The biogeochemistry of tropical lakes: A case study from Lake Matano, Indonesia

Abstract

We examined the chemical composition of the water column of Lake Matano, Sulawesi Island, Indonesia, to document how the high abundances of Fe (hydr)oxides in tropical soils and minimal seasonal temperature variability affect biogeochemical cycling in lakes. Lake Matano exhibits weak thermal stratification, yet a persistent pycnocline separates an oxic epilimnion from anoxic meta-and hypolimnions. The concentration of soluble P in the epilimnetic waters is very low and can be attributed to scavenging by Fe (hydr)oxides. Chromium concentrations in the epilimnion are high (up to 180 nmol L 21 ), but below U.S. Environmental Protection Agency guidelines for aquatic ecosystems. The concentration of chromium decreases sharply across the oxic-anoxic boundary, revealing that the hypolimnion is a sink for Cr. Flux calculations using a one-dimensional transportreaction model for the water column fail to satisfy mass balance requirements and indicate that sediment transport and diagenesis play an important role in the exchange of Fe, Mn, P, and Cr between the epilimnion and hypolimnion. Exchange of water between the epilimnion and hypolimnion is slow and on a time scale similar to temperate meromictic lakes. This limits recycling of P and N to the epilimnion and removal of Cr to the hypolimnion, both of which likely restrict primary production in the epilimnion. Owing to the slow exchange, steep concentration gradients in Fe and Mn species develop in the metalimnion. These concentration gradients are AcknowledgmentsWe thank the International Nickel Company (INCO) Canada and PT INCO Tbk. for their financial and logistical support of both field and laboratory work. Support for Sean A. Crowe was partly provided by a Natural Sciences and Engineering Research Council (NSERC) Industrial Partnership Scholarship sponsored by INCO Canada. We are grateful to Bill Napier, Les Huelett, and Matt Orr for logistical support; Jim Gowans for the use of his patio boat, the

Mixing and its effects on biogeochemistry in the persistently stratified, deep, tropical Lake Matano, Indonesia

“…The anoxic waters below 100-m depth are virtually devoid of sulfide (, 0.6 mmol L 21 ) but are rich in dissolved iron and methane (140 and 1400 mmol L 21 , respectively). Light penetration to 120-m depth allows photosynthesis by a low-light adapted community of photoferrotrophic bacteria in the upper part of the anoxic water column, in a manner similar to what has been postulated for the Archean Ocean (Crowe et al 2008b). The waters of Lake Matano may also support anaerobic oxidation of methane using ferric iron as an electron acceptor, a novel biogeochemical pathway (Crowe 2008;Beal et al 2009;Crowe et al in press).…”

“…The difference between the surface and bottom water temperatures is less than 3.5uC (Crowe et al 2008a), a feature not uncommon in tropical lakes (Lewis 1982(Lewis , 1987. Despite the weak temperature gradient, a persistent pycnocline situated at , 100-m depth separates an oxic surface layer from anoxic bottom waters (Crowe et al 2008a). Below 100-m depth, the salinity gradient is very weak (conductivity varies by less than 110 mS cm 21 over a 500-m depth interval) and the temperature gradient is less than 2uC per 500 m. The gradients are virtually constant throughout the year and do not appear to differ from year to year (Crowe et al 2008a).…”

“…Located at 2uS latitude, Lake Matano experiences minimal seasonal temperature fluctuations. The difference between the surface and bottom water temperatures is less than 3.5uC (Crowe et al 2008a), a feature not uncommon in tropical lakes (Lewis 1982(Lewis , 1987. Despite the weak temperature gradient, a persistent pycnocline situated at , 100-m depth separates an oxic surface layer from anoxic bottom waters (Crowe et al 2008a).…”

“…Recent studies of Lake Matano have revealed an environment that is geochemically and microbiologically intriguing and possibly unique (Crowe 2008;Crowe et al 2008a,b). The anoxic waters below 100-m depth are virtually devoid of sulfide (, 0.6 mmol L 21 ) but are rich in dissolved iron and methane (140 and 1400 mmol L 21 , respectively).…”

The Towuti Drilling Project: paleoenvironments, biological evolution, and geomicrobiology of a tropical Pacific lake

“…The lake is situated within the East Sulawesi Ophiolite ( Fig. 1; Kadarusman et al, 2004;Monnier et al, 1995), the third largest ophiolite in the world, which releases iron, chromium, and other metals that catalyze biogeochemical activity by a unique and diverse microbial community in the lake and its sediments (Crowe et al, 2008). Lake Towuti's Kadarusman et al, 2004), and (c) the configuration of the Malili lake system.…”