Abstract
Dziani Dzaha is a maar the age of which is close to 4000 years. While its water is thought to have originated from seawater it is now considered as an extreme environment due to its hypersaline and alkaline characteristics. Those extreme features have led to the simplification of the trophic network. Cyanobacteria account for up to 95% of the photosynthetic biomass. The main biogeochemical processes, i.e. photosynthesis, bacterial sulfate reduction and methanogenesis could explain the current water composition. As far as we know, this ecosystem could be unique on Earth, extending the nature and chemical limits of aquatic inland ecosystems.
Highlights
The Dziani Dzaha is a small tropical lake of volcanic origin hosted in a crater formed by a phreatomagmatic eruption that likely occurred between 7000–4000 years BP [Zinke et al, 2003] on Petite Terre, an island of Mayotte archipelago (Northern Mozambique Channel, Western Indian Ocean)
Lateral variations of the measured parameters were considered negligible, since the water samples collected at the CLB station (3 to 4 m depth) did not show any significant difference with those collected at the DS station, the same day at the same depth
Water temperature varied in a clear seasonal pattern (Figure 2): mean summer temperatures (January to March) were around 32 °C at 0.5 m depth, dropping down to 27–28 °C in winter
Summary
The Dziani Dzaha is a small tropical lake of volcanic origin (maar) hosted in a crater formed by a phreatomagmatic eruption that likely occurred between 7000–4000 years BP [Zinke et al, 2003] on Petite Terre, an island of Mayotte archipelago (Northern Mozambique Channel, Western Indian Ocean). The lake surface extends at ≈0 m above sea level Since this latter remained almost constant in the last 7000 years BP, some authors suggest that seawater was by far the main contributor to the initial filling of lake Dziani Dzaha [Smith et al, 2011]. Growing stromatolites are found on the shores, with Cyanobacteria (mostly Pleurocapsales and Leptolyngbia) and non-sulfur purple bacteria (mostly Rhodobacteraceae) identified in the biofilms [Gérard et al, 2018]. These modern microbially-mediated sedimentary structures are living counterparts of the oldest fossil records of life (Late-Archean, Proterozoic oceans). They are scarce in modern marine environments and are mainly encountered in locations where metazoans are excluded as a consequence of extreme environmental conditions [Rishworth et al, 2017]
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