Dynamic Nutrient Limitation in a Major Tributary to Eastern Lake Erie: The Role of Groundwater Silicon

Abstract:

Nutrient silicon (Si) limited systems tend to promote more harmful algal blooms, compared with phosphorus (P) or nitrogen (N) limited systems. In this project, we studied the biogeochemical sources and sinks of Si in the Grand River watershed (GRW), a 7000 km² basin located inthe largely agricultural region of southwestern Ontario, Canada. The river, its major tributaries, and eastern Lake Erie, into which the GRW drains, have historically been considered P limited. We collected groundwater and surface water samples at 11 locations in the lower half of the GRW at monthly to weekly intervals for one year. Samples were analyzed for dissolved and reactive particulate Si (DSi and PRSi), total dissolved P, soluble reactive P, and a suite of other macro and micronutrients including nitrate, nitrite, sulfur and iron. Results indicate that groundwater discharge to surface water provides a year-round source of DSi to surface water, with concentrations roughly equal to winter surface water concentrations. For the majority of the year, this groundwater DSi flux results in Si excess in the GRW. However, during extreme high flow events such as the spring snowmelt and long-term heavy rain events, P is flushed in high concentrations into the river, while DSi concentrations, which experience seasonal drawdown due to biological uptake, are diluted. These dynamics can lead to periods of Si limitation, which persists throughout the river and into Lake Erie.