Hydroelectric Regulation Decouples Arctic Silica Delivery from the Diatom Bloom: A Climate-Independent Causal Attribution Across Twenty Subarctic Rivers

Ali Bin Shahid 

Large subarctic rivers deliver most of their dissolved nutrients during the spring freshet, in approximate phase with the ice-edge diatom bloom that those nutrients support. Hydroelectric regulation flattens the river hydrograph, holding back the freshet and raising winter discharge, and in doing so it redistributes nutrient delivery in time. Across twenty subarctic rivers spanning the Arctic Ocean and Bothnian Bay drainages, classified by basin-cumulative Degree of Regulation (DOR), we test whether regulation shifts the delivery of dissolved silica out of phase with the shelf-specific diatom bloom window. Using the Global Aggregation of Stream Silica dataset (Jankowski et al., 2025) and the Arctic Great Rivers Observatory record, we estimate seasonal silica flux with a LOADEST-style concentration-discharge rating and map delivery onto receiving-shelf bloom climatology for eight shelves (Kara, Laptev, East Siberian, Beaufort, Bering/Chukchi, Bothnian Bay, Bothnian Sea, Gulf of Finland). Regulated rivers deliver 24.8 percent of annual silica during the open-water bloom window against 35.8 percent for controls, a synchrony gap of -11.0 percentage points that remains negative under every combination of plus-or-minus 15-day perturbations of the bloom window (49 of 49 negative, range -7.3 to -13.8 pp). The synchrony loss scales monotonically with DOR across the panel (Spearman rho = -0.80; OLS slope -0.284 per unit DOR, R-squared = 0.59, t = -5.05, two-sided p = 0.00008 from Student-t with n = 20 rivers spanning DOR 0 to 0.63), and leave-one-out cross-validation preserves a negative slope in 20 of 20 fits (slope range -0.310 to -0.250, all p less than 0.001). A placebo difference-in-differences design argues against the climate-only counterfactual on the cleanest available natural experiment: post-Krasnoyarsk Yenisei shifted winter discharge fraction by 4.96 percentage points more than the unregulated Lena (95 percent CI 4.21 to 5.72, p less than 0.0001) under identical Siberian climate. Within Yenisei, the year-by-year differential winter discharge fraction tracks the cumulative basin reservoir-filling curve across 73 years (r = +0.82), converting the binary causal test into a continuous-dose trajectory. The expanded twenty-river panel reproduces the original six-river ArcticGRO result independently (gap -10.0 vs -9.2 pp on the same six rivers using GlASS data), with multiple high-DOR anchors (Yenisei, Lulealven, Indalsalven, Skellefte, Oulujoki, Kymijoki, Angermanalven) replacing the single-river leverage of the original analysis. The contribution is the reframing of regulated-river nutrient alteration from a winter-enhancement story to a measurable loss of synchrony between SiO2 delivery and biological demand on the receiving shelves, robust across the Arctic Ocean and Bothnian Bay drainages.