Deep Sea

Views differ as to which parts of the ocean belong to the deep sea. According to a commonly used definition, it begins where the comparatively flat seafloor of the coastal regions segues into deeper and steeper areas. Depending on the respective region, this can be at very different water depths. In the Antarctic, for instance, the tremendous ice masses weigh down the continent considerably. Its margin, referred to as the continental shelf, can extend up to 500 metres below the water’s surface; only after this point does the deep sea begin. In contrast, in most other regions, this transition begins just 200 metres below sea level.

Another frequently used definition considers all waters beyond the reach of light from the surface to be part of the deep sea. This lightless deep sea encompasses ca. 88 percent of the global oceans and begins at a depth of 200 metres. Consequently, there is considerable overlap between the two definitions.

At this point, we have a fairly good idea of the seafloor’s topography, complete with plains, ridges and trenches – because experts have used echosounders to survey its depth profile and created corresponding maps. However, less than one percent of the seafloor has been examined in detail – e.g. with regard to the fauna living there. As a result, scientists working in the deep sea constantly encounter new species and other surprises. For example, in February 2021 an AWI team discovered the world’s largest fish breeding colony in the Antarctic’s Weddell Sea. Images taken with a camera system towed by the research icebreaker Polarstern captured countless nests of the ice fish species Neopagetopsis ionah on the seafloor, at depths from 420 to 535 metres. There, an estimated 60 million nests are spread across 240 square kilometres, an area the size of 36,000 football fields.

Broad expanses of the seafloor consist of seemingly unvarying, nutrient-poor sediment. Yet scattered about this desert-like landscape you’ll find oases of life, e.g. hot and cold springs. Specially adapted microorganisms feed on the chemical compounds released by the springs, and are in turn fed on by other organisms. In this way, unique biotic communities form near these springs. But species don’t just gather where there’s ample food to be found; the relatively few parts of the deep sea that have a hard seafloor are also attractive. These include places where, following landslides, bare rock is exposed. Or the so-called “dropstones” found in the polar regions: these stones, many of which were transported out to sea by glaciers, are often densely colonised by sedentary animals like sponges, bryozoans, ascidians and small corals.

For many years, the deep sea was assumed to be a hostile desert in which only few species could survive. By now, however, we know that the biodiversity down there is very high. In fact, some experts believe that parts of the seafloor have a biodiversity similar to that of the tropical rainforests. However, you have to take a very close look to find that diversity: in the seafloor sediment, there are hosts of tiny nematodes barely visible with the naked eye. As such, we only know of a fraction of the species living in the deep sea, and new species are discovered with virtually every sample taken.

All higher classes of fauna can be found in the deep sea. Nematodes make up 90 percent of the organisms living in the sediment; much more rarely, crabs and polychaetes can also be found. The seafloor is home to e.g. sponges, sea lilies, serpent and feather stars, sea urchins, starfish and sea cucumbers; the ecosystem’s mobile species include fish and squid. The biotic communities differ according to the water depth, and their occurrence chiefly depends on the available nutrients.

Though fish can be found at all depths, their density is far lower in the bottom-most layers. In the open ocean, you can find e.g. the bizarre deep-sea anglerfishes, which live at depths of ca. 300 metres and below. The females have an organic “fishing rod” complete with bait attached to their heads, and in many species, the bait actually glows. They use this feature to attract males, but also (and especially) prey species. 

But there are also fish at the seafloor. They primarily feed on carrion-eating amphipods, which can be found in abundance near their food sources. Experiments have revealed how quickly and efficiently bits of food that sink to the seafloor are put to use. When an AWI team observed the response to a new whale cadaver in the Arctic deep sea with the help of cameras, they saw that, after just a few minutes, large groups of amphipods gathered and were soon joined by fish related to eelpouts, which engaged in a veritable feeding frenzy.

One of the best-known deep-diving mammals is the sperm whale, which, at depths between 300 and 800 metres, frequently hunts squid; in some cases, even at depths surpassing 2,500 metres. But the diving record is still held by Cuvier’s beaked whale, which on rare occasions has been known to remain underwater for more than three -and-a-half hours, diving up to 3,000 metres beneath the surface. And seals’ performance in this regard is nothing to sniff at, either: the southern elephant seal can sometimes be found more than 2,000 metres beneath the surface.

Organisms that hope to survive in these habitats have to face a range of challenges – from a lack of food, to cold and perpetual darkness, to extreme pressures (e.g. 200 times surface pressure at a depth of 2,000 metres). But with the aid of specific adaptations, denizens of the deep can overcome all these problems. For example, to counter the high pressures, their bodies feature no swim bladders or other gas pockets. And, since food is often in short supply in these habitats, many species have adapted to endure extended periods with no food – e.g. by creating internal fatty reserves. When it comes to the absolute darkness, many organisms have evolved sensory solutions – like biochemical sensors that can detect scents in the water. In addition, fish rely on their lateral line organ, which allows them to detect even the smallest changes in currents and pressure to locate obstacles or other animals moving nearby. And last but not least, many deep-sea organisms have large eyes, helping them pick up the tiny amounts of residual light in the water or the light signals put out by other fauna.

Many deep-sea organisms are capable of producing light, either on their own or with the help of bacteria. This is possible thanks to biochemical reactions in which compounds known as luciferins react with oxygen and the enzyme luciferase. This leads to a molecular change that generates energy in the form of light. In this way, many jellyfish, but also some species of fish, squid, and other deep-sea fauna can emit a blue, green, or in some cases even red light. This is done e.g. to attract potential mates, lure in prey, or to illuminate their surroundings with organic “searchlights”. Some species also emit glowing substances to scare off enemies.

The most important food source for deep-sea organisms is particulate organic matter, which drifts down from the surface to the seafloor and is also known as “marine snow”. From individual cells of dead plankton to clumps of algae, to whole whale cadavers – which do not count as marine snow but are instead referred to as “large foodfalls” – there are meals of all shapes and sizes. Even zooplankton excrement contains enough nutrients for other organisms to get by on. Those species that gather near hot or cold springs pursue a different strategy. There you’ll find specially adapted microorganisms capable of extracting energy from the chemical compounds that the springs pump out into the water. In turn, many other organisms directly or indirectly live off of these bacteria, while others live in symbiosis with them. New life can spring from these deep-sea oases even after thousands of years. For example, in the central Arctic Ocean, a research team including AWI staff was surprised to discover lush gardens of sponges growing on dormant underwater volcanoes. Many creatures that lived on the volcano millennia ago are now long gone – yet their remains linger. And thanks to symbiotic bacteria, the sponges can still put these relics of the past to use. 

Using high-tech tricks, many measuring devices can be adapted for the high pressures found in the deep sea, even those in the Mariana Trench. For researchers, the much greater problem consists in reaching their destination to begin with. This is especially true for ice-covered ocean regions: the ice is constantly in motion, making it a challenge simply to keep the ship in the desired position, let alone to deploy high-tech devices far below the ice cover. Although autonomous underwater vehicles (AUVs) can easily be programmed to scan on their own and then surface at a predetermined location, ice drift means that areas that were open water just a few moments ago are now covered with ice. When this happens, valuable equipment can be lost.

That has since become a commonly used method for investigating life and processes in the bottom-most ocean. For this purpose, the AWI relies on PAUL and his “little sister” SARI – two autonomous underwater vehicles (AUVs) that can be programmed for entire missions. They are deployed from a research vessel, scan a predetermined route independently, taking readings at regular intervals, and then surface again at fixed coordinates for retrieval. Depending on the specific research goals, the AUVs can operate at various depths and be fitted with a broad range of instruments. PAUL can dive down to 3,000 metres, while the smaller SARI has to draw the line at 200 metres.

Another option: remotely operated vehicles (ROVs). Instead of operating on their own, they remain connected to the ship by a specially designed deep-sea cable. Through it, they receive electrical power and commands from the ship and can transmit images and data back to it. ROVs not only allow us to make targeted observations and collect samples with precision down to the nearest centimetre but also to conduct complex experiments in the deep sea.

For that purpose, we have crawlers – autonomous tracked vehicles that can be precisely deployed on the seafloor by free fall or in a cable-tethered frame. Once there, they use their tracks to travel to predetermined sites, where they measure e.g. the oxygen content at different sediment depths. In addition to measuring devices, they have a high-resolution onboard camera, used to capture their surroundings. The AWI crawler TRAMPER can operate at depths of up to 6,000 metres and remain submerged for up to a year. An advanced follow-up model, NOMAD, is larger but – thanks to consistent lightweight design – not heavier than the 1.5-metre-long TRAMPER and can carry four times as much weight in instruments. In the future, an even larger crawler will be available; roughly the size of a minivan, it will not only be able to monitor and record, but also to gather samples.

For observations, experiments and taking measurements directly on the seafloor, “bottom landers” are a good choice – devices that, without a cable, sink down to the seafloor, where they take pictures and conduct pre-programmed experiments. Depending on their payload, they can e.g. monitor flow direction and speed, the water’s oxygen content, or the number of particles that make their way to the deep from the surface.  If the goal is to gather this type of data not on the seafloor but at various depths, moorings are often used – extremely robust lines hanging vertically in the water, which can be several kilometres long and equipped with measuring and sample-taking devices at regular intervals.

For photo and video coverage from the depths, we employ camera systems, which are connected to the ship by a specially designed fibre optic and power cable and are towed just over the seafloor as the ship moves through the water. For example, the AWI’s Ocean Floor Observation and Bathymetry System (OFOBS) consists of a downward-pointing camera and flash, together with a sonar system that scans the seafloor topography to its left and right; all of which is housed in a protective metal frame. 

For the past 25 years, the AWI’s deep-sea focus has been on the Arctic. In 1999, the institute established a long-term observatory in Fram Strait between Greenland and Svalbard. Today, the HAUSGARTEN observatory consists of 21 monitoring stations at depths of between 250 and 5,500 metres beneath the surface. Every summer, researchers use the observatory for their work. In addition, instruments moored to the ocean floor operate year-round, while autonomous underwater vehicles (AUVs) can now be deployed there for winter surveys. 

Taken together, these assets have produced a valuable dataset that documents the long-term trends and changes in this Arctic ecosystem. It’s only with the help of long-term studies (time series) like this one that we can assess how climate change is impacting marine ecosystems in the Arctic. There are only a handful of comparable observatories worldwide, and HAUSGARTEN is the only one located in a polar region.

Indirectly, yes, absolutely. Above all, rising water temperatures are provoking rapid responses in the deep-sea ecosystem. This can already be seen in Fram Strait between Svalbard and Greenland, where the composition of the phytoplankton has changed. Whereas, in past decades, more diatoms grew in the colder water, today you’ll find more foam algae. But the latter aren’t as nutritious for the organisms that feed on them. As a result, the amount of food drifting down from the surface to the deep has declined over the past 20 years. This is turn has affected life at the seafloor. For example, there are only roughly half as many nematodes in the deep-sea sediment as in the past. And where there are fewer smaller organisms, there will eventually be fewer of the larger ones. As such, it’s quite possible that climate change will reduce diversity in the biotic communities of Fram Strait.

In 2012, an AWI team first observed that thinner sea-ice cover could mean a veritable feast for deep-sea fauna, at least in the short term. When instead of up to four metres, the ice is only 90 centimetres thick, it lets more light through. This allows more ice algae to grow on its underside. When the ice melts in the course of the summer, the algae sink to the deep sea in dense clumps. For seafloor dwellers like sea cucumbers and feather stars, this provides such an ample supply of food that they grow larger and reach sexual maturity faster. However, this effect can also be very short-lived: if temperatures rise too high for the ice algae, or if the ice melts completely, taking their habitat with it, there won’t be any more food drifting down from the surface.

The deep sea is home to natural resources that have been a subject of interest since the 1970s. These include manganese nodules, which can be found on the ocean floor at a depth of more than 4,000 metres, especially in the Pacific. In addition to manganese and iron, these clumps contain valuable metals like copper, nickel and cobalt. However, mining them is a technically complex and correspondingly expensive undertaking. As such, there have only been pilot projects; there is no commercial mining network. But many countries and private companies have already applied for exploration licenses with the United Nations’ International Seabed Authority. 

But mining in the delicate ecosystem of the deep sea can do lasting harm. This was demonstrated in the experiment DISCOL (Disturbance and Recolonization), which the AWI and a host of other European research centres contributed to. In 1989, eleven square kilometres of the Pacific seafloor were churned up in an area roughly 650 kilometres southeast of the Galápagos Islands to simulate the mining of manganese nodules. In the years since, several expeditions have returned to the site to track its development. Decades later, the scars are still clearly recognisable, and there have been lasting changes to the biotic community.

In fact, considerable amounts of litter can now be found in the deep sea. The palette ranges from plastic bags and fragments, to glass bottles and the remains of fishing nets, to paint buckets. Packages and bags have been discovered that have apparently been on the seafloor for decades, virtually untouched by time. Even in remote regions like the floor of Fram Strait between Greenland and Svalbard, the number of these large bits of litter has grown substantially in recent years. Moreover, there’s the issue of microplastic. These tiny particles can even be found in the snow cover on Arctic ice floes. When the ice melts, the plastic drifts down to the deep sea, where it can be ingested by various organisms. The ecological consequences of this development remain to be seen.