New Orleanians know how to party and do it most of the year. They don’t even need a reason. But Mardi Gras is the city’s longest party. It’s a three-month-long fête with parades of dazzling floats, their masked riders tossing strings of colorful beads and slews of trinkets to screaming crowds with outstretched arms. The revelry is contagious, but millions of non-biodegradable “throws” litter the streets, creating an environmental nightmare.