Coupled reservoir-river systems: Lessons from an integrated aquatic ecosystem assessment.

Sustainable reservoir-river management requires balancing complex trade-offs and decision-making to support both human water demands and ecological function. Current numerical simulation and optimization algorithms can guide reservoir-river operations for optimal hydropower production, irrigation, nutrient management, and municipal consumption, yet much less is known about optimization of associated ecosystems. This ten-year study demonstrates an ecosystem assessment approach that links the environmental processes to an ecosystem response in order to evaluate the impact of climatic forcing and reservoir operations on the aquatic ecosystems of a coupled headwater reservoir-river system. The approach uses a series of numerical, statistical, and empirical models to explore reservoir operational flexibility aimed at improving the environmental processes that support aquatic ecosystem function. The results illustrate that understanding the seasonal biogeochemical changes in reservoirs is critical for determining environmental flow releases and the ecological trajectory of both the reservoir and river systems. The coupled models show that reservoir management can improve the ecological function of complex aquatic ecosystems under certain climatic conditions. During dry hydrologic years, the high post-irrigation release can increase the downstream primary and macroinvertebrate production by 99% and 45% respectively. However, this flow release would reduce total fish biomass in the reservoir by 16%, providing management tradeoffs to the different ecosystems. Additionally, low post-irrigation flows during the winter season supports water temperature that can maintain ice cover in the downstream river for improved ecosystem function. The ecosystem assessment approach provides operational flexibility for large infrastructure, supports transparent decision-making by management agencies, and facilitates framing of environmental legislation.

Invasive Macrophytes Control the Spatial and Temporal Patterns of Temperature and Dissolved Oxygen in a Shallow Lake: A Proposed Feedback Mechanism of Macrophyte Loss.

Submerged macrophytes can have a profound effect on shallow lake ecosystems through their ability to modify the thermal structure and dissolved oxygen levels within the lake. Invasive macrophytes, in particular, can grow rapidly and induce thermal gradients in lakes that may substantially change the ecosystem structure and challenge the survival of aquatic organisms. We performed fine-scale measurements and 3D numerical modeling at high spatiotemporal resolution to assess the effect of the seasonal growth of Potamogeton crispus L. on the spatial and temporal dynamics of temperature and dissolved oxygen in a shallow urban lake (Lake Monger, Perth, WA, Australia). Daytime stratification developed during the growing season and was clearly observed throughout the macrophyte bed. At all times measured, stratification was stronger at the center of the macrophyte bed compared to the bed edges. By fitting a logistic growth curve to changes in plant height over time (r2 = 0.98), and comparing this curve to temperature data at the center of the macrophyte bed, we found that stratification began once the macrophytes occupied at least 50% of the water depth. This conclusion was strongly supported by a 3D hydrodynamic model fitted to weekly temperature profiles measured at four time periods throughout the growing season (r2 > 0.78 at all times). As the macrophyte height increased and stratification developed, dissolved oxygen concentration profiles changed from vertically homogeneous oxic conditions during both the day and night to expression of night-time anoxic conditions close to the sediments. Spatially interpolated maps of dissolved oxygen and 3D numerical modeling results indicated that the plants also reduced horizontal exchange with surrounding unvegetated areas, preventing flushing of low dissolved oxygen water out of the center of the bed. Simultaneously, aerial imagery showed central dieback occurring toward the end of the growing season. Thus, we hypothesized that stratification-induced anoxia can lead to accelerated P. crispus dieback in this region, causing formation of a ring-shaped pattern in spatial macrophyte distribution. Overall, our study demonstrates that submerged macrophytes can alter the thermal characteristics and oxygen levels within shallow lakes and thus create challenging conditions for maximizing their spatial coverage.