Sea otter return providing lifeline for dwindling seagrass in California
A significant improvement in the health of seagrass in a central Californian estuary is due to the return of sea otters, according to recent research.
Seagrass has been suffering drastic declines worldwide, and coastal California is no exception. Urbanisation has led to a massive increase in nutrient pollution along the state's coast, with run-off from fields treated with nitrogen-rich fertilisers being blamed for the reduction in seagrass beds in the region. However, new research has revealed that the return of sea otter populations to the area may be enabling seagrass levels to recover.
Sea otters were hunted to near extinction in the late 19th and early 20th centuries, mainly for their dense pelt which was extremely sought-after for the fur trade. This latest research, published in the journal Proceedings of the National Academy of Sciences (PNAS), suggests that the drastic reduction in sea otter numbers may have exacerbated the decline of seagrass in the region.
Sea otters are now returning to the area, and, despite the continued pollution of the ocean, the water-dwelling plants are now doing much better. It is thought that the return of sea otters has triggered a complex ecological chain reaction which favours the survival of seagrass.
Scientists assessed seagrass levels in part of Monterey Bay, California, over the past 50 years, mapping increases and declines. A whole host of factors which could potentially affect seagrass levels were studied, but the only one which matched the recorded changes was sea otter numbers. The health of the marine ecosystem relies upon a delicate balance of predator and prey species, and scientists have theorised that it is a readjustment in this balance that is now enabling seagrass to thrive.
Increased nutrients in the ocean due to fertiliser run-off have favoured the growth of a particular type of algae which grows on seagrass, shading the leaves and causing them to die off. Ordinarily, this algae is kept in check by small invertebrates which feed upon it, but with the reduction in sea otters came an increase in one of its main food sources — crabs. Crabs feed on marine invertebrates, so higher numbers of crabs meant fewer invertebrates to keep algae levels down, therefore contributing to the drastic reduction in seagrass.
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Sea otter and kelp image via Shutterstock.