Science

Scientific core mission

Much of the transport of mass and energy from the upper ocean to the sea-floor is controlled via the sinking of particles. Thus it is important to understand the functioning of this export. In particular the time scales of sinking and the origin and pathways of the particles is of interest as these factors are crucial to determine the impact of rapidly changing climate on a specific deep-sea ecosystem. The major source of the particles is the photosynthetic production of phytoplankton in the euphotic zone. The material which finally sinks can either be direct phytoplankton biomass or a conversion of it into certain aggregates. To investigate whether a region shows a strong vertical coupling between the surface ocean and the sea-floor or whether it is dominated by lateral fluxes, the variables for controlling the particle export have to be analysed: the characterisation of the particles themselves, particle composition, form and the ability of the environment to re-suspend the particles, the role of ocean transport, and finally the local stratification of the water column. In regions with low stratification and deep mixing, as the Arctic and Antarctic, a very close coupling between surface and deep ocean have been found. It is this coupling and its variability which is the major scientific objective of this proposal.

Figure 1: The ESONET Porcupine Abyssal Plain node and the MODOO deployment location (similar to EuroSITES PAP mooring location).

The Porcupine Abyssal Plain (PAP) time-series station off southwest Ireland (Figure 1) is the longest running, multidisciplinary marine time-series in Europe. Since 1989, regular measurements of a variety of physical and biogeochemical parameters have been undertaken at PAP (see http://www.noc.soton.ac.uk/pap/pubs.php for a rather complete list on publications). Of particular interest in this proposal is the seasonal to interannual variability of downward particle flux that has been documented for the PAP site. The variability has been attributed to changes in surface productivity (Figure 2, left), but longer time series of particle fluxes (Figure 2, right) also show a pronounced interannual variability which can be related to climate modes as the North Atlantic Oscillation (NAO). In most parts of the ocean outside the boundary current regions the strongest water currents and the strongest vertical stratification can be found in the upper ocean. For the sinking particles this is where most of the horizontal drift, away from the vertical and towards a lateral transport, is expected. As stronger the lateral fluxes as less-local/more-widespread is the sinking of particles. The PAP region is characterized by very deep winter mixed layers of 600 to 700m thickness. The vertical motion associated with the deepening of the mixed layer provides an 'express way' for particles to escape the normally well stratified surface ocean. Particles can quickly pass through much of the so called 'twilight zone', the depths between the euphotic zone and 1000 meters. The twilight zone is crucial for the efficiency of particle export as rapid biological consumption and re-mineralization reduce the efficiency of sequestration.

Figure 2: (left) Time series (7/2003 to 6/ 2005) of mixed layer Chl-a concentration (green line) and particle flux from trap data (black bar) at PAP site (Körtzinger et al. 2008). (right) Interannual variability of particle flux at the PAP site from 1990 to 2005.

Combining the PAP observatory with a benthic lander, MODOO is the ideal tool to monitor the critical variables that influence the vertical flux of particles. Our observation strategy comprises a combination of critical physical and biogeochemical variables: Optical instruments located in the euphotic zone and near the sea-floor will provide time series of backscatter intensity at a single point, representing ‘only’ a small volume sample. The backscatter information from four beams of an Acoustic Doppler Current Profiler (ADCP) will be used to obtain backscatter density of a much larger volume (in the order of 8 meters vertical thickness) but also over a much larger distance (up to 500m). In addition to the vertical information, the ADCP backscatter data will be used to investigate the horizontal homogeneity of the backscatter signal by comparing the time series of the four individual beams. This will allow to elaborate the near station 'patchiness' of the observations. One ADCP will be mounted at about 150m depth on the mooring in a downward looking mode. This instrument will provide information from most of the 'twilight zone'. A second, high frequency ADCP will be mounted on the lander for a detailed survey of near bottom processes. Here we are particularly interested in observing possible re-suspension of material e.g. through the influence of tidal currents, which are expected to be the most significant motions at this location.
In addition to backscatter information, the water currents and thus the horizontal transport will be obtained from the ADCP data. They will provide information about the relative importance of lateral transport processes and facilitate conclusions on how “local” and small scaled the observations are. ADCPs allow us to monitor most of the twilight zone and of the bottom boundary. To get full water depth information single point rotor current meters (RCM) placed at specific water depths will record the respective current regime. Another important physical process that controls the particle sinking is the vertical stability of the water column. Stability will be recorded with a number of MicroCat CTD's mounted on the mooring and concentrated in the upper 1000m. Another CTD will be mounted on the lander. A number of biogeochemical parameters will be available from the PAP EuroSITES configuration: Chlorophyll-a time series near the surface in the euphotic zone will give an estimate of phytoplankton biomass production. Oxygen, nutrients (Nitrate+Nitrite) and pCO2 are accompanied observations which will allow to differentiate near-surface local (biogeochemcial) and remote (physical) forced changes on the phytoplankton production. Planned in-situ measurements of oxygen consumption (as part of the EuroSITES science program) may provide local respiration and from this also re-mineralization efficiency to be estimated.
All the above listed sensor data will be oriented along a common time line which will allows to view and analyse the data in a comprehensive way. Fundamental to the integration of the datasets are the DCD nodes, one on the mooring and one on the lander. DCD nodes store data on a local disc and provide an accurate time stamp through a very precise internal clock. If required, we will also make use of additional third party data, as remotely sensed Chlorophylla, sea surface temperature data or atmospheric data. Argo float profile data will be used to bring the moorings upper ocean (2000m) T/S data into a wider spatial context.

Guest scientific mission
Deep Sea Marine life (SArea4 – Marine Ecology)
Life in the deep sea almost entirely depends on the fall out of organic matter from the surface layers. Consequently, the abundance, biomass and composition of deep sea marine life is suspected to be closely linked to the patterns of surface productivity and its link with the deep sea (see scientific core mission). In the PAP region strongest surface deposition of phytoplankton has been observed in early spring (April/May) and late summer (September). From discrete sampling of the deep sea environment at PAP sudden changes in deep sea marine life species have been observed. During the period 1997 to 2000 a sudden infestation of the Northeast Atlantic Ocean abyssal plain by sea cucumbers Amperima rosea (Figure 3) and brittle stars Ophiocten hastatum was detected.

Figure 3: Holothurian (sea cucumber) Amperima rosea on the seafloor at PAP

The changes in composition of the deep sea fauna have been attributed to changes in fluxes of organic matter to the deep sea influenced by the North Atlantic Oscillation. Although the Porcupine Abyssal Plain location is the best monitored deep sea abyssal location in Europe there is an urgent need to establish continuous monitoring at this location to record changes over time in the oceans around Europe. Within MODOO we will monitor the deep sea marine life by collection of sea floor photography and the recording of deep sea sound. Low resolution copies of the photographic images will be accessible on daily basis in quasi real-time via the acoustic underwater and surface telemetry.
There will be a technological demonstration on the remote control of the imaging system via the acoustic telemetry link (see technological advancements). A passive bioacoustic sensor will monitor the natural sounds generated by animals within its detection range, as well as the background noise level. Data from the passive bioacoustic sensor and other instruments on the Lander will allow us to investigate animal reaction to noise disturbances, and the natural acoustic behavior of deep ocean animals.

Geohazards (SArea1 – Geosciences)
As part of the MODOO deployment geodynamical sensors (ocean bottom pressure and seismometer) will be deployed with the lander. The device is a Longterm OBS for Tsunami and Earthquake Research (LOBSTER). Seismicity data from the region is sparse and the experiment will provide a baseline data set for further studies. The instrument will be tested as part of the telemetry system although not sending data in real-time but in case of an event. Here we will 'simulate' the event by accessing the LOBSTER in predefined intervals.

The image shows the Ocean Bottom Seismometer (OBS) with 3 axis seismometer and hydrophone (1&2 locator (flashlight, signal flag), 3 acoustic releaser, 4 syntactic foam, 5 data recorder, 6 batteries, 7 weight)