Anchialine system
Anchialine systems may occur in karst landscapes, regions with bedrock composed of soluble sedimentary rock, such as limestone, dolomite, marble, gypsum, or halite.
[3][2] The processes to form these karst morphological features occur on long geological timescales; caverns can be several hundred thousand to millions of years old.
[1] The marine transgression after the last glacial maximum caused saline groundwater to intrude into karst caverns resulting in anchialine systems.
[5] Horizontal white “bathtub ring” stains are observed in submerged sections of Green Bay Cave, Bermuda, indicating paleo-transition zones between freshwater and saltwater at a lower sea level.
[4] Saltwater intruded into many coastal lava tubes during the marine transgression after the last glacial maximum creating many volcanic anchialine pools observed today.
[4] In volcanic and seismically activity areas, faults in coastal environments can be intruded by saline groundwater resulting in anchialine systems.
The Ras Muhammad Crack area in Israel is an anchialine pool created by an earthquake in 1968 from the uplift of a fossil reef.
[7] Deep anchialine pools created by faulting from the uplift of a reef limestone block are also seen on the island of Niue in the Central Pacific.
However, in the same scenario in a polyhaline pool, the seawater forms a freshwater lens at the top, reinforcing the stratification and potentially creating a hypoxic environment depending on oxygen reaction rates.
[citation needed] Water chemistry of anchialine systems are directly related to the amount of connectivity to the adjacent marine and freshwater inputs, and evaporative losses.
Major nutrient compositions (carbon, nitrate, phosphate, and silicate) from the ocean and groundwater sources determine the biogeochemical cycles in an anchialine system.
[8] This stratification and available nutrient resources establishes redox gradients with depth which can support a variety of stratified communities of micro-organisms and biogeochemical cycles.
[citation needed] In deeper stratified systems water below the chemocline can be associated with an increase in dissolved hydrogen sulfide, phosphate, and ammonium, and a decrease in particulate organic carbon.
Systems further inland are more dominated by freshwater algae and terrestrial deposits but exhibit increasingly restricted diversity within algal communities.
[16][17] Due to the ephemeral nature of many anchialine systems and their limited distribution across the planet, many of their inhabitants are either well adapted to tolerate a broad range of salinity and hypoxic conditions or are introduced through tides from neighboring marine habitats.
[18] Dominant non-crustacean invertebrates groups within anchialine systems include sponges and other filter feeders (most common in Blue Holes), which thrive in moderate flow systems where the structure acts in a way to compress the water and make particulate organic matter less dilute, improving filter feeding efficacy.
[20] Hypogeal shrimps have been observed to have high population densities in anchialine ponds upwards of hundreds of individuals per square meter.
[22] Anchialine pools are considered an ecosystem that combines elements from brackish surface water bodies, subterranean systems, and terrestrial landscapes and are usually wet lit.
[17] Algal primary producers inhabit the water column and benthos, while the diversity and productivity are often influenced by geological age and connectivity to the sea.
Ecological studies of anchialine pools frequently identify regionally rare and endemic species, while primary producers in these systems are typically algae and bacteria.
[25] Fauna that reside strictly within the aphotic zone of anchialine caves typically exhibit adaptations associated with low light and food, and are often classified as stygofauna.
Stygofauna are however quite different than deep sea organisms, most of which have kept their eyes and specialized them to see bioluminescence and possibly Cherenkov radiation in their otherwise dark environments.
Because of discrepancy between warmer seawater and cooler groundwater, temperatures of the anchialine system may also increase with depth and penetration, which has implications for growth and respiration rates.
Ha Long Bay marine lakes have been exploited by residents in surrounding boat villages for fisheries and aquaculture.
[18] Some caves in Bermuda, the Canary Islands, and Mallorca are used as wishing wells which increases concentration of copper and is thought to have caused the decline of the squat lobster, Munidopsis polymorpha.
[18] Cave divers also have unintended negative impacts on these habitats by using flashlights that enable fish such as Astyanax fasciatus to feed on otherwise inaccessible prey.
[18] Due to the high endemism in these environments and limited global distribution, many species in anchialine systems are at risk of extinction.
[31] In Vietnam, green sea turtles were introduced into anchialine pools for practices related to animistic rites and consumption.
Using highly specialized techniques, divers navigate the sprawling overhead environment to form detailed maps of the underground aquifers, collect a variety of biologic, geologic, or chemical samples, and track hydrologic flow.
It is currently an area of active research to predict how climate change induced sea level rise may affect the formation and health of anchialine systems in the near future.
