By Suzi Gage - BBC News
 |
| A sea otter enjoys a crab in California, and helps seagrass in the process |
The
return of sea otters to an estuary on the central Californian coast has
significantly improved the health of seagrass, new research has found.
Seagrass
was deemed to be heading for extinction in this region before the otters
returned.
But
scientists found that the animals triggered a chain reaction of events that
boosted the water-dwelling plants.
The research is
published in the journal, PNAS.
The
urbanisation of California has led to a huge increase in nutrient
pollution in coastal waters, from increasing use of
nitrogen-rich fertilizers.
It's
almost like these sea otters are fighting the effects of poor water quality”
Brent HughesUniversity
of California
This is said to be the reason for the dieback of seagrass, which
has also been declining
worldwide.
This
research suggests that the hunting to near-extinction of sea otters in the late
19th and early 20th Century may have exacerbated the problem, and conversely
that their reintroduction is helping revive ailing seagrass populations, even
in the face of hugely nutrient-rich water.
Links in the
chain
The researchers assessed
seagrass levels over the past 50 years in the Elkhorn Slough in Monterey Bay,
and mapped their increases and declines.
They
looked at a variety of changes that may have affected the grass, but the only
factor that really matched the changes in seagrass was sea otter numbers.
They
theorised that sea otters were eating the crabs which prey upon small
invertebrates in the water.
These
invertebrates eat a type of algae which blooms when there are more nutrients in
the soil. It grows on the leaves of the seagrass, shading them from sunlight
and causing them to die back.
This is
quite a complex cascade of effects, so the researchers tested out their theory
by comparing similar estuaries with and without sea otters, and by doing
experiments in the lab, and in the field.
These
experiments, which included putting cages that sea otters either could or
couldn't access, down on the seagrass, confirmed their hypothesis.
Brent
Hughes, lead author of the study, said: "This estuary is part of one of
the most polluted systems in the entire world, but you can still get this
healthy thriving habitat, and it's all because of the sea otters.
 |
| Sea otters have been responsible for improving the health of the seagrass in these estuaries. |
"So
it's almost like these sea otters are fighting the effects of poor water
quality."
Hughes
described seagrass as "the canary in the coalmine" in terms of
predicting levels of nutrient pollution in the water.
Foundation
species
It also acts as a nursery
habitat for many species of fish and it uses CO2 from sea water and the
atmosphere, thus potentially helping with climate change.
Not
only that, but it acts as protection to the stability of the shoreline.
Hughes
said: "It's what we call a foundation species, like kelp forest, salt
marsh or coral reef. The major problem from a global perspective is that
seagrass is declining worldwide. And one of the major drivers of this decline
has been nutrient inputs from anthropogenic sources, via agriculture or urban
runoff."
These
findings are of particular interest at the moment, as a ban on sea otters
moving along the coast to southern California was lifted last year. The ban was
in place as there was a fear the sea otters would impinge on fisheries in the
area.
Hughes
told BBC news: "That's important because there's a lot of these kind of
degraded estuaries in southern California because of all the urban runoff from
places like Los Angeles and San Diego.
"Coastal
managers will now have a better sense of what's going to happen when sea otters
move in to their systems.
"There's
a huge potential benefit to sea otters returning to these estuaries, and in to
these seagrass beds that might be threatened."
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