The Deep Sea Environment – search for resources

The Chronos Group has interests in deep sea exploration. Here we explore some of the environments our clients encounter in their search for sustainable resources.

The different types of sediment which cover the sea floor

There are two main types of sediment, terragenous and bioclastic and less widespread types of sediment from volcanic and hydrothermal vent activity. Sediments can also be classified as pelagic or deep sea sediments. If we look at terragenous sediments first, these are the result of erosion from continental rocks. The material eroded is deposited on the continental shelves by run off or other physical actions and advances the continental shelf seawards by deposition of sediments. Submarine fans may form e.g. the giant Ganges Fan and currents eventually move sediments off the continental shelf and into the abyssal plain. The ocean shifts the coarser material in turbidity currents and there are occasional sudden movements e.g. 1929 Grand Banks in North America turbidity event. Bioclastic sediments are the result of biological activity and include the dead remains of pelagic plants and animals that have sunk. Pelagic bioclastic sediments are also called oozes and may be composed of calcareous or silaceous materials.

Calcareous ooze is composed of chalky remains of foraminifera and pteropods, and forms the deep ocean red clays. The silaceous material is derived from shells of radiolarians and diatoms and found mainly in tropical and polar seas. The distribution of ooze reflects primary production taking place near the surface. The thickness of the sediments also reflects the age of the ocean crust with thickness increasing as we move away from mid ocean ridges for example. Volcanic ash from eruptions can also travel large distances and end by being deposited on the ocean floor, thus contributing to sediments. Finally around hydrothermal events we have unique sediments with metalliferous muds. It should also be noted that sediments on the abyssal plains are not completely static as currents, earthquakes and tectonic activity can move them.

Food availability in the deep sea and how animals cope with limited food supplies.

Basically food availability decreases with depth as does species diversity. The supply of food to the deep sea depends on primary production in the photic zone (except for hydrothermal vent areas). However, it has been estimated that just 2% of phytoplankton sink to the bottom as they are mainly consumed above or on the way down.

Since food is relatively scarce the marine organisms have a number of ways of coping. We can loosely categorize these as

1) Energy conservation adaptations e.g. slow movements, slow metabolisms, and some fish with relatively low muscle mass compared to fish in shallower seas.

2) Related to energy conservation some fish are ambush predators e.g. deep sea Angler fish.

3) Dwarfism and gigantism are methods of coping with food availability e.g. tiny nematode worms.

4) Physiological adaptations also include distended stomachs in some species to cope with the rare chances of feeding e,g, angler fish but even bivalves in the deep ocean have been found to have longer guts to take full advantage of food availability.

5) Related to this opportunistic feeding but perhaps in a class of its own we have the animals adapted to feed on dead whales. These are very important and provide many year’s food supply to an area of the ocean floor in one moment. 43 species have been found on one whale carcass e.g. sharks, hagfish, bone eating zombie worms, snails, limpets, clams and anaerobic bacteria. Since there are many similarities with organisms found round hydrothermal vents these carcasses may have acted as stepping stones from vent to vent.

6) Deposit feeders. Since the deep sea floor is dominated by loosely compacted biogenic ooze it is dominated by deposit feeders like the deep sea cucumber (Scotoplanes).

7) Vertical migration. Some fish move upwards to feed and have replaced swim bladders with fatty deposits in order to cope with the vast differences in pressure. This is just a brief cross section of the ways in which animals cope with limited food supplies.

Some of the adaptations to the deep sea.

We only have space to discuss some of the adaptations to the deep sea. Lets us select five main categories to discuss as follows: adaptations to pressure, temperature, food availability, lack of light, and reproduction. Animals adapt to pressure in a variety of ways e.g. sperm whales have lungs that can compress to 1% of their normal volume, angler fish have reduced skeletons and other fish have reduced muscle mass. Sea cucumbers have bodies largely composed of water and others have proteins and enzymes adapted to work at pressure. Other species have dispensed with shell formation below the carbonate compensation depth. In these ways we see that there are physiological and chemical adaptations to cope with increased pressure. Secondly we have a brief discussion of temperature. The deep sea is largely isothermal with very stable temperatures prevailing that need few adaptations.

Hydrothermal vents are an exception to this rule. As far as food availability is concerned we have discussed in section three above the adaptations animals use to cope ranging from predatory and scavenging behavior, opportunistic feeding on whale carcasses to vertical migration strategies. Lack of light perhaps creates some of the most interesting adaptations. Eyes of fish in the deep sea tend to be generally larger than their counterparts above, although below 2000 meters eyes again grow smaller or are absent. Eyes contain a higher density of rods in the retina or tubular eyes are common e.g. hatchet fish. Where eyes are useless in the total darkness other methods have developed to sense the environment. Lateral lines are well developed to sense vibrations and antennae may also be used e.g. in hairy angler fish.

Bioluminescence is another adaption with 60 to 70% of deep water animals possessing this ability. Organs called photophores, sometimes using bacteria as a light source are found in many fish e.g. lantern fish. Bioluminescence can be used as a lure for food or for defence. Finally we have adaptations in reproduction in the deep sea with eggs with large yolks to combat lack of food, long lived species with slow sexual maturity may also help in this area. The relative difficulty of finding isolated mates may also have led to high degrees of hermaphroditic behavior and the famous adaptation of the tiny parasitic male in angler fish. These are just some of the adaptations to the deep sea.

Kelp forests.

Kelps are large brown algae that form giant kelp forests where there is a floating surface canopy. Kelps beds are lower lying kelps and vertical zonation of kelps helps them to receive as much sunlight for photosynthesis as possible. There are for example understory species (e.g. Nereocystis) and canopy species. Different species of kelp also predominate in different parts of the world with Laminaria sp. dominating in the North Atlantic for example and  Macrocystis kelp forests in the Southern Atlantic. Kelp forests are found throughout the world in the sublittoral zone up to 50 meters deep throughout the cold temperate regions of the Earth. Kelp forests are vital for biodiversity and possess a very high primary production with some species growing several centimeters per day. They support a very diverse ecosystem. The kelps are attached by holdfasts to the rock and even this provides a habitat for young mussels (Mytilus edulus) and starfish. Kelp supports a range of associated species ranging from polychaete worms, isopods, sea urchins and brittle stars to crabs and fish. Detritus from kelp helps support deposit feeders. Finally there are a range of other animals associated with kelp ranging from seabirds to otters. The well documented example from Nova Scotia of Sea otters keeping sea urchins in check on the kelp provides us with a good example of the complex interactions of life in the kelp forests.

Dr Simon Harding

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