Our rental unit has several anchialine ponds, as this section of Hilo seems to be dominated with them. Anchialine ponds are also a unique ecosystem, and are one of Hawaiʻi’s most threatened ecosystems. Around 700 known anchialine ponds in the state of Hawaiʻi, with most present on the youngest lava areas of the Big Island of Hawaiʻi and Maui. They form along the coast as inland lava depressions, and are brackish water with the freshwater lens floating on top fed to the ponds from ground water that moves downslope and from rainwater. Nearby warmer ocean water seeps into the ponds through underground crevices in the surrounding lava rock, and mixes (but layers) with the fresh water forming brackish condition, with the saltier water being on the bottom as salty water has a heavier density. The level of these ponds rises and falls with the tide and the salinity of the pond varies.
This unique water environment is always in flux, yet supports common plants such as seaweeds, blue-green algae, and native grasses. Native plants such as pickleweed, sedges (makaloa), and herbs (ʻakulikuli) also grow around the edges of anchialine ponds. Damselflies, eels, crustaceans, mollusks, and fish such as mullet and gobies can be found in ponds that are closer to the ocean. Inland ponds are typically dominated by tiny, endemic shrimp called ʻopae ‘ula.
People have introduced species into these ecosystems, of course, and the 2.5 foot long red koi, shown below, is just one introduced species in the ponds around us. Laura has seen inch-long shrimp, and other fish are resent that look remarkably like trout.
A black crowned night heron, doves, and a mallard duck pair are among the birds that also hang out in our yard, working these ponds.
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