Long-term spatial trends in sedimentary algal pigments in a narrow river-valley reservoir, Lake Diefenbaker, Canada

Narrow river-valley reservoirs are typically spatially heterogeneous. Little is known about how (a) water quality and algal community composition change longitudinally along a reservoir and (b) how algal composition and production change as a reservoir ages. To address these unknowns, multiple sediment cores were collected from mid-channel locations along the longitudinal axis of Lake Diefenbaker, Saskatchewan, Canada, a long, narrow river-valley reservoir on the Canadian Prairies. Profiles of concentrations of various pigments in sediment cores were measured to infer spatial and temporal trends in algal biomass and community composition. Diverse mixtures of pigments derived from filamentous and colonial cyanobacteria, diatoms, chlorophytes, and other phyla were observed. Spatial patterns of sedimentation of pigments (nmol m^-2 yr^-1) in surficial sediments suggest increases in algal biomass with distance down-reservoir, with maximum inferred biomass occurring in mid-reservoir. This is consistent with general knowledge of patterns of primary production in narrow, river-valley reservoirs. However, myxoxanthophyll, a biomarker of filamentous or colonial cyanobacteria, detected only at sites furthest down-reservoir, did not follow this general trend. Temporally, an increase in algal biomass occurred at down-reservoir locations after 1990, followed by a substantial increase after 2000 at the majority of sites. Profiles of concentrations of pigments exhibited no clear trends to support the prevailing paradigm that predicts an initial upsurge in trophic status upon formation of reservoirs. This pattern may result from limited penetration of light in the early years after reservoir formation. This study reinforces the need for paleolimnological analyses among hydrologic zones of large reservoirs.