Ridge-to-Reef Connectivity under a Changing Climate: Current and Future Nutrient Export Dynamics of the Wailuku River Watershed in Hawai'i Island, HI, USA

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2024-12

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Rivers serve as critical links between terrestrial and marine ecosystems by exporting nitrogen and phosphorus to the coastal zone. However, projected changes to precipitation patterns as a result of climate change have the potential to alter river discharge and nutrient export dynamics. By analyzing the dissolved nutrient and particulate organic matter (POM) content of river water samples over a 3-yr period in the Wailuku River Watershed (WRW) in Hawai‘i Island, HI, USA, this study: (1) quantifies N and P export in the WRW using USGS load estimation (LOADEST) software, (2) models changes to nutrient export dynamics under six future climate scenarios, and (3) examines potential sources of riverine nutrient input through stable isotope analyses of nitrate (NO3-). Results indicate that dissolved organic N (DON) and P (DOP) yields exceeded those of dissolved inorganic N (DIN) and P (DIP). However, complex temporal export patterns in dissolved P forms resulted in periods when DIP exceeded DOP. Annual nutrient export stoichiometry indicated P-limitation throughout the study period, though high contributions of DOP to P-pools lowered mean (±1 SD) annual TDN:TDP (49.1 ± 34.5) relative to DIN:DIP (72.1 ± 64.3). Yields of most nutrient forms were greater during the wet (November - April) season than the dry season (May - October), though mean DIP yields were greater in the dry season. Stormflow-driven export of POM contributed 56.6% and 38.6% to annual watershed yields (kg km-2 yr-1) of total N (82.4 ± 49.2) and total P (4.7 ± 4.9), though the timing and magnitude of stormflow nutrient pulses were highly variable across years. Under future climate scenarios, maximum changes to total N (+27.7 ± 2.7%) and total P (+24.6 ± 0.6%) annual yields occurred as modeled changes to river discharge were allocated to stormflow conditions. While modeled DIN, DON, and DIP yields displayed little variation across climate scenarios, maximum changes to DOP yields (+37.1 ± 19.2%) occurred as discharge changes were allocated to baseflow conditions. Mixing models of δ15N and δ18O-NO3- indicated that soil and groundwater are the predominant sources of riverine NO3-, though sewage and/or manure may also contribute. Overall, this study suggests that climate change may impact N and P export dynamics differently in the WRW, and these changes could impact downstream food webs and ecosystems by adding to the existing seasonal and interannual variability in nutrient export patterns.

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Water resources management, Climate change, climate change, nitrogen, phosphorus, stable isotope, tropical, watershed

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71 pages

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