Plankton respiration in the Atacama Trench region: Implications for particulate organic carbon flux into the hadal realm

Abstract

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 AE 0.4 mmol C m À2 d À1 being 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.

Trophic ecology of midwater zooplankton along a productivity gradient in the Southeast Pacific

“…Although our more coastal station SPG1 was outside the direct influence of the upwelling core, we still found 7.51 mg m -3 of zooplankton dry mass in the epipelagic layer, a figure slightly below those reported in waters over the central Chilean shelf (Escribano et al, 2012). Even at the coastal mesopelagic zone, the zooplankton biomass exceeded by an order of magnitude the one found in the surface oligotrophic waters, indicating a potential linkage between deep-sea ecosystem processes and overlying dynamics, as previously argued by Fernańdez-Urruzola et al (2021). The subsequent transport of zooplankton towards the open ocean by lateral advection might further explain the biomass peaks that we found at the offshore stations SPG2 and SPG3 (Table 1 and Figure 3A), both dominated by larger zooplankton that were likely derived from succession processes within the community (Vinogradov and Shushkina, 1978).…”

“…Still, the small amount of variation in the heterotrophic re-synthesis index (SV) suggests the assimilation of relatively fresh organic matter at depth. While the low differences in the baseline d 15 N Phe values throughout the water column (Supplementary Table S6) point to the importance of the export flux in this ecosystem (Fernańdez-Urruzola et al, 2021), the migrant zooplankton would also serve as an alternative source of energy for resident carnivores in the deep ocean (Steinberg and Landry, 2017). The active transport of photosynthetically fixed carbon to depth by eucalanid copepod species may indeed represent a potential trophic pathway below the ESSW considering the inability of other zooplankters to penetrate into these oxygen deficient waters (Escribano et al, 2009;Tutasi and Escribano, 2020).…”

“…Furthermore, whether zooplankton trophic ecology in the deep ocean mirrors that in the sunlit surface layer along the productivity gradient is yet to be resolved. Although surface productivity has been correlated with deep-sea ecosystem processes in the region (Fernańdez-Urruzola et al, 2021;Glud et al, 2021), an apparent imbalance in the carbon budget throughout the twilight zone suggests the presence of compensatory mechanisms that may shape the food web structure in the deep sea (Fernańdez-Urruzola et al, 2021). Alternative feeding strategies fueling midwater food webs might include, among others, microbial gardening, i.e.…”

Modeling present and future distribution of plankton populations in a coastal upwelling zone: the copepod Calanus chilensis as a study case

“…Historically, C. chilensis is reported in the HCS all along the Chilean and Peruvian coast from approximately 21 to 45°S, with no data available for more southern or northern regions 13 . In earlier studies conducted in Chilean waters, C. chilensis was found inhabiting oxygenated waters over the oxygen minimum zone (OMZ) 56 , although sometimes smaller portions of the population were also found below the oxycline 57 , through passive sinking 58 , diel vertical migration (DVM) 59,60 , or combining DVM (daily vertical migration) and predation. The latter and the fact that this species is restricted to oxygenated waters above the OMZ 56 , represents an important food resource for the larger animals populating this zone, makes C. chilensis a species of ecological importance in this system.…”

Bacterial and eukaryotic intact polar lipids point to in situ production as a key source of labile organic matter in hadal surface sediment of the Atacama Trench

“…A physical-chemical characterization of the water column during the ChiMeBo-SO211, LowpHOX-2, and HADES-SO261 cruises has been reported in Matys et al (2017), Cantarero et al (2020) and Vargas et al (2021), andFernández-Urruzola et al (2021), respectively. Briefly, the potential-temperature-salinity-dissolved oxygen (θ -s-O 2 ) diagrams revealed an oxygenated and well-mixed water mass occupying the deeper parts of the Atacama Trench (Fig.…”