Reactive silicon dynamics in a large prairie reservoir (Lake Diefenbaker, Saskatchewan)

There is an up-coming global surge in dam construction. River damming impacts nutrient cycling in watersheds through transformation and retention in the reservoirs. The bioavailability of silicon (Si) relative to nitrogen (N) and phosphorus (P) concentrations, in combination with light environment, controls diatom growth and therefore influences phytoplankton community compositions in most freshwater aquatic ecosystems. In this study, we quantified reactive Si cycling and annual Si retention in Lake Diefenbaker, a 394 km2 prairie reservoir in Saskatchewan, Canada. Retention estimates were derived from 7 sediment cores combined with high-resolution spatiotemporal sampling of water column dissolved Si (DSi). Current annual DSi retention in the reservoir is approximately 28% of the influx, or 2.5 × 108 mol yr− 1. The relative retention of DSi is higher than that of bioavailable N, but lower than that of bioavailable P, indicating a decoupling of the P, N, and Si cycles in the reservoir. The largest accumulation of reactive particulate Si (PRSi) is found in sediments deposited in the mid-reservoir region, which corresponds to the region of highest epilimnetic DSi decrease during the 2013 open water season. This region corresponds to the fluvial-to-lacustrine transition zone, and represents a hot spot for Si retention. Differences in retention efficiencies of macronutrients may not only affect the in-reservoir ecology and water quality, but also modifies the N:P:Si ratios exported downstream, which has the potential to alter ecosystem processes in receiving water bodies.