Ocean

Some Corals Have Started Adapting to Warmer Temperatures

This new study reveals evidence that some corals are adapting to warming ocean waters — offering great hope following recent reports of coral die-offs due to rising temperatures.

Researchers observed how reefs in two Kenyan marine national parks responded to extreme temperature exposure over time. They found that 11 of the 21 coral species that they studied showed less of the destructive coral bleaching than others.

Looking at two similarly severe warming events in 1998 and 2016, Wildlife Conservation Society zoologist Tim McClanahan found that the number of pale and bleached coral colonies declined from 73% to 27%, and 96% to 60%, in the two parks with different background temperatures. About half of the most common species did not bleach strongly in 2016.

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Sea creatures’ sticky ‘mucus houses’ catch ocean carbon really fast

larvacean
MUCUS HOUSE The pale inner house (rounded flank in foreground) as well as a big, stickier outer envelope of a larvacean’s shelter could be important in ocean carbon cycles.

Never underestimate the value of a disposable mucus house.

Filmy, see-through envelopes of mucus, called “houses,” get discarded daily by the largest of the sea creatures that exude them. The old houses, often more than a meter across, sink toward the ocean bottom carrying with them plankton and other biological tidbits snagged in their goo.

Now, scientists have finally caught the biggest of these soft and fragile houses in action, filtering particles out of seawater for the animal to eat. The observations, courtesy of a new deepwater laser-and-camera system, could start to clarify a missing piece of biological roles in sequestering carbon in the deep ocean, researchers say May 3 in Science Advances.

The houses come from sea animals called larvaceans, not exactly a household name. Their bodies are diaphanous commas afloat in the oceans: a blob of a head attached to a long tail that swishes water through its house. From millimeter-scale dots in surface waters to relative giants in the depths, larvaceans have jellyfish-translucent bodies but a cordlike structure (called a notochord) reminiscent of very ancient ancestors of vertebrates. “They’re more closely related to us than to jellyfish,” says bioengineer Kakani Katija of the Monterey Bay Aquarium Research Institute in Moss Landing, Calif.

The giants among larvaceans, with bodies in the size range of candy bars, don’t form their larger, enveloping houses when brought into the lab. So Katija and colleagues took a standard engineering strategy of tracking particle movement to measure flow rates…

Lakes worldwide feel the heat from climate change

Lake Huron
Oceans aren’t the only bodies of water affected by climate change. Lake Huron (shown) and many lakes worldwide could experience ecological, recreational and economic impacts from warming.

HOT WATER

About 40 kilometers off Michigan’s Keweenaw Peninsula, in the waters of Lake Superior, rises the stone lighthouse of Stannard Rock. Since 1882, it has warned sailors in Great Lakes shipping lanes away from a dangerous shoal. But today, Stannard Rock also helps scientists monitor another danger: climate change.

Since 2008, a meteorological station at the lighthouse has been measuring evaporation rates at Lake Superior. And while weather patterns can change from year to year, Lake Superior appears to be behaving in ways that, to scientists, indicate long-term climate change: Water temperatures are rising and evaporation is up, which leads to lower water levels in some seasons. That’s bad news for hydropower plants, navigators, property owners, commercial and recreational fishers and anyone who just enjoys the lake.

When most people think of the physical effects of climate change, they picture melting glaciers, shrinking sea ice or flooded coastal towns (SN: 4/16/16, p. 22). But observations like those at Stannard Rock are vaulting lakes into the vanguard of climate science. Year after year, lakes reflect the long-term changes of their environment in their physics, chemistry and biology. “They’re sentinels,” says John Lenters, a limnologist at the University of Wisconsin–Madison.

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Lake Tanganyika
Lake Tanganyika is a major food source for people throughout eastern Africa. Fish stocks are dwindling as water temperatures in the lake rise.

LAKE LIFE

Globally, observations show that many lakes are heating up — but not all in the same way or with the same ecological consequences. In eastern Africa, Lake Tanganyika is warming relatively slowly, but its fish populations are plummeting, leaving people with less to eat. In the U.S. Upper Midwest, quicker-warming lakes are experiencing shifts in the relative abundance of fish species that support a billion-dollar-plus recreational industry. And at high global latitudes, cold lakes normally covered by ice in the winter are seeing less ice year after year — a change that could affect all parts of the food web, from algae to freshwater seals.

Understanding such changes is crucial for humans to adapt to the changes that are likely to come, limnologists say. Indeed, some northern lakes will probably release more methane into the air as temperatures rise — exacerbating the climate shift that is already under way.

Lake layers

Lakes and ponds cover about 4 percent of the land surface not already covered by glaciers. That may sound like a small fraction, but lakes play a key role in several planetary processes. Lakes cycle carbon between the water’s surface and the atmosphere. They give off heat-trapping gases such as
carbon dioxide and methane, while simultaneously tucking away carbon in decaying layers of organic muck at lake bottoms. They bury nearly half as much carbon as the oceans do.

Yet the world’s more than 100 million lakes are often overlooked in climate simulations. That’s surprising, because lakes are far easier to measure than oceans. Because lakes are relatively small, scientists can go out in boats or set out buoys to survey temperature, salinity and other factors at different depths and in different seasons.

A landmark study published in 2015 aimed to synthesize these in-water measurements with satellite observations for 235 lakes worldwide. In theory, lake warming is a simple process: The hotter the air above a lake, the hotter the waters get. But the picture is far more complicated than that, the international team of researchers found.

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A recent survey of 235 lakes worldwide found that from 1985 to 2009 most warmed (red dots) while several cooled (blue).

C.M. O’REILLY ET AL/GEOPHYS. RES. LETT. 2015

On average, the 235 lakes in the study warmed at a rate of 0.34 degrees Celsius per decade between 1985 and 2009. Some warmed much faster, like Finland’s Lake Lappajärvi, which soared nearly 0.9 degrees each decade. A few even cooled, such as Blue Cypress Lake in Florida. Puzzlingly, there was no clear trend in which lakes warmed and which cooled. The most rapidly warming lakes were scattered across different latitudes and elevations.

Even some that were nearly side by side warmed at different rates from one another — Lake Superior, by far the largest of the Great Lakes, is warming much more rapidly, at a full degree per decade, than others in the chain, although Huron and Michigan are also warming fast.

“Even though lakes are experiencing the same weather, they are responding in different ways,” says Stephanie Hampton, an aquatic biologist at Washington State University in Pullman.

Such variability makes it hard to pin down what to expect in the future. But researchers are starting to explore factors such as lake depth and lake size (intuitively, it’s less teeth-chattering to swim in a small pond in early summer than a big lake).

Depth and size play into stratification, the process through which some lakes separate into layers of different temperatures. Freshwater is densest at 4° C, just above freezing. In spring, using the Great Lakes as an example, the cold surface waters begin to warm; when they reach 4°, they become dense enough to sink. The lake’s waters mix freely and become much the same temperature at all depths.

Some lakes stratify twice a year, separating into
layers of different temperatures. Surface waters become warm enough (in spring) or cool enough (in autumn) to reach 4° Celsius, the temperature at which these waters become dense and sink toward the lake’s bottom, mixing the waters. In summer and winter, the layers separate. Lake Superior is stratifying earlier each year, giving its surface waters more time to heat up in summer, contributing to its long-term warming.

But then, throughout the summer, the upper waters heat up relatively quickly. The lake stops mixing and instead separates into layers, with warm water on top and cold, dense water at the bottom. It stays that way until autumn, when chilly air temperatures cool the surface waters to 4°. The newly dense waters sink again, mixing the lake for the second time of the year.

Lake Superior is warming so quickly because it is stratifying earlier and earlier each year. It used to separate into its summer layers during mid- to late July, on average. But rising air temperatures mean that it is now stratifying about a month earlier — giving the shallow surface layers much more time to get toasty each summer. “If you hit that starting point in June, now you’ve got all summer to warm up that top layer,” Lenters says.

Deep lakes warm very slowly in the spring, and small changes in water temperature at the end of winter can lead to large changes in the timing of summer stratification for these lakes. Superior is about 406 meters deep at its greatest point, so it is particularly vulnerable to such shifts.

In contrast, shallow lakes warm much more quickly in the spring, so the…

The Arctic is a final garbage dump for ocean plastic

Arctic expedition
A recent expedition to the Arctic found that some areas of the seemingly pristine waters, particularly around Greenland and in the Barents Sea, are littered with copious amounts of plastic debris.

HIDDEN HEAP

The Arctic Ocean is a final resting place for plastic debris dumped into the North Atlantic Ocean, new research suggests.

A 2013 circumpolar expedition discovered hundreds of tons of plastic debris, from fishing lines to plastic films, ecologist Andrés Cózar of the University of Cádiz in Spain and colleagues report April 19 in Science Advances. While many areas remain relatively unpolluted, the density of plastic trash in the…

More than one ocean motion determines tsunami size

2011 tsunami visualized
OCEAN MOTION The 2011 tsunami that devastated Japan (forecast shown) was fueled by both the horizontal and vertical motion of the seafloor, new research suggests. Darker colors represent higher waves. Black triangles mark tsunami-measuring buoys.

Earthquake-powered shifts along the seafloor that push water forward, not just up, could help supersize tsunamis.

By combining laboratory experiments, computer simulations and real-world observations, researchers discovered that the horizontal movement of sloped seafloor during an underwater earthquake can give tsunamis a critical boost. Scientists previously assumed that vertical movement alone contributed most of a tsunami’s energy.

More than half of the energy for the unexpectedly large tsunami that devastated Japan in 2011 (SN Online: 6/16/11) originated from the horizontal movement of the seafloor, the researchers estimate. Accounting for this lateral motion could explain why some earthquakes generate large tsunamis while others don’t, the researchers report in a paper to be published in the Journal of Geophysical Research: Oceans.

“For the last 30 years, we’ve been moving in the wrong direction to do a good job predicting tsunamis,” says study coauthor Tony Song, an oceanographer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “This new theory will lead to a better predictive approach than we have now.”

The largest tsunamis form following earthquakes that occur along tectonic boundaries where an oceanic plate sinks below a continental plate. That movement isn’t always smooth; sections of the two plates can stick together. As the bottom oceanic plate sinks, it bends the top continental plate downward like a weighed-down diving board. Eventually, the pent-up stress becomes too much and the plates abruptly unstick, causing the overlying plate to snap upward and triggering an earthquake. That upward…

Food for microbes found on Enceladus

Enceladus’ plume
FINDING FOOD A deep dive into Enceladus’ plume, shown here in an artist’s illustration, reveals that the moon harbors molecular hydrogen. On Earth, the gas serves as a food source for some microbes, suggesting life could exist on Enceladus, too.

Saturn’s icy moon Enceladus packs snacks suitable for microbial life.

Data from the Cassini spacecraft show that the vaporous plume shooting out of the moon’s southern pole contains molecular hydrogen. It is probably generated when water in the moon’s subterranean ocean reacts with rock in its core, researchers report in the April 14 Science. Such reactions at hydrothermal vents and in other extreme environments on Earth produce high abundances of hydrogen, which some microbes use for food. There’s enough hydrogen on Enceladus to sustain microbial life, the team suggests.

“We are not saying Enceladus has life, but the discovery does move the moon higher on the list of potentially habitable places in the solar system,” says study coauthor J. Hunter Waite of the Southwest Research Institute in San Antonio.

Enceladus became a good target for finding life beyond Earth when researchers found a global ocean under the moon’s icy exterior and hints of hydrothermal activity (SN: 10/17/15, p. 8; SN: 4/18/15, p. 10)….