Dissolved Organic Matter Composition and Microbial Diversity In The Lake Tahoe Basin, Sierra Nevada, California.

Abstract:

Dissolved organic matter (DOM) inputs into high elevation lakes of the Sierra Nevada, California are seasonally segregated, and this enables an examination of the dominant compositional features and microbial responses associated with allochthonous versus autochthonous DOM inputs. Furthermore, because lakes within this watershed have very different hydraulic residence times, extending from days (e.g., Upper Angora Lake) to centuries (Lake Tahoe), the Tahoe Basin represents an ideal experimental system in which to characterize long-lived DOM. We used a variety of analytical tools, including elemental, stable isotope and radiocarbon measurements, nuclear magnetic resonance (NMR) spectroscopy, comprehensive 2D gas chromatography coupled to time of flight (TOF) mass spectrometry and fluorescence measurements, to characterize solid phase extracted (SPE) DOM, and in some cases, whole DOM. Our data show that DOM with typical terrestrial characteristics is quickly removed in lakes with >annual water residence time, leaving behind SPE DOM that is extremely N-rich, with a functional group distribution that is consistent with protein. Furthermore, our radiocarbon measurements estimate a 100-200 year residence time for the N-rich DOM accumulating in Lake Tahoe. All of the analytical techniques distinguish samples based on lake water residence time, which indicates that the lacustrine reactor plays an important role in determining the composition of DOM that accumulates on long timescales. We also examined temporal variations in the microbial community of Lake Tahoe to identify taxa that may be involved in processing DOM from distinct sources. Our results confirm the importance of DOM as a currency for carbon and nitrogen exchange between different compartments of the terrestrial ecosystem and argue for its inclusion in models that examine the response of lake ecosystems to global change.