The US state of Florida will place clams and plant seagrass as part of a five-year, $2.5 million plan to restore fisheries and improve water quality in the Gulf of Mexico near the city of Tampa.   The area targeted, three estuaries of national significance -- Tampa Bay, Charlotte Harbor and Sarasota Bay -- will eventually host a natural "macro-laboratory" that will allow researchers to study how to combat nutrient loading, a key element to oxygen-depletion of water, and improve water quality, state officials said in a statement [...]