Measurements Under the Ice
The underside of sea ice, which is in direct contact with the ocean below, is of particular interest to researchers. Due to desalination processes, it is here that highly concentrated brine seeps out of the ice and here that the solid ice cover slowly grows – not to mention that it’s teeming with life. After all, the complex food web begins on the underside of the ice, where countless bacteria produce biomass, and with it, sustenance for a host of worms, crustaceans, fish larvae and juvenile fish, which not only feed just below the ice, but also use it to hide from predators. Yet as important as the underside of the ice is for researchers, it is also particularly difficult for human beings to access. Here we’ll show you how cutting-edge technologies like remote-controlled underwater robots (remotely operated vehicles, ROVs) can allow us to explore the magical world below the ice without harming it.
A remotely operated vehicle (ROV) is an underwater robot that, connected to a control unit on the ice or on board a ship, is used to explore sea ice from below. The type of readings taken and parameters measured differ greatly depending on the ROV. The complexity of the measuring system also varies considerably, ranging from simple floating cameras to highly complex multi-sensor platforms. With the help of ROVs, the hard-to-reach underside of the sea ice can be repeatedly explored in a non-disruptive and non-destructive way.
In marine research, the most common areas of application for ROVs are:
- Capturing video and photographs of the underside of the sea ice
These are most often used to document ROV dives but are also directly analysed in order to e.g. quantify the distribution of biomass within and below the sea ice. Can also be used to examine pressure ridges and dynamic structures and to track their development over time. ROVs are also used to check the status of sensors and instruments operating below the sea ice, and in some cases, to deploy or retrieve them. - Light measurements under the sea ice
Transparency and light intensity are key components in determining the energy budget of the sea ice and upper ocean. Here, spectral radiometers are often used to map the lighting conditions at different depths. - Sea-ice thickness and draft measurements
Echosounder readings measure the distance between the ROV and ice and, together with the ROV’s current depth, the sea ice’s draft. These can be taken as point measurements or via multibeam echosounders and can be used to map the topography of the underside of the sea ice. The ice’s melting and freezing rate can be deduced as changes over time. - Oceanographic measurements
In addition to measuring instruments that focus directly on the properties of the sea ice, many ROV systems include onboard sensors that record the physical, biological and geochemical parameters of the ocean – e.g. the salinity, temperature, oxygen saturation or the amount of chlorophyll and dissolved substances in the water column. This yields valuable information on the ecosystem in and below the sea ice. - Hauling nets
Larger ROV systems can also haul nets to catch e.g. zooplankton. This can help to more precisely determine the composition of the ecosystem and to distinguish between depths and regions below the ice.
ROVs are most often controlled from the surface of the sea ice (for station-based work) or directly on board. ROV systems always consist of the underwater robot itself and a surface unit that essentially represents a large computer system. This computer system prepares and visualises all data for the pilot, while simultaneously recording it. One central element is the control unit, which looks a bit like a controller for a video game console. The size of the surface unit ranges from a simple briefcase to stationary systems built into control booths or containers.
The polar oceans receive heat from the temperate latitudes via ocean currents, which counteract the effects of sea-ice cover. On the coasts of Greenland and Antarctica, interactions between the ocean currents and ice sheets take place. In the process, new water masses are formed that have a major influence on global circulation in the oceans.
Physical oceanography methods are used to investigate processes in the Arctic Ocean and Southern Ocean, improving our understanding of climate change and their ecosystems.
A CTD sampler, also referred to as a rosette sampler, measures conductivity and temperature along a vertical profile in the water column. To do so, the sampler is lowered to depths of several thousand metres on a steel cable. CTD stands for conductivity, temperature and depth; the parameters measured are used to determine these three values. The salinity and density of the water are derived on the basis of the conductivity and taking into account the temperature and pressure. In addition, during a single CTD measurement, up to 24 water samples, 12 litres each, can be gathered at various depths, making it possible to gauge the presence of trace elements and microorganisms at each depth. During measurement, the recorded data is transmitted back to the ship via the steel cable, where it is monitored by a computer. The decision on which depth to sample at is also made on board. To collect a sample, the ship transmits a signal that closes the bottles on the sampler.
The system consists of the underwater CTD unit (SBE9plus with attached sensors), the CTD equipment on board the ship (e.g. the winch), the rosette carousel (SBE32) with 24 12-litre bottles, and a computer for recording the data.
The underwater CTD unit is normally equipped with two temperature (SBE3plus) and two conductivity sensors (SBE4), as well as two pumps (SBE5T), which are calibrated at regular intervals. As a rule, one or two oxygen sensors (SBE43) are also attached. Various external sensors, which can be used at depths of up to 6,000 m, are also available: fluorometers for chlorophyll a and coloured dissolved organic matter (CDOM), transmissometers, altimeters that measure the distance from the seafloor, and mechanical bottom contact detectors.
ADCPs (acoustic Doppler current profilers) were developed to provide acoustic current profiles of device-dependent depth ranges in the ocean. Their main applications include the creation of time series for fixed sites via oceanographic moorings and creation of current profiles from on board moving vessels via vessel-mounted (VM) ADCPs. The vertical range that ADCPs can potentially cover depends on the frequency and the reflecting particles in the water column. In this regard, instruments with higher frequencies (e.g. 1200 kHz) are generally used for higher vertical resolution (0.5-m measuring interval, ~10 m range), while those with lower frequencies (e.g. 75 kHz) are used for broader applications with lower vertical resolution (e.g. 16-m measuring interval) and larger range.
Vessel-mounted ADCPs (VMADCPs) are comprised of a transducer, the electronics housing, a computer located in the echosounder room, and an additional monitor located at the winch control stand. The 150 kHz transducer is mounted behind an ice-protection porthole and can penetrate water depths of up to 350 m with a resolution of 4 m (measuring interval at speeds between 1 and 5 knots) and an accuracy of ca. 0.30 m/s for a single-ping reading.
Here you can see an overview of a vessel-based ADCP reading. While underway, the research vessel measures the flow speed of the water column below using two components: the zonal (east / west) flow and meridional (north / south) flow. Here we can see the meridional component: depending on the frequency, the Doppler speed-measuring system registers signals to depths of between 250 and 1,000 metres. As can be clearly seen, the first few hours are dominated by a southward flow, which is later replace by a northward flow before reversing again. These changes could be due to tidal forces or large-scale gyres. The blue background colour at greater depths indicates that no speed readings are available.
This Doppler current profiler measures the flow speed in the water column. To do so, it emits acoustic signals at fixed intervals, which are reflected back by objects in the water like plankton and suspended particles. The current’s speed is calculated using the frequency change in the reflected signals (Doppler effect), while the travel time of the acoustic signals allows the relative distance from the transducer to be derived.
Acoustic Doppler current profilers (ADCPs) can be used on a wide range of platforms; different variants cover different physical ranges and frequency ranges. Depending on their design and configuration, ADCPs can be used to measure three-dimensional water velocity at different scales of water depth.
The MicroStructure Sonde (MSS) is an instrument used to measure small eddies and turbulences in the water column and provides the observational basis for a better grasp of mixing and small-scale processes in the ocean. The MSS is a sensor-laden, 1- to 1.25-m-long cylinder that creates profiles of the water column as it descends in freefall (at ca. 0.7 m/s). The probe is connected to a winch-driven cable and transmits the data it measures to a computer in real-time, showing the operator all relevant information (measuring depth, descent speed, temperature etc.). Depending on the site and research focus, the measurements can be gathered by lowering the probe through a hole in the sea ice or directly from on board a research vessel.
The turbulence sensors chiefly consist of two high-sensitivity velocity shear probes, a fast temperature sensor and an acceleration sensor, combined with sensors for measuring conductivity, temperature and depth (CTD). Shear is also recorded by a micro-thermistor for fast temperature measurement with a measuring frequency of 1024 Hz (1024 measurements per second), which, at a descent speed of less than 1 m/s, yields a vertical resolution in the millimetre range. The other CTD sensors use a frequency of 24 Hz. The velocity shear probes are high-sensitivity pins with specially designed tips (airfoils) which, thanks to a special coating (piezoceramic), can detect even minimal changes caused by eddies in the water. These parameters form the basis for calculating the dissipation of energy and vertical exchange of water particles. By simultaneously measuring temperature and salinity, the MSS can identify vertical heat flows, which are important to understanding how warmer ocean layers affect the ocean’s surface and sea-ice cover. In addition, the MSS can optionally be fitted with chlorophyll, oxygen or light-transmission sensors, yielding valuable data for interdisciplinary questions on how physical processes affect biological and biogeochemical conditions.
Small-scale processes in particular are often inadequately represented in ocean and climate models, which reduces the reliability of climate projections. Despite the pressing need to improve these models with the aid of targeted small-scale measurements, observations like microstructure measurements are still comparatively rare in the polar regions, due in part to the complexity of the sensors and the relatively high amount of time and effort required. The MOSAiC expedition recently laid the foundation for an unprecedentedly diverse range of observations, including a high-resolution MSS dataset.
Oceanographic moorings are autonomous long-term observing systems that are installed at key points in the global ocean circulation and collect data continuously. These marine research instruments also enable continuous data collection in the polar regions and provide valuable insights into changes in the world's oceans. The functionality and deployment of the moorings is described here. The data portal provides an overview of historical and current mooring data.
Moorings are stationary monitoring systems that continually gather key oceanographic data at a fixed location for years at a time. Each mooring consists of an anchor, a long wire, and several floats, which keep the wire positioned vertically in the water column. Various monitoring devices are mounted along the length of the wire and autonomously measure essential parameters like the water temperature, salinity, flow direction and speed. In addition, special-purpose devices can be added to measure e.g. the draft of the sea ice drifting above the mooring, to collect samples of suspended sediments in the water, gather water samples, or even record the vocalisations of marine mammals.
Moorings can be several kilometres long and, during ship-based expeditions, are deployed with the aid of cranes and winches. The anchor is installed on the seafloor, while the floats attached to it keep the wire nearly vertical in the water column. The length of the wire is measured precisely, ensuring that the topmost floats remain a safe distance from the water’s surface – and preventing them from being damaged or even torn off by drifting sea ice or icebergs. The monitoring devices are pre-programmed and attached to the wire as it is deployed. They work autonomously, recording e.g. the water temperature, salinity or flow characteristics on an hourly basis. Thanks to high-performance batteries, they can operate nonstop for years at a time.
Whenever a mooring is deployed, its precise geographic position is recorded, so that a research vessel can come back to the same spot between a year and five years later and retrieve the device, together with its precious data. But how are moorings recovered?
At the lower end of the wire, just above the anchor, there is a special device known as a “releaser”. When it picks up acoustic signals emitted from on board the ship, it opens a catch connecting the wire to the anchor. Now released, the wire, along with its floats and monitoring devices, rises to the surface.
The signal is transmitted from the ship using “Posidonia”, an acoustic positioning system which is even capable of tracking the mooring as it rises in the water. The acoustic communication can even work at depths of several kilometres. Within minutes, the floats appear on the surface, one after the other, making it possible to retrieve the mooring. The multi-metric-ton anchor, which is often made of several railcar wheels welded together, remains on the seafloor, where it will gradually be colonised by local marine organisms. Once retrieved, the monitoring devices are removed from the wire and their stored data is transferred to a computer for analysis.
Ship-based expeditions, especially in the polar regions, are extremely expensive and logistically challenging. Yet they are also essential to gaining a better grasp of our climate system and the dramatic changes it’s currently undergoing. And in marine research, there are also unique challenges when it comes to gathering observational data:
The majority of ship-based scientific expeditions take place in summer, i.e., from May to October in the Northern Hemisphere and from November to April in the Southern Hemisphere. In summer, the sea-ice cover is low, and the ice is comparatively thin. Even powerful icebreakers can only effectively traverse the polar oceans at this time; in winter, it’s far more difficult, if possible at all, which is why we have virtually no data from these regions for the winter months. Another challenge: the limited availability of research vessels. The number of suitable ships is small, which greatly limits the spatial coverage of ocean-based measurements. Although it is sometimes possible to deploy multiple ships at different places simultaneously, doing so is the exception, not the rule. Moreover, there are a diverse range of processes to investigate. Beyond physical parameters like temperature, salinity and flow characteristics, many more aspects and processes are essential to gaining a holistic understanding – such as the chemical composition of the polar oceans, marine microplastic pollution, and changing ecosystems. It is only by simultaneously observing a wide range of these aspects that researchers can gain a comprehensive grasp of the processes involved. Even so, this requires the efforts of numerous experts and specialised devices.
These challenges can only be overcome with the help of autonomous sensors and suitable platforms, like moorings or monitoring buoys. One example is the parallel operation of multiple moorings equipped with physical and biogeochemical sensors throughout a given region, e.g. in Fram Strait (between Greenland and Svalbard) or the Weddell Sea (Antarctic), over a span of several years or even decades. This method has been successfully applied at the Alfred Wegener Institute for years and forms the core of our long-term observations (time series). The long-term data obtained from moorings is invaluable for climate research and can be viewed in the data portal of the SEA ICE PORTAL and used for further research purposes.
Though deploying moorings may sound simple in theory, any number of problems can emerge in practice.
Each mooring must be painstakingly designed, as early as the planning phase. One particularly important aspect: the ratio of air-filled, pressure-resistant floats to the weight of the devices attached to them. The positions of the floats along the wire, as well as the specific sensors and their respective depths, have to be carefully calculated. When mistakes are made, it can lead to floats bursting, sensors getting waterlogged, wires snapping or metal components like shackles and rings corroding. Fortunately, these problems rarely arise.
Another common problem is known as “biofouling”: especially those sensors deployed in shallow waters are gradually covered by organisms, which can adversely affect the quality of their data.
Further, there is the risk of moorings being lost or failing to rise to the surface after being released. There can be various causes: imprecise positioning data, a broken releaser, a snapped wire, or poor visibility due to e.g. fog. But on the open water, recovery is nearly always possible, unless there are extreme weather conditions like high winds.
In ice-covered regions, however, recovery is more difficult. Here, the mooring has to be acoustically located and the surrounding sea ice broken up. With a bit of luck, the floats can find their way between the floes at the surface. Then the ship has to skilfully approach the mooring without crushing it between the floes. The last few metres are covered with the aid of the “mummy chair” (a metal basket mounted on an outboard crane and used for transporting staff and crew), which is connected to the ship by a cable.
Though deploying moorings may sound simple in theory, any number of problems can emerge in practice.
Each mooring must be painstakingly designed, as early as the planning phase. One particularly important aspect: the ratio of air-filled, pressure-resistant floats to the weight of the devices attached to them. The positions of the floats along the wire, as well as the specific sensors and their respective depths, have to be carefully calculated. When mistakes are made, it can lead to floats bursting, sensors getting waterlogged, wires snapping or metal components like shackles and rings corroding. Fortunately, these problems rarely arise.
Another common problem is known as “biofouling”: especially those sensors deployed in shallow waters are gradually covered by organisms, which can adversely affect the quality of their data.
Further, there is the risk of moorings being lost or failing to rise to the surface after being released. There can be various causes: imprecise positioning data, a broken releaser, a snapped wire, or poor visibility due to e.g. fog. But on the open water, recovery is nearly always possible, unless there are extreme weather conditions like high winds.
In ice-covered regions, however, recovery is more difficult. Here, the mooring has to be acoustically located and the surrounding sea ice broken up. With a bit of luck, the floats can find their way between the floes at the surface. Then the ship has to skilfully approach the mooring without crushing it between the floes. The last few metres are covered with the aid of the “mummy chair” (a metal basket mounted on an outboard crane and used for transporting staff and crew), which is connected to the ship by a cable.
