Spatial and temporal patterns of light attenuation among lakes of the Mackenzie Delta

SUMMARY 1. The seasonal dynamics of light attenuation, and the relative roles of total suspended solids (TSS), dissolved organic carbon (DOC) and chlorophyll as light attenuators among two sets of lakes in the Mackenzie Delta, were assessed during the open‐water periods of 1998 and 1999. 2. The first set consisted of 40 spatially discrete lakes where the frequency of flooding with river water was controlled by sill height (‘sill‐set lakes’). The second set consisted of a chain of six lakes connected to a main river channel (frequently flooded, all with same frequency), but where riverine influence was controlled by the distance from the channel connection point (‘chain‐set lakes’). 3. As the flooding frequency of lakes decreased (sill‐set), and as the distance from the channel connection point increased (chain‐set), lake water became increasingly transparent and the stability (decreasing temporal variability) of underwater light increased. 4. The effect of flooding on transparency was greater in years with a high minimum summer water level. However, the effect of river flooding on lake water transparency was damped more by an increase in the frequency and duration of flooding than by an increase in distance from the channel connection point. 5. The index of scattering was linearly related to TSS over the common range of concentrations in both sets of lakes. The specific attenuation coefficient for TSS (and scattering) increased substantially from the most turbid to the most transparent waters. 6. During the summer, DOC provided an approximate index of water colour in the sill‐set lakes but not in the chain‐set lakes, where the gradient of DOC ran counter to the gradient of water colour. The specific attenuation coefficient for water colour was roughly constant among both sets of lakes. 7. Calculations of partial attenuation show that, during the spring flood peak, TSS is the dominant attenuator among most lakes, other than those with high sills or positioned far from channel connection points. During the lengthy summer period of open water, however, water colour appeared to be the most important light attenuator among almost all of the lakes in the central delta, with chlorophyll a of only minor importance. 8. Lakes of the Mackenzie Delta may be quite sensitive to changes in climate and ultraviolet‐b (UV‐b) radiation in the circumpolar arctic because of the role of DOC as an attenuator of photosynthetically active radiation and UV‐b irradiance and as an energy source for microbial foodwebs in this system.