Limnological Aspects of Two Tropical Crater Lakes (Lago Biao and Lago Loreto) on the Island of Bioko (Equatorial Guinea)

The limnology of Lago Biao and Lago Loreto, two tropical crater lakes on the island of Bioko, was investigated for the first time in January of 2002. Lago Biao fills a caldera at 1750 m altitude and is 18.5 m deep. This lake was characterized by low conductivity (11.6-13.3 μS cm−1), low pH (5.9-6.2), an oxycline between 6 and 10 m depth (90-27% saturation), and a Secchi depth of 5.5 m. Dinoflagellates, desmids, and green algae dominated the phytoplankton community. The rotifers Anuraeopsis fissa, Polyarthra indica, Trichocerca simoneae, and T. similis were most abundant above or in the thermocline. During the day Tropocyclops prasinus tjeefi and Chaoborus cf. ceratopogones concentrated in the oxycline, whereas Daphnia laevis was most abundant between 0 and 4 m depth. In contrast, L. Loreto is 82 m deep and probably fills an explosion crater (‘maar’) at 1050 m altitude. The lake had a turbid, yellowish appearance and a Secchi depth of only 0.6 m. Oxygen content dropped from 121% at the surface to 6% in 10 m depth. The pH declined from 9.15 at the surface to 7.4 in only 2.5 m depth and became acidic at approximately 15 m depth. One species of cyanobacterium, one dinoflagellate and one green alga dominated the phytoplankton and together reached densities of 12 100 cells ml−1. Only one rotifer (Brachionus angularis) and one copepod (Tropocyclops prasinus tjeefi) species colonized the open water. Intact algal cells and zooplankton specimens were encountered near the bottom.