ABSTRACT

Lakes Nicaragua and Managua are large, rather shallow, fresh-water bodies that lie in a late Tertiary and Quaternary graben in western Nicaragua.

The bottom sediments comprise highly organic diatomaceous volcanic silts and clays in which quartz, plagioclase feldspar, dioctahedral montmorillonite and volcanic glass are the principal constituents.

Organic nitrogen content of the lake sediments ranges from 0.3-1.2 percent, about the same range as in eutrophic lakes of higher latitudes. Bitumen content is rather low, much like that of oligotrophic lakes. Hydrocarbon fractions of bitumens are similar in amount to an alkaline lake of western U. S. Non-hydrocarbon bitumens are high in proportion to hydrocarbons. Carbohydrates and amino acids sre similar in amount to those of eutrophic lake sediments. Chlorinoid pigments are similar to but flavinoid pigments are less than the typical pigments in eutrophic lake sediments.

Studies of the pH, Eh, inorganic composition and organic chemical and fossil residues of the lake sediments indicate that they are moderately productive, early eutrophic or mesotrophic lakes. Despite the presence of elasmobranches and of several marine or brackish-water types of invertebrates, no definite evidence of former marine connection has been found in the sediments. It is suggested that the lakes have been fresh-water for all or nearly all of their history and may be a little more highly mineralized now than formerly.