Lake Kauhako, Molokai, Hawaii: Biological and chemical aspects of a morpho-ectogenic meromictic lake

S. P. Donachie¹, R. A. Kinzie III², R. R. Bidigare³, D. W. Sadler¹ and D. M. Karl^1,3

¹Laboratory for Microbiological Oceanography, ²Department of Zoology and ³Department of Oceanography, SOEST, University of Hawai'i, Honolulu, Hawai'i 96822, USA

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 = 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 ammunium 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 α- and β-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 1^-l, 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.