• collapse of key kelp forests -- NASA Earth Observatory

    From MrPostingRobot@kymhorsell.com@21:1/5 to All on Wed Jun 9 09:43:53 2021
    XPost: alt.global-warming


    Monitoring the Collapse of Kelp Forests

    Changing climate and a marine epidemic have combined to decimate
    one of Northern California's most productive ecosystems.

    Laura Rocchio, Landsat Communication and Public Engagement Team,
    with Mike Carlowicz.
    4 Jun 2021
    NASA Earth Observatory

    August 31, 2008 - August 14, 2019 <https://eoimages.gsfc.nasa.gov/images/imagerecords/148000/148391/kelpdet_oli_2019226_lrg.jpg>

    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
    from space.

    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

    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 2.5C (4F) 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 Ni&ntildeo" of 2015-2016.

    July 1 - 31, 2015

    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 forests.

    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

    References & Resources

    * Inside Climate News (2021, March 16) In the Pacific, Global Warming
    Disrupted the Ecological Dance of Urchins, Sea Stars and
    Kelp. Otters Help Restore Balance. Accessed May 4, 2021.

    * 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.

    * Rogers-Bennett, L., and C. A. Catton (2019) Marine heat wave and
    multiple stressors tip bull kelp forest to sea urchin
    barrens. Scientific Reports 9 (1), 15050.

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