Indicators of Allochthony in High Mountain Lakes

1. A B C D Kevin Rose1, Craig Williamson1, Jasmine Saros2, and Carrie Kissman1 1 Department of Zoology, Miami University Oxford, OH USA 45056 2 School of Biology and Ecology & Climate Change Institute University of Maine, Orono, ME USA 04469 Indicators - Spectral slopeis a measure of the shape of the optical absorbance across the 350 – 400 nm waveband for a filtered water sample. More negative spectral slopes have been associated with water that is more allochthonous in nature. - Ln(DOC/Chl), [Ln (Dissolved Organic Carbon/Chlorophyll a)] is a traditional metric of allochthony where aquatic ecosystems that are more heavily influence by the surrounding terrestrial ecosystem have a higher Ln(DOC/Chl) value. - Seston Deuterium is a measure of the relative concentration of deuterium in seston from a water sample. Values that are less negative (enriched) indicate a greater contribution of terrestrial material in the sample whereas values that are more negative (depleted) indicate an autochthonous source. Typical ranges for seston in aquatic ecosystems are -125 to -292 ‰ - Fluorescence Index (FI) ratio is a measure of the fluorescence properties of aquatic fulvic acids where terrestrial fulvic acids have a FI ratio ~1.4 and aquatic fulvic acids have a FI ratio of ~1.9. - Transparency ratios are measures of the transparency of 320 nm UV light relative to the transparency of 380 nm UV, where transparency to these wavelengths is expressed as 1% depths, the depth to which 1% of surface irradiance remains (eg. 1% 320 nm Depth/1% 380 nm Depth). Background The degree of allochthony is an important feature of aquatic ecosystems; it is an indicator of the contribution of terrestrial ecosystem support to an aquatic ecosystem relative to internal (or autochthonous) contributions. The degree of allochthony is important in influencing biogeochemical cycles, food webs, and other factors that influence aquatic ecosystem structure and function such as lake metabolism. New techniques have recently been developed to assess the degree of allochthony within a particular lake ecosystem, however no work has yet to compare these indicators across a suite of lakes that vary in the degree of terrestrial influence. Here we compare the following indicators of allochthony: - Spectral Slope (e.g. Helms et al. 2008) - DOC/Chl ratio (e.g. Webster et al. 2008) - Seston Deuterium (e.g. Doucett et al. 2007) - Fluorescence Index (e.g. McKnight et al. 2001) - Transparency ratios (320:380 nm UV) Indicators of Allochthony in High Mountain Lakes Methods We measured transparency and collected water samples in a suite of high mountain lakes in Montana and Wyoming, U.S.A. for analysis of DOC, Chl a, dissolved absorbance, fluorescence, and seston deuterium. These lakes ranged over an elevation gradient that spanned treeline, thereby providing a wide range of terrestrial support to the lakes, where lakes below treeline likely receive much more terrestrial inputs relative to lakes above treeline. Samples using filtered lake water (spectral slope, DOC, fluorescence index) were filtered through pre-ashed 0.7 um Whatmann GFF filters and analyzed via standard methods. A Biospherical Instruments BIC radiometer was used to measure ultraviolet (UV) light (305, 320, & 380 nm) and PAR (photosynthetically active radiation, 400-700 nm) transparency. Seston deuterium samples (0.2 um – 80 um size range) were dried, weighed, and sent to the U.C. Davis Stable Isoptope Facility for analysis. Example Study Sites Results and Discussion There were significant relationships between spectral slope and Ln(DOC/Chl), seston deuterium, the 320:380 nm transparency ratio, and the fluorescence index. These results show that novel indicators of allochthony, such as deuterium seston isotopes and fluorescence, provide similar signals as a more traditional metric of allochthony, the Ln(DOC/Chl a) ratio. These indicators of allochthony use different techniques and are applied to different materials within a lake. For example, spectral slope and fluorescence both use filtered water while seston deuterium uses particulate matter and the transparency ratio takes into account both particulate and dissolved fractions. While we found significant relationships between spectral slope and these other allochthony indicators, the fact that these techniques reflect both particulate and dissolved fractions implies that in systems where the particulate and dissolved fraction sources are different the allochthony indicators will give different results. This may account for some variation shown above. Allochthony is an important aspect of aquatic ecosystems and terrestrial subsidies influence a wide array of ecosystem properties and functions. For example, most lakes are net heterotrophic which is made possible through terrestrial subsidies of carbon (Hanson et al. 2003). Indicators of allochthony may reflect other ecosystem properties, such as net ecosystem production and further research needs to be completed to examine the ecosystem properties that these indicators reflect. Study lakes ranged across a gradient of terrestrial influence from above treeline (e.g. Heart Lake, A) to below treeline (e.g. Kersey Lake, D) and included lakes right at treeline (Glacier Lake, B) and just below treeline (Beauty Lake, C). Literature Cited Helms et al 2008, Limnology and Oceanography 53(3), 955-969. Webster et al 2008, Limnology and Oceanography 53(3), 1137-1148. Doucett et al. 2007, Ecology 88(6), 1587-1592. McKnight et al. 2001, Limnology and Oceanography 46(1), 38-48 Hanson et al. 2003, Limnology and Oceanography, 48(3), 111-1119. Acknowledgements This work was supported by NSF grant DEB #0639901 and Miami University’s field workshop program.