Calcium carbonate

This is the main component of limestone. Calcium is an essential ingredient of the macro invertebrate exoskeleton. This compound dissolves in weak acid, dissociating into Ca++ and (-)CO3. In doing so, it makes calcium available to the life forms of the stream, and at the same time changes the overall pH of the environment to slightly basic in pH (typically 7.2-7.8). These are the two reasons why limestone streams are so productive.

Links
  http://www.digital-analysis.com/Limestone.htm