Quantification of the actual amount of carbon export to the mesopelagic layer by both zooplankton and micronekton is at present a gap in the knowledge of the biological pump. These organisms perform diel vertical migrations exporting carbon through respiration, excretion, mortality, and egestion during their residence at depth. The role of zooplankton in active flux is nowadays partially assessed. However, micronekton active flux is scarcely known and only a few studies addressed this downward transport. Even less is known about the capacity of both communities to export carbon in the ocean. Here, we show the results of zooplankton and micronekton active flux across a productivity gradient in the tropical and subtropical Atlantic Ocean. Biomass vertical distribution from the surface up to 800 m depth by day and night was studied during April 2015 in a transect from 9°S to 25°N, covering from the quite oligotrophic zone off Brazil to the meso- and eutrophic areas of the equator, Guinea Dome, and the oceanic upwelling off Northwest Africa. Zooplankton and micronekton migrant biomass was estimated from day and night catches at different layers of the water column using MOCNESS-1 (1 m² mouth area) and Mesopelagos (35 m²) nets, respectively. Respiratory flux was assessed by measuring the enzymatic activity of the electron transfer system (ETS) of organisms at depth. Results showed a close relationship between migrant biomass and respiratory flux in zooplankton and micronekton as expected. Using a rather conservative 50% of efficiency for the net used to capture micronekton, respiratory flux resulted in similar values for both communities. Gravitational (passive) flux measured using sediment traps increased from the oligotrophic toward the meso- and eutrophic zones. Total active flux (including respiration and estimated mortality, excretion, and gut flux) by zooplankton and micronekton accounted for about 25% of total flux (passive plus active) in oligotrophic zones. Total active flux also increased toward meso- and eutrophic zones, reaching about 80% of total flux and being at least twofold higher than passive flux. These results alert about an important underestimation of the ocean biological pump using only passive flux measurements.

Assessment of fish trophic ecology in natural environments is imperative to understand biological and ecological requirements, supporting the management and conservation of populations and environments. There are several methods available to assess fish diets, generically divided in noninvasive approaches, such as underwater observations, and invasive approaches, such as analyses of lipid biomarkers, stable isotopes and stomach contents. Each one has application assumptions and conditions that must be aligned with study objectives and methodological criteria, assuring results robustness and allowing comparability among different studies. This review addresses the advantages and limitations of these methods, highlighting those applied for stomach analysis (SA). This work presents complete updated review concerning qualitative and quantitative methods applied to SA: sixty equations were reviewed comprising four formulae to value stomachs per se and, for stomach content analysis, eight single indices, fifteen composite indices and thirty-three equations related to modeling approaches. The review presents different potentialities associated to these methods, standardizes distinct names applied to a same method, and provides clarifications on confusion sources due to the use of same names in distinct methods, a source of scientific mistakes detected in the specialized literature.