Hot spring ecosystems through time: Homes for endemics, hot spring specialists or widespread but pre-adapted generalists?

CHANNING, A.; EDWARDS, D.; ZAMUNER, A.; GUIDO, D.; MOREIRA, P.

Oslo, Noruega

Congreso; 33rd Internacional Geological Congress; 2008

Newly discovered hot spring ecosystems in Argentine Patagonia reveal how Mesozoic plant and animal communities colonised and withstood life in the harsh environments represented by a broad set of hot spring facies. Observations from the numerous, well preserved, well exposed and laterally extensive deposits provide information allowing generalisations to be made about the traits that equip organisms for life in hot springs. They provide a crucial data point between observations of active hot spring ecosystems and the Palaeozoic ecosystem preserved at Rhynie that plays a central role in our understanding of early terrestrial biotas, particularly early land plants. At active hot springs e.g. Yellowstone the plants and animals most commonly preserved, and particularly those that are permineralised to the cellular level, are those that live and die in contact with silica supersaturated water of moderate to ambient temperature. These interactions occur frequently in pools on sinter aprons, distal regions of run-off streams, on extensive geothermally influenced wetlands and where geothermal waters interact with lake-margin and river bank communities. Plants in these settings are commonly hydrophytic or flooding tolerant and capable of withstanding physiological drought brought about by immersion in mineral rich, brackish-water. Plants preserved are representatives of genera with regional, even global distributions, with adaptations to much larger and more common habitats such as saline seeps, evaporation dominated lakes, or coastal salt marshes.Similar observations can be made in Patagonia. Hot spring floras are of low diversity, comprising at present only 3 taxa. This contrasts starkly with local clastic environments where a broad range of plant groups are present. The most stressed environment observed to be colonised, sinter apron pools, are habitat for dense monotypic stands of sphenophytes, aquatic algae and cyprid ostracods. The same groups are present in more distal, marshy, geothermally influenced wetlands but may be joined in areas where these merge with fluvial and lacustrine environments by probable shrubby cheirolepidian conifers. Progradation of sinter/geothermal wetland margins across broad areas of coniferous forest appears to result in larger arborescent conifers "drowning", their fallen and decaying trunks/branches being incorporated into basal hot spring deposits. The Patagonian hot spring biota, at the generic level, has a regional, probably global, distribution, and is similar in many respects to contemporary saline lake biotas. So were the Rhynie plants truly endemics specially adapted to life in hot springs or typical elements of coeval clastic environments? We hypothesise, that more likely, they were flooding tolerant and capable of withstanding alkalinity, salinity and heavy metal stress and were pre-adapted to hot spring environments on the margins of ephemeral, evaporation driven inland water bodies or coastal fringes.