In a 5-qt. pot over medium-high heat, melt butter. Add onion, carrots, bay leaf, white rice, salt, and black pepper. Cook, stirring, until onions are light golden, about 6 minutes. Add 5 cups water and bring mixture to a boil, then reduce heat and simmer 25 minutes. Remove bay leaf.
Working in batches, purée soup in a blender until smooth. Return soup to pot, stir in lemon zest, and keep warm. In a small bowl, toss crab with lemon juice and minced chives. Put an equal mound of crab mixture in the center of each of 4 soup bowls, then ladle soup around crab. Garnish with chopped chives.