Otters busy helping environment
Fertilizer runoff has led to a global decline in seagrass meadows, which provide crucial habitat for fish. But thanks to sea otters, these meadows are flourishing in Elkhorn Slough, a major estuary in Monterey Bay, Calif., scientists say.
Fertilizer from farms in Salinas flows into Elkhorn Slough, carrying phosphates and other nutrients that fuel the growth of algae on seagrass leaves. As the algae blooms, it shades seagrass from sunlight it needs to grow.
In fact, nutrient levels are so high in Elkhorn Slough that scientists wouldn't expect seagrasses to survive there. Yet in recent years, the estuary bed has been teeming with the lush, green grass.
“There's more seagrass in the slough than there's ever been,” said study leader Brent Hughes, a marine ecologist at the University of California, Santa Cruz.
To find out why, he and his colleagues combed through surveys conducted by the California Department of Fish and Wildlife and other agencies that tracked the growth of seagrass in Elkhorn Slough during the last 50 years. Fertilizer runoff nearly wiped out the seagrasses, but they began expanding in 1984 — right when sea otters began recolonizing Elkhorn Slough, the researchers reported on Monday in the Proceedings of the National Academy of Sciences.
Sea otters once thrived along the California coast, but the fur trade nearly drove them to extinction in the early 1900s. Since then, conservation efforts have helped them recover and repopulate their former habitats, including Elkhorn Slough.
Since sea otters returned to the estuary, the seagrass has expanded sevenfold. But how did the otters boost its numbers? The researchers suspected that the otters fed on a predator that affected the plant's growth.
The only predators sea otters eat are crabs, which feed on invertebrates — such as sea slugs — that nibble algae off seagrass leaves. With fewer crabs to prey on them, these grazers grow more abundant, keeping seagrass leaves clean and healthy.
To test whether crabs really were the missing link, the researchers compared seagrasses in Elkhorn Slough with those in Tomales Bay, an estuary that has a similar ecosystem, but with low levels of nutrient runoff and no otters.
The researchers also simulated the effects of otters in the ecosystem using enclosures in Elkhorn Slough. One of the enclosures contained the large crabs one would expect to find when otters aren't around to eat them; the other had the smaller crabs found when otters are present. As expected, the seagrass grew larger and faster in the enclosure with the smaller crabs.