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Climate change in Hawaiʻi is usually told through coral bleaching and rising seas. Its effect on streams gets less attention, though their native animals may be among the most exposed. The forecast for the islands is less rain overall but more frequent and severe storms, with more rain in the summer "dry" season and less in the winter "rainy" season. That means much wilder swings in stream discharge — how much water a stream carries.
A native Eleotris sandwicensis in a Hawaiʻi stream. NPS
A life split between stream and sea
Every native stream animal in Hawaiʻi — fish, shrimp and snails alike — is amphidromous:
- Larvae hatch in the stream and wash out to the ocean.
- They grow at sea for up to a few months.
- Juveniles return and swim back up the stream to mature into adults.
The whole cycle depends on the stream staying connected to the ocean. In drought, a stream mouth can close off: larvae cannot get out to sea, and juveniles cannot get back in. Adults, meanwhile, get stranded in shrinking pools.
Normal flow near a stream mouth at Haleakalā National Park. NPS
The same stream mouth in drought. NPS
Running out of oxygen
Stream animals take all their oxygen from what is dissolved in the water, and low flow attacks that supply several ways at once:
| What changes | Why oxygen falls |
|---|---|
| Warmer water | gases dissolve better in cold water, so warm water simply holds less |
| Less tumbling | less water pouring over falls and around rocks means less air mixed in |
| More plants and algae | still, warm water makes them grow, and they consume oxygen |
| Decay | when the algae die, their decomposition uses up what oxygen is left |
Some animals survive very low oxygen, but even moderately low levels can hurt them, because making eggs takes a lot of oxygen. Many — fish above all — can die outright if there is too little.
Keeping watch
The National Park Service's Pacific Island Inventory & Monitoring Network follows the health of these stream communities in parks across the Pacific, including Haleakalā, Kalaupapa, the National Park of American Samoa and War in the Pacific, so that changes can be seen as the climate shifts. Its monitoring reports are in the NPS DataStore.
Sources
Written from "Stream Life in Hawaiʻi National Parks," by A. Farahi, Biological Science Technician, Pacific Island Inventory & Monitoring Network, National Park Service (2015); a work of the United States government in the public domain.
In these publicationsKalaupapa National Historical ParkHaleakalā National ParkWar In The Pacific National Historical ParkNational Park of American Samoa
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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