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.
Testing the hypothesis
To test their theory, the researchers set up experiments in similar estuaries with and without sea otters, and carried out other tests in the field as well as in the lab. One experiment involved putting cages on the seagrass, with some being accessible to sea otters and some not. The results of the tests confirmed the hypothesis.
Fighting climate change
Brent Hughes, lead author of the study, described seagrass as being ‘the canary in the coalmine’, as it can be used to predict the levels of nutrient pollution in the water. He marvelled at the positive effect the return of the sea otters is having, saying, “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. So it’s almost like these sea otters are fighting the effects of poor water quality.”
Seagrass plays an extremely important role in the marine ecosystem, acting as a nursery habitat for a wide variety of fish species, and taking in carbon dioxide from the water and the atmosphere, therefore potentially helping in the fight against climate change. In addition to this, seagrass contributes to the stability and protection of the shoreline.
“It’s what we call a foundation species, like kelp forest, salt marsh or coral reef,” said Hughes. “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.”
A ban on sea otters that was in place to prevent them from impinging on fisheries in the southern California area was lifted last year, and so the findings from this latest research are particularly relevant.
“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,” said Hughes. “Coastal managers will now have a better sense of what’s going to happen when sea otters move into their systems. There’s a huge potential benefit to sea otters returning to these estuaries, and into these seagrass beds that might be threatened.”
Source : Arkive
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