Strategic Monitoring And Resilience Training in the Ala Wai Watershed, Oahu: Seasonal and Episodic Variability

The Ala Wai Canal is an artificial estuary connecting three major streams (Makiki, Palolo, Manoa) and numerous urban drainages to the ocean in Honolulu, Hawai'i. Design of the canal and extensive urbanization has led to severe pollution and eutrophication in the Ala Wai canal. The lack of previous systematic water quality monitoring data has inhibited restoration and resilience efforts in the canal and watershed. In order to understand the biogeochemical interactions and anthropogenic inputs within the watershed, this study aimed to construct high­-resolution spatial and temporal characterization of existing physical and chemical conditions prior to near-future dredging activities (State DLNR) and watershed flood mitigation actions (US Army Corps of Engineers). Monthly water quality surveys and sampling were conducted at 12 sites along three major streams in the watershed, water column sensor profiling, and discrete water sampling were conducted in the canal and in the nearshore water. A high-resolution bathymetric survey was conducted in the canal at the beginning of the study period. Sensor data included temperature, salinity, dissolved oxygen, chlorophyll, and turbidity. Discrete samples were analyzed for dissolved inorganic nutrients and dissolved inorganic carbon. Vertical stratification and horizontal gradients in the Ala Wai Canal were strongly affected by runoff from episodic rain events, tidal mixing, and canal bathymetry. Bathymetric mapping and sensor surveys indicate that lateral mixing is primarily inhibited by sediment shoals, while vertical mixing was largely affected by salinity gradient. Data also showed that the restricted end of the canal was largely anoxic in the bottom water due to limited ventilation and enhanced organic matter respiration. Quantification of physical/chemical parameters can assist in understanding urbanization and ecological impacts, which is especially important for future planning, legislation, engineering, and resilience decisions.