NASA Earth Observatory images by Lauren Dauphin, using Landsat
data from the US Geological Survey and topographic data from
the USGS 3D Elevation Program (3DEP). Historical sea surface
temperature image by Jesse Allen, using microwave and infrared
multi-sensor SST data from Remote Sensing Systems. Photo by Steve
Lonhart, NOAA Monterey Bay National Marine Sanctuary.
Off the coast of Sonoma and Mendocino counties, changing climate and a
marine epidemic have combined to decimate one of California's most
productive ecosystems. In the span of a single year, the region's
renowned kelp forests almost completely collapsed, and they are still struggling. Floating forests that once harbored and fed many marine
species have turned into barrens devoid of biodiversity.
Using 34 years of Landsat imagery, a team of researchers led by
Meredith McPherson of the University of California, Santa Cruz,
documented the fast and catastrophic collapse of the once hardy kelp
forest, as well as its struggle to regenerate. The research team found
that the Northern California kelp canopy declined more than 95 percent
in 2014-15, and the effects persisted for 5 years.
The map above, based on data from McPherson and colleagues, shows the
location of bull kelp forests in 2008 and 2019. The images below show
the same areas as observed in shortwave infrared, near-infrared, and
red light by Landsat 5 (bands 7,5,3) in 2008 and Landsat 8 (bands
7,5,4) in 2019. The combination helps make some kelp forests visible
Bull kelp is a canopy-forming macroalgae that flourishes in
nutrient-rich, cool water and grows as much as 60 centimeters (nearly
2 feet) per day. The kelp is considered an "ecosystem engineer"--the foundational species of a nearshore ecosystem that feeds and shelters
other marine life. It is the dominant kelp species north of Monterey
Bay, California, with underwater forests thriving along 160 kilometers
(100 miles) of rocky reefs from Fort Bragg to Jenner.
August 31, 2008 - August 14, 2019
[NO URL -- SEE LINK AT TOP OF ARTICLE]
Unlike the giant kelp more common to the south, bull kelp is an annual
species that grows vigorously from June through August. It then
disperses its spores before fall and winter storms dislodge the mature
plants from their rocky perches. While the exact location and extent
of the bull kelp can change from year to year (based on spore
dispersal and environmental factors), the underwater forest has
historically regenerated regularly.
Looking across several decades of Landsat observations, McPherson and colleagues found that the geographic distribution of bull kelp
contracted, first receding in 2008 in the sandier regions north of
Fort Bragg, and then in 2012 in sandier sections south of
Jenner. (These areas are just north and south of the map area shown.)
But along the rocky substrate in the middle, the bull kelp held strong.
Then came "the blob." In 2013, a marine heatwave started warming the
Bering Sea, and by 2014 the warm waters reached the California
coast. Water temperatures rose as much as 2.5°C (4°F) above normal off
the US and Canadian coast and stayed high for 226 days--the longest
marine heatwave ever recorded. (Sea surface temperatures from July
2015 are shown below.) "The blob" eventually merged with warm waters
from the "Godzilla El Niño" of 2015-2016.
July 1 - 31, 2015
[NO URL -- SEE LINK AT TOP OF ARTICLE]
The nutrient-poor waters associated with marine heatwaves hinder kelp
growth, leading to smaller canopies. Historically kelp have been
resilient, though, coming back in force once waters have cooled
down. But this time, a cascading series of environmental and
biological events--exacerbated by climate change--combined to decimate
The delicate interplay of species that safeguards kelp forest
biodiversity was shifted in 2013 when more than 20 sea star species
from Alaska to Mexico started wasting away. In particular, sunflower
sea stars, the primary predator of kelp-devouring purple sea urchins,
were ravaged by a mysterious wasting syndrome. Renowned regenerators
known to grow back entire limbs, the sea stars (starfish) looked as if
they had melted to goo.
With this pivotal predator functionally extinct, and bull kelp growing
poorly due to the warm water, the balance of predators and feeders was
thrown off. Purple sea urchins that had previously occupied shallow
tidal pools and ate kelp leaf litter were suddenly eating growing kelp
stalks, or stipes. Urchins climbed down the stipes all the way to the
seafloor, eating until there was nothing left.
By 2015, the kelp forests were mostly gone, replaced by urchin
barrens. Divers described the conversion of once-rich kelp forests
into spiky purple carpets. With no kelp left to eat, the purple sea
urchins now mostly subsist in a starvation state, rousing occasionally
to eat any nascent kelp that tries to establish itself. These zombie
urchins are effectively killing the chances of kelp recovery.
The loss of bull kelp forests has meant the loss of the ecosystem
services they rendered. California's recreational abalone fishery--the
world's largest, with over 35,000 fishers--was closed in 2018 after
more than 80% of these edible sea snails died for lack of kelp
sustenance. Kelp harvesting and recreational diving have been
clobbered, too. The ecosystem also lost capacity to sequester
carbon--kelp are 20 times more efficient than their terrestrial counterparts--and to temper the destructive power of waves.
Restoring the kelp forests is a priority for marine managers, but it
is a massive challenge. The purple urchins are of little nutritional
interest to most predators or fishermen in their diminished state, yet
they have still been observed spawning. A group of citizen scientists
known as Reef Check has taken to diving to remove the urchins manually
in an effort to create small urchin-free oases where new kelp can
grow. In 2020, they scooped, hauled, and composted 20,000 pounds of
urchins. Some innovative conservationists also have been removing
emaciated urchins to onshore tanks to fatten them up for humans to eat.
The dire kelp situation is an expression of catastrophic tipping
points and ecosystem shifts that climate change can bring. The
collapse of Northern California's kelp forests was quick and nearly
total. Meanwhile, marine heatwaves are increasing in intensity and
frequency, making the long-term recovery of kelp forests uncertain.
Yet there are some hopeful signs. Closer to Alaska, sunflower sea
stars are starting to recover. Near Monterey Bay, urchin-eating sea
otters have been able to protect local kelp forests. And in spring
2021, Reef Check reported new bull kelp growing at one of the
surviving patches off the Mendocino coast.
Freely available satellite data can provide insights about the
environmental drivers influencing kelp productivity, potentially
helping managers time their restoration efforts for years when
conditions will best support kelp growth, McPherson
explained. "Landsat has allowed managers to observe regional trends in
kelp canopy area and biomass across more than 30 years," she
said. "This is very valuable."
Landsat 5 -- TM
Landsat 8 -- OLI
* McPherson, M. L., (2021) Large-scale shift in the structure of a
kelp forest ecosystem co-occurs with an epizootic and marine
heatwave. Communications Biology, 4 (1), 298.
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