LONGITUDINAL SAMPLING REVEALS LOCAL IMPACTS TO STREAM BIOGEOCHEMISTRY IN A TROPICAL WATERSHED

Abstract

The Ala Wai Watershed on the island of O‘ahu, Hawai‘i is 42.4 square kilometers in area, drained by three stream systems, Makiki, Mānoa, and Pālolo discharging into the Ala Wai Canal, is densely urbanized in the mid and lower reaches, and has forested areas along the upper valleys and valley ridges. The Ala Wai Canal was constructed in the early 20th century to drain the Waikīkī wetland and the three streams were subsequently altered to direct flow and prevent flooding. Measuring stream nutrient concentrations and understanding longitudinal patterns in this highly modified watershed is important to better determine stream, canal, and nearshore ecosystem health. From July of 2018 to March of 2019, measurements of nutrients and environmental parameters were taken approximately monthly from thirteen sites along the three tributaries draining the Ala Wai Watershed. The Makiki Stream consistently registered elevated silicate, total phosphorus, and conductivity, suggesting a geological source. Samples from below Lyon Arboretum in the upper Mānoa Valley measured an increase of total organic nitrogen and ammonium. In the lower Mānoa Valley, the outflow water from Lo‘i o Kānewai taro patch was the most undersaturated in dissolved oxygen of the entire watershed while also exhibiting a drawdown of nitrate and nitrite with a concomitant increase in total organic nitrogen, total phosphorus, and ammonium concentrations. In the adjacent Pālolo Valley, an upstream tributary carried the highest concentration of total organic nitrogen, nitrate, and total phosphorus concentrations, consistent with agricultural practices and high-density onsite sewage disposal systems. A heavily channelized site near the lower stretches of Pālolo Stream exhibited the highest temperatures and concentrations of dissolved oxygen, pH, and ammonium in the system, highlighting the ecosystem impacts of waterway hardening. This study exposed the heterogeneity of nutrient and environmental parameter patterns in a relatively small watershed, demonstrating the value of longitudinal and time-series sampling to identifying nutrient sources, both natural and anthropogenic, between sites and streams. Future work investigating other streams should include high spatial and temporal resolution sampling to better understand intricate nutrient sources and sinks unique to those systems. Increased knowledge of the processes affecting the composition and concentrations of nutrients will aide in environmental management and policy making.