Hydrobiologia 381: 1–7, 1998. Zheng Mianping, S. H. Hurlbert & W. D. Williams (eds), Saline Lakes VI. © 1998 Kluwer Academic Publishers. Printed in Belgium. 1 Isabela Crater-Lake: a Mexican insular saline lake Javier Alcocer 1 , Alfonso Lugo 1 , Mar´ ıa del Rosario S´ anchez 1 & Elva Escobar 2 1 Limnology Lab., Environmental Conservation and Improvement Project, Interdisciplinary Research Unit for Health & Environmental Sciences, Universidad Nacional Aut´ onoma de M´ exico Campus Iztacala. Av. de los Barrios s/n, Los Reyes Iztacala, Tlalnepantla, Edo. de Mexico 54090, Mexico City 2 Benthic Ecology Lab., Institute of Marine Sciences and Limnology, Universidad Nacional Aut´ onoma de M´ exico. Apdo. Postal 70-305, Ciudad Universitaria, 04510, Mexico City Corresponding author: Javier Alcocer, Fax: (525) 390.5900; E-mail: jalcocer@servidor.unam.mx Key words: Tropical limnology, meromixis, nutrients, plankton, Nayarit, Mexico, Isabela island Abstract 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 salient features were documented in November 1993. It appears meromictic, with three well-defined strata separated by sharp pycnoclines. Surface water was warm (32 ◦ C) reaching a subsurface (0.5–1 m) maximum temperature (33 ◦ C), declining gradually to 26.7 ◦ C at maximum depth (17.5 m). Dissolved oxygen was near saturation at the surface, attained 145 percent 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 about –240 mV from 3.5 m to the bottom. The pH varied from 9.3 in surface waters to slightly acid (6.4) in deep anoxic layers. Atypically, NO 3 was more abundant than NH 4 in both aerobic and anaerobic strata. PO 4 and SiO 2 concentrations were extremely high. The planktonic microbial community was formed by four groups: bacteria (photosynthetic sulfur bacteria and heterotrophic bacteria), phytoflagellates (mainly cryptomonads), heterotrophic nanoflagellates (Spumella spp., Bodo spp.) and ciliates (Hypotricha and Oligotricha). Bacteria occurred throughout the water column, but other biota were restricted to surface waters. Introduction Isabela Island (also named Isabel) is at 21 ◦ 52 ′ N and 105 ◦ 54 ′ W (Anon, 1987), 32 km off the Nayarit coast in the Mexican Pacific (Figure 1). This small (2 km 2 ), recently formed, basaltic, island arose by the align- ment of stratovolcanoes along the Cocos plate. The presence of peridotite (a variety of olivine) nodules on Isabela characterizes it as an oceanic Quaternary is- land (Ortega-Gutiérrez & González-González, 1980). Isabela was declared a National Park (‘Ecological Reserve’) by the Mexican government in 1980. Addi- tional information on Isabela is given by Cervantes et al. (1986), Medrano (1979), Ruíz (1977), and Sánchez (1980). According to Cervantes et al. (1986), two distinct plant assemblages occur: deciduous trees (Crataeva tapia Linnaeus, and Euphorbia schlechtendalii Boiss) and grasses (Cenchrus viridis Spreng, Choris chlo- ridae (Presl.) Hitche, Dactylotenium aegiptrium (L.) Rich, and Eleusine indica (L.) Gaertn) are the main components of the forest; the grassland is domi- nated primarily by the grasses Jouvea pilosa Scribn, Cenchrus viridis Spreng, Trichachne insularis (L.) Hitche, and Digitaria sanguinalis (L.) Scop. The most important species of birds and reptiles are (Cervantes et al., 1986): the resident birds Sula leucogaster (Boddaert), S. nebouxii Milne-Edwards and Fregata magnificens Mathews, the migratory birds Anous stolidus (Linnaeus), Phaethon aethereus (Lin- naeus), S. sula (Linnaeus), Larus heermanni (Cassin), Sterna fuscata (Linnaeus) and Pelecanus occidentalis (Linnaeus), two iguanas, Iguana iguana (Linnaeus) and Ctenosaura pectinata (Wiegmann), three other hy-sh11.tex; 24/11/1998; 8:05; p.1 Article: hy-sh11 Pips nr.: 180270 (hydrkap:bio2fam) v.1.1