant crustacean taxa by gelatinous predators. This study indicates that zooplankton FPC production in estuaries can surpass that in oceanic systems and suggests that fecal pellet export is important in benthic-pelagic coupling in estuaries.

Hydrodynamic interactions between swimming or flying organisms can lead to complex flows on the scale of the group. These emergent fluid dynamics are often more complex than a linear superposition of individual organism flows, especially at intermediate Reynolds numbers. This paper presents an approach to estimate the flow induced by multiple swimmer wakes in proximity using a semianalytical model that conserves mass and momentum in the aggregation. The key equations are derived analytically, while the implementation and solution of these equations are carried out numerically. This model was informed by and compared with empirical measurements of induced vertical migrations of brine shrimp, Artemia salina. The response of individual swimmers to ambient background flow and light intensity was evaluated. In addition, the time-resolved three-dimensional spatial configuration of the swimmers was measured using a recently developed laser scanning system. Numerical results using the model found that the induced flow at the front of the aggregation was insensitive to the presence of downstream swimmers, with the induced flow tending towards asymptotic beyond a threshold aggregation length. Closer swimmer spacing led to higher induced flow speeds, in some cases leading to model predictions of induced flow exceeding swimmer speeds required to maintain a stable spatial configuration. This result was reconciled by comparing two different models for the near-wake of each swimmer. The results demonstrate that aggregation-scale flows result from a complex, yet predictable interplay between individual organism wake structure and aggregation configuration and size.

It is unknown how many spawning areas exist for tropical South Pacific eels (Anguilla marmorata, A. megastoma, A. obscura) populating island archipelagos between Papua New Guinea and French Polynesia. They could spawn at single centralised eastern and western locations, implying long-distance migrations by some eels, or at several local spawning areas. Larval catches, morphological and genetic investigations, and tagging experiments have provided no unequivocal answer. In this study, A. marmorata and A. megastoma were tagged with pop-up satellite archival transmitters at Samoa, in the centre of their distribution ranges. Tags surfaced prematurely after 11 to 25 d, 91 to 345 km from the point of release. One A. marmorata and one A. megastoma came within 180 and 230 km, respectively, from where a small A. marmorata leptocephalus was caught north of American Samoa during a recent research cruise, suggesting that eels may spawn near the archipelago. Silver eels exhibited diel vertical migrations between 180 m during the night and more than 700 m during the day. At their upper migration depths, eels migrated towards increasing salinity and towards local eddies, raising the question of whether they may actively search for these oceanographic features. Up to 15% of virtual larvae released near Samoa were retained within local eddies and could have recruited back to the archipelago. The remaining larvae drifted as far as Fiji and the Cook Islands to the west and east, respectively. The exchange of leptocephali probably connects several local spawning areas throughout the South Pacific Ocean, causing genetic exchange among areas.

Pelagic organisms inhabiting coastal upwelling regions face a high risk of advection away from the nearshore productive habitat, potentially leading to mortality. We explored how animals remain in a productive yet highly advective environment in the Northern California Current System using the cabled observatory system located off the Oregon coast. Acoustic scatterers consistent with swimbladder‐bearing fish were only present during the downwelling season as these animals avoided the cold waters associated with strong upwelling conditions in summer and fall. Fish responded to short‐term upwelling events by increasing the frequency of diel vertical migration. Throughout the study, their vertical positions corresponded to the depth of minimum cross‐shelf transport, providing a mechanism for retention. The observed behavioral response highlights the importance of studying ecological processes at short timescales and the abilities of pelagic organisms to control their horizontal distributions through fine‐tuned diel vertical migration in response to upwelling. Plain Language Summary Coastal upwelling regions are characterized by high nearshore productivity supported by wind‐driven nutrient supply. This high productivity cascades through the food web supporting high abundance of fish and providing feeding habitat for seabirds and marine mammals. Organisms living in the upwelling ecosystems constantly face challenges of being swept offshore into habitat with fewer food resources due to strong offshore‐moving currents near the surface. Studying how pelagic organisms remain in these nearshore productive waters despite the highly advective physical processes has been difficult. We used simultaneous observations of biological and physical properties to quantify how upwelling variability affects fish behavior and distributions in the Northern California Current System on the Oregon shelf throughout the year. Fish appeared at the nearshore study site during the downwelling season (fall—spring), avoiding cold waters associated with the summer upwelling season. Within the downwelling season, fish responded to short‐term upwelling events by conducting diel vertical migration more frequently during upwelling than downwelling conditions. Regardless of the upwelling strength, fish positioned themselves at the depth of minimum advection risk which aids in retention at the nearshore habitat. These observations highlight the survival strategies of animals in the environment where physical forcing exceeds their swimming capabilities. Key Points Acoustic scatterers responded to short‐term upwelling events outside of the summer upwelling season on the Oregon shelf Diel vertical migration occurred more frequently during upwelling than downwelling Acoustic scatterers positioned themselves at the depth of minimum cross‐shelf transport during both upwelling and downwelling