Biological, chemical, and physical monitoring of a protected Hawaiian stream subject to water diversion, 2007-2021

Waikolu Stream in Kalaupapa National Historical Park, Moloka‘i, Hawaii, USA, was monitored from 2007 to 2021 to assess spatial and temporal trends, inform the park of natural resource issues, improve monitoring protocol design, and quantify the “wilderness character” of physical habitats, water quality, and animal populations in a windward-side Hawaiian stream free of many stressors common in other Hawaiian streams. Fish, shrimp, snails, total dissolved nitrogen and phosphorus, nitrate + nitrite, oxygen, pH, temperature, turbidity, specific conductance, chlorophyll, canopy cover, substrate, channel width, and discharge were measured. The physical habitat remained stable. Nitrate + nitrite regularly exceeded state water quality criteria. Abundance of Kuhlia sandvicensis (Āholehole), Eleotris sandwicensis (‘O‘opu ‘akupa), the native shrimp Atyoida bisulcata (ʻŌpae kalaʻole), and the threatened Neritona granosa (hīhīwai) decreased. Lentipes concolor (‘O‘opu ‘alamo‘o) abundance remained stable. Abundance of Awaous stamineus (‘O‘opu nakea), Sicyopterus stimpsoni (‘O‘opu nōpili), and the non-native Macrobrachium lar (Tahitian prawn) increased. Cohorts were observed in temporal patterns of M. Lar and N. granosa population structures suggesting lifespans of 5–6 years and 10 years, respectively. An historic low-head dam near the stream mouth did not impede amphidromous animal migration. The Molokaʻi Irrigation System had measurable impacts on discharge and spatial patterns of water quality and animal distribution, restricting available habitat. Water quality monitoring can be achieved by reducing the number of sample sites. Management by the national historical park maintained Waikolu Stream at high conservation value, and the stream can serve as a benchmark for comparison to anthropogenically impacted windward-side Hawaiian streams.