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Highlights • Zooplankton vertical distribution, migrant biomass, and respiratory flux were studied. • Migrant biomass was estimated by sampling during day and night in 0–900 m depth. • The electron transfer system enzymatic activity was measured as a proxy for respiration. • Migrant biomass and the respiratory flux showed the strongest correlation. • Active flux estimated suggests a significant transport to the mesopelagic zone. [ABSTRACT FROM AUTHOR]

• arbitrarily high-order accurate numerical method for conservation laws. • based on a continuous approximation of the solution. • the degrees of freedom are point values at cell interfaces and moments of the solution inside the cell. • to lowest (3rd) order this method reduces to the Active Flux method. • the update of the moments is achieved immediately by integrating the conservation law over the cell. • two ways how the point values can be updated in time. We propose an arbitrarily high-order accurate numerical method for conservation laws that is based on a continuous approximation of the solution. The degrees of freedom are point values at cell interfaces and moments of the solution inside the cell. To lowest (3 rd) order this method reduces to the Active Flux method. The update of the moments is achieved immediately by integrating the conservation law over the cell, integrating by parts and employing the continuity across cell interfaces. We propose two ways how the point values can be updated in time: either by first deriving a semi-discrete method that uses a finite-difference-type formula to approximate the spatial derivative, and integrating this method e.g. with a Runge-Kutta scheme, or by using a characteristics-based update, which is inspired by the original (fully discrete) Active Flux method. We analyze stability and accuracy of the resulting methods. [ABSTRACT FROM AUTHOR]

• Migrant biomass and respiratory flux were estimated in mesopelagic organisms. • The largest migrant biomass were mesopelagic fishes followed by decapods. • Fishes showed high values of respiratory flux at the centre-eastern coast of the gulf. • Midriff Islands region had one of the highest values of crustacean respiratory flux. • Micronekton respiratory flux was higher during June than during October. The biological carbon pump is the process that transports carbon vertically out of the mixed layer in the ocean. Besides the sinking flux of organic particles, active flux due to the daily vertical migration of zooplankton and micronekton promotes a significant carbon transport not fully accounted for or understood in the world's oceans. The diversity and abundance of epipelagic and mesopelagic species in the Gulf of California has been extensively studied, but the role of micronekton in carbon export has not yet been investigated. We studied the carbon flux promoted by juvenile and adult mesopelagic fishes and crustaceans (Decapoda and Euphausiidae) during the transition from the cold to warm period (June) and the onset of the warm season (October) in 2018. We provide the first estimation of migrant biomass and respiratory flux of the most abundant migratory species of mesopelagic fishes, decapods and euphausiids in the Gulf of California. The micronekton species collected accounted for a large biomass of mesopelagic fishes and pelagic crustaceans. The average migrant biomass estimates were 151.5 ± 101.2 mg C·m−2 during June and 90.9 ± 75.3 mg C·m−2 during October. The enzymatic activity of the electron transfer system (ETS) was measured as an estimate of their respiratory rates. Average specific ETS activity was significantly different between fishes and decapods, and between fishes and euphausiids (p < 0.05). The respiratory flux of fishes was predominant in the Gulf of California, followed by pelagic decapods and euphausiids. Seasonal changes in respiratory flux were observed for fishes (June: 6.1 ± 1.5 mg C·m−2·d−1; October: 3.2 ± 1.8 mg C·m−2·d−1) and decapods (June: 0.4 mg C·m−2·d−1; October: 0.7 ± 0.05 mg C·m−2·d−1). Respiratory flux estimation by crustaceans (decapods and euphausiids) and fishes together was 6.86 mg C·m−2·d−1 during June, and 4.21 mg C·m−2·d−1 during October 2018, suggesting a functional role of this large micronektonic fauna in the biological carbon export in this region. [ABSTRACT FROM AUTHOR]

Abstract: Seasonal change in the downward carbon transport due to respiration and mortality through diel vertical migration (DVM) of the calanoid copepods Metridia pacifica and Metridia okhotensis was estimated in the Oyashio region, western subarctic Pacific during six cruises from June 2001 to June 2002. M. pacifica (C4, C5 and adult females) was an active migratory species throughout the year though its DVM amplitude varied among seasons and stages. The mean distribution depths of adult females during the daytime were positively related with the illumination level in the water column, being shallowest in April and deepest in January. M. okhotensis generally showed less-extensive migrations than M. pacifica. Therefore, together with their lower abundance, this species is considered to be a less-important mechanism of downward transport of carbon except for April when their DVM was more active and descended deeper than M. pacifica, which remained in the upper 150m even during the daytime. The mean migrating biomass of the two Metridia species was 558mgCm−2 d−1 and was high during summer to winter (263–1676mgCm−2 d−1) and low during spring (59–63mgCm−2 d−1). Total downward flux through DVM fluctuated between 1.0 and 20.0mgCm−2 d−1 with an annual mean of 8.0mgCm−2 d−1. Contribution of the respiratory flux was greater than the mortality flux and accounted for 64–98% of total migratory flux throughout the year except for January when contribution of both fluxes was equal. Overall the annual carbon transport by DVM of Metridia spp. was estimated as 3.0gCm−2 year−1, corresponding to 15% of the annual total POC flux at 150m at the study site, suggesting that DVM is a significant process for carbon export in the subarctic region as well as that in tropical and subtropical oceanic regions. Since DVM in M. pacifica is more active during the non-bloom season when the gravitational flux of particulate matter is low, this species plays an important role in driving the biological pump in the subarctic Pacific during summer to winter. [Copyright &y& Elsevier]

Diel vertical migration of zooplankton profoundly impacts the transport of nutrients and carbon through the water column. Despite the acknowledged importance of this active flux to ocean biogeochemistry, these contributions remain poorly constrained, in part because daily variations in metabolic rates are not considered or are modeled as simple functions of temperature. To address this uncertainty, we sampled the subtropical copepod Pleuromamma xiphias at 4- to 7-h intervals throughout the daily migration and measured rates of oxygen consumption, ammonium excretion, fecal pellet production and metabolic enzyme activity. No significant patterns were detected in rates of oxygen consumption or ammonium excretion for freshly caught animals over the diel cycle. Fecal pellet production was highest during mid-night, consistent with several hours of feeding near the surface. Surface feeding resulted in fecal pellet production at depth in the morning, providing direct evidence that active flux of particulate organic carbon occurs in this region. Electron transport system activity was highest during the afternoon, contrary to our prediction of reduced daytime metabolism. Activity of both glutamate dehydrogenase and citrate synthase increased during early night, reflecting higher capacity for excretion and aerobic respiration, respectively. Overall, these results show that activities of metabolic enzymes vary during diel vertical migration. The surprising observation of elevated afternoon enzyme activity coupled with daytime fecal pellet and ammonium production suggests that additional characterization of the daytime activity of migratory zooplankton is warranted. • Glutamate dehydrogenase and citrate synthase were elevated during early night. • Electron transport system was elevated during afternoon. • Oxygen consumption rate and ammonium excretion showed no significant pattern. [ABSTRACT FROM AUTHOR]

• Migrant biomass and mesopelagic respiration are needed to estimate respiratory flux. • We compared respiration rates in the mesopelagic zone using different approaches. • The different methods provided good agreement in the mesopelagic zone. • We observed higher ETS enzymatic activities during daytime at depth. Respiration in the mesopelagic layer is rather difficult to estimate but is of paramount importance to assess active flux, the downward carbon transport carried out by diel vertical migrants in the ocean. Migrant biomass and respiration in the mesopelagic zone are required to estimate respiratory flux, an important component of active flux. Three main approaches were used in the past to assess respiration in the mesopelagic zone: (1) The use of Q 10 values, (2) published equations relating respiration, body mass, and temperature, and (3) the electron transfer system (ETS) enzymatic activity, calibrated to obtain respiration rates. The latter method is also a commonly used approach for measuring mesopelagic community respiration. Here we report respiration rates and ETS activity of migrant copepods captured at night in the epipelagic layer. The estimated mesopelagic respiration obtained from conversion of epipelagic respiration at night using a Q 10 value for mesopelagic zooplankton was compared to the estimated respiration obtained using the equations given by Ikeda (1985), Ikeda (2014), and ETS activity using a respiration to ETS (R/ETS) ratio of 1. They were not statistically different. However, differences were observed between those latter values and the R/ETS ratio of 0.5. These results show that the commonly used R/ETS ratio of 0.5 promotes quite conservative values of the respiratory flux, and the equation of Ikeda (2014) displayed values in the middle of both R/ETS ratios, and it may be used with confidence. Relatively high specific ETS activities were observed in the mesopelagic zone, also coinciding with a previous observation in Eastern Equatorial Pacific. We discuss this observation as an adaptation of migrant zooplankton to endure the adverse conditions of low temperature and low oxygen in deep waters. [ABSTRACT FROM AUTHOR]