For sea turtles, there's no place like magnetic home

This is a loggerhead sea turtle nesting in the Archie Carr National Wildlife Refuge in Melbourne Beach, Florida.

Adult sea turtles find their way back to the beaches where they hatched by seeking out unique magnetic signatures along the coast, according to new evidence reported in the Cell Press journal Current Biology on January 15.

"Sea turtles migrate across thousands of miles of ocean before returning to nest on the same stretch of coastline where they hatched, but how they do this has mystified scientists for more than fifty years," says J. Roger Brothers of the University of North Carolina, Chapel Hill. "Our results provide evidence that turtles imprint on the unique magnetic field of their natal beach as hatchlings and then use this information to return as adults."

While earlier studies have shown that sea turtles use the Earth's magnetic field as a guide while out at sea, it has remained unclear whether adult turtles also depend on magnetic features to recognize and return to the nesting sites chosen by their mothers before them, the researchers explain.

Several years ago, UNC's Kenneth Lohmann, the co-author of the new study, proposed that animals including sea turtles and salmon might imprint on magnetic fields early in life, but that idea has proven difficult to test in the open ocean. In the new study, Brothers and Lohmann took a different approach by studying changes in the behavior of nesting turtles over time.

"We reasoned that if turtles use the magnetic field to find their natal beaches, then naturally occurring changes in the Earth's field might influence where turtles nest," Brothers says.

To investigate, the researchers analyzed a 19-year database of loggerhead nesting along the eastern coast of Florida, the largest sea turtle rookery in North America. They found a strong association between the spatial distribution of turtle nests and subtle shifts in the Earth's magnetic field.

In some times and places, the Earth's field shifted so that the magnetic signatures of adjacent locations along the beach moved closer together. When that happened, nesting turtles packed themselves in along a shorter stretch of coastline, just as the researchers had predicted. In places where magnetic signatures diverged, sea turtles spread out and laid their eggs in nests that were fewer and farther between.

Brothers says that little is known about how turtles detect the geomagnetic field. Most likely, tiny magnetic particles in the turtles' brains respond to the Earth's field and provide the basis for the magnetic sense, but no one knows for sure.

Sea turtles likely go to great lengths to find the places where they began life because successful nesting requires a combination of environmental features that are rare: soft sand, the right temperature, few predators, and an easily accessible beach.

"The only way a female turtle can be sure that she is nesting in a place favorable for egg development is to nest on the same beach where she hatched," Brothers says. "The logic of sea turtles seems to be that 'if it worked for me, it should work for my offspring.'"

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Coral reefs threatened by a deadly combination of changing ocean conditions

To study corals in the South China Sea, Woods Hole Oceanographic Institution scientists set up a makeshift floating lab, strapping wooden planks across plastic tubes.

The lowering of the ocean's pH is making it harder for corals to grow their skeletons and easier for bioeroding organisms to tear them down. Erosion rates increase tenfold in areas where corals are also exposed to high levels of nutrients, according to a study published January 2015 in the journal Geology. As sea level rises, these reefs may have a harder time growing toward the ocean surface, where they get sunlight they need to survive.

The study, led by scientists at Woods Hole Oceanographic Institution (WHOI), highlights the multiple threats to coral reef ecosystems, which provide critical buffers to shoreline erosion, sustain fisheries that feed hundreds of millions of people, and harbor 25 percent of all marine species. And it points to a key management strategy that could slow reef decline: reducing the input of nutrient pollution to the coastal ocean from human activity such as runoff from sewers, septic tanks, roads, and fertilizers.

Corals make their skeletons out of calcium and carbonate ions from seawater, constructing massive colonies as large as cars and small houses. As the ocean absorbs excess carbon dioxide from fossil-fuel burning, it spurs chemical reactions that lower the pH of seawater, a process known as ocean acidification. The process removes carbonate ions, making them less available for corals to build skeletons.

"A healthy coral reef ecosystem exists in a constant and often overlooked tug-of-war. As corals build their skeletons up toward the sea surface, other organisms--mollusks, worms, and sponges--bore into and erode the skeletons to create shelters," said lead author Thomas DeCarlo, a graduate student in the WHOI-MIT Joint Program in Oceanography, working in Anne Cohen's lab at WHOI.

This process, called bioerosion, reduces skeletons to rubble, which is transported offshore during fierce storms or gradually dissolved in the sediments. On healthy reefs today, calcium carbonate production barely exceeds the loss by erosion, dissolution, and offshore transport. As a result of this delicate balance, coral reefs grow very slowly, if at all, when sea level is stable.

The new study shows that additional nutrients provide a dramatic boost for bioeroders that, combined with lower pH conditions, will tip this balance in favor of erosion. The bioeroders are filter feeders, sifting particles of food out of seawater. Nutrients spur the growth of plankton, supplying food for large populations of bioeroders that burrow into coral skeletons.

When corals and bioeroders are in balance, the former grow just fast enough to stay near the sea surface, while the latter are busily sculpting the coral skeletons into an intricate, three-dimensional habitat full of nooks and hiding places for fish, urchins, and other marine life.

In waters with fewer carbonate ions and more nutrients, corals may not be able to build new skeleton fast enough to keep pace with bioeroders cutting down the reef. The result would be "flatter" coral reefs with less of the three-dimensional structure responsible for the rich biodiversity found on coral reefs.

To conduct the study, the research team investigated coral reefs spanning the Pacific Ocean, from the west coast of Panama to Palau. The reefs also spanned a range of different naturally occurring pH and nutrient conditions in the ocean, including several reefs in seawater with pH levels today that are as low as those expected for much of the tropical ocean by 2100. That allowed the scientists to examine how bioeroders are affected by the isolated and combined influences of pH and nutrient conditions.

The scientists used underwater drills to collect cores of coral skeletons. They put the cores through the CAT scanner at the Computerized Scanning and Imaging Facility at WHOI to get 3-D images of tunnels and borings made by bioeroders with a resolution of about the width of a human hair. That allowed them to calculate precisely how much skeleton the bioeroders had removed.

The researchers found that relatively acidic (lower-pH) reefs were more heavily bio-eroded than their higher-pH counterparts. But their most striking finding was that in waters with a combination of high nutrient levels and lower-pH, bio-erosion is ten times higher than in lower-pH waters without high nutrient levels.

"The ocean will certainly absorb more CO₂ over the next century, and ocean acidification is a global phenomenon that reefs cannot escape," DeCarlo said. "But the encouraging news in our findings is that people can take action to protect their local reefs. If people can limit runoff from septic tanks, sewers, roads, farm fertilizers, and others sources of nutrient pollution to the coastal ocean, the bioeroders will not have such an upper hand, and the balance will tip much more slowly toward erosion and dissolution of coral reefs."

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Distribution of fish on northeast US shelf influenced by both fishing, climate

Summer flounder (Paralichthys dentatus).

Scientists studying the distribution of four commercial and recreational fish stocks in Northeast U.S. waters have found that climate change can have major impacts on the distribution of fish, but the effects of fishing can be just as important and occur on a more immediate time scale.

The four species studied -- black sea bass, scup, summer flounder, and southern New England/Mid-Atlantic Bight winter flounder -- have varied in abundance and have experienced heavy fishing pressure at times over the past 40 years. Scientists examined the distribution of the four species using Northeast Fisheries Science Center (NEFSC) research trawl survey data collected between 1972 and 2008. Generalized additive models were used to determine if the distributions of the four species had changed over time, and if these changes reflect changes in temperature or fishing pressure.

The researchers found that black sea bass, scup, and summer flounder exhibited significant poleward shifts in distribution in at least one season. The shifts in black sea bass and scup were related to temperature, while the shift in summer flounder was related to a decrease in fishing pressure and an expansion of the population age structure. The southern New England/Mid-Atlantic Bight stock of winter flounder showed no change in distribution.

"The study combined a range of resources at the Center, long-term oceanographic data and trawl survey data," said Richard Bell, a National Research Council research associate working at the NEFSC's Narragansett Laboratory in Rhode Island and lead author of the study. "Using these data, we demonstrated how a combination of fishing and climate can influence the distribution of marine fish. It is not one or the other."

Increasing ocean temperatures have significantly affected marine life, inducing shifts in distribution and changes in abundance. Climate change alters the distribution of suitable habitats, forcing organisms to move to a more favorable area of their range or attempt to survive under less than ideal conditions. Fishing reduces the abundance of marine populations and truncates their size and age structure, which can lead to range contractions or shifts.

Fishing typically removes the larger fish from a population. Larger, older summer flounder are typically found further north, and as exploitation reduced the numbers of summer flounder in the 1980s and 1990s, larger fish were preferentially harvested by the fishery. The remaining summer flounder population, dominated by smaller fish, subsequently became centered further south. The northward shift of the stock in recent decades was linked to an increase in the number of larger, older fish as the population has rebuilt.

"The fish were not shifting northward with warmer conditions, but simply re-colonizing their former habitat areas," said Bell.

Northerly shifts in scup and black sea bass are linked to increases in temperature and are more tied to climate than fishing.

The study suggests multiple factors specific to individual species need to be considered when developing management regulations for living marine resources. The management of each of the four species analyzed in this study is based on spatial allocations, and shifts in stock distributions can cause a mismatch between the distribution of fish and the catch allocations for different regions and states.

Findings from the study were published online in the ICES Journal of Marine Science.

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