“Algae is one of the few winners in a world dominated by humans. It thrives on our waste, such as sewage and runoff from farmland, which is full of nitrogen and phosphorous — nutrients that algae need to grow.
As pollution runs into the ocean, algae booms.
Meanwhile, animals that eat algae have declined precipitously in recent decades. In the 1980s, an unknown pathogen wiped out longspined sea urchins in the Caribbean. These marine invertebrates — which take the shape of an overfilled pin cushion — eat loads of algae. Similarly, overfishing and the loss of various ecosystems has caused declines in algae-eating fish, such as parrotfish.
Like a fertilized pasture with no cows, a field of algae on a reef with no herbivores grows unencumbered. In the last decade, the extent of algae on reefs globally increased by roughly 20 percent, turning them from brilliant fields of color to monochrome patches of green.
This is a serious problem for coral.
When a thick layer of seaweed covers the reef, it’s hard for baby corals — which spend their early days as larvae swimming in the ocean — to find a spot on the seafloor and start a colony. This seaweed not only takes up floor space but it can also limit the amount of sunlight that reaches the bottom (coral needs light to grow) and produce chemicals that dissuade corals from settling. Abundant algae also competes with adult colonies for space, crowding them out.”