One of two environmental organizations opposed to a proposed seawater desalination plant in Carlsbad has withdrawn its lawsuit.

The Surfrider Foundation had challenged a Superior Court ruling that upheld a 2008 decision by the State Lands Commission to approve the project by Poseidon Resources Corp. Construction began on the project in November.

Surfrider's withdrawal leaves San Diego Coastkeeper as the remaining litigant against the plant. It will have the capacity to turn seawater into 50 million gallons of fresh drinking water a day ---- enough for 300,000 people ---- when it goes on-line in two years.

While still opposed to the project, Surfrider decided its efforts could be better spent elsewhere, said Gabriel Solmer, an attorney for San Diego Coastkeeper, Surfrider's partner in the lawsuit.

Poseidon Resources, of Stamford, Conn., issued a press release Friday applauding the decision and urging Coastkeeper to also drop its litigation.