BASIC MORPHOMETRIC AND LIMNOLOGICAL PROPERTIES OF LAGUNA-HULE, A CALDERA LAKE IN COSTA-RICA

Laguna Hule, a caldera lake at 740 m above sea level, is situated on t he Atlantic slope of the Cordillera Volcanica Central in Costa Rica. T he lake area is 56.9 ha, the volume 7.8 x 10(6)m(3) and the maximum de pth 22.5 m. At the time of the study (January/February 1989) the bound ary between the oxic and anoxic layer was at 10 m, i.e. 33 % of the la ke's volume was anoxic. Relatively low concentrations of H2S (up to 4. 1 mu mol l(-1)) were present in the hypolimnion simultaneously with la rge amounts of Fe and Mn (114 and 21.5 mu mol l(-1) resp.). The thermo cline was very weakly developed (total temperature difference between surface and bottom water layer amounted to only 1.3 degrees C). This s mall difference, the relatively high concentrations of ammonia and iro n in the epilimnion and the low O-2 saturation (81 %) of the surface l ayer indicate that, during or shortly before the study time, a partial circulation of the lake had occurred causing a substantial entrainmen t of hypolimnetical water into the epilimnion. The mixing process did not produce a total circulation but was probably the reason for the in trusion of small amounts of O-2 into layers below the chemocline, whic h enabled the production of nitrite by nitrifying bacteria within the hypolimnion. On Feb 7 the gross primary productivity was 2.74 g C m(-2 )d(-1) and the community respiration between surface and 6 m amounted to 1.98 g Cm(-2)d(-1) concluded from the results of the present as wel l as from an earlier study (May 1979) and from comparisons with the ne arby located Laguna de Rio Cuarto, that the productivity pattern of La guna Hule is very much governed by its mixing behavior.