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Project title and acronym

SEAsonal food web interactions with the biological pump at the PAP site – SEAPAP

Host facility

PAP (ID No. 5) – Porcupine Abyssal Plain

Modality of Access

MoA2 – Partially remote (the presence of the user is required at some stage, e.g. for installing and uninstalling an instrument).

Description

The objective of the proposed work is to study food web interactions with the biological pump at high temporal resolution (hours to days) on long-term time scales (year(s)). The PAP site has long-term measurements of vertical particle flux for over 20 years and has observed clear inter-annual and seasonal variation in the mass fluxes (Lampitt et al. 2001; Progress in Oceanography 50: 27-63). Still, no link between surface ocean processes and deep ocean flux was so far established (Lampitt et al. 2010; Deep-Sea Res II 57: 1267-1271). This shows the need of detailed process oriented studies investigating the biologically mediated particle transformations occurring in the twilight zone – the depths between the surface ocean and the deep ocean. The proposed work will study how food web induced changes in sinking aggregate composition and size spectra influence the export flux to the deep ocean. This will be done through the deployment of a novel Multi-Sensor-Platform (MSP) with the already existing PAP#1 and PAP#3 deep ocean sediment trap moorings. The MSP quantify in situ particle abundance and size distribution at high temporal resolution (hours to days) and collect and preserve intact fragile sinking organic aggregates. The deployment of the MSP together with the PAP#1 and PAPä3 deep ocean traps provide long-term information on how changes in particle types and size distribution influence the export flux on high temporal resolution. The infrastructure at PAP site offers a unique study site where the link to the long- term monitoring of the surface ocean processes such as temperature, salinity, CO2, chlorophyll, and irradiation will make it possible to relate the fluxes and particle composition in the deep ocean to the surface properties shaping the flux. Additionally, combining long- term data sets from the time-series measurements with snap-shot studies during the deployment and recovery cruises will allow in-depth process studies of both short and long- term controls on the biological pump. The short-term processes will focus on the relative importance of particle transformations such as aggregate sinking velocities, microbial degradation and zooplankton flux feeding for POC flux attenuation and export to the epipelagic, mesopelagic, and deep ocean. The findings from the short-term processes will be related to the aggregate transformations obtained from the MSP, the PAP#1 and PAP#3 moorings, and the biological, chemical, and physical sensors deployed as part of the PAP site infrastructure (moorings and gliders) to identify the controlling processes for the biological pump.

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Project title and acronym

Benthic organic matter cycling dynamics at PAP (PAP‐Dynamic)

Host facility

Porcupine Abyssal Plain Observatory (PAP)

Modality of Access

MoA2 – Partially remote (the presence of the user is required at some stage, e.g. for installing and uninstalling an instrument).

Description

We propose to add time series of seafloor oxygen microprofiles to the established long term benthic observatory at PAP. Based on this we would address seasonal changes in benthic carbon mineralization and connect it to seasonality in organic matter supply t o the deep‐sea floor. Benthic organic matter remineralization is a bulk measure of the metabolic activity of seafloor communities. It is considered a key function of abyssal ecosystems with strong implications for the global carbon cycle. Remineralization rates at the seafloor determine the amount of organic carbon supply from the upper water column that is returned to the s ea and eventually the atmosphere as inorganic carbon and the amount that is sequestered in the sediments and removed from earth’s active carbon pool. Organic matter supply to the deep sea is not constant but varies with seasonal and interannual forcing. For the PAP region this has been shown with sediment trap studies of vertical fluxes of sinking particles and marine snow imaging (Lampitt et al., 2001, Smith et al., 2009). Time‐lapse photography series of the sediment surface at PAP obtained with the free vehicle camera system ‘Bathy snap’ proved the occurrence of phytoplankton bloom‐related sedimentation events at the seafloor and documented the immediate utilization of the food supply by the seafloor megafauna community. In situ ‘food pulse experiments’ have shown the ability of the sediment community at PAP to quickly respond to organic matter input and the transfer of biomass trough the sediment infauna community’s food chain (Witte et al., 2003). The quantification of overall benthic remineralization in response to natural sedimentation events, however, is still not fully achieved. This knowledge is relevant for a general understanding of temporal dynamics in benthic activity and a prerequisite in order to determine annual organic matter turnover and close the carbon budget for the deep ocean. Uptake of oxygen by the sediment is a standard measure to quantify organic matter remineralization because oxygen as the terminal electron acceptor integrates any metabolic pathway involved. Traditionally, oxygen uptake is measured by benthic enclosures (chamber incubations) or by assessing oxygen distribution in the diffusive boundary lay er (DBL) at the sediment water interface and in the pore waters of the uppermost sediment horizon based on oxygen microprofiles (Fig. 1). Although the microprofile approach misses the contribution of the macrofauna, the flux rates obtained with the two methods in deep sea environments are typically very similar as the microbial communities largely dominate benthic remineralization (Wenzhöfer & Glud, 2002). Moreover, previous studies have also found no clear influence of macrofauna on sediment community oxygen consumption (Ruhl et al., 2008). A big advantage of the microprofiler approach is its minimal invasiveness that allows for repeated measurements at almost the same spot which reduces the influence of spatial variability. Additionally, microprofiles provide information on the oxygen distribution within the sediment (oxygen penetration depth) which may reflect varying food availability during a seasonal cycle. Connected to this, the effort needed for repeated flux measurements is comparably small. While repeated chamber incubations call for mobile platforms (i.e., benthic crawlers) time series of microprofiler measurements may be obtained using simple translating or rotating units attached to stationary benthic installations.

 

Fig. 1: Example of an oxygen microprofile. The thin line marks the oxygen gradient across the DBL

 

We propose to deploy an oxygen microprofiler at PA for a full year or longer to study seasonality in diffusive benthic oxygen fluxes as a proxy of organic matter remineralization (4‐8 replicate 20 cm long profiles at weekly intervals). Results obtained would be compared to studies of seasonality in organic matter supply carried out at NOC. This includes quantifications of particle fluxes studied with trap moorings and qualitative time series observations of dynamics and occasional peaks in organic matter supply and benthic megafauna activity obtained with Bathysnap.