Comparison of attenuation characteristics between glacially- and non-glacially fed lakes

Transparency is a fundamental characteristic of aquatic ecosystems and is highly responsive to climate and land use changes. Transparency of glacially-fed (GF) lakes may be a particularly sensitive sentinel characteristic. However, relatively little is known about the light attenuation characteristics of GF lakes, how these characteristics compare with non-GF (NGF) systems, and the variability in these characteristics across systems. We sampled a series of 7 GF and 8 NGF lakes in New Zealand and 13 GF lakes in North and South America to characterize how dissolved organic carbon (DOC), algal biomass (approximated by chlorophyll a) and turbidity contributed to attenuation of ultraviolet (UV) and photosynthetically active radiation (PAR). Attenuation in NGF lakes was regulated primarily by variation in the percent aromaticity and molecular weight of DOC, while algal biomass played a smaller role. In contrast, variation in both UV and PAR attenuation in GF lakes was related to variation in turbidity, a proxy for glacial flour. Contributions to attenuation from glacial flour increased with increasing distance from glaciers, possibly a product of changing particulate sources and size-dependent settling and scattering efficiency. Estimates of chlorophyll a-specific and glacial flour-specific attenuation coefficients derived in New Zealand lakes produced attenuation coefficient estimates of within 4-8% of in situ measurements for the 13 North and South American glacially-fed lakes. Understanding the optical characteristics of substances that regulate light attenuation in GF and NGF lakes will help elucidate the sentinel signals these ecosystems provide and forecast the effects of climate change, glacial recession, and changing land-use on these critical freshwater ecosystems.