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Respiration is a key process in the cycling of particulate matter and, therefore, an important control mechanism of carbon export to the ocean's interior. Most of the fixed carbon is lost in the upper ocean, and only a minor amount of organic material sustains life in the deep‐sea. Conditions are particularly extreme in hadal trenches, and yet they host active biological communities. The source of organic carbon that supports them and the contribution of these communities to the ocean carbon cycle, however, remain uncertain. Here we report on size‐fractionated depth profiles of plankton respiration assessed from the activity of the electron transport system in the Atacama Trench region, and provide estimates of the minimum carbon flux (FC) needed to sustain the respiratory requirements from the ocean surface to hadal waters of the trench and shallower nearby sites. Plankton < 100 μm contributed about 90% to total community respiration, whose magnitude was highly correlated with surface productivity. Remineralization rates were highest in the euphotic zone and declined sharply within intermediate oxygen‐depleted waters, remaining fairly constant toward the bottom. Integrated respiration in ultra‐deep waters (> 1000 m) was comparable to that found in upper layers, with 1.3 ± 0.4 mmol C m−2d−1being respired in the hadopelagic. The comparison between our FC models and estimates of sinking particle flux revealed a carbon imbalance through the mesopelagic that was paradoxically reduced at greater depths. We argue that large fast‐sinking particles originated in the overlying surface ocean may effectively sustain the respiratory carbon demands in this ultra‐deep marine environment.

The role of zooplankton in microbial nitrogen turnover in marine environments is poorly understood. Here, we present results from two experiments designed to determine the excretion rate of ammonium and dissolved organic nitrogen (DON) by dominant copepods, Acartia tonsaand Paracalanuscf indicus, fed with two natural sized‐fractioned diets (20–150 μm and < 20 μm), and its possible effects on the transcriptional activity of ammonia monooxygenase subunit A (amoA), a functional marker for ammonia‐oxidizing archaea (AOA) and bacteria (AOB), as a response to the input of ammonium and DON by copepod excretion, during autumn and winter in central/southern Chile. Our results reveal that DON was the main excretion product, with rates up to 3.7 μmol L−1h−1. DON production increased in copepods fed with the small‐sized food. Ammonium was also excreted, with rates up to 0.08 μmol L−1h−1in autumn and 0.4 μmol L−1h−1in winter, and rapidly consumed by the microbial community, decreasing down to 0.07 μmol L−1between the initial time to 4 h. Ammonium consumption coincided with increased AOB and AOA amoAtranscript copies in copepods fed with the larger‐sized food, while a different microbial community, probably heterotrophic, reacted to nitrogen input via excretion by copepods fed with the smaller‐sized food in autumn. AOA‐A was transcriptionally active in winter with nearly zero ammonium concentration, suggesting that AOA outcompete AOB when ammonium becomes limited. We conclude that nitrogen excreted by copepods can be used directly by microbial communities, including nitrifying ones. Zooplankton excretion may thus provide significant remineralized nitrogen for new and regenerated production in the upper ocean.

The Gulf of Lion is an important area of deep convection, where intense winter vertical mixing brings nutrients up from deeper layers, promoting the largest bloom in the Mediterranean at the end of winter/early spring. The DEWEX program conducted cruises in February and April 2013 to investigate the ecosystem level impacts of deep water convection. Zooplankton data were collected through net sampling and imaging with an Underwater Vision Profiler. In winter, low zooplankton abundance and biomass were observed in the Deep Convection Zone (DCZ) and higher values on its periphery. In spring, this pattern reversed with high biomass in the DCZ and lower values on the periphery. On average for the whole area, the potential grazing impact was estimated to increase by one order of magnitude from winter to spring. In April, all areas except the DCZ incurred top‐down control by zooplankton on the phytoplankton stock. In the DCZ, the chlorophyll‐avalues remained high despite the high zooplankton biomass and carbon demand, indicating a sustained bottom‐up control. The zooplankton community composition was comparable for both periods, typified by high copepod dominance, but with some differences between the DCZ and peripheral regions. In spring the DCZ was characterized by a strong increase in herbivorous species such as Centropages typicusand Calanus helgolandicus, and an increase in the number of large zooplankton individuals. Our study indicates that the DCZ is likely an area of both enhanced energy transfer to higher trophic levels and organic matter export in the North Western Mediterranean Sea. In winter, phytoplankton and zooplankton biomasses were low within the Deep Convection Zone (DCZ) and higher on its peripheryIn spring, zooplankton biomass dominated by herbivorous species increased over the whole area especially in the DCZIn spring, all areas except the DCZ presented a top‐down control by zooplankton on the phytoplankton stock

Mesoscale eddies are prominent structures in the world's oceans generating a high degree of spatial and temporal heterogeneity that influences zooplankton distribution. Euphausiids (krill) are a key zooplankton group mainly inhabiting coastal upwelling areas where high productivity, advection, and eddy kinetic energy (EKE) play pivotal roles in the distribution and structure of zooplankton. We analyzed the spatial distribution of the Humboldt Current krill, Euphausia mucronata, in relation to environmental variability and mesoscale circulation during the 2007 austral spring. Using net‐based zooplankton samples, remotely sensed environmental conditions, multivariate analysis, and generalized additive models, we described and tested the effect of oceanographic variability and mesoscale eddies on E. mucronataabundance and biomass. E. mucronatawas significantly more abundant in coastal (97%) than oceanic habitats, and more abundant in cyclonic cores (mean: 76 indiv. m−2) than in surrounding waters (mean: 13–29 indiv. m−2). Abundance correlated to current and EKE fields at >10–20 cm s−1and >50–200 cm2s−2, respectively, and biomass correlated negatively to sea level anomaly and positively to alongshore winds. Krill abundance and biomass were also strongly coupled to both eddy dynamics and the coastal upwelling regime in spring 2007. Mesoscale eddies may doubly influence the E. mucronatapopulation dynamic by retaining krill within them and, by advection from coastal to oligotrophic regions. Eddy kinetic energy field provides strong heterogeneity to krill habitatCyclonic cores concentrate higher krill abundance than their surrounding watersEddy circulation and coastal upwelling impact the krill dynamics

The recruitment rate of anchovy in the Peru–Chile upwelling system was studied by testing sensitive to environmental variability when the spawning stock is low in abundance. Times series of sea surface temperature, salinity, depth of the 15°C isotherm, the upper limit of the oxygen minimum zone, upwelling indices, the Southern Oscillation Index, and indices El Niño 1+2 and El Niño 3.4 were summarized trough Principal Component Analysis (PCA). The first PCA (PC-I) explained 57% of variance and was related to interannual variability driven by the El Niño-Southern Oscillation. The second Principal Component explained 15% of variance and was linked to upwelling indices. Anchovy recruitment rate anomalies were correlated with PC-I scores on the basis of a 6-year-moving-window and accumulated correlation as time progressed. Significant correlation coefficients were found when the spawning stock biomass was low in abundance before 1990. Once sufficient spawning biomass was accumulated (after 1990), the stock was less sensitive to environmental variability. Thus, sensitivity to environmental changes in anchovy, and probably in other small pelagic fishes, is stock-size dependent. This is a key aspect to be considered in the management of transboundary fish stock shared between Chile and Peru, upon current efforts to develop an ecosystem approach to fisheries management.