There is a
soda lake in the middle of the island, occupying two intersecting craters 39 and 69 meters deep as determined by echo-sounding. The southern crater is 950 m in diameter and the northern one is 400 m in diameter; at the bottom they are separated by a 10 m high ridge. The lake is surrounded by sandy beaches. At 13 sites around the lake, large calcareous reefs extrude from rocky points; they are submerged for at least 23 m, are 1 to 2 m thick with very steep walls, and their tip emerge by about 50 cm at the end of the dry season. They are made of brittle, cavernous limestone composed of
aragonite and low-Mg-
calcite, partly
silicified. Their structure alternates between layers of
in vivo calcifying
Pleurocapsales cyanobacteria and of
red algae (
Peyssonnelia sp.,
Lithoporella sp.), often separated by accumulations of gastropod fecal pellets settled in cyanobacterial
micrite — although the red algae are present only in the first 1 cm of the reefs. The pellets are produced by the
Cerithium species; these and the gastropods' shells contribute significantly to the mass of the reef. The fauna in the lake is extremely poor in species; contrary to what one could expect, hardly any colonization seems to issue from the nearby reef only 100 m away and boasting a thriving diversity of tropical marine reef species. In 1990 the following species were noted (some of which may be endemic): one species of thin-shelled
cerithiid gastropod; one species of monaxonid
demosponge; one species of amphipod crustacean; one species of small fish; one species of
hydrozoa; one species of infaunal
oligochaet; and three species of green algae. There were also, in reef and sediment samples, subfossil shells of two bivalve species (
Lioconcha sp.?,
Pinctada sp.?); three gastropod species (
Cerithium sp., common;
Ocenebra sp., seldom; and
Neritina sp., rare); and dense aggregates of
serpulid tubes. A population of
monaxonid sponges (
Suberites sp.) colonizes the reefs surfaces, intertwining with the green algae; and a dense population of
Oligochaeta (worms) lives in the black sandy mud on the lakeshore. suggested instead that it was more likely due to changes in the saturation index of calcite (both ideas are not mutually exclusive, because the solubility of calcite increases when its magnesium content rises — see page "
Marine biogenic calcification"). Thus the lake has been closely linked with the "Soda Ocean Hypothesis"; Kempe & Kazmierczak have qualified lake Satonda as "a recreated model of late Precambrian ocean chemistry" — that is, the "
soda lake" environment that prepared the great explosion of life during the
Cambrian. == Notes ==