Hawaiian Stream Diversions Alter Aquatic Ecology

For more than 100 years, Hawaiian streams have been diverted to irrigate agricultural plantations. But, until now, the potential costs in terms of Hawaii’s endemic stream life were undocumented. A recent study reports drastic changes in stream communities below the diversion of a Maui stream. Below the diversion of Iao Stream, Maui; three macroinvertebrate groups (which include insects, crustaceans, and snails) disappear. Densities of other macroinvertebrate groups drop by 46 percent below the diversion. The changes in the macroinvertebrate community could have serious impacts on endemic fish that rely on them for food.From the Earthwatch:Hawaiian Stream Diversions Alter Aquatic Ecology

For more than 100 years, Hawaiian streams have been diverted to irrigate agricultural plantations. But, until now, the potential costs in terms of Hawaii’s endemic stream life were undocumented. A recent study published by former Earthwatch principal investigator Dr. Eric Benbow (Michigan State University) and his colleagues’ reports drastic changes in stream communities below the diversion of a Maui stream.

The article in River Research and Applications documents that below the diversion of Iao Stream, Maui; three macroinvertebrate groups (which include insects, crustaceans, and snails) disappear. Densities of other macroinvertebrate groups drop by 46 percent below the diversion. The changes in the macroinvertebrate community could have serious impacts on endemic fish that rely on them for food.

“Reduced stream flow lowers the velocities and depth important for many of the endemic and native stream species that are rheophilic, preferring fast moving water, such as fly larvae,” said Benbow. “When the densities of these non-charismatic critters are reduced, this has negative effects on the food resources for the more charismatic critters, such as fish, found no where else in the world but Hawaii.”

The Hawaiian Islands are known for their endemic species. The only native stream fish are four species of goby and one eleotrid, collectively known as o’opu, small, bottom-dwelling marine species that have taken up the habits of stream life in their adult stage. These endemic stream fish are potentially threatened by the changes in the macroinvertebrate community, as well as by stream diversions themselves.

Extensive irrigation systems transporting stream water from the wet, windward sides of Hawaiian Islands to plantations on the dry sides draw nearly 100 percent of the median flow of many streams. Team members working with Benbow on the Earthwatch-supported Hawaii’s Mountain Streams project from 1999 to 2001 collected data on the impact of these diversions on stream communities.

“These data could not have been collected without the help of Earthwatch volunteers,” said Benbow, referring to the recent paper. “Not only did individuals contribute their time and relentless effort in the field, they also provided valuable insight into things that may have been overlooked in the sea of details and logistics necessary for making a project like this a success.”

Benbow’s study is one of the first to directly assess the effects of stream flow diversion on the macroinvertebrate community of a Hawaiian stream. Another paper, in review, documents the change in biomass of macroinvertebrates below stream diversions, which translates directly to calories available for larger animals such as endemic fish. These findings are an important step toward more effective stream management and conservation of Hawaii’s endemic species.


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