S1 Subterranean lithic systems, S2 Anthropogenic subterranean voids, SF1 Subterranean freshwaters, SF2 Anthropogenic subterranean freshwater, SM1 Subterranean tidal systems

Subterranean lithic systems

biome is comprised these Ecosystem Functional Groups (EFG): Aerobic caves, Endolithic systems, Anthropogenic subterranean voids. The subterranean lithic biome includes non-aquatic lithic systems beneath the earth’s surface. Sunlight is absent or of insufficient intensity to sustain photosynthesis. There is no standing water and moisture is supplied primarily by seepage through the substrate and may be lost by slow diffusion through the atmosphere to cave openings or by vertical or lateral seepage through the substrate. These physically stable systems exhibit low levels of environmental variability. Rarely, mass movements, for example rock falls, may re-organise the physical structure of subterranean ecosystems. Subterranean ecosystems have truncated trophic structures with no photoautotrophs and few obligate predators. Heterotrophic microbes and invertebrates dominate the biota, while chemoautotrophs are the primary energy assimilators. Most have low metabolic rates and prolonged life histories in response to resource limitations, resulting in low overall productivity. The subterranean biome includes dry caves and endolithic systems distributed throughout the earth’s crust. Incursions of fresh or marine waters generate transitional biomes (SF, SM).

Anthropogenic subterranean voids

biome is comprised one Ecosystem Functional Groups (EFG): Anthropogenic subterranean voids. The Anthropogenic subterranean voids biome includes asingle functional group of ecosystems that owe their genesis to excavation by humans. They include underground mines, transport tunnels, tombs, defence and energy installations, and other infrastructure. Most are very recent ecosystems constructed with earth-moving machinery during the industrial era, but some were constructed manually up to several millennia ago. Productivity is low and energy generally comes from allochthonous sources via connections to the surface, either by atmospheric diffusion or seepage, but some energy is contributed by chemoautotrophic microbes. While sunlight is absent or highly diffuse, some active voids are artificially lit and this may provide sufficient energy to sustain algal autotrophs. Trophic webs are simple and dominated by opportunistic microbes and invertebrates introduced by machinery or directly by humans, or else colonising spontaneously through openings to the surface. The latter may include small mammals that use the voids as refuges or breeding sites. Microbes from external and endolithic sources rapidly colonise newly exposed lithic surfaces and create biofilms that support detritivores and enhance substrate weathering. The stability of artificial subterranean voids varies depending on their substrate and management, with some prone to collapse and structural change after active use ceases.

Subterranean freshwaters

biome is comprised these Ecosystem Functional Groups (EFG): Underground streams and pools and Groundwater ecosystems. The Subterranean freshwaters biome includes streams, small lakes and aquifers beneath the earth’s surface and potentially has the largest volume of water of all the freshwater biomes. In the absence of sunlight, these ecosystems rely on allochthonous energy sourced from surface ecosystems via connected waters and in situ chemoautotrophs. Depending on the mode of connectivity to the surface, water flow- through varies from extremely rapid to slow. Highly connected subterranean streams in monsoonal climates undergo seasonal flooding and drying cycles. In contrast, paleo- aquifers are characterised by slow, low-variability seepage over millennial time scales. Inflowing water is the principal source of dissolved oxygen and mineral nutrients, although some nutrients are liberated by in situ weathering of lithic substrates. The water regime largely determines environmental variability in subterranean freshwaters, but these systems may occasionally be influenced by mass movements. The trophic structure of subterranean waters is typically truncated, although photosynthetically inactive, algae and higher-plant propagules may be transient occupants in systems that are connected to the surface. Chemoautotrophic and heterotrophic microbes in biofilms and the water column dominate the trophic web, supporting small invertebrate detritivores and predators. Small predatory fish may occur in streams and lakes, where voids in the subsurface are of sufficient size. Productivity, metabolic rates, life histories and the diversity of the biota all reflect resource scarcity but may vary depending on water source. Insular systems exhibit high levels of endemism.

Anthropogenic subterranean freshwater

biome is comprised these Ecosystem Functional Groups (EFG): Water pipes and subterranean canals, Flooded mines and other voids. The Artificial subterranean freshwaters biome includes aquatic systems in underground canals, drains, sewers, water pipes and flooded mines constructed by humans. These are usually well connected to surface waters. The availability of resources is largely a function of source waters and the water regime, which varies from permanent to intermittent with low to high flow velocity or, in the case of flooded mines, negligible flow. Sunlight is absent or, if it diffuses through vents and portals (as in some canals), it is generally too dim to support photosynthesis. Algae may nonetheless be transported through these systems depending on the water of source. Although primary productivity is low and energy is supplied from allochthonous sources, secondary productivity by heterotrophic microbes in biofilms and in the water column may be high in sewers and drains where organic Carbon, nutrients and dissolved oxygen are abundant. This may support several tiers of detritivores and predators, including microscopic invertebrates, macro-invertebrates, and small vertebrates, including rodents and fish. Anaerobic bacteria may be important components of the trophic network where organic Carbon and nutrients are abundant but dissolved oxygen is scarce due to either low aeration or high microbial activity. In water supply pipes, low levels of organic carbon and nutrients exacerbate constraints on productivity imposed by the absence of light. Trophic webs within pipes are truncated and simple, and the mostly transitory biota reflects that of source waters.

Subterranean tidal systems

biome is comprised these Ecosystem Functional Groups (EFG): Anchialine caves, Anchialine pools, and Sea caves. The subterranean tidal biome includes coastal pools and subterranean voids with a partially or entirely submerged connection to marine waters. Like all other subterranean ecosystems, sunlight is absent or too dim to sustain photosynthesis. Marine shelf ecosystems (M1), terrestrial aquifers (SF1) and surface coastal systems (T, MT) connected to these subterranean systems are their sources of allochthonous energy, nutrients and oxygenation. Food and energy availability are influenced by in situ microbial processing (biogeochemical transformation) of these allochthonous organic matter inputs. The marine interface, a typical feature of coastal aquifers and subterranean estuaries, also generates a marked salinity gradient is the primary zone of biogeochemical cycling. In carbonate and volcanic geologies, the salinity gradient can often be observed in the flooded pools, voids, and caves as a halocline (a sharp salinity gradient in the water column), which is not present in other subterranean environments. In comparison to other subterranean ecosystems, diverse assemblages of chemoautotrophic and heterotrophic microbes, as well as scavengers, filter feeders and predators. Physiological traits enabling osmotic regulation allow some species to transit across haloclines between the fresh- and saline waters. In dark sections of the subterranean marine systems where photoautotrophs are absent, trophic webs are truncated. Some of the subterranean marine biota belong to lineages otherwise restricted to the deep sea floor (M3), and share traits with those in other low- productivity, dark biomes, including depigmentation, reduced visual organs, increased tactile and chemical sensitivity, low fecundity, long lifespans, and slow metabolism and growth rates. Tides are an important means of hydrological mixing, resource flux, biotic dispersal and perturbation. In subterranean tidal systems with more direct connections to the sea, marine suspension feeders, particularly sponges and other sessile invertebrates, are dominant. Farther into marine and anchialine caves, where tidal flushing and water exchange diminishes or disappears, the fauna consists of stygobitic crustaceans, annelids and several other faunal groups (i.e. strictly subterranean aquatic fauna that complete their entire life in this environment).


Inland Un- or Sparsely Vegetated biome is comprised of Underground systems, Inland rock formations, and Other (inland un- or sparsely vegetated)


Not defined as a land cover / ecosystem type.

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