Print PageThe degree of allochthony is an important property of aquatic ecosystems as it regulates many limnological characteristics including transparency to UV (ultraviolet radiation, 280-400nm) and PAR (photosynthetically active radiation, 400-700nm), net ecosystem production, and energy flow through food webs. Even small changes in allochthony, concurrent with environmental changes such as global climate change, have the potential to induce large net changes in the ecosystem structure and function of lakes. Here, we test the utility of several traditional and emerging technologies and techniques to characterize allochthony in freshwater ecosystems, including DOC/Chl and a440nm/Chl ratios, naturally occurring stable hydrogen isotopes, the fluorescence index, spectral slopes S275-295nm and S350-400nm, and 320:380nm UV diffuse attenuation coefficient (Kd) ratios as indicators of allochthony.
Despite the number of allochthony indicators in use, we know of no systematic study comparing these indicators. The lack of an empirical comparison makes inferences into the relative strengths, weaknesses, and applicability of these indicators difficult. In addition, the various indices measure different properties and thus are likely to be differentially responsive to various aspects of allochthony. For example, some parameters characterize dissolved properties (e.g. spectral slope) while others characterize particulate properties (e.g. seston deuterium) and still others integrate both of these pools of material (e.g. 320:380nm UV Kd ratios). We measured these indicators across 38 lakes in the Beartooth Mountains (Montana and Wyoming, USA) and Glacier National Park (Montana, USA). In these montane to alpine lakes allochthony can vary widely within small geographic distances as lakes are found above and below the tree-line along gradients of terrestrial inputs.
Results/Conclusions
We found significant relationships between almost all pairings of allochthony indicators where more allochthonous lakes had a steeper (more negative) S350-400nm spectral slope, higher Ln(DOC/Chl) and Ln(a440nm/Chl) ratios, less negative deuterium enrichment, lower fluorescence index ratios, and higher 320:380nm UV Kd ratios relative to autochthonous lakes. Residuals of regressions between several indicators were significantly related to dissolved absorbance coefficients. Several indicators provide the ability to characterize allochthony in situ. For example, Ln(a440nm/Chl) may be estimated by in situ fluorometers measuring CDOM and chlorophyll and 320:380nm UV Kd ratios can be quantified with in situ radiation sensors. These new technologies provide the opportunity for using high frequency lake observatories to characterize allochthony to provide new understanding into global carbon cycling and better understanding of how allochthonous inputs subsidize and regulate recipient ecosystems.
