Australia: The Land Where Time Began

A biography of the Australian continent 

Tethys Ocean Productivity & Recycling

About 6,000 billion tons of phytoplankton grows per year in the photic zone around the world's oceans. All are single-celled protists that contain a tiny amount of chlorophyll that utilises sunlight to build their bodies from chemical nutrients in the water, though this production in marine plankton is not at a constant rate in time or space. The same process is believed to have almost certainly been occurring in the Tethys. When the conditions are optimum they reproduce at extremely high rates to produce large concentrations covering huge areas of the oceans, spring blooms.

Diatoms are one of the most abundant groups, very small organisms that live in silica glass boxes of various shapes and designs. They reproduce asexually every 12-24 hours, the cells becoming smaller after each division until they can no longer reproduce asexually, at this point they emerge from their glass cases and reproduce sexually, growing to their full size, then secrete a new home. Most diatoms store food as small oil drops and fatty acids, that also assist with the maintenance of buoyancy to try to gain a prime position as they jostle for the best position to receive the maximum amount of sunlight.

Coccoliths and dinoflagellates also inhabit tropical seas. Their complex spherical homes are constructed of tiny patterned calcium carbonate (lime) discs. They can multiply at phenomenal rates in certain seasons, producing a spring bloom of milky turquoise patch on the ocean surface containing 25 billion billion individuals. These spring blooms have been observed from satellites to cover 250,000 km² of ocean, about the size of Great Britain.

Another type of photosynthetic organism are dinoflagellates, organisms that can be seen at night as they glow green-blue by bioluminescence, often being seen in tropical oceans in the bow waves of moving ships. They are active swimmers, added by 2 long whip-like flagella, among the plankton with a tough cellulose coat. Some species of dinoflagellate have lost their coating, living as symbionts in the bodies of certain jellyfish, corals and molluscs. They produce food for their host in exchange for nutrients and protection. They are believed to have lived in similar circumstances in the Tethys.

For many years cyanobacteria were not recognised, and so unsampled, in the phytoplankton, as they are of an ultramicroscopic size. They are so small the author³ says more the 500,000 could fit on the head of a pin, yet they are responsible for 80 % of the primary production in the surface waters of the ocean. They also form dark dense green mats, that are rather slippery, on the seafloor where the water is shallow enough for the bottom to be in the photic zone. They are also responsible for the stromatolites as seen at Shark Bay in Western Australia.

Together with the phytoplankton are the zooplankton, the grazers and predators, also single-celled organisms that secrete sculptured shells to live in, about the size of grains of sand. To make the most of the spring blooms of the phytoplankton they also reproduce at a rapid rate to keep pace with the spring bloom. They are comprised of foraminiferans with calcium carbonate shells that are multichambered, and radiolarians that produce shells of silica glass spheres that are spiky. Filopeda, fine strands of their cell material ('feet of thread') are extended through tiny pore in the shells of both organisms, that ensnare other microscopic organisms that are then drawn into the parent cell where it broken down with enzymes. Organisms of similar size to the predators and grazers can also be caught, in which case it is broken up externally, the parts of the body then being drawn into the cell as usual.

The rest of the ocean animals are fed by the phytoplankton and their grazers and predators, passing up short food chains and webs, that are commonly complex, to the top marine predators. Whether eaten or dying naturally, the lifespan of these microscopic organisms is short, from a few hours to a few weeks, the dead organisms and their debris slowly falling to the ocean floor in a constant shower in all parts of every ocean. It has been found that there is a constant drift of dead and decaying organic matter, faeces of various sizes, from that of whales to that of the tiniest of organisms, as well as organic detritus that has been either washed or blown into the ocean from that land. As many of the particles of this rain are extremely small it would take a very long time for them to reach the sediment surface, but when they collect in conglomerates of particles of various sizes they sink faster, all falling as 'marine snow'.

When the surface productivity is high in particular areas of the ocean and at certain times of year the debris falls faster, reaching the floor of the deepest parts of the ocean, the seafloor being blanketed in a carpet of organic rich mud.

There are also microbial breakdown organisms in the plankton at the surface, attaching to any dead cells or faecal material they encounter and begin the process of decay and recycling. They are also found in the twilight zone of the oceans where they attach to particles of falling debris, falling with it to the ocean floor. They are also present on the parts of the ocean floor that are too deep to receive any light. They are essential to the health of the oceans and all that lives in it, decomposing and recycling all organic martial and releasing the nutrients to the environment to be reused.

Most parts of the oceans are well aerated and the waste products, including carbon, are efficiently recycled. There are some regions where productivity is excessively high, such as in areas where deeper water is upwelling, in places such as the coastal waters off Peru and Namibia, or in stagnant waters that are greatly enriched by the decaying organisms and their waste, in places such as the Black Sea and the Gulf of California, places were conditions are locally reversed. In locations such as these the recycling capabilities of the microbial recyclers are overwhelmed by the organic material being deposited. These conditions come about because the first wave of aerobic microbes can't cope, using up large amounts of oxygen from the water, and when additional microbes begin work they use up even more oxygen. Once the oxygen has been depleted anaerobic bacteria take over the decomposition. The organic matter is then decomposed more slowly, part of the carbon being left in the sediment.

During the Middle Cretaceous these conditions of productivity and recycling existed in the Tethys Ocean.

Sources & Further reading

Stow, Dorrik, 2010, Vanished Ocean; How Tethys Reshaped the World, Oxford University Press.