Los Cerritos Wetlands Stewards: A local organization working to protect the Los Cerritos Wetlands, and other natural areas in Long Beach.
Los Cerritos Wetlands Trust Actively working to acquire, protect, and restore lands around the Los Cerritos Wetlands.
Surfrider Foundation. Has a Long Beach chapter.
California Native Plant Society: Long Beach is part of the South Coast chapter.
Friends of the Los Angeles River: They focus on restoring the entire watershed of the LA River, including the "downstream" end in Long Beach.
Sierra Club. Has an Angeles Chapter, which includes a Long Beach club.
The Audubon Society: The El Dorado chapter includes Long Beach and surrounding communities.
The Trust for Public Land helps protect open space for both recreation and preservation. They helped protect a portion of the Los Cerrtios wetlands (you can read that story here).
The Aquarium of the Pacific is not really an environmental organization in the traditional sense. But it is a world-class institution and a great place to learn about the marine environment in Long Beach. Your support helps their local and global research efforts for marine conservation.