Saltwater and freshwater meet underwater forest

Searching for Life in Mexico's Underwater Caves |

saltwater and freshwater meet underwater forest

Although the surface appears calm, the underwater intersection of When river water meets sea water, the lighter fresh water rises up and over. Iliffe, an underwater cave biologist, discovers the life scattered It can act like a barrier, preventing leaf litter from the surrounding forest — along with plastic When saltwater and freshwater mix together, the result is reactive. This river, where salt water meets fresh water, is blanketed by a thick cloud the dense, tropical jungle is littered with sinkholes called cenotes.

Monodactylus argenteus Brackish water condition commonly occurs when fresh water meets seawater. In fact, the most extensive brackish water habitats worldwide are estuarieswhere a river meets the sea. The River Thames flowing through London is a classic river estuary.

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  • The transition from salt to fresh water is turbulent, vulnerable, and incredibly bountiful

The town of Teddington a few miles west of London marks the boundary between the tidal and non-tidal parts of the Thames, although it is still considered a freshwater river about as far east as Battersea insofar as the average salinity is very low and the fish fauna consists predominantly of freshwater species such as roachdacecarpperchand pike.

The Thames Estuary becomes brackish between Battersea and Gravesendand the diversity of freshwater fish species present is smaller, primarily roach and dace; euryhaline marine species such as flounderEuropean seabassmulletand smelt become much more common. Further east, the salinity increases and the freshwater fish species are completely replaced by euryhaline marine ones, until the river reaches Gravesend, at which point conditions become fully marine and the fish fauna resembles that of the adjacent North Sea and includes both euryhaline and stenohaline marine species.

A similar pattern of replacement can be observed with the aquatic plants and invertebrates living in the river. River estuaries form important staging points during the migration of anadromous and catadromous fish species, such as salmonshadand eelsgiving them time to form social groups and to adjust to the changes in salinity.

Salmon are anadromous, meaning they live in the sea but ascend rivers to spawn; eels are catadromous, living in rivers and streams, but returning to the sea to breed. The evidence still floats in its water and litters the perimeter.

The Underwater Forest

Most tourists never see the likes of Crustacea. Aside from Iliffe and a small cadre of explorers and scientists, who would want to? The fecal-brown water and smell of stagnant, decaying vegetation would be enough to turn away even the most intrepid adventurer.

But about 33 feet below the murky surface, the water becomes beautifully transparent. Like oil on water, freshwater that seeped into the cave from the surface floats above a denser layer of saltwater from the ocean. These layers meet and mix in a lens of brackish water called the halocline. If left undisturbed, the halocline can be paper thin, yet still visible to the naked eye.

Whereas saltwater and freshwater are clear, the halocline appears somewhat like a hazily defined liquid body rippling within the water. It can act like a barrier, preventing leaf litter from the surrounding forest — along with plastic bags, soda cans and other light trash — from sinking into the denser saltwater.

Along with the debris, the upper freshwater layer contains more oxygen than the saltwater below, and it supports a completely different set of life-forms. The halocline divides the freshwater and saltwater throughout the cave system of limestone passageways — so narrow in parts that a diver must detach from his oxygen tanks to fit through — and caverns sometimes large enough to drive a semi-truck through them.

Only a few known species live here: Life is impossible for all other organisms. In an environment devoid of sunlight, with no obvious energy source for the most basic life-forms, it remains a mystery how life can exist here at all. Iliffe and student David Brankovits, holding a dive tank, prepare to dive into a coastal cave on the Yucatan.

saltwater and freshwater meet underwater forest

At Pennsylvania State University, where he studied biochemistry, Iliffe saw a scuba class among the course offerings and jumped on the opportunity. He fell in love with scuba diving and started planning his life around it.

Diving technology was now sophisticated enough to support long dives during which divers could be disconnected from an air supply above water.

Legendary cave diving pioneers like Sheck Exley began breaking deepwater diving records. Iliffe was simply in the right place at the right moment to find his calling.

Then he met Iliffe.

Where the Rivers Meet the Sea

He needed a diving partner for an upcoming research trip. A few weeks later, even though they barely knew each other, Pohlman and Iliffe hopped in a van and drove south through Mexico. On more than one occasion, Pohlman recalls waking to an unwelcome surprise in his face. One time, it was the snout of a horse. Another time, while they camped outside an abandoned research station, it was the gun barrel of a concerned security guard. During those long hours on the road, Iliffe instructed Pohlman, reviewing scientific diving techniques like how to drag a plankton net to collect microscopic crustaceans.

He spent the next two years working with Iliffe to figure out what powered life in the deep caves. When he analyzed isotopic signatures of carbon and nitrogen found in the tissues of some of the small cave crustaceans, he discovered that the animals had a food source fueled by something other than the sun.

saltwater and freshwater meet underwater forest

Pohlman believed chemosynthetic bacteria, like the kinds found around deep-ocean hydrothermal vents, must be the answer. The few seaweed species that grow in a hard sand or clay bottom use a root-like structure called a rhizoid to penetrate the substrate.

These seaweeds with rhizoids are in the green algae group and are probably distantly related to land plants. The simple parts of seaweeds begin with the stipe, a stalk that lifts up from the holdfast. It looks much like the thin trunk of a young sapling, or the stem of a large grass plant.

It takes the seaweed into the light. The blade or frond or thallus often more than one word in the seaweed lexicon can be used for the same thing is what the stipe brings to the light. The blade is the equivalent of branches and leaves on a tree. A seaweed blade may branch, and air bladders may punctuate it, especially in seaweeds that are long or that grow in quiet waters where currents may not lift it high enough to the surface and into the light.

The bladders are simple buoys carrying the blades aloft. Bathed in water and sunlight, the blades have two important jobs: Some Sargassum species can reproduce by fragmentation, but most species of seaweed reproduce by alternating generations.

They have a sporophyte phase, which sends out a drift of tiny spores, as mushrooms and ferns do. And they have a phase of sexual reproduction, as flowers do.

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With the diploid form of reproduction — the spore phase — a seaweed can replicate itself, but this allows for no genetic variation. Most of them never find each other, never join. As a result, the solution for most, but not all, seaweed species is to depend on both means, which allows for genetic change as well as a chance at abundance.

Underwater Forest Aquarium - Tanked

Ascophyllum nodosum, or knotted wrack, is the tough, familiar seaweed along our shore. It reproduces only through the haploid form. Its blade is multi-branched, shaped like a hardwood tree. If you go to a shore with a marked incline, you will find that the native seaweeds arrange themselves neatly into bands.

The green algae live closest to shore, the brown seaweeds inhabit the inshore waters and can also thrive in subtidal depths where sunlight reaches, but it is the red seaweeds that can live in the deepest water with the least light.

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saltwater and freshwater meet underwater forest