Multi-Year Nutrient and Organic Carbon Mass Balance of a Young Boreal Hydroelectric Reservoir Complex

Reservoir construction can alter the transport and export of nutrients and organic matter by rivers to coastal areas. However, influences from construction within the first years after flooding are not well understood. Here we present a 9-year study of La Romaine Hydroelectric Complex, composed of four cascading boreal reservoirs sequentially commissioned along La Romaine River in Northeastern Québec, Canada. We followed longitudinal and temporal patterns in concentrations of total nitrogen (TN), total phosphorus (TP), dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and particulate organic matter (POM) in the river above, within each reservoir, and downriver into the estuary, during and after reservoir construction. TN concentrations varied greatly within and between successive reservoirs, suggesting that reservoir habitats can be sources and sinks of N, but concentrations below the Complex remained on average like those upriver. In contrast, TP consistently increased longitudinally and were greater below the reservoirs than upriver, suggesting that these young boreal reservoirs are net sources of phosphorous. DOC and DIC concentrations were relatively constant through the reservoir continuum, suggesting no net change despite evidence of variable but intense C processing seasonally and annually. POM was highly dynamic but consistently declined through the reservoirs. Although reservoirs were influenced by upstream conditions, each had their own distinct nutrient and carbon dynamics, likely influenced by morphometry, residence time, and pre-flood landscape. As reservoirs and the Complex age, they can individually or collectively shift from becoming less of an enhanced source of materials relative to pre-flooding, to sinks of transported material.