Description: Carbon fixation by phytoplankton plays a key role in the uptake of atmospheric CO2 in the Southern Ocean. Yet, it still remains unclear how efficiently the particulate organic carbon (POC) is exported and transferred from ocean surface waters to depth during phytoplankton blooms. In addition, little is known about the processes that control the flux attenuation within the upper twilight zone. Here, we present results of downward POC and particulate organic nitrogen fluxes during the decline of a vast diatom bloom in the Atlantic sector of the Southern Ocean in summer 2012. We used thorium-234 (234Th) as a particle tracer in combination with drifting sediment traps (ST). Their simultaneous use evidenced a sustained high export rate of 234Th at 100 m depth in the weeks prior to and during the sampling period. The entire study area, of approximately 8000 km2, showed similar vertical export fluxes in spite of the heterogeneity in phytoplankton standing stocks and productivity, indicating a decoupling between production and export. The POC fluxes at 100 m were high, averaging 26±15 mmol C m−2 d−1, although the strength of the biological pump was generally low. Only 〈20% of the daily primary production reached 100 m, presumably due to an active recycling of carbon and nutrients. Pigment analyses indicated that direct sinking of diatoms likely caused the high POC transfer efficiencies (~60%) observed between 100 and 300 m, although faecal pellets and transport of POC linked to zooplankton vertical migration might have also contributed to downward fluxes.

Description: We compared particle data from a moored video camera system with sediment trap derived fluxes at ∼1100 m depth in the highly dynamic coastal upwelling system off Cape Blanc, Mauritania. Between spring 2008 and winter 2010 the trap collected settling particles in 9-day intervals, while the camera recorded in-situ particle abundance and size-distribution every third day. Particle fluxes were highly variable (40–1200 mg m−2 d−1) and followed distinct seasonal patterns with peaks during spring, summer and fall. The particle flux patterns from the sediment traps correlated to the total particle volume captured by the video camera, which ranged from1 to 22 mm3 l−1. The measured increase in total particle volume during periods of high mass flux appeared to be better related to increases in the particle concentrations, rather than to increased average particle size. We observed events that had similar particle fluxes, but showed clear differences in particle abundance and size-distribution, and vice versa. Such observations can only be explained by shifts in the composition of the settling material, with changes both in particle density and chemical composition. For example, the input of wind-blown dust from the Sahara during September 2009 led to the formation of high numbers of comparably small particles in the water column. This suggests that, besides seasonal changes, the composition of marine particles in one region underlies episodical changes. The time between the appearance of high dust concentrations in the atmosphere and the increase lithogenic flux in the 1100 m deep trap suggested an average settling rate of 200 m d−1, indicating a close and fast coupling between dust input and sedimentation of the material.

Description: Phytoplankton blooms in surface waters of the oceans are known to influence the food web and impact microbial as well as zooplankton communities. Numerous studies have investigated the fate of phytoplankton-derived organic matter in surface waters and shelf sediments, however, little is known about the effect of sinking algal biomass on microbial communities in deep-sea sediments. Here, we analyzed sediments of four regions in the Southern Atlantic Ocean along the Antarctic Polar Front that had different exposures to phytoplankton bloom derived organic matter. We investigated the microbial communities in these sediments using high-throughput sequencing of 16S rRNA molecules to determine microorganisms that were active and catalyzed reporter deposition fluorescence in situ hybridization to infer their abundance and distribution. The sediments along the Antarctic Polar Front harbored microbial communities that were highly diverse and contained microbial clades that seem to preferably occur in regions of high primary productivity. We showed that organisms affiliated with the gammaproteobacterial clade NOR5/OM60, which is known from surface waters and coastal sediments, thrive in the deep-sea. Benthic deep-sea NOR5 were abundant, diverse, distinct from pelagic NOR5 and likely specialized on the degradation of phytoplankton-derived organic matter, occupying a similar niche as their pelagic relatives. Algal detritus seemed to not only fuel the benthic microbial communities of large areas in the deep-sea, but also to influence communities locally, as we found a peak in Flavobacteriaceae-related clades that also include degraders of algal biomass. The results strongly suggest that phytoplankton-derived organic matter was rapidly exported to the deep-sea, nourished distinct benthic microbial communities and seemed to be the main energy source for microbial life in the seafloor of vast abyssal regions along the Antarctic Polar Front.

Description: Until the 1980s, the deep sea was generally considered to be a particularly stable environment, free from major temporal variations (Sanders, 1968). Studies in the abyssal northeast Atlantic by Billett et al. (1983), and subsequently Lampitt (1985) discovered seasonal pulses of surface primary production-derived particulate organic matter (phytodetritus), and hence carbon, at abyssal depths. These early observations were subsequently extended to the central oceanic region of the NE Atlantic (Pfannkuche, 1993; Thiel et al., 1989), and prompted the establishment of more concerted time series studies in the Porcupine Abyssal Plain area. Today, the Porcupine Abyssal Plain Sustained Observatory (PAP–SO) is a multidisciplinary open-ocean time series site in the NE Atlantic (48°50′N 16°30′W, 4850 m water depth; Fig. 1), focused on the study of connections between the surface and deep ocean. In situ measurements of climatically and environmentally relevant variables have been made for more than 30 years. This represents an exceptionally long time series - a recent compilation of biological time series data, across terrestrial, freshwater, and marine realms, indicates an average duration of only 13-years (Dornelas et al., 2018). Long-term time series in the deep sea are rare, particularly those collecting data from surface to seabed. The PAP-SO is one of two abyssal long-term time series sites globally (Smith et al. 2015), the other being a thirty-year time series at Station M in the northeastern Pacific Ocean (34°50′N, 123°00′W, ~4000 m water depth), maintained by the Monterey Bay Aquarium Research Institute (Smith et al., 2020). This ‘sibling’ abyssal time series site also aims to understand the connections between the surface ocean and the seabed, using many similar techniques (Smith et al., 2017), facilitating comparisons between the two sites (e.g. Durden et al., 2019; Durden et al., 2020a; Laguionie-Marchais et al., 2013; Smith et al., 2009). Another source of extended comparison is the 21 year time series Long-Term Ecological Research Observatory HAUSGARTEN, Frontiers in Arctic Marine Monitoring (FRAM) in the Fram Strait between the North Atlantic and the central Arctic Ocean (78.5°N–80°N, 05°W–11°E, 250–5500 m water depth), maintained by the Alfred Wegener Institute for Polar and Marine Research (Soltwedel et al., 2016; Soltwedel et al., 2005). Much of our understanding of temporal variation in the deep sea, and connections between the surface ocean and the seabed have been derived from research conducted at these observatories.