The Effects of Environmental Forcing on the Water Quality in Ke‘ehi Lagoon O‘ahu, Hawai‘i

ABSTRACT The Ke‘ehi Lagoon Beach Park is situated on the south shore of O‘ahu, Hawai‘i, USA; it is located to the east of the Daniel K. Inouye International Airport and south of the Nimitz Highway. For over 15 years, Keʻehi Lagoon, Hawai‘i, and the two streams that discharge into the lagoon have been deemed by the Hawaiʻi Department of Health Clean Water Branch (HDOH CWB) to be impaired, according to the 2023 HDOH CWB report. Impaired waters are defined as a water source that has been threatened by runoff pollutants. The Hawai‘i Department of Health contracted the Pacific Islands Ocean Observing System Near Shore Sensor Group (PacIOOS-NSSG) to investigate the water characteristics of Keʻehi Lagoon. We deployed oceanographic instrumentation in Keʻehi Lagoon that measured conductivity (salinity), temperature, pressure (depth), chlorophyll a, and turbidity. We downloaded the United States Geology Survey (USGS) stream gauge and stream discharge data, as well as data on wind speed and direction, air temperature, and precipitation from the Visual Crossing Weather Station, located at the Daniel K. Inouye International Airport (site # 91182022521). Water characteristics were compared across spring and neap tides and diurnal heating. Results from this work indicate that the Keʻehi Lagoon is most heavily influenced by the diurnal tide, stream flow, weather, and solar insolation. The two streams that flow into Keʻehi Lagoon traverse heavily urbanized areas; thus, the potential for the transport of heavy metals, toxins, and pathogens is high. Salinities increased on the flood tide and decreased on the ebb tide. Temperatures varied diurnally and seasonally, with warmer temperatures during daylight hours and summer months, respectively. Chlorophyll a was elevated when temperatures were warmer. In addition, turbidity was lowest when ocean waters were in the Keʻehi Lagoon, and highest when stream waters were predominant in the lagoon. Finally, we discuss how these data sets can be used to illustrate how the system will respond as our climate changes.