Isabela Crater-Lake: a Mexican insular saline lake

The Isabela Crater-Lake is a bright-green, hypersaline lake (68-112.5 mS cm (-1)) on Isabela Island off the Pacific coast of Nayarit, Mexico. Some sali ent features were documented in November 1993. It appears meromictic, with three well-defined strata separated by sharp pycnoclines. Surface water was warm (32 degrees C) reaching a subsurface (0.5-1 m) maximum temperature (3 3 degrees C), declining gradually to 26.7 degrees C at maximum depth (17.5 m). Dissolved oxygen was near saturation at the surface, attained 145 perce nt saturation at 0.5 m, but was completely absent by 2.5 m. Eh was maximum at the surface (123 mV), declining to a minimum at 3 m (-261 mV), and was a bout -240 mV from 3.5 m to the bottom. The pH varied from 9.3 in surface wa ters to slightly acid (6.4) in deep anoxic layers. Atypically, NO3 was more abundant than NH4 in both aerobic and anaerobic strata. PO4 and SiO2 conce ntrations were extremely high. The planktonic microbial community was forme d by four groups: bacteria (photosynthetic sulfur bacteria and heterotrophi c bacteria), phytoflagellates (mainly cryptomonads), heterotrophic nanoflag ellates (Spumella spp., Bodo spp.) and ciliates (Hypotricha and Oligotricha ). Bacteria occurred throughout the water column, but other biota were rest ricted to surface waters.