AME 19:93-103 (1999)  -  doi:10.3354/ame019093

ABSTRACT: We undertook the first combined microbiological and hydrochemical study of the 248 m deep meromictic Lake Kauhako-. Situated at sea level 1.6 km from the sea in the crater of an extinct volcano on the island of Moloka'i, Hawai'i, USA, Lake Kauhako- has the highest relative depth (ratio of depth to surface area, z_r = 374%) of any lake in the world. The upper 4.5 m were stratified (T = 23 to 26°C; salinity = 6 to 24.5), but below a pycnocline at ~4.5 m temperature and salinity were uniform (T = ~26.25°C; salinity = 32). Seawater likely intrudes by horizontal hydraulic conductivity through rock separating the lake and the Pacific Ocean. Anoxia commenced below 2 m. Hydrogen sulfide was undetectable at 4 m, but averaged ~130 μM between 5 and 28 m. Dissolved inorganic carbon concentrations ranged from ~1.50 mM at the surface to ~3.3 mM below 5 m. Total organic carbon peaked at 0.94 mM above the pycnocline but remained about 0.30 mM below 5 m. Soluble reactive phosphorus and ammonium concentrations, nanomolar above the pycnocline, increased to ~28 and 175 μM, respectively, at greater depth. Nitrate attained 3.7 μM in shallow water, but was ~0.2 μM from the pycnocline to 100 m. Leucine aminopeptidase (LAPase) activity at the surface exceeded 1100 nmol of substrate hydrolyzed l^-1 h^-1. Activities of a- and b-glucosidase were lower, but showed depth distributions similar to that of LAPase. Surface waters hosted large and diverse picoplankton populations; chlorophyll a (chl a) exceeded 150 μg l^-1, and heterotrophic bacteria and autofluorescent bacteria attained 2 x 10⁹ and 9 x 10⁹ l^-1, respectively. Filamentous cyanobacteria and 'Prochlorococcus'-like autotrophs occurred only in the upper 2 m. Chl a was the dominant pigment above 2 m, but pigment diversity increased markedly in anoxic waters between 3 and 5 m. Lake Kauhako- is a unique habitat for further studies, particularly of interactions among flora and fauna restricted to a shallow water column within a single basin.

KEY WORDS: Lake Kauhako · Meromixis · Moloka'i · Carbon · Nutrients · Pigments · Ectoenzyme · Bacteria