The importance of transparent exopolymer particles over ballast in determining both sinking and suspension of small particles during late summer in the Northeast Pacific Ocean

Gravitational sinking of particles is a key pathway for the transport of particulate organic carbon (POC) to the deep ocean. Particle size and composition influence particle sinking velocity and thus play a critical role in controlling particle flux. Canonically, sinking particles that reach the mesopelagic are expected to be either large or ballasted by minerals. However, the presence of transparent exopolymer particles (TEP), which are positively buoyant, may also influence particle sinking velocity. We investigated the relationship between particle composition and sinking velocity during the Export Processes in the Ocean from RemoTe Sensing (EXPORTS) campaign in the Northeast Pacific Ocean using Marine Snow Catchers. Suspended and sinking particles were sized using FlowCam for particle imaging, and their biogeochemical composition was assessed by measuring the concentration of particulate organic carbon (POC) and nitrogen, particulate inorganic carbon, biogenic and lithogenic silica, and TEP. Sinking fluxes were also calculated. Overall, both suspended and sinking particles were small (<51 μm, diameter) in this late summer, oligotrophic system. Contrary to expectation, the ratio of ballast minerals to POC was higher for suspended particles than sinking particles. Further, suspended particles showed TEP-to-POC ratios three times higher than sinking particles. These ratios suggest that TEP content and not ballast dictated whether particles in this system would sink (low TEP) or remain suspended (high TEP). Fluxes of POC averaged 4.3 ± 2.5 mmol C m−2 d−1 at 50 m (n = 9) and decreased to 3.1 ± 1.1 mmol C m−2 d−1 at 300–500 m (n = 6). These flux estimates were slightly higher than fluxes measured during EXPORTS with drifting sediment traps and Thorium-234. A comparison between these approaches illustrates that small sinking particles were an important component of the POC flux in the mesopelagic of this late summer oligotrophic system

Sinking Organic Particles in the Ocean—Flux Estimates From in situ Optical Devices

Optical particle measurements are emerging as an important technique for understanding the ocean carbon cycle, including contributions to estimates of their downward flux, which sequesters carbon dioxide (CO2) in the deep sea. Optical instruments can be used from ships or installed on autonomous platforms, delivering much greater spatial and temporal coverage of particles in the mesopelagic zone of the ocean than traditional techniques, such as sediment traps. Technologies to image particles have advanced greatly over the last two decades, but the quantitative translation of these immense datasets into biogeochemical properties remains a challenge. In particular, advances are needed to enable the optimal translation of imaged objects into carbon content and sinking velocities. In addition, different devices often measure different optical properties, leading to difficulties in comparing results. Here we provide a practical overview of the challenges and potential of using these instruments, as a step toward improvement and expansion of their applications

Slow-sinking particle export measured in the North Atlantic during the M87/1 cruise in April 2012

Slow-sinking particles were sampled using the Marine Snow Catcher (MSC). For a full description of the MSC and flux calculations see Riley et al. (2012). The MSC was deployed at four depths between 50 - 650 m during four visit at Stations 1 (63°3' N 11°0' W) and three visits at Station 2 (62°5' N 2°3' W) to obtain depth profiles of slow-sinking material. The MSC was further deployed at 50 m during two visits at Station 3 (60°2' N 1°0' E). A total of 33 MSC were deployed. Slow-sinking particles were analysed for particulate organic carbon (POC), particulate inorganic carbon (PIC), biogenic silica (BSi), and Chlorophyll a (total, >10 µm)

Meteorological data from RRS Discovery cruise DY086 at site P3

Meteorological data were collected across three visits to site P3 during COMICS cruise DY086 aboard the RRS Discovery in November and December, 2017. Measurements included: air pressure, temperature and humidity; solar and photosynthetically active radiation at both Port and Starboard sides. Data were provided by the British Oceanographic Data Centre and funded by the National Environment Research Council. BODC advised that the ship's anemometer shows inconsistencies and so data from the instrument were not included

Vertical imbalance in organic carbon budgets is indicative of a missing vertical transfer during a phytoplankton bloom near South Georgia (COMICS)

The biological carbon pump, driven principally by the surface production of sinking organic matter and its subsequent remineralization to carbon dioxide (CO2) in the deep ocean, maintains atmospheric CO2 concentrations around 200 ppm lower than they would be if the ocean were abiotic. One important driver of the magnitude of this effect is the depth to which organic matter sinks before it is remineralised, a parameter we have limited confidence in measuring given the difficulty involved in balancing sources and sinks in the ocean's interior. One solution to this imbalance might be a temporal offset in which organic carbon accumulates in the mesopelagic zone (100–1000 m depth) early in the productive season before it is consumed later. Here, we develop a novel accounting method to address non-steady state conditions by estimating fluxes of particulate organic matter into, and accumulation within, distinct vertical layers in the mesopelagic zone using high-resolution spatiotemporal vertical profiles. We apply this approach to a time series of measurements made during the declining phase of a large diatom bloom in a low-circulation region of the Southern Ocean downstream of South Georgia. Our data show that the major export event led to a significant accumulation of organic matter in the upper mesopelagic zone (100–200 m depth) which declined over the following weeks, implying that temporal offsets need to be considered when compiling budgets. However, even when accounting for this accumulation, a mismatch in the vertically resolved organic carbon budget remained, implying that there are likely widespread processes that we do not yet understand that redistribute material vertically within the mesopelagic zone

Acoustic backscatter data from two visits to site P3 (P3A and P3B) during COMICS cruise DY086 in November to December, 2017

Acoustic backscatter data was collected at five frequencies (18, 38, 70, 120 and 200 kHz) across two visits to site P3, South Georgia, aboard the RRS Discovery during DY086. Acoustic backscatter was measured with the Simrad EK60. The data consistently shows no evidence of synchronised diel vertical migration